Singlet triplet absorption spectra of charge transfer complexes of naphthalene
by Raymond Graham
A thesis submitted to the Graduate Faculty in partial fulfillment of the requirements for the degree of
DOCTOR OF PHILOSOPHY in Chemistry
Montana State University
© Copyright by Raymond Graham (1970)
Abstract:
The singlet-triplet spectra of charge-transfer complexes with arsenic trichloride, antimony trichloride,
bismuth trichloride, mercuric acetate, mercuric chloride, and mercuric bromide were examined. The
intensities of these enhanced singlet-triplet bands were found to depend on spin-orbit coupling in the
metal ion of the inorganic salt and the strength of the charge-transfer complex. The order of
effectiveness of these salts as heavy-atom perturbers was found to be: bismuth trichloride > mercuric
bromide > mercuric chloride > antimony trichloride > mercuric acetate > arsenic trichloride.
The singlet-triplet spectra of phenylmercurie acetate, diphenyImercury, and the mercuric
chloride-benzene complex were also studied. The heavy-atom effect was found to be greater in the
covalently bonded mercury compounds. SINGLET T R IPL E T ABSORPTION SPECTRA OF CHARGE TRANSFER
COMPLEXES OF NAPHTHALENE
by
RAYMOND GRAHAM
A thesis, subm itted to the G raduate F aculty in p a rtia l
fulfillm ent of the re q u ire m e n ts fo r the deg ree
of
DOCTOR OF PHILOSOPHY
in
C h em istry
Approved:
Head, M ajor D epartm ent
C hairm an, Exam ining C om m ittee
G raduate Dean
^
MONTANA STATE UNIVERSITY
B ozem an, M ontana
M arch, 1970
ACKNOWLEDGMENT
I w ish to e x p re ss m y thanks to. M ontana State U n iv ersity and the
D epartm ent of H ealth, Education and W elfare fo r th e ir fin an cial support
during my long g raduate c a r e e r.
. To the faculty and g radu ate students who helped m e, e sp ecially to
D r. R eed A. Howald, I w ish to e x p re ss m y appreciation.
TA B LE OF CONTENTS
Page
V ita ...................
ii
A ck n o w led g m en t.....................
iii
T able of C ontents................................................................................... ..................... '
iv
L is t of F ig u r e s ..................................................................................................
vi
L is t of T a b l e s .................................................................................................................
L ist of S p ectra . ..........................................................
vii
viii
A b stra ct
Introduction. ............................................ ... . . .......................................................
I.
H.
III.
N ature of P ro b lem
. .........................................................................
S in g let-T rip let T r a n s itio n s ..............................
3
The E ffect of Spin-O rbit Coupling on S in g let-T rip let
T r a n s i t i o n s ...........................
IV.
I
6
A ssignm ent of T rip le t Energy L evels in A rom atic
Compounds...............................................................................................
12
V.
H eavy-A tom Enhancem ent Techniques ........................................
14
VI.
C h arg e-T ran sfer, C o m p le x e s ............................................................
16
E xperim ental
I.
n.
Low T e m p e ra tu re S tu d ies..................................................
25
. S tru ctu re of N aphthalene and N aphthalene Com plexes . . . .
28
III.
S p ectra of Benzene and B enzene Com plexes / . . ...........................34
IV.
S p ectra of D iphenylm ercury and P h e n y lm e rc u ric A cetate
V.
In te rp re ta tio n of S pectra ...................................... ...................... . ., . 36
D iscu ssio n
I.
II.
Low T e m p e ra tu re S tu d ies..........................................
43
D iscu ssio n of S p e c tr a ...................
45
IE .
D iscu ssio n of N aphthalene C o m p le x e s ................................................. 51
IV.
The N ature of C h a rg e -T ra n sfe r In te ra c tio n s .......................................54
V.
S in g let-T rip let E n h a n c e m e n t.................................................................. 57
Appendix A
C alculation of Spin-O rbit C o n s ta n ts ...............................................................63
Appendix B
S in g let-T rip let Spectra. .................................................................................... 66
L ite ra tu re C ited ............................................................................................................
78
LIST OF FIGURES
F igure
No.
1.
T itle
Page
Naphthalene Bond D istances fo r the Antimony
T r i chloride -N aphthalene Com plex . . ........................................
2.
Schematic- R ep resen tatio n of the E lectro n ic T ran sitio n s
in a C h a rg e -T ra n sfe r C o m p lex .................. ....................................
3.
20.
22
Schem atic D iagram of the A pparatus U sed in the Study of
the Effect of M ercu ry on the S p ectra of A rom atic Hydro­
carbons .....................................................................................................
27
4.
C alculation of Enhancem ent F a c to rs ( I ) ........................................
39
5.
C alculation of Enhancem ent F a c to rs ( 2 ) ........................................
40
6.
. M olecular O rbital D iagram fo r B enzene, P h e n y lm e rc u ric
A cetate, and Dipheny!m e rc u ry ....................... -...............................
7.
50
The M anner in Which S in g let-T rip let T ran sitio n s May
Steal Intensity fro m a C h a rg e -T ra n sfe r C o m p le x ......................
61
LIST OF TA BLES
Table No.
I.
T itle
Page
E ffect of D egassing on 0.1 M olal M ercu ric C hloride
in N aphthalene
S i­
ll.
Enhancem ent F a c to rs .........................................................................
m.
O sc illato r Strengths fo r S in g let-T rip let T ran sitio n s . . . .
IV.
P o sitio n s of Enhanced S in g let-T rip let Bands of B enzene.
V.
Bond Lengths and Bond O rd e rs fo r N aphthalene and
38
42
.
48
Com plexed N a p h th a le n e .......................................................... ... .
55
VI.
C om parison of H eavy-A tom P e r t u r b e r s ............. ...................
57
VII.
T abulation of Spin O rbit Coupling C onstants and
M ultiplet Spapings ,
............................. ................................ . . .
65
LIST OF SPEC TR A
Spectrum
No.
T itle
Page
1.
N aphthalene P lu s Oxygen and D egassed N aphthalene . . . .
67
2.
0.15 M olal M ercu ric C hloride in N aphthalene...................... . .
68
3.
0.15 M olal M ercu ric B rom ide in N aphthalene.................
4.
0.10. M olal M ercu ric A cetate in N a p h th a le n e ................... 70
5.
0.50. M olal Antimony T ric h lo rid e in Naphthalene . . . . . . .
6.
3.25 M olal A rsen ic T ric h lo rid e in N ap h th alen e.............
7.
0.01 M olal B ism uth T ric h lo rid e in N aphthalene................73
8.
0.0023 M olal Stannic C hloride in N aphthalene....................74
9.
0.02 M olar M ercu ric C hloride in B e n z e n e .....................
10.
0.10. M olar P h e n y lm e rc u ric A cetate in G lacial A cetic
11.
69
71
72
75
A c i d ......................................................................................................
76
0.05 M o lar D ip h e n y lm e rc u ry in C h lo ro fo rm
77
ABSTRACT
The s in g le t-trip le t s p e c tra of c h a rg e -tra n s fe r com plexes w ith a rse n ic
tric h lo rid e , antim ony tric h lo rid e , bism uth tric h lo rid e , m e rc u ric acetate,
m e rc u ric ch lo rid e, and m e rc u ric bro m id e w ere exam ined. The in te n sitie s
of th e se enhanced s in g le t-trip le t bands w ere found to depend on sp in -o rb it
coupling in the m etal ion of the in o rg an ic s a lt and the stre n g th of the c h arg etra n s fe r com plex. . The o rd e r of effectiv en ess of th e se s a lts as heavy-atom
p e rtu rb e rs was found to be: bism uth tric h lo rid e > m e rc u ric brom ide >
m e rc u ric chloride > antim ony tric h lo rid e > m e rc u ric a cetate > a rse n ic t r i ­
chloride.
The s in g le t-trip le t s p e c tra of p h en y lm ercu rie a ce ta te, dipheny!m e r­
cury, and the m e rc u ric ch lorid e-b en zen e com plex w ere a lso studied. The
heavy-atom effect was found to be g re a te r in the covalently bonded m e rc u ry
compounds.
INTRODUCTION
I.
NATURE OF PROBLEM
T h e re a re many conflicting arg u m en ts proposed concerning the location
and assignm ent of trip le t energy lev els in aro m a tic com pounds.
The p rin c ip a l
m ethods of exam ining th e se tr ip le t s ta te s a re e ith e r by phosphorescence o r
s in g le t-trip le t ab sorption m e a su re m e n ts.
Since s in g le t-trip le t tra n sitio n s a re
highly forbidden (o scillato r stre n g th s of le s s than ca. IO- 5 ), e ith e r long p a th lengths o r in ten sity enhancem ent techniques m u st be u sed fo r s p e c tra l a b so rp ­
tion.
In long pathlengths of a ro m a tic compounds im p u rity ab so rp tio n s and hot
band s tru c tu re fro m nearby inten se sin g le t-sin g le t tra n s itio n s becom e dom inant.
Hot bands a re v ib ratio n ally allowed tra n s itio n s fro m th e rm a lly populated h ig h er
v ib ratio n al s ta te s .
S in g le t-trip le t enhancem ent techniques a re c la ssifie d as e ith e r heavy- .
atom o r p aram ag n etic p e rtu rb a tio n s.
Two d ifferen t m ech an ism s a re proposed
fo r th e se enhancem ents.
One is an exchange m echanism and.the o th er is of a
c h a rg e -tra n s fe r n a tu re .
It is not c le a r if one of th e se m ech an ism s w ill have
g re a te r im p o rtan ce than the o th e r.
In e ith e r c ase , s p in -o rb it coupling seem s
to be th e m a jo r fa c to r affecting th e se enhancem ents.
The n a tu re of th is coupling,
fo r a m any e le c tro n sy ste m , is not fully e stab lish ed , and a p re c is e sp in -o rb it
in te ra c tio n o p e ra to r is not known.
Since th e re is som e doubt about the n a tu re of s in g le t-trip le t in ten sity
2
enhancem ents, it w as decided to exam ine the effect of c e rta in heavy atom s
(either in m olecu les o r in the atom ic state) on the in ten sity of th e se tra n sitio n s
in c e rta in a ro m a tic h y d rocarbo n s.
This r e s e a r c h o riginally co n sisted of ex ­
am ining the effect of atom ic m e rc u ry on th e s in g le t-trip le t tra n s itio n s of
benzene at low te m p e ra tu re s .
F o r v ario u s re a so n s, to be d iscu ssed la te r, no
s in g le t-trip le t tra n s itio n s w e re o b serv ed by th is m ethod.
Since naphthalene
and benzene fo rm c h a rg e -tra n s fe r com plexes w ith som e s a lts of Group 2b, 4a,
and 5a elem en ts, the effects of th e se s a lts on the s in g le t-trip le t tra n sitio n s of
benzene and naphthalene w e re studied.
A lso the s p e c tra of diphenyl m ercu ry
and pheny!m e rc u ric a cetate w e re exam ined in o rd e r to co m p are the enhance­
m ent effects of a covalently bonded heavy atom to one th a t w as weakly bonded
(m ercu ric ch lo rid e-b en zen e com plex).
II.
SIN G LET-TR IPLET TRANSITIONS
The in ten sity of a s p e c tra l line is p ro p o rtio n al to the tra n s itio n m om ent
in te g ra l Mlk (4, 38),
Mi k =
( h i® I *k )
.
w here A and ^ a re the. wave functions fo r the final and in itia l s ta te s , re s p e c ­
tively, and "o'is the o p e ra to r re p re se n tin g the m echanism (e le ctric o r m agnetic
dipole o r quadrupole, fo r instance) re sp o n sib le fo r the change of state of the
m olecule.
Selection ru le s a re p red ictio n s concerning the value of th is in te g ra l.
T hese selectio n ru le s d e term in e the conditions fo r which th is tra n s itio n m om ­
ent in te g ra l w ill have e ith e r z ero o r nohezero values.
One of the well-known
sele c tio n ru le s of atom ic and m o lecu lar sp ectro sco p y is AS = 0 ; that is , only
tra n s itio n s betw een s ta te s w ith the sam e m u ltip licity a re allow ed.
This s e le c ­
tion ru le , called the "prohibition of in te r com binations11, only applies fo r
R u ssell-S au n d ers coupling, w here S is a w ell defined quantum num ber.
The sele c tio n ru le , AS = O, m ay be d eriv ed in the following m anner.
If s p in -o rb it coupling is ignored, the wave function fo r an elec tro n ic state may
be w ritte n as a product of an o rb ital and a spin function
♦es = y
w here f
is the o rb ita l p a r t and /3 is the spin p a rt of the wave function.
The
tra n s itio n m om ent in te g ra l fo r a tra n s itio n betw een s ta te s I and k m ay be
w ritte n as
4
( W
d l W ) = (*e, * |d l*e,k> < W >
w here M is the dipole m om ent v e cto r.
Since the spin functions a re orthogonal,
the second in te g ra l on the rig h t van ish es fo r s ta te s w ith d ifferen t spin.
tra n s itio n s betw een s ta te s w ith d ifferen t m u ltip licities a re forbidden.
sectio n ru le only applies if s p in -o rb it coupling is ignored.
Thus
This
When sp in -o rb it
coupling is c o n sid ered , the wave function m ay no longer be w ritte n as a sim ple
product of an o rb ita l and a spin function.
T his m eans th at the above equation
no longer holds and the tra n s itio n m om ent (M ^ ) m ay be n o n -z e ro fo r spin
forbidden t r ans itio n s .
In atom ic sp ectro sco p y , th e re a re two coupling sch em es u sed to explain
the in te ra c tio n of o rb ita l angular m om entum w ith spin angular m om entum .
R u sse ll-S a u n d e rs coupling applies fo r lig h t atom s, while th e second schem e,
(j, j) coupling, applies fo r the h e av ie r ato m s.
The R u sse ll-S a u n d e rs schem e
a ssu m e s th a t the in te ra c tio n betw een Ij (orbital angular m om entum ) and si
(spin angular m om entum ) a re so stro n g betw een the individual Ii te rm s and
betw een the vario u s Si te rm s th a t they com bine to give re s u lta n t L and S (Zli =
L and E s i = S).
m om entum ).
The L and S then com bine to give the re s u lta n t J (total angular
This coupling schem e holds fa irly w ell fo r second row atom s
(sp in -o rb it coupling energy is very sm all) but th e validity of th e R u sse llSaunders coupling schem e d e te rio ra te s rap id ly w ith in c re a sin g atom ic num ber.
The (j, j) coupling schem e a ssu m e s th a t th e re is co n sid erab le in te ra c tio n
5
betw een each individual Ij and s i.
T hese com bine to give a re s u lta n t jj.
ji then com bine to give the to ta l angular m om entum (J) of th e atom .
The
P u re (j, j)
coupling seldom o c cu rs and m o st atom s a re in te rm e d ia te c a s e s of th ese two
coupling, sch em es.
It can be seen fro m the above d iscu ssio n th at L and S a re "good" quan­
tum. n um bers only when the R uss e l l - Saunders coupling schem e is valid.
For
light e lem en ts, the validity of th e se quantum num bers is not se rio u sly im p aired .
On the o th er hand, fo r the elem en ts of row s V and VI of the p erio d ic table (for
elem en ts like cadm ium , tin , antim ony, m e rc u ry , lead, and bism uth), L and S
a re not good quantum n u m b ers.
energy is quite la rg e .
differen t m u ltip lic itie s.
In th e se elem ents., the s p in -o rb it coupling
T his h as the effect of scram b lin g to g e th e r s ta te s of
B ecause of th is, "sp in forbidden" tra n s itio n s becom e
allowed in th e se heavy atom s.
in.
THE E F F E C T OF SPIN-ORBIT COUPLING ON SING LET-TRIPLET
TRANSITIONS
The d e riv a tio n of the sele c tio n ru le , ZiS = O, contains a m a jo r approxi­
m ation concerning the wave functions u sed in the evaluation of the tra n sitio n
m om ent (38).
T hese wave functions a re eigenfunctions of th e o n e-electro n
H am iltonian hj,
hj = (h2 / 2m)Y^ - Ze 2 /rj.
The H am iltonian hj is not com plete and in the absence of e x te rn a l fields should
contain te rm s th a t a re due to e le c tro n spin.
In one e le c tro n s p e c tra , the effect
of spin a r is e s fro m an in te ra c tio n betw een the m agnetic m om ent of th e spinning
e le c tro n and the m agnetic field due to its o rb ita l m otion about the nucleus (spino rb it in teractio n ).
F o r m olecules w here re la tiv is tic effects a re sm a ll, th e p a rt of the
H am iltonian th a t depends on e le c tro n sp in s m ay be w ritte n as the sum of two
in te ra c tio n s, |—|' and }—] (38).
|—| and j—j a re the sp in -o rb it and the sp in -sp in
p a rts of the H am iltonian, re sp ec tiv e ly .
and |—|
The c o rre c te d H am iltonian now beco m es
a re m uch s m a lle r th an |—jQ , the wave functions of H
m aY
be obtained fro m the eigenfunctions of |—Jq by m eans of p e rtu rb a tio n theory.
The n atu re of the s p in -o rb it p a rt of the H am iltonian o p e ra to r m ay be
obtained fro m c la s s ic a l s p in -o rb it th eo ry .
The m agnetic m om ent of a spinning
7
e le c tro n is re la te d to its angular momentum, by (9)
JU-= - ( |e |/ m c ) s
The in te ra c tio n energy betw een th is m agnetic dipole m om ent and th e m agnetic
field due to the o rb ita l m otion of an e le c tro n w ith velocity v and in the e le c tric
field (E) of the nucleus is c la s s ic a lly given by
H 1 = TjU- (E xv)
o r H 1 = + (|e | / 2 m c 2 ) ( E x v ) 's
w here the fa c to r 1/2 is included fo r re la tiv is tic p u rp o ses.
Since the m om entum
p is equal to m v and E is the g ra d ie n t of the s c a le r potential. V of. the e le c tro n
in the n u c lea r field, the s p in -o rb it H am iltonian m ay be w ritte n as
H' = (gradv x p ) / (2m 2 c2)
F o r a g e n e ra l m any e le c tro n potential field V = V(r^), th is sp in -o rb it H am iltoni­
an becom es
H' = ( I ^ m 2C2)E(IZri) (3V(ri) LiSi - E f (H )L i ^ i
w here the sum is over a ll the e le c tro n s.
The above d eriv atio n does not give the com plete s p in -o rb it H am iltonian.
The to ta l s p in -o rb it H am iltonian is given by (56)
Mi
ri M
i,j
ri j
U
ri j
w here a is the fine s tru c tu re constant (a = e 2 /h c ), P is the lin e a r m om entum
th
o p e ra to r, Zp is the effective n u c lea r ch arg e of the /l— n ucleus.
This f ir s t
te rm in the b ra c k e t is the previo u sly d eriv ed p a rtia l sp in -o rb it. H am iltonian and
8
re p re s e n ts the coupling of the spin and the o rb ital m om entum of an e lec tro n
in the p re se n c e of an a ttra c tiv e n u c le a r field Z^.
The coupling of the spin and
the o rb ita l m otions of e le c tro n i w ith the rep u lsiv e field of e le c tro n j is r e p r e ­
sented by the second te rm .
The th ird te rm re p re s e n ts the in te ra ctio n betw een
the spin of one e le c tro n and the spin of another e lec tro n and is usually very
sm all.
The im p o rtan ce of the second te rm d e c re a se s w ith in c re a sin g n u c le a r
ch arg e.
H ere, it w ill be assum ed th at the p a rtia l s p in -o rb it H am iltonian (Ht)
is the controlling fa c to r in the sp in -o rb it in te ra c tio n and the o th e r te rm s in th e
s p in -o rb it H am iltonian w ill be ignored.
F o r an atom ic sy ste m , the effect of sp in -o rb it coupling m ay be in v e sti­
gated w ith f ir s t- o r d e r p e rtu rb a tio n th e o ry .
The approxim ate sp in -o rb it coupling
energy m ay be obtained by evaluating the m a trix elem en ts of H' w ith the eig en ­
functions of |—|q (the H am iltonian w ithout sp in -o rb it in teractio n ) (9, 70). The
o n e -e le c tro n eigenfunctions of f^ |0 m ay be w ritte n in te rm s of the quantum
num bers n, mq, and m s ;
<p (n, I, m i, m s ) = R(n, I) F (I, m i, m s )
w here R(n, I) and F (I, m l, m s ) a re the ra d ia l and angular p a rts of the onee le c tro h eigenfunction, re sp e c tiv e ly .
The m a trix elem en ts of H' may then be
w ritte n as
(<p(n, I, m p m s ) | 'f (r)L . S | <p (n', I ', m ^, m ' ^
(R (n, 1)| j (r)|R (n ', I ') ) ( f (1, m i, m s ) | L. S |F (1 ', rn 'i, m 'g))
9
The L. S m a trix is equal to zero un less 1 = 1 ' and
+ m s = rn 'i + m 's . In the
case of a Coulomb p o ten tial, V (r) = - Z e 2/ r , the m a trix elem en t ^ (r) m ay be
evaluated in te rm s of the s p in -o rb it coupling constant
-f
J nl
-
477-2 ^
^2
^ nl
e 2 Z4_________
2 m 2 c 2 aa n 3 l(l + I) (I + 5 )
The re la tiv e im p o rtan ce of s p in -o rb it coupling in an atom is given by the m ag ­
nitude of the sp in -o rb it coupling constant.
The im p o rtan t point to notice h e re
is the stro n g dependence of 'S Qi on the atom ic num ber (Z) (heavy-atom effect).
The effect of s p in -o rb it in te ra c tio n of the wave functions of a polyatom ic
m olecule is to m ix functions w hich have d ifferen t m u ltip lic itie s (29, 56). Thus
the sin g let s ta te s of a m olecule m ay have a sm a ll amount of tr ip le t c h a ra c te r
and the tr ip le t s ta te s a sm all am ount of sin g let c h a ra c te r.
A ccording to the
f i r s t o rd e r p e rtu rb a tio n th eo ry , the sin g let ground state wave function fo r a
polyatom ic m olecule m ay be w ritte n as
w here
.
N
_
_
( 1 Itn0 I H ' I 3 i>j°’)
, I1Eo - IjEjI
H ere the sum m ation is c a r rie d out o v er th e trip le t m anifold, and 60j is the
m ixing coefficient betw een H 00 and ^ f j0. S im ilarly , the low est trip le t state
w ave function is given by
' t l - 3 t l ° * ' 2 6l k "Sk 0
k
10
w here the sum m ation is c a r rie d out over a ll the sta te s in th e sin g let m anifold.
The p ro b ab ility of a ra d ia tiv e tra n s itio n betw een
I
\|r and
3
ijr is p ro ­
p o rtio n al to the sq u are of the tra n s itio n m om ent M(Sqj T j ).
w here M is the dipole m om ent o p e ra to r.
The f ir s t and second te rm s in the
expanded e x p re ssio n vanish b ecau se of sp in orthogonality.
The tra n sitio n
m om ent now becom es
The la s t two te rm s in th is e x p re ssio n depend on the ground sta te and
the excited s ta te dipole m om ents and v an ish fo r nonpolar m o lecu les (naphtha­
lene and benzene). T hus fo r nonpolar m o lecu les th e re a re two so u rces of
in ten sity fo r s in g le t-trip le t tra n s itio n s .
The f ir s t so u rce of in ten sity is
c h ara c te ristiz e d , by a s p in -o rb it in te ra c tio n betw een the f i r s t excited trip le t
s ta te and those excited sin g let sta te s to w hich tra n s itio n s fro m the ground s ta te
a re allow ed.
The in ten sity is "sto len " fro m allowed sin g le t-sin g le t tra n sitio n s
w ith tra n s itio n m om ent M(Sq) S .). The second so u rce of in ten sity is the p ro c e s s
in w hich th e re is a s p in -o rb it in te ra c tio n betw een the ground s ta te and those
excited trip le ts in which tra n s itio n s fro m the low est tr ip le t a re allowed.
T his
11
p ro c e s s ste a ls in ten sity fro m the tr ip le t- tr ip le t tra n sitio n s w ith tra n sitio n s
m om ents M (T^, T ).
T hese two so u rc e s of in ten sity fo r s in g le t-trip le t tra n s itio n s depend
on two fa c to rs : the in te n sitie s of n e a r-b y sin g le t-sin g le t and tr ip le t- trip le t
tra n s itio n s and the am ount of s p in -o rb it coupling p re s e n t in the m olecule.
Since th is s p in -o rb it in te ra c tio n in c re a s e s w ith atom ic n u m ber, the p re sen c e
of a heavy atom , e ith e r in te rn a l o r e x te rn a l, should in c re a s e the probability
of. a s in g le t-trip le t tra n s itio n in a polyatom ic m o lecule.
heavy-atom effect.
T his gives r is e to the
IV. ASSIGNMENT OF TR IPLE T ENERGY LEVELS IN AROMATIC
COMPOUNDS
Even with s p in -o rb it interaction?jconsidered, the tra n s itio n p ro b a b ilities
of s in g le t-trip le t tra n s itio n s in m o s t a ro m a tic compounds a re g en erally sm all.
B ecause of th is , the identification of trip le t sta te s fo r m o st aro m atic com ­
pounds h as been difficult.
K earns (52) review ed th is p ro b lem fo r catacondensed
aro m a tic hydrocarbons, and P la tt (76) gives arc ritic a l review of the p a rtic u la r
p ro b lem of locating the second trip le t of benzene.
Both P la tt and K earns lis t
the p o ssib le c r ite r ia fo r determ ining the assig n m en ts of trip le ts sta te s of
aro m a tic compounds.
stro n g .
They divide th e se c r ite r ia into two c a te g o rie s : weak and
W eak c r ite r ia include ag reem en t of the position and in ten sity of the
o bserved band w ith the calculated location and intensity.
Strong c r ite r ia in ­
clude the M cC lure (57-59) enhancem ent of s in g le t-trip le ts by heavy-atom o r
p aram ag n etic su b stitu en ts and K asha enhancem ent (50) of s in g le t-trip le t tr a n s i­
tions by heavy-atom o r p aram ag n etic solvent m olecules.
P la tt (76) has review ed the evidence fo r the assig n m en t of the low est two
trip le t sta te s of benzene.
m ent of the low est as
p o ssib le am biguity).
3
B^
He concludes th a t th e re is little doubt in the a ss ig n (although H o c h stra sse r (42) has re c en tly indicated a
Although th e re is som e doubt about the assignm ent of the
second trip le t, P la tt in d icates th at it is v ery probably the
m entally the
3
3
E^ state.
E x p e ri-
B^ s ta te has. been identified w ith observed bands at ca. 31kK.
P la tt points out th a t the proof is unanim ous fo r this; assig n m en t.
The location
13
of the sec o n d .trip let has been m o re difficult.
T h eo retically the second trip le t
is p re d ic ted to lie betw een 33 and 40 kK (I, 36, 52, 72, 76).
R ecently, Colson
and B ern ste in (8 ) have o bserved som e bands at ca. 36.6 kK (both in thick s in ­
gle c ry s ta ls of p u re benzene and in p aram ag n etically enhanced thin film s).
T hese bands have been tentatively assig n ed to the
3
-
I
A.
The low est trip le t of naphthalene is p re d ic ted to be a
tran sitio n .
3
species (49,
62).
This trip le t sta te has been identified w ith the ob serv ed bands at ca. 21.,3
kK.
E xperim entally, th e se bands have.been studied by s e v e ra l m ethods.
K asha
(50) f i r s t observed th e se bands using a heavy-atom enhancem ent technique, and
Evans (24) used a p aram ag n etic enhancem ent technique to lo cate them .
Hanson
and Robinson (40) have m e asu re d the p ositions of th e se s in g le t-trip le t bands in
c ry s ta llin e naphthalene.
In 1966, M arch etti and K earns (65) used the phospho­
re s c e n c e ex citation m ethod to /o b serv e th e se bands.
--------
-
'v
.
.
------- —- - A . i
,
-• - ‘
,-V - .- " ;
V.
HEAVY-ATOM ENHANCEMENT TECHNIQUES
K asha (50) f ir s t noticed th a t when the c o lo rle ss p u re liq u id s, cc-
chloronaphthalene and ethyl iodide w ere m ixed, a yellow co lo r was produced
even though no chem ical re a c tio n o c cu rred .
The explanation w as that the
low est s in g le t-trip le t absorption band of the naphthalene m olecule was g re a tly
enhanced in in tensity in the p re se n c e of ethyl iodide.
That is , according to th e
previous d iscu ssio n , the proxim ity of the heavy atom (iodine) in th e solvent
p e rtu rb s the R u ssell-S au n d ers coupling valid in low Z e le c tro n ic sy ste m s.
T his
cau ses a re la x atio n of spin quantization and in c re a s e s the s in g le t-trip le t tr a n s i­
tio n p ro b ab ility .
M cGlynn, A zum i, and K asha (61) exam ined the effects of a v ariety of
solvents on the in te n sitie s of the low est s in g le t-trip le t tra n s itio n of se v e ra l
organic m o lecu les.
Some of th e solvents used contained atom s of la rg e atom ic
num ber (heavy atom s).
It w as shown th a t th ese heavy-atom solvents in c re a se d
the s in g le t-trip le t ab so rp tiv ity , and th at th is enhancem ent w as unique fo r
s in g le t-trip le t tra n s itio n s (that is , no enhancem ent of sin g le t-sin g le t bands
occurred),.
They also v e rifie d th a t th is p e rtu rb a tio n w as of a sp in -o rb it n a tu re
(by varying the s p in -o rb it n atu re of the heavy-atom solvent).
Studies have been re p o rte d on the heavy-atom enhancem ent of sin g le ttrip le t tra n s itio n s in benzene, both as in tra m o le c u la r su b stitu tio n (57) and as
in te rm o le c u la r solvent effects (8 , 19, 59, 60, 61, 62, 77, 81).
Robinson (77)
has studied the low est s in g le t-trip le t tra n s itio n of benzene in c ry sta llin e r a r e
15
gas solvents.
He found th a t the o rd e r of effectiveness of the r a r e g ases as
heavy-atom p e rtu rb e rs w as : X e> K r> A r> N e.
This o rd e r a g re e s with the
supposition th at n u c lea r ch arg e is th e lim itin g p a ra m e te r fo r effecting mixing
of singlet and trip le t s ta te s .
M u rre ll (70, 71) and H oijtink (44) have shown th at th e re a re two m ech ­
anism s w hereby the s in g le t-trip le t tra n s itio n s can becom e allow ed through the
influence of a heavy atom .
The f ir s t m ech an ism involves the m ixing of ex­
cited singlet and excited trip le t s ta te s of the organic m olecule.
Excited s ta te s
of the h eavy-atom p e rtu rb e r a re m ixed w ith excited sin g let s ta te s of the organic
m olecule (mixing involves tw o -e le ctro n exchange in te g ra ls betw een the o rb ita ls
of the hydrocarbon and those of th e h eavy-atom p e rtu rb e r) w hich a re in tu rn
m ixed w ith the trip le t s ta te .
The second m echanism involves m ixing in which
both the sin g let and trip le t s ta te s of hydrocarbon m ix w ith a c h a rg e -tra n s fe r
sta te .
M u rre ll (70, 71) has shown th at both m echanism s p re d ic t th at the in te n ­
sity of the s in g le t-trip le t tra n s itio n w ill be p ro p o rtio n al to the fourth power of
the bv 6r.lap in te g ra l betw een o rb ita ls of the p e rtu rb e r and th e p e rtu rb e d m o le­
cule.
C onsidering th is , one cannot ru le out one m echanism in favor of the o th e r.
N evertheless,. M u rre ll (70, 71) and T subom ura and.M ulliken (80) have indicated
th a t the c h a rg e -tra n s fe r m echanism is likely to be m o re im p o rtan t fo r the b en zenoid hydrocarbons.
VL
CHARGE-TRANSFER COMPLEXES
The te rm c h a rg e -tra n s fe r com plex is used to d e sc rib e c e rta in types of
weakly bonded com plexes.
T hese com plexes a re form ed by w eak in teractio n s
betw een two su b stan ces one of which acts as e lec tro n donors and the other as
an e le c tro n a ccep to r.
Many d o n o r-ac ce p to r com plexes a re unstable and only
e x ist in equilibrium in solutions.
The ra te s of fo rm ation of th e se com plexes
a re very ra p id and g e n erally k in etics stu d ies cannot be m ade by o rd in ary p ro ­
c ed u res.
The h e ats of in te ra c tio n a re g e n erally sm all and the degree of e le c tro n
tr a n s f e r to the acceptor m olecule is m uch le s s than when o rd in a ry covalent com ­
pounds a re form ed.
Donor m olecules can be e ith e r in organic or organic in n a tu re , how ever
com plexes w ith organic m olecules only a re of in te re s t h e re .
O rganic su b stan ces
th a t can s e rv e as e le c tro n donors can be c la ssifie d into two groups (2).
The
f i r s t group c o n sists of compounds having non-bonding (lone p a ir) electro n s
available fo r coordination (n-donors).
This group includes alcohols, organic
sulfides and iodides, and n itro g en b a se s.
The second group, which is of in te r ­
e st h e re , c o n sists of compounds having p i-e le c tro n s available fo r sharing (pidonors).
Exam ples of th is type a re alk en es, alkynes, and aro m a tic h y d ro c a r­
bons and th e ir su bstitution prod u cts.
A cceptor m olecules include a wide ran g e of ch em ical compounds.
T hese
include organic hydroxylic com pounds, organic p i-a c id s, F r ie d e l-C ra fts c a ta ­
ly s ts , and o th e rs.
H ydroxylic compounds fo rm hydrogen bonded com plexes w ith
17
many n -d o n o rs.
plex es.
T hese can be loosely c la ssifie d as c h a rg e -tra n s fe r com ­
One of the stro n g e st p i-a c id s is tetracy an o eth y len e.
This substance
fo rm s b rillan tly colored solutions in aro m a tic hydrocarbons and other donor
solvents (2 ).
F rie d e l-C ra fts c ata ly sts a re Lewis acids and th e re fo re read ily accept
e le c tro n s fro m many donor com pounds.
The m ost common F rie d e l-C ra fts
c a ta ly st, alum inum tric h lo rid e , fo rm s com plexes w ith both n and p i-d o n o rs (2 ).
Alum inum trib ro m id e fo rm s a faintly yellow co lo red com plex w ith benzene (7).
The m o le c u lar fo rm u la fo r th is com plex w as re p o rte d as CgHg- AlgBrg.
The inorganic s a lts of the elem en ts of Groups 2b, 4a, and 5a (only the
s a lts of zinc, cadm ium , m e rc u ry , tin , lead, a rse n ic , antim ony, and bism uth
a re of in te re s t h e re ) can be c la ssifie d , to v arin g d e g re e s, as Lewis acids and
th e re fo re act as e le c tro n a cc e p to rs.
C h a rg e -tra n sfe r com plexes of sa lts of a
few of th e se elem ents w ith both n and p i-d o n o rs have been re p o rte d .
H ere, the
in te re s t lie s in com plexes of s a lts of th e se elem en ts w ith aro m a tic hydrocarbons
(mainly naphthalene and benzene).
M olecular com plexes of naphthalene and benzene w ith a few of th ese
Lewis acids have been known fo r quite som e tim e .
In 1882, Sm ith and Davis (79)
re p o rte d th a t antim ony tric h lo rid e fo rm ed c ry sta llin e m o le c u lar addition com ­
pounds w ith naphthalene and benzene.
They re p o rte d the m o le c u lar fo rm u las of
th e se compounds as SSbClg' 2C^QHg and SSbClg*4CgHg.
T hese fo rm u las now
18
seem to be in c o rre c t. . Around the tu rn of the century,. M enshutkin (66 ) studied
sy ste m s of antimony trib ro m id e and tric h lo rid e w ith s e v e ra l organic compounds
(now known as M enshutkin com plexes). . M enshutkin exam ined the phase d ia­
g ra m s of th e se sy ste m s and found th a t sy ste m s of both antim ony tric h lo rid e
and antim ony trib ro m id e w ith naphthalene form ed compounds w ith congruent
m elting points (stable com plexes w ere form ed).
T hese compounds„diad com ­
positions of 28bClg- CioHg (m elting point 86 °C) and 2SbBr 3 - CiQHg (melting
point 66 °C)..
The e lec tro n ic absorptio n sp ec tru m and dipole m om ent of a one to one
m o lecu lar com plex betw een stannic ch lo rid e and naphthalene have been re p o rte d
(82). A stro n g absorption in the n e a r u ltra v io le t was in te rp re te d as being
caused by a c h a rg e -tra n s fe r tr a n s itio n .. Kawalec (51) re p o rte d th at both I - and
2 -m ethy!naphthalene and antim ony tric h lo rid e form ed stab le compounds w ith
the com position 2SbClg" CHgC I qH^.
pale yellow in co lo r.
P u re c ry s ta ls of th e se compounds w ere
Much w ork has been done on the n u c lea r quadrupole
reso n an ce (NQR) s p e c tra of the com plexes of naphthalene w ith antimony t r i ­
chloride and trib ro m id e (30-36).
G rechishkin and K yuntsel (30) concluded fro m
NQR s p e c tra th a t the antim ony atom in the antim ony trib ro m id e-n ap h th alen e
com plex is , to a la rg e d e g re e, the a ccep to r.
They also found th at th e re a re
th re e nonequivalent brom ide atom s in the la ttic e of th is com plex.
The. in fra re d and R am an s p e c tra of the antimony trich lo rid e-b en z e n e
com plex have been in v estig ated (11, 27, 28).
The in fra re d sp ec tru m w as
19
in te rp re te d as showing a d e c re a s e in sy m m etry fro m Dgj1 fo r p u re benzene to
Cgv fo r the com plex.
This in te rp re ta tio n seem s to be in c o rre c t, and D aasch
(11) has re p o rte d th at the sy m m etry of the com plex is m o st probably Cgv (in
a stag g e re d form ).
Hulme and Szym anski (48), fro m X -ra y c ry stallo g rap h y data, re c en tly
re p o rte d the c ry s ta l s tru c tu re of the 2:1 com plex of antim ony tric h lo rid e and
naphthalene.
The s tru c tu re c o n sisted of antim ony tric h lo rid e m olecules
stacked in th e be plane alte rn atin g w ith la y e rs of naphthalene m olecules in the
plane x = 1 /2 .
The antim ony atom s e x isted in a 4 -co o rd in ated , d isto rte d sp^d
trig o n al b ip y ra m id a l•enviro n m en t, w ith two chlorine atom s in eq u ato rial p o s i­
tio n s and the th ird chlorine atom occupying an axial position.
The th ird equ­
a to ria l p o sitio n w as filled by the antim ony lone p a ir, and th e o th er axial p o s i­
tion w as u sed in the bonding to the p i-s y s te m of the naphthalene m olecule.
The
d istan ce fro m the antim ony atom to the plane of the naphthalene m olecule w as
3.2 A.
The naphthalene bond d istan ces and angles w ere d is to rte d from those of
c ry s ta llin e naphthalene (see fig u re I).
C h a rg e -tra n s fe r com plexes have been d e sc rib e d using m o lecu lar o rb ita l
theory in two re la te d but slightly d ifferen t w ays.
T hese d e sc rip tio n s a re due to
. M ulliken (68 ) and Dewar (12, 13, 14). . M ulliken re p re s e n ts a c h a rg e -tra n s fe r
com plex as a reso n an ce hybrid of a "no bond” type s tru c tu re and an ionic
s tru c tu re .
The ground s ta te wave function fo r such a com plex is given by
20
= a |(A , B) + b |(A +B~), w here a » b .
I (A, B) is the wave function fo r th e "no bond" s tru c tu re , and f (A+B- ) is the
wave function fo r the ionic s tru c tu re ; . The accep to r A and donor B m ay in
g e n eral be any two suitable sp ecies in.w hich the ionization potential of A is
g re a te r than th a t of B.
The excited sta te wave function is given by
I e = -b |(A , B) + a | (A+B - ), w here a » b
The "no bond" wave function, | (A, B), has the fo rm
t (A > B ) = (ft
+• • •
H ere ^ .d e n o te s th at the product | |
of the wave functions of A and B a re to
f
A B
be m ade a n tisy m m etric w ith re s p e c t to all e le c tro n s.
The Wave function |(A +
B - ) re p re s e n ts a tr a n s f e r of an e le c tro n fro m A to B accom panied by the fo rm a ­
tio n of an ionic bond:
. 1.44
1.43
1.42:
F ig u re I ;
N aphthalene bond d ista n c e s fo r the antim ony tric h lo rid e -
naphthalene com plex (from the s tru c tu ra l d a ta of Hulme and
Szym anski (48) ).
21
Using. M ulIiken's schem e, th e e n erg ies of the ground s ta te and th e f ir s t
excited s ta te of a c h a rg e -tra n s fe r com plex m ay be approxim ated using second
o rd e r p e rtu rb a tio n th e o ry (67).
The ground s ta te energy is given by
WN = w Q- (H0 1 -W0 S)2/ (W1-W0 )
and the excited sta te energy by
WE = W i + (H0 1 T-W1S)2 (W1-W 0 )
w here
W0 -
^ (A , B) IH H (A, B))
W1 =. Ct(A+ET) ( K U (A4B -))
H01 -
(t(A , B ) | K l t(A +B -))
K is the com plete H am iltonian fo r th e com plex, and W q - Wn is the ground
s ta te re so n an c e energy and u sually ra n g e s fro m 0 to 10 K cal/m o le fo r th is type
of com plex.
C h a rg e -tra n sfe r com plexes a re c h a ra c te riz e d 3 by th e appearance of a
new e lec tro n ic ab sorption band w hich does not appear in the s p e c tra of the
accep to r o r donor alone.
A ccording to. M ulliken's th eo ry th is new band is a t t r i ­
buted to a tra n s itio n fro m the ground s ta te ( i y which has v ery little ionic
c h a ra c te r to an excited s ta te (i|f^) which is m ostly ionic.
The energy of th is
tra n s itio n is equal to differen ce in energy of the ground s ta te and the excited
22
to excitation of the a ccep to r o r donor alone.
The ab so rp tio n bands caused by
ex citation to th e se excited s ta te s w ill hot d iffer v ery m uch fro m the bands of
'-'rI
the se p a ra te m olecules.
D ew ar (12, 13, 14, 15) used m o le c u lar o rb ital th eo ry to d e sc rib e the
charge - tr a n s f e r complex; A, B in te rm s of in te ra c tio n s betw een th e o rb itals
of A and B.
T hese in te ra c tio n s m ay be c h a ra c te riz e d by considering the
o rb ital d iag ram in fig u re 2 .
HOMO
X -X
B
F ig u re 2.
X -X
X -X
A
Schem atic re p re se n ta tio n of the elec tro n ic tra n sitio n s
in a c h a rg e -tra n s fe r com plex.
HOMO equals h ig h est occu­
pied m o le c u lar o rb ital and LVMO equals low est vacant
m o lecq lar o rb ital.
23
In te ra c tio n s of the filled bonding o rb ita ls of A and B w ill not lead to any
change in th e ir to tal energy and to no net tra n s fe r of charge betw een A and B.
In te ra c tio n of the filled o rb ita ls of A (or B) w ith the em pty antibonding o rb ita ls
of B (or A) d e p re ss the fo rm e r and r a is e the la tte r w ith a sim ultaneous tr a n s ­
f e r of charge fro m A (or B) to B (or A). T h ese in te ra ctio n s a re in v ersely
p ro p o rtio n al to the differen ce in energy betw een the in te ra c tin g o rb ita ls.
Since
th e donor has filled o rb ita ls of re la tiv e ly high energy and th e accep to r had
em pty o rb ita ls of re la tiv e ly low en erg y , the m ain in te ra c tio n is betw een the
filled o rb ita ls of the donor and the em pty o rb ita ls of the a cc e p to r.
This w ill
give a net tr a n s fe r of ch arg e fro m th e donor B to the acc e p to r A.
D ew ar explains the appearance of a new band in the e lec tro n ic sp ec tru m
of the com plex as a tra n s itio n betw een the highest occupied m o lecu lar o rb ital
(HOMO) of the donor (B) to the low est vacant m o lecu lar o rb ita l (LVMO) of the
accep to r (A). The lo cally excited tra n s itio n s of the donor and the acceptor
should not d iffer m uch fro m the tra n s itio n s of th e se p a ra te m o lecu les.
U sing th is schem e, the energy of a c h a rg e -tra n s fe r com plex (for p i-p i
com plexes) m ay be w ritte n in te rm s of e n erg ies of the o rb ita ls of the accep to r
and the donor (14).
This energy is given by
Ec t =
- -Pi = Aj - ft
jSXj
a is the coulomb energy of carbon, )3 is th e c arb o n -ca rb o n reso n an ce energy and
Xj is a quantity calcu lated th e o re tic a lly w hich re la te s to the energy of the h ig h est
24
filled o rb ita l of the donor.
The energy of a c h a rg e -tra n s fe r tra n s itio n m ay be re la te d to the donor
ionization potential (Id ) an^ the e le c tro n affinity of the acc e p to r (E&). Q uali­
tativ e evidence on the dependence of th is tra n s itio n energy on the ionization
p o ten tial of the donor m ay be obtained by considering the p o sitio n s of c h a rg e tr a n s fe r bands of c e rta in a ro m a tic com plexes w ith iodine (2).
C onsidering the
following iodine com plexes, benzene (Am ax = 2900A), toluene (A.m ax = 3000A),
and m esity len e (Am ax - 3300A), one can see th at the en erg ies of the ch arg etr a n s f e r tra n s itio n s in c re a s e w ith in c re a sin g ionization p o ten tial of the donor.
The quantitative re la tio n sh ip h t^ rp = Ip,
- W was obtained by considering
the plot of Ip, v e rs u s I u ^ ^ fo r 18 iodine com plexes (W is a coulom bic te rm ) (6 ),
Since th is plot is lin e a r, it was concluded th a t W fo r th e se iodine com plexes is
independent of the n a tu re of the donor. . Since e le c tro n affin ities a re not known
fo r m o st com pounds, no quantitative re la tio n sh ip betw een
a ch arg e--tran sfer tra n s itio n can be obtained.
and the energy of
EXPERIMENTAL
I.
LOW TEMPERATURE STUDIES
The purpose of th e se stu d ies was to te s t atom ic m e rc u ry as a-heavy-
atom p e rtu rb e r of the s in g le t-trip le t tra n s itio n s of benzene and re la te d a ro m a ­
tic com pounds.
(77).
The m ethod th at w as used m odified the p ro c e d u re of Robinson
This m ethod c o n sisted of depositing m e rc u ry and an aro m atic compound
sim ultaneously on a cold q u a rtz p late and taking the sp e c tru m of th is film
through q u a rtz windows.
A sp ecial sta in le s s ste e l low te m p e ra tu re dew ar and
a vacuum m anifold w e re co n stru cted fo r th is purpose (figure 3).
The following p ro c e d u re was u sed fo r the plating out of m ercu ry and
aro m a tic compound.
nitrogen.
The sy ste m (figure 3) w as evacuated and flushed with
The dew ar was filled w ith liquid a ir and the te m p e ra tu re allowed to
com e to eq uilibrium (ca. one hour).
M ercu ry w as lo cated in the heated p y rex
boat at C. A v a ria c was connected to the nichrom e w rapped g la ss boat in o rd e r
to control the r a te of deposition of m e rc u ry on the cold q u a rtz p late.
The
aro m a tic compound (benzene in th is case) was placed in a s m a ll flask at A, and
the m ic ro m e te r valve w as opened slightly in o rd e r to get a slow ra te of d ep o si­
tio n of benzene.
The r a te s of deposition of both m e rc u ry and benzene w ere
v a rie d in o rd e r to get th in film s of good optical q u alities.
The deposition of
m e rc u ry w as also re g u la ted so that the solid thin film contained ca. 5% m e rc u ry .
Only v e ry th in film s could be obtained th a t had reaso n ab le optical p ro p e rtie s .
26
The u ltra v io le t sp ec tru m of each filrm w as then re c o rd ed .
. A 1.5 m e te r B ausch & Lomb sp ec tro g rap h was f ir s t u sed to re c o rd
th e se s p e c tra . ,\A deuterium ' lam p w as u sed as a light so u rc e , and the s p e c tra
w e re re c o rd e d on Kodak 103 ao photographic film . . A v a rie ty of s lit widths and
exposure tim e s w ere used fo r the re c o rd in g of th e se s p e c tra .
trip le t tra n s itio n s of benzene w ere observ ed in th is m an n er.
No sin g letFollowing the
sam e p ro c e d u re as above, s p e c tra of th e thin film s of m e rc u ry and benzene
w ere also re c o rd e d using a 0.25 m e te r B ausch & Lomb hionochrom ator (using
the deu teriu m lam p a s a light so u rce).
This m onochrom ator was equipped w ith
a P-28 photom ultiplier and a D-C A m p lifier and E le c tro m e te r (type 1230-rA).
T hese s p e c tra w ere re c o rd e d on a Brow n s trip c h a rt re c o r d e r (1 - 5 mv ran g e).
Again no s in g le t-trip le t tra n s itio n s w e re ob serv ed using th is technique.
27
LIQUID AIR
COOLED WINDOW
LENS
LENS
LIGHT
SOURCE
TO VACUUM
F ig u re 3.
Schem atic d ia g ra m of the ap p aratu s u sed in the study
of the effect of Hg on the s p e c tra of aro m atic hydrocarbons.
M icro m eter needle valves w ere located at A and B so that
rep ro d u cib le am ounts of the aro m atic h y d rocarbon could
be condensed on th e cold su rfa c e .
The h eated p y re x boat
at C w as u sed fo r depositing th e m e rc u ry onto th e cold
q u a rtz su rfa c e .
The c a p illa ry flow re s is ta n c e (D) was used
to keep a slight p re s s u re d ifferen tia l betw een th e m anifold
and the dew ar.
The aro m a tic compound w as then sprayed
through the nozzle (D) and condensed on th e cold su rfa ce .
II.
SPECTRA OF NAPHTHALENE AND NAPHTHALENE COMPLEXES
The s in g le t-trip le t s p e c tra of naphthalene and the naphthalene com ­
plexes w ere taken at 93 + 2°C (liquid s ta te sp e c tra ) on a C ary m odel 14 sp ec­
tro m e te r.
A tungsten lam p w as used as th e light so u rc e.
taken w ith e ith e r liquid naphthalene o r a ir as a re fe re n c e .
T hese sp e c tra w ere
In both c ases e ith e r
q u artz o r p y rex ground g la ss -s to p p e re d c e lls (one, five, and te n c en tim ete r
c ells) w ere used.
When a ir was used as a re fe re n c e the c e ll holder co n sisted
of a m e ta l tube w rapped w ith in su lated m a te ria l and re s is ta n c e w ire.
This tube
w as p laced in an in su la ted alum inum box designed to be u se d in a C ary 14 sp ec ­
tro m e te r.
A v a ria c w as adjusted to hold the te m p e ra tu re at ca. 93°C.
The s p e c tra w ith liquid naphthalene as a re fe re n c e w e re taken in five
c e n tim e te r g la ss -s to p p e re d q u a rtz cells coated w ith In sta th e rm (a c o m m ercial
low re s is ta n c e coating u sed fo r heating).
The c ell h o ld ers co n sisted of in su ­
la te d alum inum boxes (equipped w ith p y re x windows) c o n stru cte d to be u sed in
the C ary 14 s p e c tro m e te r.
The te m p e ra tu re w as co n tro lled w ith a variac:
connected to a YiSl T h e rm iste m p T e m p e ra tu re C o n tro ller (model 71).
The
th e rm is to r w as located in the sam ple c e ll ho ld er (in contact w ith the In sta th erm
coated cell) and the c o n tro lle r was ad ju sted to hold the te m p e ra tu re at 93 + 2°C.
B ak er and A dam sen r e sublim ed naphthalene w as u sed fo r all s p e c tra
w ithout fu rth e r p u rificatio n .
gas chrom atography.
The p u rity of th is naphthalene w as checked using
T hese ru n s w ere m ade in the F & M (Model 400) gas
29
chrom atograph at 175°C w ith helium as c a r r i e r gas and u sin g flam e ionization
detection.
The colum n w as 16% R eoplex on Gas C hrom G, and re d is tille d
hexane w as used as a solvent.
The gas ch ro m ag ram s tak en in th is m anner
contained only one peak (other than th e solvent peak) and in d icated that the
naphthalene was reaso n ab ly p u re .
R eagent g ra d e m e rc u ric ch lo rid e, m e rc u ric b ro m id e, m e rc u ric ace­
ta te , cadm ium ch lo rid e, and lead ch lo rid e w e re used w ithout fu rth e r p u rific a ­
tion.
R eagent g rad e antim ony tric h lo rid e and bism uth tric h lo rid e w ere p u rified
by sublim ation under a vacuum at ca. 80 and 200°C, re sp e c tiv e ly .
R eagent
g rad e zinc chlo rid e w as pu rified by dissolving in co n cen trated hydrochloric
acid and evaporating down to a p a ste .
T his p a ste was then d rie d under a
vacuum at 120°C fo r approxim ately two w eeks.
tric h lo rid e w ere p u rified by vacuum d istilla tio n .
d istille d tw ice before u sing.
Stannic ch lo rid e and a rsen ic
The a rs e n ic tric h lo rid e w as
The stannic chloride was d is tille d through a
colum n packed w ith phosphorus pentoxide on g la ss wool (46).
S p ectra w ere taken of both d eg assed and nondegassed solutions.
The
sam p les w ere d eg assed by bubbling co m m e rcial g rad e n itro g en through them
fo r ca. on e-h alf hour.
The efficiency of degassing fo r th is tim e perio d was
checked by re p e titio n of th is p ro c e ss .
A fter the f ir s t d eg assin g no change
w as o b serv ed in the sp e c tru m of p u re naphthalene upon re p e a te d d egassings.
The following s p e c tra w ere taken in the above m an n er.
30
N aphthalene and N aphthalene P lus Oxygen
N aphthalene w as sa tu ra te d w ith oxygen and the s in g le t-trip le t sp ectru m
observed.
T his solution had a faint yellow co lo r.
The liquid naphthalene was
then d eg assed and the s in g le t-trip le t sp ec tru m was taken again.
These s p e c tra
w e re tak en in 10 cm c e lls w ith a ir as a re fe re n c e .
N aphthalene P lu s M ercu ric C hloride
S in g le t-trip le t s p e c tra w ere taken of 0.05, 0.08, 0.09, 0.10, and 0.15
m olal solutions of m e rc u ric chlo rid e in naphthalene.
T hese s p e c tra w ere
tak en of nondegassed solutions w ith a ir as a re fe re n c e and u sin g 10 cm c ells.
The s in g le t-trip le t sp ec tru m of a 0.15 m olal solution w as also tak en w ith liquid
naphthalene
,as re fe re n c e and using 5 cm c e lls .
re fe re n c e w e re degassed.
N either th e sam ple n o r the
The 0.10 solution w as d eg assed fo u r tim e s w ith the
s in g le t-trip le t sp e c tru m being taken a fte r each d egassing.
T able I shows the
r e s u lts of th e se deg assin g s.
The d egassings in tab le I w ere c a r rie d out in th e following m anner.
The
sam p les w ere d eg assed (as d iscu ssed previously) and the s p e c tra taken im m ed i­
ately except a fte r the second degassing.
T h ere the sam ple w as d eg assed and
allow ed to stand overnight before the sp ec tru m w as taken.
N aphthalene P lu s M e rcu ric B rom ide
The following s in g le t-trip le t s p e c tra w ere tak en of th e m e rc u ric brom ide
31
com plex in 10 cm c e lls and w ith a ir as a re fe re n c e : 0.05, 0.01, 0.15, and
0.20 m olal solutions.
T hese s p e c tra w ere tak en of nondegassed solutions.
The s in g le t-trip le t sp ec tru m of 0.15 m o lal m e rc u ric b ro m id e w as also taken
w ith naphthalene as re fe re n c e and in 5 cm c e lls.
Table I
E F F E C T S OF DEGASSING ON 0.1 M MERCURIC CHLORIDE IN NAPHTHALENE
Sam ple (tim es degassed)
A bsorbance
5500 A
4730 A
4630 A
4430 A
0
0.00
0.113
0.118
0.204
I
0.006
0.112
0.118
0.196
2
0.007
0.135
0.140
0.223
3
—0.010
0.110
0.116
0.195
4
-0.010
0.109
0.115
.
0.193
Naphthalene P lus M ercu ric A cetate
S in g le t-trip le t s p e c tra w ere tak en of 0.05, 0.10, 0.15, and 0.20 m olal
solutions of m e rc u ric a ce ta te in naphthalene w ith a ir as a re fe re n c e and 10 cm
c e lls.
The solutions w ere not d eg assed .
N aphthalene P lu s A rsen ic T rich lo rid e
Singlet s p e c tra w ere tak en of 1.17, 1.68, and 3.25 m o lal solutions (nondegassed) of a rse n ic tric h lo rid e in naphthalene in 10 cm c e lls w ith a ir as a
32
re fe re n c e .
taken again.
The 1.17 m olal solution was d eg assed one tim e and the sp ectru m
This s in g le t-trip le t sp ec tru m did not show any ap p reciab le
change in absorption when com p ared to the sp ec tru m of the nondegassed so lu ­
tion.
Naphthalene P lu s Antimony T rich lo rid e
The following s in g le t-trip le t s p e c tra w ere tak en of nondegassed solutions
of antim ony tric h lo rid e in naphthalene: 0.10 m (10 cm c e lls), 0.20 m (5 cm
c e lls), 0.50 m (5 cm c e lls), and 1.0 m (I cm c e lls).
w ith a ir as a re fe re n c e .
T hese s p e c tra w ere tak en
A s in g le t-trip le t sp ectru m of 0.50 m olal antimony
tric h lo rid e w as also ta k en in 5 cm c e lls w ith naphthalene as a re fe re n c e .
The
0.50 m olal solution w as d eg assed and an o th er sp ectru m tak en (air re fe re n c e ).
No change in the in ten sity of the s in g le t-trip le t band w as o b serv ed .
This 0.50
m olal solution w as allow ed to stand (at ca. 93°C) fo r ca. 24 h o u rs.
Spectra
w e re th en tak en b efo re and a fte r deg assin g .
T hese s p e c tra w e re alm ost id e n ti­
cal, both showing a la rg e in c re a s e in ab so rp tio n in the s in g le t-trip le t reg io n
w ithout any in c re a s e in the s in g le t-trip le t bands.
T his in c re a s e in absorption
w as a ttrib u te d to the fo rm atio n of an o th er absorbing sp ec ie s w hen naphthalene
and antim ony tric h lo rid e w ere heated to g e th e r.
N aphthalene P lus B ism uth T rich lo rid e
S in g le t-trip le t s p e c tra of 0.001 m (10 cm c e lls), 0.002 m (10 cm c e lls),
0.010 m (I cm c e lls), and 0.030 m (I cm c ells) solutions of b ism u th tric h lo rid e
33
in naphthalene w ere taken w ith a ir as re fe re n c e .
of nondegassed solutions.
T hese s p e c tra w ere taken
A s in g le t-trip le t sp ec tru m of 0. 01 m olal bism uth
tric h lo rid e w as taken w ith naphthalene as a re fe re n c e (nondegassed sam ple
in 5 cm c ells).
N aphthalene P lus Stannic C hloride
Solutions of stannic chloride in naphthalene w ere b rig h t yellow in
c o lo r and absorbed stro n g ly in the s in g le t-trip le t region.
B ecau se of th is,
no m e asu re m e n t of s in g le t-trip le t ab so rp tio n could be obtained.
N aphthalene P lus Lead C hloride
Lead te tra c h lo rid e is yellow in co lo r (absorbs stro n g ly in the sin g le ttrip le t region) and could not be used as a heavy-atom p e rtu rb e r.
Lead (II), Zinc, and Cadm ium C hloride P lu s Naphthalene
Lead (II), zinc, and cadm ium ch lo rid e a re all quite insoluble in naph­
thalene and did not cau se any o b serv ab le enhancem ent of the s in g le t-trip le t
tra n s itio n s of naphthalene.
The so lu b ilitie s of zinc and cadm ium chloride in
naphthalene w e re checked using EDTA titra tio n s .
E rio ch ro m e black T was
used as an in d ic a to r and the solutions to be titra te d w ere buffered with a pH 10
am m onium buffer.
No zinc o r cadm ium could be detected by th is method.
in .
SPECTRA OF BENZENE AND BENZENE COMPLEXES
T hese s p e c tra w ere taken on e ith e r a Beckm an DK-2 o r a C ary 14
sp e c tro m e te r using a deu teriu m light so u rc e .
Reagent g ra d e benzene and
m e rc u ric chloride w ere used without fu rth e r p u rificatio n .
M ercu ric chloride
w as the only s a lt, of the s a lts te ste d , th a t w as found su itab le as heavy-atom
p e rtu rb e r of the s in g le t-trip le t tra n s itio n s of benzene.
Solutions of the other
s a lts (m ercu ric bro m id e, th alliu m ch lo rid e, antimony tric h lo rid e , and b is ­
m uth tric h lo rid e ) in benzene ab so rb ed light too strongly in th e sin g le t-trip le t
reg io n of benzene.
B enzene P lu s Oxygen
Oxygen w as bubbled through benzene fo r ca. o n e-h alf hour and the
s in g le t-trip le t sp ec tru m taken in 10 cm c e lls .
Upon bubbling nitrogen through
th is benzene solution fo r ca. o n e-h alf hour and again taking a sp ectru m , the
s in g le t-trip le t bands d isap p eared .
B enzene P lus M ercu ric C hloride
The s in g le t-trip le t sp ectru m of 0. 02 m o la r m e rc u ric ch lo rid e in
benzene w as taken in 10 cm c e lls w ith benzene as a re fe re n c e .
~
IV. . SPECTRA OF DIPHENYLMERCURY AND PHENYLMERCURIC ACETATE
The s in g le t-trip le t sp ec tru m of dipheny !m ercu ry w as tak en in c h lo ro ­
fo rm (0.050 m o lar) w ith the B eckm an DK-r2 sp e c tro m e te r. . T hese sp e c tra
w e re taken in 10 cm c e lls using ch lo ro fo rm as a solvent.
The s in g le t-trip le t
sp ec tru m of p h en y lm ercu ric a cetate w as tak en in the sam e m an n er except that
g la cial a ce tic acid w as u sed as th e solvent (0.10 m o lar).
In both c a s e s, E a s t­
m an ch em icals w ere u sed w ithout fu rth e r p u rificatio n .
The s in g le t-trip le t bands of p h en y lm ercu ric acetate (in g lacial acetic
acid) Were shifted ca. 30 A to the blue of the diphenylm ercury (in chloroform )
bands. . To see if th is w as a solvent effect, th e s in g le t-trip le t sp ectru m of
diphenylm ercury w as again taken in g la c ia l a cetic acid.
Except fo r a b ro ad en ­
ing of the p eak s, th is sp ec tru m w as id en tical (within e x p erim en tal e r r o r ) to
the diphenylm ercury sp ec tru m in ch loroform .
V.
INTERPRETATION OF SPECTRA
The in te rp re ta tio n of the s in g le t-trip le t s p e c tra of th e naphthalene com ­
plexes w as com plicated by two d ifferen t fa c to rs .
F ir s t, it w as observed th a t
a change in the optical d e n sitie s of th e se liquid naphthalene solutions o c cu rred
w ith tim e .
The actual siz e and shape of th e s in g le t-trip le t peaks did not change
w ith tim e ; they w ere only disp laced upw ard on the c h a rt p a p er.
The second
fa c to r affecting th e se s p e c tra dealt w ith th e positions of the c h a rg e -tra n s fe r
bands of th e se com plexes.
The ta ils of th e se intense bands, to varying d e g re e s,
overlapped the w eak s in g le t-trip le t bands.
The s in g le t-trip le t bands appeared on the ta ils of th e se c h a rg e -tra n s fe r
bands.
In the c ase of stannic ch lo rid e-n ap h th alen e com plex, th e c h a rg e -tra n s ­
fe r band com pletely co v ered up the s in g le t-trip le t bands (see sp ectru m no. 8).
In the other com plexes w here at le a s t a p o rtio n of the s in g le t-trip le t bands w ere
o b servable, the contribution of the c h a rg e -tra n s fe r bands to the optical den­
s itie s of th e se com plexes (in the s in g le t-trip le t region) could not be d eterm ined.
B ecause of th e se two fa c to rs , m eaningful in te g ra te d extinction co effici­
ents (as o s c illa to r stre n g th s) could not be obtained fo r all th e com plexes
studied.
However, using s e v e ra l approxim ations, o s c illa to r stren g th s w ere
calculated fo r the s in g le t-trip le t tra n s itio n s of se v e ra l of th e se com plexes.
m ethod used w ill be d isc u sse d la te r .
Since o sc illa to r stre n g th s could not be
obtained fo r all the enhanced s in g le t-trip le t bands, a m ethod of com paring
The
37
s in g le t-trip le t in te n sitie s of th e se com plexes had to be obtained.
This m ethod
co n sisted of finding enhancem ent fa c to rs fo r a s e r ie s of co n cen tratio n s of
heavy-atom s a lts in naphthalene.
C alculation of Enhancem ent F a c to rs
Enhancem ent fa c to rs w ere obtained in the following m an n er.
Singlet-
tr ip le t s p e c tra w e re taken of s e v e ra l d ifferen t co n cen tratio n s of each heavyatom s a lt (m ercu ric ch lo rid e, m e rc u ric b ro m id e, m e rc u ric a c e ta te , a rse n ic
tric h lo rid e , antim ony tric h lo rid e , and b ism u th tric h lo rid e ) in naphthalene.
The approxim ate peak heights of the 4730 A peak (of th e s in g le t-trip le t tr a n s i­
tio n of naphthalene) w ere then found f o r each of the above s p e c tra .
The 4730 A peak w as chosen because it was affected le s s by the positions
of the c h a rg e -tra n s fe r bands of the com plexes.
The approxim ate peak heights
w ere then found fo r each sp ec tru m by assum ing th at the background peaks
(these w ere com binations of c h a rg e -tra n s fe r bands and the sin g le t-sin g let band
of naphthalene) w ere stra ig h t lin e s (see fig u re 4).
peak w as th en found fo r each sp ec tru m .
The cen ter, of the 4730 A
The sp e c tra w e re th en m ark ed at
points 180 A on both sid es of the c e n te r of the peak.
Then, fo r each sp ectru m ,
the absorbance at th e se two points w ere av erag ed and su b tra c te d fro m the ab­
so rb an ce reading at the c e n te r of the peak.
These approxim ate peak heights,
obtained fro m the d ifferen t s p e c tra , w ere then c o rre c te d so th a t they w ere all
based on the sam e pathlength of solution.
38
T hese c o rre c te d peak heights w ere then plotted v e rs u s concentration
of s a lt fo r each of the d ifferen t com plexes studied.
The slo p es of th ese plots
(see figure 5) w ere th e enhancem ent fa c to rs and gave a rough indication of
s in g le t-trip le t in ten sity enhancem ent ab ility of each of in o rg an ic s a lts studied
(table II).
Table II
Enhancem ent F a c to r
P e rtu r b e r
A sC l3
Hg(C2H3O2)2
6 + 3 x IO"3
’
1156 + .2 x IO"1
SbCl3
1.7
+ .1 x IO"1
HgCl2
2.4
+ .2 x 10 1
HgBr2
3.1
+ .2 x IO"1
B iC l3
2.6
+ .1
A bsorbance
39
i 180 A
180A I
w avelength (m/i)
F ig u re 4.
C alculation of enhancem ent fa c to rs (I).
T his is the
sp ec tru m of 0.15 m olal solution of m e rc u ric chloride in
naphthalene (10 cm pathlength).
A bsorbance
40
m olality
F ig u re 5.
C alculations of enhancem ent fa c to rs (2).
P lo ts of
c o rre c te d peak heights v e rsu s co n cen tratio n s fo r AsClg,
SbClg, and BiClg com plexes.
41
C alculation of O scillatorVStrengths
O sc illa to r stre n g th s w ere calcu lated using the fo rm u la (78),
■
fgT - 4.32 x IO-9 jg d f
The in te g ra l in th is e x p re ssio n is th e in te g ra te d m o lal extinction coefficient.
This is the a re a u n d er the s p e c tra l band when the sp ec tru m is plotted as e x ­
tin ctio n coefficient v e rs u s wave n u m b ers.
The s in g le t-trip le t bands studied
h e re w ere located on the ta ils of in ten se s p e c tra l bands.
T h ese intense bands
w ere a com bination of the sin g le t-sin g le t band of naphthalene and the c h arg etr a n s f e r band of th e com plex in question.
The contribution of the background
peak to the ab sorption w as obtained by ex trapolating a lin e a r plot of log e v e r ­
sus, p, fro m s h o rte r w avelengths, w h ere th e contribution of the sin g le t-trip le t
band is assum ed to be negligible.
T his ex trap o lated ta il is th en plotted on the
s in g le t-trip le t' sp ec tru m and the a re a betw een the actual ab so rp tio n band and
th is exponential ta il is the in te g ra te d extinction coefficient (this is only tru e
when the sp ec tru m is plotted in te rm s of extinction coefficient and wave num ber).
The a re a betw een the c u rv es ’was obtained using a g ra p h ic al in teg ratio n
m ethod (54). The following p ro ced u re w as used.
in te rv a ls along the sp e c tru m .
Points w e re picked at clo se
T hese points w ere re c o rd e d in w avelengths and
then converted to w avenum bers (U). The in te rv a ls betw een the points w ere
clo se enough to g e th e r so th a t the sp ec tru m approxim ated a s tra ig h t line betw een
the points.
F o r each w avenum ber (U) re c o rd e d , th e d ifferen ce betw een the
42
absorbance of the o rig in al sp ec tru m and th e exponential ta il (Adiff) was found.
Next the differen ces in the w avenum bers (A^f) and the av erag e of the Ar]iff
(Adiff) w ere found fo r each su cc e ssiv e p a ir of points.
The sum m ation of all
the p roducts ArHff A p w as then com puted. . This re s u lt (divided by the fa c to r
pathlength tim e s concentration) gave the approxim ate extinction coefficient.
T able III
OSCILLATOR STRENGTHS FOR SING LET-TRIPLET TRANSITIONS
Species
O sc illato r Strength (f)
A s CI3-C io H8
1.9 x 10™8
SbCl3-C io H 8
9.6 x 10“7
HgCl2-C io H 8
1.2 x 10™G
HgCl2-C 6H6
4.4 x 10“6
(C6H5)2Hg
1.5 x 10™G .
C6H5HgOCOCH3
7.9 x 10-G
DISCUSSION
I.
LOW TEM PERATURE STUDIES
The purpose of th is low te m p e ra tu re w ork was to te s t atom ic m ercu ry
as heavy-atom p e rtu rb e r of the s in g le t-trip le t tra n s itio n s of c e rta in aro m atic
compounds (only benzene w as used).
T h ere w ere se v e ra l re a so n s fo r testin g
m e rc u ry as a heavy-atom p e rtu rb e r. . M ercu ry is re la tiv e ly in e rt, has a high
atom ic num ber (Z = 80), and does not absorb light appreciably in the near
u ltra -v io le t reg io n of the sp ec tru m . . M ercu ry is also m onoatom ic in the gas
phase and has high enough vapor p re s s u re so th at it could be deposited on a
cold q u a rtz p late in the m an n er d e sc rib e d in the ex p erim en tal section.
The s p e c tra of the film s of benzene and m e rc u ry only showed the 2600
A bands of benzene.
No absorption bands w ere observed in the reg io n of the
low est s in g le t-trip le t tra n s itio n of benzene.
T h ere w ere s e v e ra l re a so n s fo r
not being able to ob serv e any enhancem ent of the low est s in g le t-trip le t tr a n s i­
tion of benzene.
m ade v e ry thick.
F i r s t of all, the film s of benzene and m e rc u ry could not be
Only v ery th in film s had reaso n ab le o ptical p ro p e rtie s , and
the th ic k e r film s w ere opaque to light.
B ecause of th is, th e in stru m e n ts used
in attem pting to d etect th e se w eak tra n s itio n s w ere not se n sitiv e enough.
A lso
a d eu teriu m lam p w as used in th e se e x p erim en ts; a b rig h te r lig h t source would
have allowed th ic k e r film s to be used.
V ery th in film s of m e rc u ry w e re opaque to light.
B ecau se of th is, the
44
co ncentration of m e rc u ry in the film s had to be kept sm all (less than ca. 5%).
The id e al situation fo r the m ost efficient enhancem ent would o ccu r when each
benzene m olecule was surround ed by the heavy m e rc u ry ato m s.
When the
film s contained le s s than 5% m e rc u ry , only a few of the benzene m olecules in
the film had m e rc u ry atom s as n e a re s t neighbors.
Since th e re m u st be som e
m ixing of the o rb ita ls of the p ertu rb in g atom and the p e rtu rb e d m olecule, th is
did not lead to v ery efficient enhancem ent of the s in g le t-trip le t tra n sitio n s of
benzene.
Thus v e ry thick film s (which would be opaque) would have to be u sed
to observe any enhancem ent of the s in g le t-trip le t tra n s itio n s in benzene by th is
m ethod.
II.
DISCUSSION OF SPECTRA
Oxygen Enhanced S in g let-T rip let S pectrum of N aphthalene
The sp ec tru m of a sa tu ra te d solution of oxygen in liquid naphthalene
(spectrum no. I) showed w eak ab sorption bands in the 4000-5000 A region of
the sp ec tru m .
T hese bands d isap p eared when the solution w as degassed, and
they w ere in te rp re ta te d as being the oxygen enhanced s in g le t-trip le t bands of
naphthalene.
The oxygen (param agnetic) enhancem ent of s in g le t-trip le t tr a n s i­
tio n s has been w ell c h a ra c te riz e d (I, 17, 19, 20-26, 43, 47, 70, 80) and has been
u sed as a strong c rite rio n fo r the location and assignm ent of trip le t states (52,
76). . As fu rth e r evidence of th is assig n m en t, th e se oxygen enhanced bands
c o rresp o n d to those re p o rte d in the lite ra tu re as being cau sed by a spin fo r­
bidden e lec tro n ic tra n s itio n fro m the sin g let ground s ta te to th e f ir s t excited
3
trip le t s ta te of naphthalene (
-
I
A, ) (24, 40, 50, 65).
The s p e c tra of the solutions of naphthalene with m e rc u ric chloride,
m e rc u ric b rom ide, m e rc u ric a ce ta te, a rs e n ic tric h lo rid e , antimony tr ic h lo r ­
ide, and bism uth tric h lo rid e contained w eak absorption bands th at co rresponded
to the oxygen enhanced bands.
T hese bands did not d isap p e a r upon degassing
and w ere also in te rp re ta te d as being due to th e
thalene.
3
-
I
A
tra n sitio n of naph­
The enhancem ent of th e se s in g le t-trip le t bands w as explained as being
due to a heavy-atom p e rtu rb a tio n caused by the heavy m etal s a lts .
S p ectra of C om plexes of M ercu ric C hloride, B rom ide, and A cetate
The s p e c tra of solutions of th e se s a lts in naphthalene had absorption
bands at 473, 462, 443, 434, and 417 m/j, (the 417 band was a shoulder that
was not o bserved in the m e rc u ric brom ide sp ectru m ).
T hese bands c o r r e s ­
ponded fa irly closely to the re p o rte d bands fo r the low est s in g le t-trip le t tr a n s i­
tio n of naphthalene (60).
D egassing of th e se solutions did not cause any a p p re c i­
able change in the in te n sitie s of the bands.
R eferrin g to tab le I (degassing d ata
fo r m e rc u ric chlo rid e solutions), th e ab so rb an ce reading fo r th e 473 band only
changed ca. 1% a fte r the f ir s t d egassing and the to tal change in absorbance
a fte r four deg assin g s w as only ca. 4%. T h is 4% change is w ell w ithin the un­
c e rta in ty of the absorbance read in g s (ca. 8%).
Table I also shows an abnorm al reading a fte r the second degassing., T his
read in g w as tak en a fte r the solution w as allowed to stand heated overnight.
The
absorbance reading re tu rn e d to its o rig in al value a fte r th e solution was again
degassed.
It w as su sp ected th at chlorine gas w as form ed by a d isp ro p o rtio n a­
tio n re a c tio n of the m e rc u ric ch lo rid e.
The p re sen c e of th is gas in solution
would cause an in c re a s e d absorption th at should d isap p ear upon degassing.
The optical density of the m e rc u ric acetate solution also in c re ase d w ith
tim e .
This in c re a s e w as m o re ra p id th an th a t fo r the m e rc u ric chloride so lu ­
tion.
F u rth e rm o re , the in c re a s e in optical density of th is solution did not d im ­
in ish w ith degassing.
This fa c t seem ed to indicate th a t another absorbing
47
sp ec ie s (not gaseous) w as being fo rm ed . . This would' probably be som e s o rt
of substitu tio n compound of naphthalene.
S p ectra of C om plexes of A rsen ic, A ntimony, and B ism uth T rich lo rid e
A bsorption bands w ere observ ed at 473, 462, 443, and 417 mf/ in the
s in g le t-trip le t s p e c tra of a rs e n ic tric h lo rid e and antim ony tric h lo rid e com ­
plex es.
Due to the position of the c h a rg e -tra n s fe r band in the bism uth t r i ­
chlo rid e com plex only the 473 band (shoulder) was observed.
F o r a ll th re e
of th e se com plexes the optical density in the s in g le t-trip le t reg io n of the
sp ec tru m of the heated solutions changed w ith tim e .
The actu al in ten sities
of the s in g le t-trip le t bands did not seem to change, only the base lin es of
the s p e c tra w ere disp laced upward.
th e se in c re a s e s in absorption.
D egassing did not have any effect on
T hese in c re a s e s in optical d e n sitie s w ere
in te rp re te d as being caused by the fo rm atio n of new absorbing sp ecies.
The
n atu re of th e se sp ecies w as not d eterm in ed .
S pectra of D iphenylm ercury, P h en y lm ercu ric A cetate, and th e M ercu ric
C hloride-B enzene Complex
.
The s in g le t-trip le t tra n s itio n s of dipheny !m e rc u ry , p h en y lm ercu ric a c e ­
ta te , and the m e rc u ric chloride-b en zen e com plex appeared as bands on the ta ils
of in ten se sin g le t-sin g le t bands.
The s p e c tra of dipheny !m e rc u ry and phenyl­
m e rc u ric a cetate w ere taken in ch lo ro fo rm and g lacial a ce tic acid, re sp ec tiv e ly .
T hese solvents w ere chosen because of solubility re q u ire m e n ts.
The sin g le t-
48
trip le t bands of diphenylm ercury appeared at the sam e p o sitio n s as those r e ­
p o rte d by M archetti and K earns (65) using a phosphorescence excitation te c h ­
nique. . A lso th e se bands w ere located at the sam e positio n s as La P ag lia (55)
re p o rte d fo r the s in g le t-trip le t bands of tetrap h en y l lead (also in chloroform ).
The s in g le t-trip le t bands of pheny !m e rc u ric acetate appeared shifted
ca. 30 A to the blue fro m the diphenylm ercury bands (see tab le IV).
The
s in g le t-trip le t sp ec tru m of diphenylm ercury in g la cial acetic acid was alm ost
id en tical (except fo r co n sid erab le broadening of the bands) to the sp ectru m in
chloroform . . T his se e m s to ind icate th a t the s p e c tra l shift of the pheny !m e rc u ric
> acetate bands w as not a n o rm al solvent shift.
Table IV
POSITIONS OF ENHANCED SIN G LET-TRIPLET BANDS OF BENZENE
System
Band positions (mju)
ca. 312
Benzene + oxygen
B enzene + m e rc u ric chloride
Dipheny Im e rc u ry a
P h en y lm ercu ric acetate'3
ac L
ehloform solvent,
f
320
330
340
ca. 320
330
340
■ 305
314
322
331
342
ca. 305
312
318
328
338
hO
g la cial a cetic acid solvent
A p o ssib le explanation of th is s p e c tra l sh ift can be obtained w ith the
help of the m o lecu lar o rb ita l d iag ram in F ig u re 6.
The f ir s t sin g le t-sin g le t and
49
s in g le t-trip le t bands of dipheny lm ercu ry a re shifted to the re d of the bands in
benzene.
cury.
This shift can be explained by a reso n an ce effect in diphenylm er­
T his reso n an ce (m esom eric) effect in te ra c tio n c au ses a splitting of the
excited s ta te s of diphenylm ercu ry w hich le ad s to excited s ta te s of low er
e n e rg ie s.
This lead s to a re d shift in the observ ed bands.
The sp ec tru m of the pheny!m e rc u ric acetate is com plicated by th e fact
th a t th e f ir s t sin g le t-sin g le t band is low ered in energy w hile the f ir s t sin g le ttrip le t is ra is e d in energy (again com p ared to benzene).
The shift in the
sin g le t-sin g le t band can be explained in the following m an n er.
The f ir s t ex ­
cited singlet s ta te is stab ilized (low ered in energy) by the delocalization of the
p i-e le c tro n s of the phenyl rin g on to the m e rc u ry ion.
This delocalization of
e le c tro n s can also cause the energy d ifferen ce in the f ir s t excited singlet sta te
and the f ir s t excited trip le t state to be le s s sin ce e le c tro n re p u lsio n would be
le s s in the m o re d elocalized sy ste m .
The o b serv ed s p e c tra l sh ift of the
s in g le t-trip le t bands could po ssib ly be due to th is lessen in g of e lec tro n re p u l­
sion, but m o re e x p erim en tal d ata is needed before th is qu estio n can be settled .
The s in g le t-trip le t bands in the oxygen and the m e rc u ric chloride en­
hanced sy ste m s w ere located at the sam e positio n s.
D egassing had no effect on
the m e rc u ric chlo rid e enhanced bands, w hile the oxygen enhanced bands d is a p ­
p e a re d upon degassing.
50
S1
S,
I /
T
Sc
P h e n y lm e rc u ric acetate
Benzene
D iphenylm ercury
F ig u r e ' 0 .. M olecular o rb ital d ia g ra m re p re se n tin g the f ir s t
sin g le t-sin g le t and th e f i r s t s in g le t-trip le t tra n s itio n s
of benzene, dipheny!m e rc u ry , and p h en y lm ercu ric ace­
ta te . . The a rro w s re p re s e n t observ ed tra n s itio n s .
III. . DISCUSSION OF NAPHTHALENE COMPLEXES
Solutions of liquid naphthalene w ith c e rta in inorganic s a lts (m ercu ric
ch lo rid e, m e rc u ric b ro m id e, m e rc u ric a c e ta te , a rse n ic tric h lo rid e , antim ony
tric h lo rid e , bism uth tric h lo rid e , and stannic chloride) w e re yellow in co lo r
and absorbed strongly in the n e a r u ltra -v io le t reg io n of the sp ec tru m .
The
c o lo r of th e se solutions ranged fro m b rig h t yellow fo r bism u th tric h lo rid e and
stannic chlo rid e to pale yellow fo r th e a rs e n ic tric h lo rid e solution.
These
co lo rs w e re attrib u te d to two fa c to rs : the enhanced s in g le t-trip le t tra n sitio n
of naphthalene and th e fo rm atio n of c h a rg e -tra n s fe r com plexes.
T hese c h a rg e -tra n s fe r com plexes w ere c h a ra c te riz e d by the appearance
of in ten se charge,-^transfer bands.
The exact positions of th e se bands could not
be d eterm in ed since both naphthalene and som e of the in o rg an ic s a lts absorbed
strongly in the s p e c tra l re g io n w here the c h a rg e -tra n s fe r bands appeared.
The
re la tiv e positions of th e se bands w e re obtained fro m the s in g le t-trip le t s p e c tra
by considering the w avelengths w here the a b so rb ance ■re a d in g s w ere equal to
one.
The following o rd e r w as obtained (increasing energy of th e c h a rg e -tra n s ­
f e r band fro m left to rig h t): stannic ch lo rid e< b ism u th tr ic h lo r id e <m e rc u ric
brom ide < m e rc u ric chloride <antim ony tric h lo rid e (m e rc u ric acetate< a rse n ic
tric h lo rid e .
Since the c h a rg e -tra n s fe r bands a re not the only contributing fa c ­
to r s to the optical d e n sitie s in th is reg io n , th is order, of c h a rg e -tra n s fe r band
e n erg ies cannot be co n sid ered absolute;
52
The energy of a c h a rg e -tra n s fe r com plex can be approxim ated by the
e x p re ssio n (2)
e CT = 1D "" e A ~ W
F o r th e se naphthalene com plexes, th e e le c tro n affinities of the acceptor (Ea )
and the coulom bic te rm (W) a re not known.
F o r a s e r ie s of 18 iodine com plexes,
B riegleb and C zekalla (6) found th a t W w as constant.
T h ere does not seem to be
any evidence in the lite ra tu re that W w ill be constant fo r a s e r ie s of com plexes
w ith the sam e donor.
Since E a is not known fo r any of th e a c c e p to rs, the
n atu re of W fo r th e se naphthalene com plexes could not be a sc e rta in e d .
Even
w ith the n a tu re of W unknown, the energy of th e se c h a rg e -tra n s fe r com plexes
should be d ire c tly re la te d to th e e le c tro n affinities of the a c c e p to rs.
Thus the
o rd e r of e le c tro n affinities should be th e sam e as the o rd e r th a t was observed
fo r the e n e rg ie s of th e c h a rg e -tra n s fe r bands.
The o rd e r of e le c tro n affinities (more a cc u ra te ly , th e accep to r stre n g th s)
of th e se inorganic s a lts m ay be d iscu ssed in te rm s of soft acid s and b ases (in­
duced p o larizin g ab ilitie s) (74, 75).
The concept of h a rd and soft acids and
b a se s is b ased on e x p erim en tal o b serv atio n s.
It has been found th at c e rta in
cations (hard acids) fo rm the m o st stab le compounds w ith th e lig h test m e m b e rs
of a group (hard b a se s).
O ther cations (soft acids) fo rm the m o re stable com ­
pounds w ith the h e av ie r m e m b e rs of a group (soft b a se s).
b a se s the m ain in te ra c tio n s a re e le c tro s ta tic in n atu re.
In h a rd acids and
P o la riz a tio n effects
53
a re m o re prom inent in the- soft acid s and b a se s.
Soft b a se s a re c h a ra c te riz e d by re la tiv e ly high energy filled o rb itals
th a t a re e asily p o la riz ed . . N aphthalene and benzene a re included in this c a te ­
gory. . Soft acids a re m olecules o r cations th at a re usually la rg e in size and
a re e a sily p o la riz ed . . The stre n g th of th e se soft acids u su ally in c re a s e s as
you go down a c e rta in group.
F o r th e group 5a plus th re e ions, th is o rd e r
would b e: Bi(III) > Sb (III) > As (III).
T his should also be th e o rd e r of the t r i ­
c h lo rid es of th is group as e le c tro n a cc e p to rs.
Both the a cetate and the ch lo rid e ions a re h ard b a se s (acetate > chloride)
and should d e c re a se the soft c h a ra c te r of the m e rc u ry (II) ion.
On the other
hand, the brom ide ion should in c re a s e the soft c h a ra c te r of the m e rc u ry (II)
ion.
F ro m th e se soft acid co n sid e ra tio n s, th e following o rd e rs of e lec tro n
a ccep to rs can be obtained.
BiClg > SbClg > AsClg and HgBrg >HgClg >Hg(CgHgOg)Z
T h e ;m e rc u ry s a lts and the tin s a lt should have co n sid erab le soft c h a ra c te r, but
th e ir re la tiv e p o sitions in the o rd e r of e le c tro n accep to rs cannot be p red icted
fro m the known d a ta on soft acids.
IV.
THE NATURE OF CHARGE-TRANSFER INTERACTIONS
Bonding in te ra c tio n s in c h a rg e -tra n s fe r com plexes a re classifie d as
c h a rg e -tra n s fe r fo rc e s and c la s s ic a l e le c tro s ta tic fo rc e s (these include d ip o ledipole and dipole-induced dipole in te ra c tio n s).
M ulliken (68), in his original
tre a tm e n t of th e se com plexes, assu m ed th a t the c h a rg e -tra n s fe r fo rces would
be predom inant.
In a la te r publication (69), M ulliken has questioned this
assum ption and pointed out the evidence in support of the im p o rtan ce of c la s s i­
cal in te ra c tio n s in the energy of c h a rg e -tra n s fe r com plexes.
S everal authors
(15, 39, 64) have indicated a belief in the p reponderance of Coulomb and p o la ri­
zation (dipole-dipole and dipole-induced dipole) fo rc e s in the stab ility of c h a rg e tr a n s fe r com plexes, e sp ecially th o se of p i-p i n atu re.
P u re quadrupole re s o n ­
ance sp ectro sco p y d ata (45) and d a ta on th e B r-B r in te ra to m ic d istan ces (41),
fo r the b enzene-brom ine com plex in the so lid s ta te , in d ic a tes th a t the amount
of c h a rg e -tra n s fe r to the brom ine is v ery sm all.
The n atu re of the bonding in te ra c tio n s in the com plexes studied h e re can
be exam ined by co nsidering the changes in th e bond lengths in com plexed naph­
thalene (from the c ry s ta l s tru c tu re of the antimony trich lo rid e-n ap h th alen e
com plex as re p o rte d by Hulme and Szym anski (48) ) fro m th a t of p u re c ry s ta l­
lin e naphthalene (10).
Since bond lengths a re re la te d to bond o r d e r s , th e se
bond o rd e rs w ere calcu lated and com pared fo r both c a se s of bonding in naph­
thalene.
55
The bond o rd e r (n) fo r a p a rtic u la r bond is re la te d to. the num ber of
e le c tro n s a sso c iate d w ith th a t bond.
Pauling (73) used the following fo rm u la
to re la te th e se bond o rd e rs to the lengths of the bonds (bonds m u st be of a
s im ila r type).
D(n) = D]_ - 0.71 log n
H ere D (n) is the ex p erim en tal bond length and D1 is a c o rre c te d single bond
length (1.504 A).
Using th is fo rm u la and the bond lengths fro m X -ra y c ry s ta l
s tru c tu re d ata (10, 48), bond o rd e rs fo r the c ry sta llin e antim ony tric h lo rid e naphthalene com plex and fo r c ry sta llin e naphthalene w ere calcu lated and com ­
p a re d (table V).
Table V
Bond lengths and bond o rd e rs fo r naphthalene and com plexed naphthalene
(see fig u re I).
Bond
I
2
3
4
5
6
D(n) naphthalene
1.420
1.359
1,395
1.359
1.420
1.395
n
1.30
L 60
1.43
1.60
1.30
1.43
D(n) com plex
1.44
1.42
1.34
1.31
1.42
1.43
n
1.23
1.31
1.70
1.88
1.31
1.27
naphthalene
com plex
The calcu lated bond o rd e rs show a shift of e le c tro n d ensity to bonds 3
and 4 (the re g io n in which the naphthalene is weakly bonded to the antimony
56
tric h lo rid e m olecule).
C om paring the bond lengths and bond o rd e rs fo r p u re
naphthalene and naphthalene in the com plex, th e re seem s to be little o r no
significant shift of e le c tro n s out of the p i-s y s te m of the naphthalene to the
antim ony tric h lo rid e m olecule.
Thus it see m s re a so n a b le to assu m e th a t the bonding in the antimony
trich lo rid e-n ap h th a le n e com plex is p rin cip ally a c la s s ic a l e le c tro s ta tic in te r ­
action (dipole-induced dipole).
Since all the naphthalene com plexes studied
h e re a re of a s im ila r n a tu re , it is expected th a t the p rin c ip a l bonding in te ra c ­
tion in th e se com plexes is of the dipole-induced dipole type.
T h ere may be
m o re c h a rg e -tra n s fe r in te ra c tio n s in the s tro n g e r com plexes (stannic ch lo rid e
and bism uth tric h lo rid e ), but th is should account fo r v ery little of the bonding
even in th e se com plexes.
IV.
SIN G LET-TR IPLET ENHANCEMENT
The effectiveness of an a ccep to r as a heavy-atom p e rtu rb e r was d e te r­
m ined by m eans of enhancem ent fa c to rs and o sc illa to r stre n g th s (see e x p e ri­
m ental section).
T his o rd e r fo r th is effectiv en ess was found to be: bism uth
tric h lo r id e > m e rc u ric brom ide > m e rc u ric chloride > antim ony tric h lo rid e £5
m e rc u ric ace ta te> a rs e n ic tric h lo rid e .
atom effect is of a s p in -o rb it n a tu re .
It is g en erally a g re ed th at th is heavyT his sp in -o rb it n a tu re is observed when
com paring the s p in -o rb it constan ts (£) w ith the enhancem ent fa c to rs (see tab le
VI). . Except fo r the c ase of the m e rc u ric a cetate com plex, the enhancem ent
fa c to rs in c re a s e w ith in c re a sin g *§.
Table VI
C om parison of h eav y -ato m p e rtu rb e rs
A sC l3
. Enhancem ent facto r
O scillato r stre n g th
6 + 3 x 10-3
1.89 x 1 0 '8
Hg (CgH3 ©2)2
1.56 + .2 x I O '1
SbCl3
1.7
HgCl2
2.4 + .2 x IO"1
H gB r2
3.1 + .2 x IO"1
B iC l3
2.6
+ ,1 x IO"1
+ .1
i
1960
6080
9.55 x IO '7
4380
1.19 x IO"6
6080
6080
13900
58
The effectiv en ess of an accep to r as a heavy-atom p e rtu rb e r also in ­
c re a s e s w ith the stre n g th of the c h a rg e -tra n s fe r com plex fo rm ed .
The en­
hancem ent fa c to r fo r the m e rc u ric brom ide com plex is about double that fo r
the m e rc u ric a cetate com plex.
This in c re a s e in enhancem ent is attrib u ted to
the hig h er e le c tro n affinity (form s a s tro n g e r c h a rg e -tra n s fe r complex) of
m e rc u ric brom ide.
It is assum ed h e re th a t the heavy m etal of the sa lt m akes
the p rin c ip a l contribution to the s p in -o rb it coupling.
This m ay not be e n tire ly
tru e and som e of the effect observ ed h e re m ay be due to the brom ine atom s.
This sam e effect is o b serv ed when one co m p ares the enhancem ent fa c to rs fo r
m e rc u ric acetate and m e rc u ric ch lo rid e.
H ere the re la tiv e ly light chlorine
atom s should not contribute m uch to the sp in -o rb it coupling.
In unperturbed naphthalene, s in g le t-trip le t tra n s itio n s a re made slightly
allow ed through the m ixing (spin-orbit) of excited sin g let and trip le t states (by
stealin g in ten sity fro m sin g le t-sin g le t and tr ip le t- trip le t tra n sitio n s).
The
am ount of m ixing is in v e rse ly p ro p o rtio n al to the d ifference in energy of the two
s ta te s . In a heavy-atom p e rtu rb e d m o lecu le, it is also expected th at the s in g le ttrip le t tra n s itio n w ill gain inten sity through the mixing of e lec tro n ic sta te s . But
in the p e rtu rb e d sy ste m , one m u st also c o n sid er th e m ixing of the excited s ta te s
of naphthalene w ith the excited sta te s of th e heavy-atom p e rtu rb e r.
As d is ­
cu ssed p rev io u sly , it is th e o re tic a lly p re d ic ted th at th is m ixing may occur by
two differen t m ech an ism s (c h a rg e -tra n sfe r and exchange).
The s in g le t-trip le t
59
in ten sity p re d ic ted by both of th e se m ech an ism s depends on the fo u rth pow er
of the overlap in te g ra l betw een th e o rb ita ls of the heavy-atom p e rtu rb e r and
th e naphthalene m olecule.
B ecause of th is one m u st assu m e th a t both of th e se
m ech an ism s contribute to the in ten sity of the c h a rg e -tra n s fe r tra n sitio n s.
T h ere a re s e v e ra l fa c to rs th at indicate th at the c h a rg e -tra n s fe r m ech ­
anism w ill be m o st im p o rtan t in the sy stem s studied h e re .
F ir s t of all, it w as
found th a t the s in g le t-trip le t enhancem ent in c re a s e s w ith in c re a sin g stren g th
of the com plex.
Second, all the com plexes exam ined h e re have intense c h a rg e -
tr a n s f e r bands in the re g io n of th e f i r s t sin g le t-sin g le t band of naphthalene.
T hese c h a rg e -tra n s fe r s ta te s should have m uch of the c h a ra c te r of the excited
sta te s of the heavy-atom p e rtu rb e rs (large sp in -o rb it coupling).
This should
lead to c o n sid erab le m ixing (spin-orbit) of the excitfed s ta te s of naphthalene
(both sin g let and trip le t) w ith th e se c h a rg e -tra n s fe r s ta te s (see fig u re 4).
T his
s p in -o rb it m ixing p ro c e ss w ill s te a l in ten sity fro m the c h a rg e -tra n s fe r tr a n s i­
tio n s.
In the exchange m echanism , only the m ixing of the o rb ita ls of the heav y -
atom p e rtu rb e r and the naphthalene m olecule is con sid ered .
The p o ssib ility of
stealin g in tensity fro m the in ten se c h a rg e -tra n s fe r tra n s itio n s is not con sid ered .
T hese c o n sid eratio n s a re supported by s im ila r argum ents fo r the c h a rg e -tra n s ­
fe r m echanism in the enhancem ent of s in g le t-trip le t tra n s itio n s of aro m atic
compounds by heavy-atom solvents (63).
A ssum ing th at c h a rg e -tra n s fe r is the p rin cip al so u rc e of s in g le t-trip le t
enhancem ent in th e se com plexes, the way in which the in ten sity is borrow ed
60
fro m the charge-^transfer tra n s itio n s is su m m a riz ed in fig u re 7 .. The r e p r e ­
sen tatio n of the e le c tro n ic sta te s in th is fig u re is an o v ersim p lificatio n .
The
s ta te s re p re se n te d as p u re donor and acc e p to r s ta te s re a lly have som e c h a rg e tr a n s f e r c h a ra c te r in them .
c h a rg e -tra n s fe r; s ta te s .
This is due to the m ixing of th e se s ta te s w ith the
In the com plexes studied h e re , th e amount of c h a rg e -
tr a n s f e r c h a ra c te r in the f i r s t trip le t s ta te of naphthalene see m s to be sm all
since th e re is little change in the positio n s of the o b serv ed s in g le t-trip le t bands
com pared to the bands in the un p ertu rb ed m olecule.
Since the am ount of m ixing is in v e rse ly p ro p o rtio n al to the difference
in energy of the two s ta te s , th e re w ill be m o re m ixing of the naphthalene tr i p ­
le t s ta te w ith the f i r s t excited c h a rg e -tra n s fe r state in th e s tro n g e r com plexes.
Thus the s tro n g e r the c h a rg e -tra n s fe r com plex the m o re enhancem ent of the
donor s in g le t-trip le t tra n s itio n is expected (this is only tru e fo r accep to rs w ith
com parable sp in -o rb it coupling).
The o b serv ed enhancem ent of the sin g le t-
tra n s itio n s a g re e s w ith th is expected tre n d .
B enzene System s
The in ten sity of the diphenylm ercury sin g le t-trip le t band is approxim ately
double th a t of phenyIm e rc u rie acetate (see ta b le III).
The d ifferen ce in in te n s i­
tie s is explained by the p re se n c e of two benzene rin g s in th e diphenylm ercury
sy ste m . , C onsidering th a t the s p in -o rb it coupling in the m e rc u ry atom should
be the p rim a ry fa c to r affectin g th ese in te n s itie s, th is re s u lt is to be expected.
61
s —>'s
ACCEPTOR
F ig u re 7.
WEAK COMPLEX
NAPHTHALENE
The m an n er in which s in g le t-trip le t tra n s itio n s may
ste a l in ten sity fro m a c h a rg e -tra n s fe r tra n s itio n .
The
a rro w s re p re s e n t o b serv ed tra n s itio n s and th e wavy lines
indicate a sp in -o rb it in te ra c tio n betw een two s ta te s .
62
The in te n sity of the m e rc u ric chloride enhanced bands a re approxim ately half
that of the p h en y lm ercu ric a cetate bands.
T his differen ce is probably due to
d ifferen t am ounts of m ixing of the m e rc u ry (or m e rc u ry ch lo rid e) sta te s w ith
the 77-states of benzene.
In the m e rc u ric ch loride-b en zen e com plex, the m ixing of th e se s ta te s
m o st likely o ccu rs by a c h a rg e -tra n s fe r m echanism (sam e argum ents apply
h e re as did fo r the naphthalene com plexes).
The m ixing of e lec tro n ic s ta te s
of benzene and m e rc u ry in the covalently bonded compound w ill probably in ­
volve m ixing w ith in te rm e d ia te states*
The sig m a bonded m e rc u ry should lead
to m o re overlap of the o rb ita ls of th e benzene m olecule and. the m e rc u ry atom .
C onsidering th is alone, the covalently bonded m e rc u ry should cause m ore en ­
hancem ent of the benzene s in g le t-trip le t tra n s itio n s th an th e c h a rg e -tra n s fe r
bonded m e rc u ry .
APPENDIX. A
C alculation of Spin-O rbit C onstants
64
Spin-O rbit Coupling C onstants
The approxim ate values of th e sp in -o rb it coupling co n stan ts w ere ob­
tain ed using the Lande in te rv a l ru le (9). T h is ru le sta te s th a t the in te rv a l
betw een two adjacent energy le v e ls of an atom ic state (term ) is p ro p o rtio n al
to the hig h er J value of the two te r m s .
In th e f ir s t approxim ation, th is con­
stan t is the sp in -o rb it coupling constant.
constants
The approxim ate sp in -o rb it coupling
) w ere obtained fro m th e m u ltip let spacings in the P - sta te s of the
m etal atom s of the v a rio u s s a lts .
T hese m u ltip let spacings w ere obtained
fro m a com pilation of atom ic energy le v e ls by M oore (67).
The sp in -o rb it
coupling constant,, is the o bserv ed m u ltip let spacing divided by J .
T hese
m u ltip let spacings and the observ ed sp in -o rb it coupling co n stan ts fo r th e se P s ta te s a re tabulated i n ta b le I VIL
6'5-
Table VII
TABULATION OF SPIN ORBIT COUPLING CONSTANTS AND
M U L Tli5LET SPACINGS
Ion
Zri(II)
Cd(II)
Hg(II)
AS(IlI)
Sb(III)
Bi(IH)
Sn(IV)
L evel
J
E nergy .(K)
2P
1/2
48480
3 /2
49354
1/2
44136
3 /2
46618
1/2
51485
3 /2
60608
1/2
0.000
3 /2
2940
1/2
0. 000
3 /2
6576
1/2
0. 000
3 /2
.20788
1/2
0.000
3 /2
6508
2P
2P
2P
2P
2P
2P
In te rv a l
3 (K)
788
3583
2482
1650
9183
6080
2940
1960
6576
4380
20788
. 13900
6508
4340
/'
APPENDIX B
Singlet-rT rip le t S pectra
OQ
.2
4000
3600
4400
4800
W avelength (A)
Spectrum No. I .
(I) D egassed naphthalene in 10 cm c e lls, re fe re n c e a ir.
thalene plus oxygen in 10 cm c e lls, re fe re n c e a ir.
(2) Naph­
4000
4500
W avelength (A)
Spectrum No. 2.
naphthalene
0.15 m olal m e rc u ric chloride in naphthalene, 5 cm c e lls, re fe re n c e
4300
4500
4700
W avelength (A)
S p ectru m No. 3.
c e lls.
0.15 m o lal m e r c u r ic b ro m id e in nap h th alen e, naphthalene re fe re n c e , 5 cm
70
4000
4300
W avelength (A)
Spectrum No. 4.
0.10 m olal m e rc u ric acetate in naphthalene,
10 cm c e lls, a ir re fe re n c e .
4500
5000
W avelength (A)
Spectrum No. 5.
5 cm c e lls.
0.50 m olal antimony tric h lo rid e in naphthalene, naphthalene re fe re n c e ,
4500
5000
W avelength (A)
S p ectru m No. 6.
3.25 m o lal a r s e n ic tr ic h lo r id e in nap h th alen e, a ir re f e r e n c e , 10 cm c e lls .
4700
5000
5500
W avelength (A)
Spectrum No. 7.
re fe re n c e .
0.010 m olal bism uth tric h lo rid e in naphthalene, 5 cm c e lls, naphthalene
4500
5000
S p ectru m No. 8 .
re fe re n c e .
2.77 x 10 3 m o lal stan n ic c h lo rid e in n ap h th alen e, 10 cm c e lls , a ir
3200
3300
W avelength (A)
S p ectru m No. 9.
0.02 m o la r m e r c u r ic c h lo rid e in b en zen e, 10 cm c e lls .
2900
3200
W avelength (A)
S p ectru m No. 10.
0.10 m o la r p h e n y lm e rc u ric a c e ta te in g la c ia l a c e tic acid , 10 cm c e lls .
2900
3200
3500
W avelength (A)
S p ectru m No. 11.
0.05 m o la r dipheny!m e rc u ry in c h lo ro fo rm , 10 cm c e lls .
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M O N TA N A S T A T F u u n / c o c t ™
__ ________
Iu / a o
O