The methyl rotor as a probe of electronic character via fluorescence excitation of para-substituted
toluenes
by Steven Gregory Mayer
A thesis submitted in partial fulfillment of thei requirements for the degree of Master of Science in
Chemistry
Montana State University
© Copyright by Steven Gregory Mayer (1990)
Abstract:
The laser induced fluorescence excitation spectra of p-toluidine and p-methylstryrene, were obtained
by the method of supersonic jet molecular spectroscopy. In this work, the methyl rotor was used as a
probe of electronic character. The spectrum of p-toluidine revealed the effect of time dependent
coupling of the methyl rotor and NH2 motion through the benzene ring. The spectral manifestation of
this interaction is a splitting of the peaks involving amine motion at 709 cm^-1 (assigned as T0^2) and
736 cm^-1 (assigned as I0^2) which was absent in the aniline spectrum. The p-methylstyrene spectrum
revealed a paradox when compared to previous work done on styrene. The p-methylstyrene spectrum
showed significantly more Franck-Condon activity in several modes than did styrene which would
indicate that the methyl group in the para position had a significant effect on ring-double bond
conjugation. This is exactly opposite to the effects seen in trans-stilbene and p-methyl-trans-stilbene
which have relative intensities that are very similar to each other. However, when the p-methylstyrene
and p-methyl-trans-stilbene spectra are compared, the methyl rotor structure is markedly different.
Therefore, it is concluded that the remote ring in stilbene had a significant effect on π electron
interaction in the para position on the main ring.
THE M E THYL R O T O R AS A PROBE OF ELECTRONIC CHARACTER
VIA
FLUORESCENCE EXCITATION OF PARA-SUBSTITUTED TOLUENES
by
Steven Gregory Mayer
A thesis submitted in partial fulfillment
of thei requirements for the degree
Of
Master of Science
in
Chemistry
M O N T A N A STATE UNIVERSITY
Bozeman, Montana
May 1990
/2?W f
ii
APPROVAL
of the thesis submitted by
Steven Gregory Mayer
This thesis has been read by each member of the thesis
committee and has been found to be satisfactory regarding
content,
English usage,
format,
citations,
bibliographic
style, and consistency, and is ready for submission to the
College of Graduate S t u d i e s .
D Ca'
—'
—-
Chairperson^Graduatfe Committee
A p p roved for the Major Department
IolJc
Date
Head, Major Department
A p p roved for the College of Graduate Studies
Dat-
Gz.CiwtClUT= WTCLll
ili
STATEMENT OF PERMISSION TO USE
In presenting this thesis in partial fulfillment of the
requirements
University,
for
a
m a s t e r rs
degree
at
Montana
State
I agree that the Library shall make it available
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from this
thesis
are
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permission,
provided that accurate acknowledgement of source is made.
Permission for extensive quotation from or reproduction
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either,
by
the
purposes.
for
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Any copying of use of the material xn this thesxs
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Date
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iv
ACKNOWLEDGEMENTS
The completion of a thesis is the final culmination of a
series of events in one's academic career.
without
its hardships
Min e has not been
or its private victories
and I w o u l d
like to express m y deepest appreciation and love to those who
were most vital in seeing me to m y destination, m y family. It
has been their unending love and support that allowed me to
successfully
complete
dedicate this thesis.
my
academic
pursuits.
To
them
I
V
TABLE OF CONTENTS
Page
A C K N O W L E D G E M E N T S ....................................
TABLE OF C O N T E N T S ..............
LIST OF T A B L E S ......
LIST OF F I G U R E S . . ......................
A B S T R A C T ....................... ..... ................
INT R O D U C T I O N ......................
iv
v
v ii
v iil
ix
I
Supersonic Jet Expansion T h e o r y ..............
I
Laser System. .........
2
The Methyl Rotor. ..............................
3
Time Dependent Coupling Between Para
S u b s t i t u e n t s ..............
13
E X P E R I M E N T A L ........................
14
THE ELECTRONIC CHARACTER OF P A R A - TOLUIDINE
EVIDENCE F O R TIME DEPENDENT COUPLING BETWEEN
PAR A S U B S T I T U E N T S ..............
18
Results and Interpretation. ................
18
Totally Symmetric Bands ....................
23
Methyl Rotor S t r u c t u r e .....................
23
Amine Inversion and Torsion M o d e s ........
24
D i s c u s s i o n ..............
32
C o n c l u s i o n ................................
35
vi
TAJBLE OF CONTENTS - Continued
Page
THE PARA-METHYLSTYRENE S P E C T R U M .... ..........
37
I n t r o d u c t i o n . .....................
37
E x p e r i m e n t a l ..................
33
Results and D i s c u s s i o n ............ . . . ....... .
41
C o n c l u s i o n .......................................
43
R E F E R E N C E S ............................................
45
vii
LIST OF TABLES
Table
Page
1.
The C 2v point group character t a b l e . . ; ...............
20
2.
The G 6 molecular symmetry
20
3.
The vibrational and methyl rotor b and assignments
of p - t o l u i d i n e ..........................
group character t a b l e ....
22
4.
The normal modes of vibration for a molecule with
C 2v s y m m e t r y ....................... ................... . . 25
5.
The methyl rotor structure interpretation and
analysis of the p-toluidine spectrum
31
viii
LIST OF FIGURES
Figure
1.
Page
Neuman projections of the methyl rotor showing 3
and 6 fold p o t e n t i a l s ................................
5
Electronic transition on Morse potential curves
illustrating the Franck-Condon p r i n c i p l e ...........
10
The thirty benzene-like modes of vibration for any
six m e m b e r e d ring s y s t e m .............................
12
6.
Block diagram of the a p p a r a t u s ......................
16
7.
Ellipsoidal reflector light collection s y s t e m .......
17
8.
The jet-cooled vibronic spectrum of p - t o l u i d i n e ...
21
9.
Expanded view of
the spectrum
at the o r i g i n .......
26
10.
Ex p anded view of
the spectrum
at the 6ag b a n d .....
27
11.
Expan d e d vie w of
the spectrum
at the I q b a n d ......
28
12.
The "Hot" vibronic spectrum of p - t o l u i d i n e ........
29
13.
The spectrum of p-toluidine
(ND2). . ; . . ;...........
30
14.
The p-methylstyrene spectrum (a) at 60°C after ~5
minutes and (b) at 60°C for ~ 2 h r s ................ .
39
The jet-cooled p-methylstyrene spectrum with
sample at room t e m p e r a t u r e ....................
40
2.
3.
15.
ix
ABSTRACT
The laser induced fluorescence excitation spectra of ptoluidine and p - m e t h y l stryrene, were obtained b y the method of
supersonic jet m olecular spectroscopy.
In this work, the
methyl rotor was use d as a probe of electronic c h a r a c t e r . The
spectrum of p-toluidine revealed the effect of time dependent
coupling of the meth y l rotor and N H 2 m o tion through the
benzene ring.
The spectral manifestation of this interaction
is a splitting of the peaks involving amine m o t i o n at 709 cm-1
(assigned as T^) and 736 cm-1 (assigned as l|) w h i c h was absent
in the aniline spectrum.
The p-methylstyrene spectrum
revealed a paradox when compared to previous work done on
styrene.
The p-methylstyrene spectrum showed significantly
more Franck-Condon activity in several modes t han did styrene
which w o u l d indicate that the methyl group in the para
position h a d
a significant
effect
on ring-double b o n d
conjugation.
This is exactly opposite to the effects seen in
trans-stilbene and p-methyl-fcrans-stilbene which have relative
intensities that are very similar to each other.
However,
when the p-methylstyrene and p-methyl-trans-stilbene spectra
are
compared,
the
methyl
rotor
structure
is
markedly
d i f f e r e n t . Therefore, it is concluded that the remote ring in
stilbene h a d a significant effect on it electron interaction in
the par a position on the main ring.
I
INTRODUCTION
Laser induced fluorescence excitation spectroscopy is an
extremely
useful
method
for
determining
conformations
and
barriers to internal rotation for molecules which contain a
methyl rotor.
The results of such experiments are of interest
not only to physical chemists but also to synthetic chemists
who
are in
upon
the
search of exact mechanistic pathways
accurate
knowledge
of
electron
and depend
densities
in
the
molecule of i n t e r e s t .
A
state
multitude
of
conformational
spectroscopic methods
studies
microwave spectroscopies.
exist
for ground
such as R a m a n f F T I R f N M R f and
H o w e v e r f excited state spectra of
molecules were found inaccessible by these means due to the
large
spectral
congestion
caused by
hot bands
(transitions
originating from vibrationally p opulated ground state levels) .
Recently,
the excited state has b een accessed v i a the method
known as supersonic jet molecular spectroscopy.
Supersonic Jet Expansion Theory
When
a
gas
is
expanded
the
t hrough
temperature
a
of
small
the
orifice
at
molecule
is
supersonic
speeds,
decreased.
A ccording to the Boltzmann distribution N 2ZN1 =
2
e -E/kT,
the
vibrational
states
above
v =0
in
the
electronic state are not appreciably p o p u l a t e d .1
ground
There are
many bina r y collisions at the orifice and the thermal energy
prod u c e d is converted into directed flow as. the
through
the
orifice
into
the
v a cuum
chamber.
gas passes
Since
the
velocity distribution is narrow, the translational temperature
of the gas is l o w e r e d .2
Acco r d i n g to statistical mechanics theory, molecules can
be cooled by c o l l i sionally imparting energy to the carrier gas
atoms
inside
the
vacuum
chamber.
accelerating the molecule to the
This
same
has
the
effect
speed as the
of
carrier
g a s . By not populating the energy levels corresponding to the
internal degrees of freedom of the molecule,
the rotational
transitions are "cooled o u t " which simplifies the spectrum due
to line narrowing in the vibrational b a n d s .2
Laser System
The frequency range necessary to excite the transition
S 1 «— S0 in para-substituted toluenes is on the order of 32000
- 36000
cm""1 .
molecules,
The use
In order to resolve
structure
of
a high resolution excitation source is required.
of tuneable
molecular
rotational
dye
transitions
offers
n eeded for this method.
application
is
lasers
as the pump
the high
resolution
for the
and power
The system of choice for the present
a N d :YAG p u mped dye
operate at either 1064nm,
source
532nm,
laser.
The N d : YAG
can
or 355nm depending upon the
3
dye pum p i n g r e q u i r e m e n t s .
Since the molecular signal is relatively weak compared to
the laser light,
scattered light inside the detection system
presents a major problem.
Therefore, great care must be taken
to ensure that scattered light is reduced to a minimum.
The
two main factors which cause this problem are b e a m quality and
poor alignment along the vacuum chamber axis.
is
a p r o b l e m that
laser.
arises
Theoretically,
should have near
uniform.
in the
the
beam
TEM0^o mode
amplifier
from
structure
the
cell
Beam quality
in the
oscillator
dye
cell
if the p ump bea m is
If this is the case, then bea m quality is strictly
a function of amplifier alignment and pumping scheme.
Careful
optimization of the dye laser will yield a comet shaped beam.
The comet shape is due to the configuration of the amplifier
cell
and
the
discussion
side
of
pumping
this
of
p roblem
the
will
N d :YAG
be
beam.
addressed
Further
in
the
EXPERIMENTAL section.
The Methyl Rotor
The methyl rotor is a CH3 group attached to the molecule
of interest.
The quantum mechanical treatment of this system
uses the particle in a ring wavefunction (exp(±im©)) perturbed
by a
(1 /2 )V n [1-cos (n©) ] p otential4'5 which has the form
J [exp-(±im©) ] (1 /2 )V n (1-cosn©) [exp (±im©) ]d ©
(I)
4
Since the methyl rotor has three equivalent positions for the
h y d r o g e n s r the potential must go in multiples of t h r e e .
(1/2)V 3 [ 1 -cos(30) ] +
(1/2) Vg [I-cos (60) ]
+
+ ...
(1/2)V 3 [I- c o s (90)]
(2)
The source of the hindering potential is us e d to describe
the interaction of molecular orbitals between the methyl group
and the ring.
This phenomenon is known as hyperconjugation.
Several
g r o u p s , b oth
nature,
have
influence
internal
experimental21
identified
and
hyperconjugation
on the p referred conformation
rotation
computational22
for methyl
groups
as
the
in
dominant
and the barrier to
attached to
it electron
containing s y s t e m s . Since hyperconjugation is the interaction
between Tt-Iike orbitals on the methyl group and the TC electron
density
in the
vicinity
of the
attached C H 3 , spectroscopic
determination of the methyl behavior
(conformational changes
upon excitation and barriers to internal rotation) can be used
as a probe of local TC electron density.
be
extracted
from
excitation
and
This information can
dispersed
fluorescence
spectra by fitting the experimental frequencies for the methyl
rotor to those calculated from the matrix elements of equation
I .23
The exponentials are the unperturbed particle in a ring
wavefunctions and the cosine ter m represents the barrier to
internal rotation.
Intensities can be fit via Franck-Condon
5
factors
determined
^i^^onsiization
of
using . eigenvectors
the
aforementioned
obtained
matrix.
from
The
conformational change between the ground and e xcited states is
obtained b y phase
until
a
best
shifting the
fit
for
spectrum is obtained.
excited
relative
state wavefunctions
intensities
in
the
rotor
The FORTRAN pro g r a m R O T I N running on a
MicroVAX was u s e d for this p u r p o s e .17
Figure
la
and
lb
are
the
Neuman
projections
which
physically illustrate the concept of interchange of hydrogen
nuclei.
The point at the center represents the carbon and the
lines are the C-H bonds.
Two equivalent positions are shown,
and there are' a total of six equivalent positions which are
obtained by rotating the methyl group b y ± 1 2 0 °.
(A)
(B)
Neuman projections of the methyl rotor showing 3 and 6 fold
potentials
FIGURE I
6
There
correctly
tunneling
internal
is
no
point
describes
the
group
permutation
of the Me-hydrogens
rotation.
The
symmetry
of
through the
correct
treatment
operation
nuclei
which
caused
by
small barrier to
is the molecular
symmetry group approach used by Longuet-Higgins6 and B u n k e r .7
The molecular symmetry groups allow one to classify the spin
states,
states of nuclear motion,
non-rigid molecule.
and electronic states of a
Since the molecules of interest can pass
from one conformation to another,
set
of
all
identical
permutations
nuclei
corresponding
which
of
the
appear
i n v e r s i o n s . As
it is necessary to define a
reasonable
m entioned
influence on the methyl behavior
and barrier height)
positions
and
spins
along
above,
with
the
of
the
dominant
(conformational preference
is hyperconjugation,
and since these
properties of the rotor can be m e a sured spectroscopically, the
C H 3 group can be developed as a sensitive probe
electron density.
F u r t h e r m o r e , extracting this
of local TC
information
from the spectrum is often straightforward since the methyl
torsion
is typically the
lowest
frequency vibration
in the
molecule.
The energy levels for the methyl rotor go as Oa1, I e , .
2e,
3ai
4e,
Se,
Sa1, Sa2 ,
where
a and ©
are the
first two torsional energy levels of the methyl rotor and the
0,
I,
2,
levels.
are B1
3,
...
are
the
quantum
numbers
of
the
torsional
The selection rules for the methyl rotor transitions
B 1 , a2
a2 r e
e.
The nuclear spin statistical
7
weights are 2 and I for the a and o levels respectively.
This
means that there are twice as man y molecules with a symmetry
as there are with <a symmetry.
The contour of the methyl rotor structure in the spectrum
reveals the degree of conformational change in the molecule
upon e x c i t a t i o n .
The molecular conformation is dependent upon the change
in the equilibrium
internuclear distance
ground to the excited state.
in going
from the
If the change is small then the
only vibrational b a n d with appreciable intensity will be the
origin.
If the
change
is large then there will be
a long
progression in the intensities of the bands w ith respect to
frequency.
principle.
are
This
phenomenon
is
known
as
the
Franck-Condon
The relative intensities of the vibrational bands
directly proportional
to the
square
of the vibrational
overlap i n t e g r a l .8
U V'vib'Yvib" ^ ! 2 = U v ' Iv">l2
The
square
of the
overlap
O)
integral between the vibrational
wave functions of the upper and lower states of the electronic
transition
is
Franck-Condon
called the Franck-Condon
factor
is
a
quantitative
factor,
qv ,v„.
assessment
of
The
the
degree of vibrational overlap between two electronic s t a t e s .
If we sum over all the vibrational states, equation 3 becomes
8
Z
|<v' Iv ")|2
= Z (vr Iv")(v" Iv r)
(4)
= (vr Iv f) = I
Figure
2
is
a
physical
illustration
of
the
Franck-Condon
principle.
If
integral
the
Born-Oppenheimer
containing
wavefunctions
coordinates,
can
bot h
be
approximation
electronic
written
in
is
and
terms
of
Q and the electronic coordinates,
(Ve" (CfQ)Vv" (Q)
assumed,
the
vibrational
the
nuclear
ar.
I H r I Ye' (CfQ)Vv' (Q))
(5)
The standard approach to evaluating this integral in order to
find the relative vibronic intensities is to use the Condon
a p p r o x i m a t i o n .9
integral
over the
nuclear position.
The
Condon
electronic
approximation
coordinates
implies
is
that
independent
the
of
Equation 5 can then be written as f o l l o w s .
<Vv" (Q) I <Ve»I Hr I Ve'> I Vv'(Q)>
= <Vv»lHe"e' (Q) I Vv')
(6)
I
9
If |Ae"er (Q) is assumed to be a constant then equation 6 can be
written as follows.
<Ve"IHxyzl V e ' X Y v " l Y v ' >
(?)
The second integral of equation 7 is the Franck-Condon overlap
integral.
2 where
The result of this overlap is illustrated in Figure
the
electron
undergoes
a vertical
transition
to
a
higher electronic state on the potential curves with the same
internuclear separation as it h a d in the ground state.
In
order for the transition to be allowed there must be a finite
amplitude
in
the
excited
vibrational w a v e f u n c t i o n .
state
for
the
ground
state
The degree of overlap between the
ground and excited state vibrational wavefunctions determines
the
conformational
change
in
the
excited
state
of
the
when
the
molecule.
Acco r d i n g
to
the
Boltzmann
distribution,
temperature is approximately 20 K or lower,
only the v=0
level in the ground state has appreciable population.
This
temperature range can be achieved for a molecule when i t .is
jet cooled.
Therefore, unless vibronic coupling is occurring
in the molecule,
only the totally symmetric vibrations will
.appear in a jet cooled spectrum.
integral
(equation
7)
conditions are implied.
If the Condon approximation
is evaluated,
the
following
symmetry
ENERGY
10
INTERNUCLEAR SEPARATION
Electronic transition on Morse potential curves illustrating
the Franck-Condon p r i n c i p l e .
FIGURE 2
11
TeM ®
^xyz ®
^e'
(8 )
F v ii ®
However,
F v , CZ
since
each
vibration
corresponds
to
a
different
dimension in the S1 state, the v=0 vibration in the S q state
can populate two quanta of an asymmetric vibration which gives
a totally symmetric band.
F vi, ®
This can be shown mathematically by
F vl, c A 1
(9)
It follows then that the vibrations can go as multiples of two
quanta in each dimension and appear in the spectrum.
In
the
importance,
group.
case
it
However,
is
where
the
necessary
methyl
rotor
to
the
use
structure
molecular
is
of
symmetry
standard point group theory is sufficient to
describe the molecule for symmetry assignments of the normal
modes of vibration.
Figure 3 lists the 30 benzene-like modes
of vibration for any six membered ring s y s t e m .10
Time Dependent Coupling Between Para Substituents
Analogous to the methyl rotor are the interactions of the
functional group para to the methyl group.
amine
group
(p-toluidine)
the
hydrogens
In the case of an
can
invert
or
the
entire group can have a torsion much like the methyl torsion.
12
^ 3fC
The
thirty
benzene-like
modes
of
membere d ring system
FIGURE 3
vibration
for
any
six
13
^ r ev**-0118 work done on aniline has shown that the amine group
is non-planar in the ground state and that the NH 2-inversion
has a two-fold potential barrier.
The
interesting
possibility
is
that
a
time
dependent
wavefunction can couple the two para-substituents through the
TC system on the ring.
The results of a series of studies on
P ara-su^sti t u t e d toluenes promises to reveal information about
electron densities on aromatic rings and ultimately lead to
quantitative results regarding r e g i o - s p e c i f i c i t y .
deals with the initial
and p - m e t h y l s t y r e n e .
investigations
This paper
involving p-toluidine
14
EXPERIMENTAL
The m e t h o d of Supersonic Jet Molecular Spectroscopy was
us e d to investigate the electronic character and methyl rotor
structure
apparatus
nozzle
of
p-toluidine
and
p-methylstyrene.
consisted of a General Valve
mou n t e d
on
one
axis
of
a
six
series
The
9 pulsed
jet
jet
inch n o m i n a l 7 six-way
cross purc h a s e d from M D C . The pumping system was a Varian VHS6 , six inch diffusion pump w ith a Varian
backing p u m p .
SD 450 mechanical
The capability of the system was on the order
of IO"7 torr without the p u lsed jet nozzle operating and IO"5
torr wit h the nozzle o p e r a t i n g .
The molecular excitation
source was
a Lumonics
HY
750
N d : YAG p u m p e d dye laser model HD 300 with a Hypertrack 1000
autotracker
for
frequency
doubling.
The
laser after frequency doubling was 0.14cm
frequency doubled to
afforded
efficient
linewidth
.
of
The Nd.YAG was
obtain the green bea m at 532nm.
pumping
of
the
dyes
doubled region 33000 cm "1 to 35000 cm"1 .
the p-tol u i d i n e and p-toluidine
in
the
the
This
frequency
The dyes used for
(ND2) experiments were Kiton
R e d 620, Rhodamine 590/Rhodamine 610, and Rhodamine 590.
The
dyes us e d for the p-methylstyrene experiment were Rhodamine
590 and Coumarin 540A.
All dyes were obtained from E x c i t o n .
15
Beam quality of the laser was greatly improved with the
use of a Bethune cell in place of the amplifier cell in the
dye
laser
perfect
cavity.
The
p umping
cylindrical b e a m at the
uniform
near
Ultimately,
Gaussian
scheme
provided
laser o u t p u t .
far-field
energy
a
nearly
This gave
a
distribution.
the scattered light inside the detection system
was reduced and the signal-to-noise ratio was increased.
The detection
normal
to
system
the
system was
laser
consisted
on the
excitation
of
a
Melles
axis
axis
Griot
of
jet expansion
(see Figure
REM
014
7) .
The
ellipsoidal
reflector, an EMI 9813 QB photomultiplier tube, and supporting
electronics.
p laced
in
An
front
iris
of
and
the
appropriate
cutoff
photomultiplier
filters
tube
to
were
reduce
scattered light from the fundamental excitation wavelength.
Data aquisition was by a Zenith ZF 248 microcomputer with
supporting
software
by
Stanford
Research
accompanied the SRS modular boxcar a v e r a g e r .
Systems
which
Synchronization
was pro v i d e d by a homebuilt signal delay g e n e r a t o r .
The p-toluidine and p - m e t h y lstyrene were obtained from
Aldrich
(99.9%).
The deuterated p-toluidine was synthesized
in the lab by our g r o u p .
in
a
stainless
steel
The molecule of interest was placed
pressure
sealed tube.
. The
tube
was
connected to the backside of the nozzle and pressurized with
4 atmospheres high purity grade He.
Both p-toluidine samples
were he ated to 60° C and the p-methylstyrene was run at room
temperature.
16
fokpzns
SYSTEM
DRIVER
SAMPLE CHAMBER
VACUUM CHAMBER
NOZZLE
ELLIPSOIDAL
FAST PHOTODIODE
BEAMSTEERER
LENS
OPTICAL
BREADBOARD
COMPUTER
DYE LASER
Block Diagram of the Apparatus
FIGURE 6
17
Reflector
Laser
Ellipsoidal reflector light collection system
FIGURE 7
18
THE ELECTRONIC CHARACTER OF PARA-TOLUIDINE
EVIDENCE F O R TIME DEPENDENT COUPLING BETWEEN P A R A SUBSTITUENTS
Results and Interpretation
The
jet-cooled
toluidine
is
fluorescence
shown
in
assignments in Table 3.
excitation
Figure
8
with
spectrum
the
of p-
vibrational
A qualitative assessment can be made
regarding the methyl rotor structure,
and the normal modes of vibration.
conformational changes
The methyl rotor structure
at the 0§, 6a J , and IJ ban d regions exhibit 0 ° conformational
changes
this
(see Figures
lack
of
change
10,
11,
is
that
12) .
The evidence to
nearly
all
the
support
intensity
is
contained in the first b a n d of the methyl rotor structure.
A n interesting feature of the p-toluidine spectrum is the
two
quartet
These
two
structures
structures
centered
are
at
proposed
709
to
cm 1 and
be
the
736
two
cm
quanta
inversion and the two quanta torsion of the amine g r o u p .
peak
at
297
cm"1
is
a
hot
band
determined
to
be
The
ij; by
comparison with the high temperature.spectrum of p-toluidine
(see Figure 13) .
The ban d at 376 cm”1 is u n a s signed but does
form combination bands.
It is labeled ? in the spectrum.
If the C H 3 and N H 2 groups are assumed to be point masses,
p-toluidine has C 2v s y m m e t r y .
This is a reasonable assumption
19
since
the
normal
mode
vibrations
are
not
expected
to
be
strongly coupled to the rotation of the methyl group or the
tunneling of the N H 2 protons.
for the C2v point group.
Table I is the character table
This symmetry condition was found to
be reasonable for assignment of the normal modes of vibration.
However,
taking
in
into
order
to
account
properly
describe
the methyl
rotor,
the
it
molecule
was
while
necessary
to
define the appropriate molecular symmetry group (see Figure 9,
Table 2 12) .
Table 313 lists the normal mode assignments made
according to previous ground state w o r k ,14,15,16
20
Table I .
The Cov point group
^ X Z -- S y 2-
Table 2.
1
I
I
I
I
I
-I
-I
I
■1
I
-I
I
■1
-I
I
The G 6 molecular symmetry group
(123)
(23) *
I
I
I
1
I
- I
2
-I
O
E is the identity operation,
(123) is permutation of three identical nuclei
(hydrogens of the methyl rotor), and (23)* is permutation of hydrogens 2 and
3 followed by inversion of all coordinates■
33100
33300
FIGURE 8
33500
33700
Tha Jet Cooled Vibronlc Spectrum of p-Toluidine
33900
22
Table 3.
Vibrational band assignments of p-toluidine
BAND
FREQUENCY*
BAND
°0
+ 2e
0 cm-1
18
51
297
IoTo
7*0
6a&12&
+ 3*1
?
6aJ
+ 2e
+ Sa 1
(NO
SaJlJ
Io
12l
Io
+ 2e
6a§
1 6b*
12lll
6ajTg
6ajl§
? 1 2
*
q
376
432
450
481
709
731
736
792
820
838
864
898
1089
1142
1168
1168
FREQUENCY
1176
1195
1222
1248
1296
1329
1418
1501
1532
1548
1574
1588
1600
Gaolo
6aj*
6ajl6b§
Tq
IZ1
0T2
0
I q tO
-
GaoT o
I2O
GaoIo
12olo
6aj7aj
IGboIo
7a^
7a%l3
lo7a%
1610
1627
1633
1902
1930
2011
All frequencies are given as displacements from the origin.
23
Totally Symmetric Bands
It
was
exception
stated
of
earlier
in
this
paper
modes
assigned to
with
combination b a n d s r only the t otally
bands appear in the jet-cooled spectrum.
normal
that
and
the
combination
spectrum.
bands
The
symmetric
Therefore, only the
of
symmetry
the
symmetry
condition
were
for the
combination bands to appear in the spectrum is given by
r VX
®
r VZ
<= A 1
(11)
The normal modes of vibration for any benzene-like molecule
are given in Figure
3.
The correct
symmetries
for the
point group are given in Table 4.
Methyl Rotor Structure
The barriers to internal rotation for all regions in the
p-toluidine spectrum were fit using a FORTRAN pro g r a m on the
MSU V A X 8550 c l u s t e r .17
The barrier heights for the Og and
6aJ regions were determined to be 3-fold potentials with no 6fold
character
potential
is
exhibited
indicative
by
of
the
an
methyl
rotor.
inequivalence
of
A
3-fold
TC electron
densities in the two positions met a to the CH3 w h i c h is caused
by the asymmetry of the para substituents
and CH3) .
(in this case, NH2
24
Amine Inversion and Torsion Modes
Vibrational modes due to motion of the amine group have
been assigned to three peaks in the spectrum.
These.also form
various combination bands with other normal mode vibrations.
The
pea k
at
concluded b y
297
cm-1 is
comparison
currently
with the
assigned
.
This
high temperature
was
spectrum
where the intensity of the peak was muc h larger than in the
jet-cooled spectrum (see Figure 13) .
The two sets of quartets
with
cm-1
max i m u m
peak
heights
at
709
and
736
cm-1 were
concluded to be motions of the amine group by comparison with
the spectrum of the deuterated molecule
two
quartets
were
observed
to
undergo
(see Figure 14).
a
large
red
The
shift
relative to the origin and also a shift with respect to each
other.
Only modes which involve nearly exclusive motion of
the amine protons should show this large a deuterium shift.
Two such motions proposed in this region are inversion of the
amino hydrogens and torsion of the entire group with respect
to
the
ring.
Previous
work
by
Brand
et
al.
on
aniline
revealed that these modes do exist, although, it has not been
determined which p eak belongs to which motion.
Tentatively,
the peaks are labeled as 709 cm-1 = T q and 736 cm -1 = I^.
25
Table 4.
Normal modes of vibration for a C->„ m o l e c u l e .
MODE
SYMMETRY
MODE
SYMMETRY
I
Al
11
Bi
2
Ai
12
Al
3
B2
13
Al
4
bI
14
B2
5
Bi
15
B2
6a
Ai
16a
A2
6b
B2
16b
B1
7a
Al
17a
A2
7b
B2
17b
Bi
8a
A1
18a
Al
8b
B2
18b
B2
9a
Al
19a
Al
9b
B2
19b
B2
IOa
A2
20 a
Al
IOb
B1
20b
B2
33070
33090
FIGURE 9
33110
Expanded View of the Spectrum at the Origin
33130
33490
33510
FIGURE 10
33530
33550
Expanded View of the Spectrum at the 6a$ Band
33570
Ze
I
33860
'
T
I
33880
FIGURE 11
I
33900
1
33920
Expanded View of the Spectrum at the Ij Band
I
33940
33050
33150
FIGURE 12
33250
33350
33450
The "Hot" Vibronic Spectrum of p-Toluidine
33550
33100
33300
FIGURE 13
33500
33700
The Spectrum of p-Toluidine (ND2)
33900
31
Table 5.
Methyl rotor structure of the p-toluidine spectrum
Tho Oq band (origin)
barrier heights:
ground state = 25 cm-1
excited state = 20*8 cm""1
conformational change:
0°
calculated
observed
Transition_____ V____ Int_______ V____ Int
2e <- Ie
19
0.75
18
0.5
Sa1*- Oa1
51
0.21
51
0.5
4e <— Ie
83
0.04
—
0
Tho 6qq band
barrier heights:
ground state = 25 cm""^
excited state = 20.8 cm™1
conformational change:
0°
O
ID
O
in
calculated
observed
Transition_____ V____ Int_______ V____ Int
2e <— Ie
451
450
0.75
0.21
481
Sa1*— Oai
481
—
—
0.04
0
515
4e *— Ie
Tho Iq band
barrier heights:
ground state = 25 cm""1
excited state = 20.8 cm"
conformational change: 0 °
Transition
2e *— Ie
Say*- Oa1
4e *— Ie
*
calculated
Int
V
0.75
839
0.21
871
0.04
903
observed
V
Int
838
0.5
871
*
_
*
The absol u t e intens i t i e s w e r e i mpossible to determine due t o t h e congestion in
th e spectrum.
32
Discussion
The general trends pertaining to intermolecular motion in
the
excited
state
observed
by
Tembreull
et
a l . in
para-
disubstituted benzenes supports the assignments and theories
contained
h e r e i n .18
The
only
mi n o r
discrepancies
are
the
peaks in the region from 690 - 850 cm-1 which they assumed to
all be due to ring bending modes.
In the case of p-toluidine,
it is true that the most prominent peaks from 790 - 900 cm "1
are due to ring m o d e s .
centered
modes,
at
709
rather,
However,
cm"1 and
736
cm"1 are
definitely
not
they are due to amine group m o d e s .
concluded by
examining the
relative
the
to
the intense quartet groups
deuterated
protonated
amine
spectrum.
group
Tembreulle
assigned the p eak at 707 cm"1 (709 cm"1 in this work)
This
was
determined
to
be
incorrect
from
the
ring
This was
spectrum
et
al.
as 12 q .
information
afforded by the spectrum of the deuterated amine g r o u p .
The
spectrum of the deuterated amine group revealed a significant
red shift
of the two quartets
relative to the
origin.
The
peak assigned as I1 was also significantly red shifted as were
all the combination bands of amine group f u n d a m e n t a l s .
The
peaks at 792 cm”1 and 820 cm"1 did not move u pon deuteration
and
were
determined
to
definitely
be
ring
modes.
By
comparison with several aniline spectra, it was concluded that
the lower frequency pea k must be 12 q and the h i gher frequency
Iq «
In general, the ring modes were all relatively unaffected
33
upon deuteration of the amine g r o u p .
It is certain that the peaks at 709 cm-1 and 736 cm-1 are
amine group modes,
however,
it has not been determined which
motion belongs to which peak although tentative assignments
have bee n made where 709 cm-1 = T§ and 736 cm -1 =
motions are described as a torsion
an inversion
.
The two
(T§) of the N H 2 group and
(Iq ) of the amino hydrogens.
Calculations first carried out by Brand et al. and later
b y Ito et a l .19 both revealed that the amine group lies in the
plane of the benzene ring or the quasi-planar structure in the
excited state.
However,
it was determined by Christoffersen
et a l .20 from the rotational constants that the
has an out of plane angle of 30° ± 10°.
puzzling and has not been resolved.
amine group
This discrepancy is
Thus, a good picture of
aniline or p-toluidine is not yet available.
The work by Brand et a l .11 revealed that aniline exhibits
an inversion and a torsion of the amine group.
the inversion was assigned
However, only
(I1 at 325 cm-1 and I0 cm-1) and it
was unclear what peak was being referred to as the t o r s i o n .
The
amine
group
different than
splittings
modes
in
p-toluidine
in aniline.
indicate
that
appear
The quartet
some
other
to
be
vastly
structure and peak
effect
must
be
taking
place.
Vibronic coupling m a y be suspected as the cause from the
two quartet splittings and subsequent combination bands but it
is not
likely since this
structure
is not
seen
in a n i l i n e .
34
This
leads to a very
quartet
methyl
splittings
group.
important
are
A
due
assumption,
solely
reasonable
to
the
explanation
that
is the two
addition
of
is
a
that
the
time
dependent coupling between the methyl group and amine group is
taking
place
through
the
ring
via
hyperconjugation.
H y p e r c o n jugation has been observed in toluene where the methyl
hydrogens interact wit h the TC system on the ring to create a
potential barrier to internal rotation.
This effect may occur
in aniline but should be more pronounced in p-toluidine where
the TC density is affected by one group which, in turn, affects
the group on the opposite end of the ring.
Thus,
vibronic
interactions between the methyl rotor potential and the amine
group
potential
wo u l d
be
expected
to
exhibit
a
complex
splitting of p e a k s .
The methyl
bands
exhibit
However,
group,
a
rotor
no
structure
change
in
at the
origin,
conformation
of
6a£,
the
and
l£
molecule.
due to the effects of hyperconjugation with the NH2
definite
change
expected in the T§ and
in the
regions.
methyl
rotor
structure
is
While it is apparent that
the methyl rotor structure is different in this region, it is
not clear which peaks are affected.
Further investigations on
the CD3C 6H 4N H 2 molecule may reveal the methyl rotor structure
when it is affected by hyperconjugation with the N H 2 g r o u p .
35
Conclusion
Time dependent coupling b etween the methyl rotor and NH 2
group on p-toluidine is readily apparent in the fluorescence
excitation
spectrum.
This
phenomenon
was
revealed by
the
quartet splittings at 709 cm-1 and 736 cm"1 which were assigned
as T§ (NH2 torsion) and
(NH2 inversion) .
It is certain then
that these peaks are due to N H 2 group i n t e r a c t i o n s .
The major evidence
for this proposed mech a n i s m is the
fact that aniline does exhibit a torsion and inversion of the
NH2 group.
However,
the peaks attributed to the N H 2 motions
in aniline look nothing like those in p - t o l u i d i n e . Therefore,
the
quartet
splittings
in
the
p-toluidine
spectrum
are
proposed to be mostly a function of hyperconjugation through
the ring between the methyl rotor and N H 2 g r o u p .
There
structure
is
at
clearly
the
a
T^ and
determined which peaks
difference
peaks
are methyl
in
the
although
methyl
rotor
it has
not been
rotor structure
and more
importantly what the physical significance of this difference
reveals.
Further experiments with the deuterated molecules
CD3C 6H 4N H 2 and CD3C 6H 4N D 2 may lead to a conclusion.
The major point of this paper was to expose the newfound
information
regarding
hypercon jugation
between
the
methyl
rotor and p ara substituents through the TC system on benzene
rings.
Ultimately,
the
hope
is
to
predict
the
electron
densities on the benzene ring by examining the methyl rotor
36
structure.
This
w o r k f together
with
several
other
recent
discoveries has afforded a greater understanding of the final
goal.
I
37
THE PARA-METHYLSTYRENE SPECTRUM
Introduction
Styrene and stilbene have bo t h attracted the interest of
chemists for a variety of reasons.
studying
photoisomerization
aromaticity
in
conjugated
They are model systems for
and
for
systems.
consideration
In
spite
of
of
the
significant interest in these molecules, structural questions
remained until very recent studies which have shown that both
molecules
are planar in both the
order
probe
to
aromaticity,
the
we
issue
have
of
obtained
S0 and S 1 s t a t e s .21'22
resonance
the
interactions
jet-cooled
In
and
fluorescence
excitation spectrum of p - m e t h y l s t y r e n e .
Recent work on p-methyl-trans-stilbene26 indicated that
the
difference
methyl
in
the
two
ring
positions
adjacent
to
the
substituted position is significantly increased upon
excitation to S1 as determined b y the CH 3 barrier height, V 3"
=
28
c m -1, V 3 ' =
150
cm -1.
Furthermore,
a
conformational
change of -30° was observed which could not be explained.
In
an attempt to further understand the interactions which could
influence the methyl behavior in such systems, investigations
into the similarities and differences between p-methyl-trahsstilbene and p-methylstyrene are in p r o g r e s s .
38
Experimental
The procedure for obtaining the p-methylstyrene spectrum
was given in the general experimental section of this paper.
The
purpose
of this
addendum
is
to
cover the
experimental
difficulties peculiar to p - m e t h y l s t y r e n e .
When
the
p-methylstyrene
polymerization began
mixture of monomer,
to
occur
dimer,
was
in the
heat e d
sample
three
different
spectral
congestion.
~60°C,
chamber
and
a
and possibly t rimer were released
from the jet nozzle and excited by the laser.
exciting
to
species
In fact,
caused
as the
a
The effect of
great
concentration
polymer species increased relative to monomer,
deal
of
of the
the spectrum
changed shape with respect to peak intensities and exhibited
some br o a d b a c k ground fluorescence.
Figures 14a and 14b are
spectra of the region surrounding the origin at ~ 5 minutes
(14a)
and
~
2
hours
(14b)
equilibrium temperature
of
under the same conditions
after
6 0 0C.
the
sample
Figure
14c
came
is
to
the
a spectrum
as 14b at the region to the blue
where most of the combination bands occur.
The pro b l e m of polymerization was nearly eliminated by
keeping
the
sample
at
room
temperature
and
lining
the
stainless steel sample chamber wit h a N a I gene® plastic tube to
prevent
catalysis by the nickel present
in the
steel.
The
spectrum in Figure 15 is the result of operating under these
conditions.
•
0°o
(A)
A
Y
--------------------
(C )
.. .
I--- ---- -- --- l-vuVAL —w —w
j
l
l
|
The p-methylstyrene spectrum (a) at 60°C
for -5 minutes and (b) at 60°C for -2 hrs
, L I
FIGURE 14
FIGURE 15
The jet-cooled p-methylstyrene spectrum with sample at room
temperature
41
Results and Discussion
The low frequency region of the jet cooled fluorescence
excitation spectrum of p - m e t h y lstyrene is shown in Figure 13.
The raised baseline in the vicinity of the origin is due to
presence of the dimer which formed in the liquid phase in the
sample container via Ni catalyzed polymerization as discussed
in the experimental section.
The spectrum shows no sign of
methyl rotor activity indicating that the conformation of the
methyl group is identical in b oth states.
no
observable
methyl
rotor
peaks,
we
Because there are
cannot
determine
the
/
barrier to internal rotation.
Considering that the dominant
influence on the methyl behavior in this molecule is expected
to be hyperconjugation,
it would appear that the influence of
the vinylic K electrons on the local TC electron density in the
vicinity
of
electronic
the
methyl
states.
group
This
result
compared
to
molecule
p - m e t h y I- trans-stilbene
change
previously
is
p ublished
in barrier height
comparable
is
most
the
two
interesting
results
for
in
on
which
(V3"= 2 8 cm"^1-, V g f=ISO
the
a
when
related
significant
c m -1) and an
~30°
change in conformation were observed upon e x c i t a t i o n .
This
dramatic
change
in
CH 3
behavior
is
indicative
of
a
significant change in the TC electron density n e a r the methyl
group and is,
in part,
due to the asymmetry w ith respect to
the para axis of the TC electron containing s u b s t i t u e n t s .
methylstyrene
exhibits
a
similar
asymmetry
but
p-
displays
42
significantly different methyl rotor b e h a v i o r .
Thus,
it can
be concluded that the dramatic behavior observed in stilbene
is predominately due to the
second phenyl group which is
6
carbons away from the methyl probe!
Additional
insight
to
the
differences
in
these
two
conjugated TC electron systems may be gained by considering the
vibronic
nature
of
the
fluorescence
excitation
spectra,
trans-stilbene and p - m e t h y I -trans-stilbene show very similar
relative intensities for all vibrations which appear in both
spectra
rotor
(once intensity distribution over all of the methyl
peaks
spectrum
is
taken
into
of p - m e t h y I styrene
different
than
published
a c c o u n t ) .21^26
However,
shown in Figure
spectra
for
13
the
is markedly
s t y r e n e .^7
The
most
intense ba n d of the styrene spectrum is the origin and the 18 q
is second with 25% of the O q intensity.
spectrum
is
dominated by
the
modes
The p - m e t h y lstyrene
appearing
at
775,
1194, and 1211 cm-1 and,their combinations and overtones.
799,
The
775 cm-1 peak is assigned as 25 q and the 1194 c m -1 peak as 18 q
b y analogy with s t y r e n e . Relative intensities of 0 q , 18 q , and
18 q for p-methy I styrene
Karplus
and
vibrational
are
coworkers2® have
analysis
of
100,
H O , and 50,
performed
styrene
which
an
respectively.
e x t ended
describes
fairly democratic mix of the C=CHg b o n d angle,
ring-ethylene stretching and a ring stretch.
V 18
PPP-CI
as
a
ring bending,
This moeity will
clearly be affected by the resonance between the phenyl and
ethylenic groups and the relative intensities in the styrene
43
and p-methylstyrene spectra indicate that the methyl group has
a
strong
influence
on
that
resonance
interaction.
The
torsional mode between the ring and substituent will give the
most direct measure of the b o n d order and resonance between
the phenyl and ethyl groups.
The ring torsion mode in trans-
stilbene is very low frequency and enharmonic in the ground
state
but
is
state.
higher
frequency
and harmonic
p-methyl-fcrans-stilbene
frequencies
containing
observed
and
this
for
similar
moeity
symmetry
shows
relative
(only
the
nearly
intensities
even
reasons).
in
quanta
excited
identical
for
bands
transitions
Similar but
are
less dramatic
behavior is seen in styrene for the ring-ethyl torsion (38 cm"
1 and anharmonic in S0, 97 cm-1 in S1)2^ but we are unable to
assign this mode in the p-methylstyrene spectrum.
Conclusion
An
apparent paradox has
methylstyrene
when
the
resulted from the
results
are
compared
study of p-
to
those
styrene, trans-stilbene, and p-methyl-trans-sti l b e n e .
methylstyrene
extremely
and
different
p-methyl-trans-stilbene
methyl
rotor
spectra
signatures
for
Thepshow
indicating
a
conformational change in the m e t h ylated stilbene on excitation
which does not
occur in p-methylstyrene.
behavior
is
determined,
electron
density
which
to
a
would
large
The methyl rotor
degree,
indicate
by
that
the
the
local
Tt
striking
changes in the p-methyl-trans-stilbene spectrum are due mainly
44
to resonance effects caused by the remote ring which is not
present
in
the
styrene molecule.
comparing p-methylstyrene
to
its
The paradox
parent
arises
molecule,
when
styrene.
The p-methylstyrene spectrum shows significantly more FranckCondon
activity
in
several
modes,
one
of
which
should
be
heavily affected by resonance between the ethylenic group and
the
ring.
Thus,
the
presence
of
the
methyl
in
the
para
position seems to have a significant effect on the TT electron
interactions
substituted
at
the
styrene
substituted stilbene.
the methyl
molecules,
rotor
as
opposite
but
end
showed
no
of
the
such
ring
in
the
effect
in
the
While this casts some doubt on use of
a probe
of
TU electron density
in
such
it also confirms that the remote ring in stilbene
has a significant effect on TC electron interaction in the para
position
by
showing
that
p ara
substitution
in
these
two
molecules has a measurable effect on the opposite end of the
ring.
A more detailed picture can be obtained via studies of
the deuterated molecule and dispersed fluorescence experiments
which will allow for identification of the ring-vinyl torsion,
the mode
therefore,
which
should be most
sensitive to b o n d
order
and
resonance between the phenyl and ethylene groups.
These studies are currently underway in our laboratory.
45
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