MICROWAVE
SPECTRA AND WEAK INTRAMOLECULAR
HYDROGEN
BONDING IN 3-BUTENF.-1-THIOL AND N-METHYLALLYLAMINE
K.-M. Marstokk
Department
and Harald MØllendal
of Chemistry
The University
of Oslo
P. o. Box 1033 Blindern
N-0315 Oslo 3
Norway
ABSTRACT.
The microwave spectra of 3-butene-1-thiol,HSCH222
CH CH=CH ,
and one deuterated speeies, DSCH222
CH CH=CH , reveal the existence of at
least
three
conformations. The
intramolecular
hydrogen bond
hydrogen atom
and
the
heavy-atom qauche
formed between the
w-elect~ons of
extended conformations were
also
the
form
has
mercapto. group
double bond. Two other
identified. The
hydrogen-bonded
qauche conformation is 2.9(5) kJ mol-1 more stable than Extended
-
3.6(6) kJ mol
1
more stable
N-methylallylamine,
than
Extended
CH 3 NHCH 2 CH=CH,2
CH3NDCH2CH=CH2'
demonstrate
conformation.
This
rotamer
and
an
Il.
The
MW
one
deuterated
l and
spectra
of
speeies,
the existence
ot
only
one stable
is also stabilized by a very weak H bond
formed between the methylamino
group hydrogen
atom and the w electrons
of the double bond.
1.
3-BUTENE-1-THIOL
1.1.
Introduction
The
conforrnational
properties
of 3-buten-1-01, HOCH2CH2CH=CH2
57
A. Weber (ed.),
Structure
and Dynamics
~
1987 by D. Reidel Publishing
of Weakly Bound
Company.
Molecular
Complexes,
57-68.
r
have
K.-M. MARSTaKK AND H. M0LLENDAL
58
been studied
as
well
by infraredl)2
as byeleetron
and in the
diff
gas phase3) 4 that
compound
has
hydroxyl
group
an
any other
Jn.4
the
that
was found
stable
hydrogen
atom and the
this
(MW)3 spectroscopic
It
most
intramoleeular
hydrogen
It was estimated3
than
and microwave
rotamer
in sol uti on l) 2
conformation
of
(H) bond formed
u eleetrons
of the
state.
this
between
double
3 kJ mol-I
is as least
form in the free
hypothetical
both
methods
the
bond.
more stable
3-Butene-1-thiol,
HSCH2CH2CH=CH2' was ehosen for study in order to eompare the ability
of the mereapto and the hydroxyl
groups to form internal
H bonds
with
eleetrons.
u
mereapto
Hydroxyl
groups
with
the
groups
generally
same acceptors.
form stronger
This
is also
H bonds
found
than
in
this
study.
1.2.
Experimental
3-Butene-1-~hiol
literature,5;6
was
purified
PMR and
NMR speetroseopy.
13C
synthesized
largelyas
by gas ehromatography
and
Extensive
deseribed
identified
speetral
in
by
measurements
the
IRr
were
made in the 26.5-38 GHz speetral region using an improved version of
the speetrometer
deseribed
about
vapour
-60 DC. The
briefly
in Ref.
pressure
was
7. The eell
was eooled
to
1-2 Pa.
The
approximately
deuterated speeies, DSCH2CH2CH=CH2' was produced
with heavy water in the wave guide.
by direet
exchange
1.3. Results
The MWspectrum
the
speetrum
Q-branch
turne d out
lines
Fig. 1. The
is quite
of
to
the
strongest
coefficients of roughly
-----
dense
at
be
-60 DC. The strongest
g-type
hydrogen-bonded
of
qauehe
as
well
conformation
as
of
Q-type
shown
in
have peak absorption
cm-I at this temperature.
these
4*10-7
R-branch
transitions
transitions
MICROW A VE SPECTRA AND WEAK INTRAMOLECULAR
59
HYDROGEN BONDING
H
H
H
5
H
H'
H /C~/H C
I
H
ill.
1.
H-bonded
The
qauche
conformation
bond.
The C-S and -H C-CH= bonds
2
C-C-C-S
dihedral
angle is 65(3)0
is
twisted
to
be
from anti
57(3)0
formed
between
bond w electrons.
the
are
viewed
qauche
from ~.
in order
mercapto
to allow
group
The H-S-C-C dihedral
to
the
on e
-H2 C-CH2 -
another.
angle
an intramolecular
H bond
is
atom and the
50(5)0
double
from ~.
.
8
.
-30
d
Bond-moment calculat10ns
pre 1ct ~a=3.3*10
C m, ~b=
-30
-30
2.6*10
C m, and ~ =1.7*10
C m, respectivel
y . The J=8 ~
~
and J=9 ~ 8 ~-type R-branch
lines were first
found, and the
transitions
were then
although
their
~-component
smaller
no dipole
of
This
predicted
J
transitions
table
species
also
which
is
by the
be determined
No ~-type
very
thus
due to
with
a J value
the
assigned
in
-----
presumed
Q-type
were
found
predicted.
The
to
be
somewhat
Unfortunately,
insufficient
intensities
of maximum 30,
a
7
method.
spectroscopic
spectroscopic
lines
accurately
bond-moment
The resulting
includes
was
be
moment
above
transitions.
assigned.
can
dipole
moment could
low
using
of the
then
readily
positions
The
The C-C-C=C dihedral
hydrogen
angle
along
is shown in
constants
straightforward
constants
of the
manner.
derived
Table
l.
deuterated
Several
K.-M. MARSTOKK AND H. M0LLENDAL
60
vibrationally
excited states were also assigned and they are discussed
in a forthcoming
paper9 which will give a
more
complete
account
of
this work.
TABLE I.
Spectroscopic constants of the ground vibrational state
of the hvdroqen-bonded qauche conformation of 3-butene-1-thiol.
Species
HSCH2CH2CH=CH2
DSCH2CH2CH=CH2
117
Number of transitions
78
0.065
Root-mean-square dev./MHz
0.078
AO/MHz
6894.4387(34)
6703.6341(87)
Bo /MHz
2308.11700(84)
2302.6352(41)
1882.05016(78)
1868.0054(41)
Co /MHz
2.2710(26)
t.J/kHz
t.JK/kHz
t.K/kHz
118
( 30
)
-13.346(14)
-12.478(29)
33.22(13)
28.69(39)
6J/kHz
6K/kHz
Uncertainties
2.
0.70608(68)
0.7076(11)
3.885(25)
3.747(37)
represent one standard deviation.
Another prominent feature of the spectrum is the lumps of lines
occurring every 2.8 GHz. They are modulated at low Stark voltages and
were
assigned as
the
g-type R-branch
prolate extended conformations depicted in
conformations have
the
pile-ups of the two highly
Fig.
asymmetry parameter10
2.
method8
them. It is also
to
be
approximately
possible for
the
calculated by
to
-
the
C m for each of
4.4*10-30
molecule
these two
-0.999 as wellas
K
similar dipole moment components along the g-axis
bond-moment
Both
take an
extended
conformation in which the S-H bond is anti to the -H C-CH - bond. This
hypothetical
rotamer
and is predicted8
third
hypothetical
conformation
---
-
2
would also be very prolate
to have a dipole moment component
C m along the g-axis. The reason why a
this
2
(not shown in Fig. 2)
successful
of about 2.6*10-30
identification
of
was not made is of course the
MICROW A VE SPECfRA AND WEAK INTRAMOLECULAR
fact that ~~
In addition,
about
of
HYDROGEN BONDING
is considerably smaller than for Extended l and Il.
the anti arrangement of S-H bond is less favourable
1.5 kJ mol-I than the qauche arrangement,
ethanethiol.11
The
ground-state
pile-up
species, and
2833.1 MHz for the parent
deuterated
61
as shown in
of
the
by
case
Extended l has ~+~
2795.5 MHz for the
~+~
species. The values found for Extended Il were 2838.2 MHz
and 2793.4 MHz, respectively. The results for
states are
vibrationally excited
discussed in Ref. 9. No dipole moment could be determined
for any of the two extended forms due to low intensities.
S
H-S
H
H
H
H
H
H
H
H
H
H/C~/H C
H/C'c
I
I
H
H
Fiq.
2.
/H
Extended l and Il differs from the H-bonded qauche of Fig. 1
in having an anti arrangement for the C-S and the -CH2-CH= bonds.
H-S-C-C dihedral angles are 60° from syn.
The energy differences
determined
from relative
between
the three identified
intens ity measurements.
The
The
rotamers were
H-bonded
qauche
conformation
I
was found to be 2.9(5) kJ mol-I more stable than Extended
-1
and 3.6(6) kJ mol
more stable than Extended Il. The dipole moments
calculated
by the band-moment
method8 were used to derive
---
these energy
62
K.-M. MARSTOKK AND H. M0LLENDAL
The quoted
differenees.
deviation. The
uneertainties
uneertainties arising
one
represent
from
using caleulated dipole
moments are presumed to have been allowed properly for in
uneertainties.
The
moments.
weaker transitions. It is also
would
possess sizable dipole
The faet that there are no relatively
makes us conelude
are also the three
eonservatively
kJ
the stated
assignments reported above and in Ref. 9 inelude all strong
lines of the speetrum as well as many
that further conformations
likely
left,
standard
mol-I
most
that the three rotamers
stable
estimated
more
stable
strong unassigned
forms
of
any
in this work
3-butene-1-thiol.
that the H-bonded
than
assigned
lines
It
is
qauche form is at least 3
further
hypothetical
unassigned
eonformahon.
The
rotational
the two extended
strueture
eonstants
(or the ~+~ combination
forms) do not suffiee to determine
for the three eonformations.
order to derive
parameters
interesting
struetural
taken from reeent aecurate
which depend strongly on the
dihedral angle
C-C-C=C
S-C-C-C
was
determined
to
found
is
50(5)°
have to be made in
be
seleeted
fits of some parameters
eonstants,
be
Using
were
made.
The
57(3)° from anti for the
and 62(3)° for both extended
forms.
The
65(3)° from syn, while this angle is
almost exaetely anti for both extended
angle
to
for
a full geometrieal
parameters.
studies9
rotational
was
H-bonded qauehe conformation
Assumptions
determined
rotamers.9
The H-S-C-C dihedral
from syn in the qauehe rotamer. No fit was made for
this angle for the two extended
eonformers.
1.4. Discussion
There
is a eonsiderable
amount of evidence
that a weak intramoleeular
H bond is indeed formed in the qauche conformation. The fact that this
rotamer
actually
most favourable
has the mercapto
orientation
of evidence. Moreover,
group hydrogen
atom direeted
for this kind of interaction,
the distance
between this hydrogen
in the
is one piece
atom and the
nearest carbon atom of the double bond is 260 pm which is about 30
--
- -
-
----
pm
MICROW AVE SPECfRA AND WEAK INTRAMOLECULAR
shorter
than
aromatic
carbon.12
the sum of the van der Waals distances
Another evidence
63
of
hydrogen and
is that the qauche conformation
is
stable than any other form of the molecule. This
more
considerably
HYDROGEN BONDING
fact would be hard to explain if one
had
to
exclude a
stabilizing
interaction between the mercapto group and the double bond. The H bond
in the title compound is not as strong as that
alcohol, and
this
has
the
in
the
corresponding
following two consequences: The C-C-C=C
dihedral angle is 75(3)° in the H-bonded conformation of 3-buten-1-ol3
in contrast to 57(3)° in 3-butene-1-thiol.The angle difference in the
two molecules of about 18° thus leads to a
proton and the
the
IT electrons
stronger interaction in
in the alcohol and consequently
HOCH
results in a much higher population
forms in the thiol than in the alcohol.
of extended
conformers
MW spectroscopy,3
extended
2.
a
The
2CH2C=CH2 than in HSCH2CH2CH=CH.
2
bond in 3-buten-1-ol than in the qauche conformer of
stronger hydrogen
3-butene-1-thiol
shorter distance between
of
In 3-buten-1-ol
was 50 low that the y could not be
while large fractions
the
extended
the population
detected
by
of the gas consists of the two
forms for the 3-butene-1-thiol.
N-METHYLALLYLAMINE
2.1. Introduction
Allyl
derivativ es of the general
'
or 5kew conformat10ns as
form, the
,
form X-CH 2 -CH=CH 2 normal ly takes ~
'
13
d1scussed in arecent
paper.
In
the
syn
X-C-C=C skeleton is planar and the c-x bond eclipses the
double bond, whereas in the skew form the c-x bond is rotated 120° out
of
this plane. If additional rotational isomerism around the c-x bond
is
possible, several ~
or
skew
forms may
exist. Allylamine,
H NCH CH=CH,
is one such example. Five rotameric forrns,two ~
and
2
2
2
three skew, are conceivable for this compound. The two syn and two of
the
three possible skew
conformations have indeed been assigned by
Botskor and coworkers.14 There are small
----
energy differences between
64
the
K.-M. MARSTOKK AND H. M0LLENDAL
four different rotamers of allylamine. In the related molecule
N-methylallylamine,CH3NHCH2CH=CH2 ' the total number ot possible ~
and skew rotamers are in tact no less than nine. Three ot these are
~
contormations; three skew conformations arise when the CH3NHmoiety is rotated 1200 out of plane in a clockwisemanner,and the
final three skew conformations are formed when
rotated
1200
in a counter-clockwise
rotamer shown in Fig. 3 is the
Additonal
rotamers,
which
manner.
the
said moiety is
It was found that the skew
most stable form of the
may
or
molecule.
may not exist, are at least 3 kJ
mol-1 less stable than this form. The p~ew rotamer ot Fig. 3
has
the
methyl group anti to the CH2-CH bond. A very weak H bond is presumably
tormed between the methylamino group hydrogen atom and the ~ electrons
of the double bond
~H3
H
\
DN"'- H
,/
H---C
H
/
H/
\
\=C
Fiq. l.
The most
stable rotameric torm of N-methylallylamine.The
N-C-C=C dihedral angle is 123(3)° from syn and
the
-anti to the CH2 -CH bond. This conformation is at
stable than any other rotameric
--------
- -
-
--
-
- --
-
is
least 3 kJ mol-1 more
form of the molecule.
-
methyl group
MICROWAVE SPECTRA AND WEAK INTRAMOLECULAR
2.2
HYDROGEN BONDING
65
Experimental
N-methylallylaminewas purchased from Fluka A. G., Buchs, Switzerland.
The compound was purified by gas chromatography
were
made
dry-ice
in
the
12.4-26.7
temperature
speetrometer
and
described
a
and 28.0-38.0
pressure
of
before
GHz
spectral
about
above.1 The deuterated
use.
1
Pa
Studies
regions at
using
the
speeies was produced by
direct exchange with heavy water in the wave guide.
2.3.
Results
The
microwave
spectrum of N-methylallylamine
is rich and of moderate
intensity. The strongest lines which turned out to be
high-J g-
and
~-type Q-branch transitions of the conformation shown in Fig. 3, have
peak
absorption coefficients of
roughly 4*10-1
cm-1
at
dry-ice
temperature. Over 200 ground-state transitions were assigned for this
rotamer with a maximum J-value of 74. More details are given
15. None
of
the
interactions of
observed transitions were
the
14N
nucleus. The
split by
in
Ref.
quadrupole
ground-state spectroscopic
constants of the parent and deuterated speeies are shown in Table II.
Results for several excited states are
published elsewhere.15 No
dipole moment could be determined due to insufficient intensities of
low-J transitions. A total of about 600
transitions
were
assigned
ground and
for this conformer.
Searches
excited-state
for the other
possible rotamers among the relatively few and rather weak
unidentified transitions were
are
predicted to
considerations of
possess
remaining
negative. Other hypothetical rotamers
sizable
unassigned lines
dipole
lead
us
moments.
to
Intensity
conclude that the
assigned conformer is at least 3 kJ mol-1 more stable than any further
unidentified conformation. Only the N-C-C=C dihedral angle was f~tted
to the rotational constants with the remaining structural parameters
taken from
related highly accurate structures of related compounds.
The N-C-C=C dihedral angle was found to be 123(3)°.
66
K.-M. MARSTOKK AND H. M0LLENDAL
TABLE II.
qround
Spectroscopic
vibrational
constants
state
of the
of N-methvlallvlamine.
Speeies
CH3 NHCH2 CH=CH2
CH3 NOCH2 CH=CH2
Number of transitions
227
54
Root-mean-square
0.087
0.103
19998.241(12)
18687.038(52)
dev./MHz
Ao/MHz
B/MHz
o
C/MHz
o
2235.7349(14)
2220.8662(46)
2203.1495(16)
2189.5416(47)
0.6441(16)
6J/kHz
0.560
( 14 )
6JK/kHz
-23.329(33)
-19.774(69)
~K/kHz
452.69(25)
429(11)
6J/kHz
-0.104892(76)
-0.093241(75)
6K/kHz
38.65(27)
12.58(37)
4>J/HZ
0.00079(69)
4>JK/Hz
0.029(27)
g
4>KJ/Hz
1.18(27)
g
-0.178(14)
-37.3(65)
4>K/Hz
g
'PJ/Hz
-0.000547(18)
'PJK/Hz
-0.462(53)
the value found
g
O.O.Q
O.O.Q
'PK/HZ
Uncertainties
-0.000377(15)
represent
one standard
deviation.
g Preset at
for the parent speeies. .Q Preset at zero.
2.4. Oiscussion
This
group
study shows
that
substitution of a hydrogen atom in the amino
in allylamine with a
methyl
group
conformational consequences. Instead of
four
has
rather
large
rotameric forms with
rather similar energies as found for allylamine,14 the
one
rotamer
shown in Fig. 3 is the predominating
form of N-methylallylamine.This
conformation is characterized by having ideal
~~-
-------
-~~
steric conditions in
MICROWA VE SPECfRA AND WEAK INTRAMOLECULAR
that the large methylamino
HYDROGEN BONDING
67
group is twisted out of the C-C=C plane and
bond.
In
in having the methyl group in anti position to the -CH2-CH=addition, a weak H bond may be formed between the amino group hydrogen
atom and the double bond
between
this
atom
TT electrons,
the
and the nearest double-bond
which is about 30 pm shorter
of
as
non-bonded
carbon atom is 263 pm
than the sum of the van der
aromatic car bon and hydrogen.12
distance
Waals
radii
Finally, in the conformation
shown
in Fig. 3 (the most stable rotamer) repulsion between the lone pair of
the
nitrogen nucleus and
conditions
are
TT electrons
favourable,
minimal, and because a weak hydrogen
of
the double bond is
this conformation
thus prefers
minimal. N-Methylallylamine
steric
the
lone pair TTelectron
bond
may stabilize
because
repulsion
is
it.
2.5 Acknowledgment
Mr.
Marko
Opresnik is
thanked for
synthesizing 3-butene-1-thiol.
Norges Teknisk Naturvitenskapelige Forskningsråd is
travel grant to the NATO advanced
research workshop
thanked for a
at Maratea,
Italy.
2.6. References
1. M. Oki and H. Iwamura Bull. Chem. Soc. Jpn.
d~
(1959)
2. W. Ditter and A. P. Luck Ber. Bunsenqes. Phvs. Chem.
567.
J~
(1971)
163.
3. K.-M. Marstakk
and H. MØllendal
Acta Chem. Scand.
~d~
(1981)
395.
4. M. Trætteherg
and H. 0stensen Acta Chem. Scand.
AJ~
(1979) 491.
5. J.-M. Suzur, M.-P. Crozet and C. Dupuy C. R" Acad. Sc. Paris.
Series C. ~g~ (1967) 610.
6. C. Walling
7. K. -M.
and M. S. Pearson J. Am. Chem. Soc.
Marstakk
and
H. MØllendal
8. C. P. Smyth Dielectric
J. Mol.
Struct.
Behavior and Structure.
,§§.
l
(1964)
2262.
(1970)
205.
McGraw-Hill,
New York 1955, p. 244.
9. K.-M. Marstakk
and H. MØllendal Acta Chem. Scand.. in press.
68
K.-M. MARSTOKK AND H. M0LLENDAL
10. W. Gordy and R. L. Cook Microwave
New York 1984,
11. F. rnagaki,
(1973) 381.
12. L. Pauling
University
13. Z. Smith,
r.
Molecular.Spectra.
Wiley,
p. 324.
Harada
The Nature
Press,
and T. Shimanouchi
of the
Chemical
New York 1960,
N. Carballo,
J.
Bond.
Mol. Spectrosc.
3rd Ed.,
~~
Cornell
p. 260.
E. B. Wilson,
K.-M. Marstokk
and H.
MØllendal J. Am. Chem. Soc. J~1 (1985) 1951.
14. ~. G. Roussy, J. Demaison, l. Botskor and H. D. Rudolph ~
Mol. Spectrosc.
J~
G. Roussy
J~ (1974) 457; ~. r. Botskor, H. D. Rudolph and
2~ (1974) 15; g. r. Botskor and H. D. Rudolph
rbid.
G. Roussy rbid.
(1971)
535;
Q. r.
Botskor,
H. D. Rudolph
and
Jh
rbid.
(1978) 430.
15. K.-M. Marstokk and H. MØllendal
publication.
------
Acta Chem. Scand..
submitted
for