Journal of Molecular Structure 567±568 (2001) 41±57
www.elsevier.nl/locate/molstruc
Microwave spectrum, molecular structure, conformational
equilibrium, vibrational frequencies and quantum chemical
calculations for methyl vinyl sul®de q
K.-M. Marstokk a, H. Mùllendal a, S. Samdal a,*, D. Steinborn b
b
a
Department of Chemistry, University of Oslo, P.O. Box 1033 Blindern, N-0315 Oslo, Norway
Institut fuÈr Anorganische Chemie, Martin-Luther-UniversitaÈt Halle-Wittenberg, D-06099 Halle, Germany
Received 5 May 2000; accepted 18 July 2000
Abstract
The microwave (MW) spectrum of methyl vinyl sul®de (CH2yCHSCH3) has been investigated in the 20.0±60.5 GHz
spectral region at dry ice temperature. The most stable rotamer is planar with the methyl and vinyl groups in the syn
conformation. A less stable skew rotamer has been assigned for the ®rst time. Its CyC±S±C torsional angle is approximately
1548 from syn. Isotopic species of all the heavy atoms of the skeleton of the syn conformation have been assigned. This allowed
the following structure parameters to be calculated as: C1±C2 ˆ 133.15 (18) pm, C1±S3 ˆ 174.11 (9) pm, S3±C4 ˆ 179.70 (9)
pm, C2±C1±S3 ˆ 130.23 (11)8 and C1±S3±C4 ˆ 101.99 (9)8. The uncertainties are one standard deviation. Ten vibrationally
excited states of the syn conformer were assigned, while the ground state and two excited states of the skew conformer were
assigned. The frequencies of several of these excited states have been determined by relative intensity measurements. The syn
form was found to be 5.0 (3) kJ mol 21 more stable than the skew conformer by relative intensity measurements. The uncertainty
is one standard deviation. Quantum chemical calculations at HF, MP2 (full), DFT and QCISD levels of theory using the 631111G pp, cc-pVTZ and AUG-cc-pVTZ basis sets have been made to assist the experimental work. q 2001 Elsevier Science
B.V. All rights reserved.
Keywords: Methyl vinyl sul®de; Microwave spectrum; Conformational equilibrium; Structure; Quantum chemical calculations
1. Introduction
Methyl vinyl sul®de (MVS) may exist in the syn
(the vinyl group syn to S±CH3), skew or anti conformations. The ®rst attempt to determine the conformation of MVS was a Raman study [1] from 1961 where
a normal coordinate analysis was made for all these
q
Dedicated to Professor Marit Trñtteberg on the occasion of her
70th birthday.
* Corresponding author. Tel.: 147-22-855-458; fax: 147-22855-441.
E-mail address: svein.samdal@kjemi.uio.no (S. Samdal).
three conformations, but lack of suf®cient
experimental data made it impossible to determine
the preferred conformations.
The microwave (MW) spectrum was assigned [2] in
1967, and a planar syn conformation was consistent
with the rotational constants. An infrared and Raman
study [3] in 1968 revealed that MVS exists in two
conformations with the syn conformation as the
most stable one, but they could not determine whether
the second conformation was skew or anti.
An electron diffraction (ED) investigation [4] in
1971 determined the average molecular structure of
syn MVS. The second conformation was shown to be
0022-2860/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved.
PII: S 0022-286 0(01)00532-4
42
K.-M. Marstokk et al. / Journal of Molecular Structure 567±568 (2001) 41±57
the skew form. This ®nding was con®rmed in another
ED investigation [5]. The ED analysis for MVS was
improved in 1975 and 1979 [6,7]. The use of a
dynamic model [7] gave a more reliable analysis.
The CyC±S±C dihedral angle of the skew conformation was found to be 1368 in this analysis, which was
an increase of 208 compared to that found using a
static model [4]. However, this angle is very dependent on the assumption made for the molecular
geometry during the internal rotation.
The ®rst ab initio calculations from 1978 [8]
predicted that the less stable conformation is a skew
form. The energy difference between skew and syn
was calculated to 3.2 kJ mol 21. Considerable uncertainty has existed for this energy difference. The
experimental determinations ranging in fact from 0
[1] to 9.6 kJ mol 21 [9]. In the ED work from 1979
this difference was judged to be between 4.2 and
9.6 kJ mol 21 from available experimental information. A recent extensive spectroscopic investigation supplemented with ab initio calculations
[10] determined the energy difference to be 8.9
(8) kJ mol 21.
The MW spectrum of the skew conformer has not
been assigned previously. If the energy difference
between the two conformations is not too large, it
should be possible to assign the MW spectrum of
the skew conformer and thereby get useful information about: (1) the energy difference between the two
conformers, (2) the torsion angle for the skew
conformer which is uncertain and very dependent of
the model used in the analysis of the ED data [6,7], (3)
the lowest vibration frequencies of the two
conformations.
Another aspect of this work was to test how accurate the quantum chemical calculations could predict
rotational constants and the energy difference between
the two conformations.
2. Experimental
2.1. Synthesis
Methyl vinyl sul®de was prepared as described in
Ref. [11]. The substance was checked by NMR spectroscopy and found to be pure. No impurities were
found in the MW spectrum.
2.2. MW spectroscopy
The MW spectrum was studied using the Oslo
spectrometer [12]. The 12±40 GHz spectral range
was investigated thoroughly. Selected regions of the
40±60.5 GHz spectral range were also studied. The
MW absorption X-band brass cell was held at dry
ice temperature (2788C) in the experiments. The
pressure was about 4±6 Pa when the spectra were
recorded, and stored electronically using the computer
programs written by Waal [13]. The accuracy of the
frequency measurements is presumed to be better than
^0.10 MHz. Radio frequency-MW-frequency double
resonance (RFMWDR) experiments were carried
out as described in Ref. [14] using the equipment
mentioned in Ref. [15].
3. Results and discussion
3.1. Quantum chemical calculations
The quantum chemical computations have been
made with the gaussian94 program package [16]
using the IBM RS6000 cluster in Oslo. The calculations were performed at four different levels of theory;
Hartree±Fock (HF), Mùller±Plesset second order
perturbation calculations (MP2) [17] with all electrons included, density functional theory (DFT)
employing the B3LYP method [18] as well as
QCISD [19]. The basis sets utilized were
6-31111G pp, cc-pVTZ [20] and AUG-cc-pVTZ [20].
The structural parameters for the syn and the skew
rotamers from the different calculations are given in
Tables 1 and 2, for the transition state between syn and
skew in Table 3, and for the anti transition state in
Table 4. The numbering of the atoms is given in Fig. 1.
All computations predict the syn rotamer to be the
most stable form. The CyC bond length (Tables 1±4)
is underestimated at the HF level of theory as
compared with the bond lengths predicted when
correlation is included. However, the CyC bond
length decreases by about 1 pm when going from
the 6-31111G pp basis set to the cc-pVTZ basis set.
The two C±S bond lengths and the two CSC and CCS
bond angles are also varying more than expected both
with respect to the level of theory and the basis set.
However, as shown explicitly in Table 5 and
K.-M. Marstokk et al. / Journal of Molecular Structure 567±568 (2001) 41±57
43
Table 1
Molecular structures of the most stable syn conformation of methyl vinyl sul®de from quantum chemical calculations (energies (hartree) for
column 1±5 are: 2514.6347334, 2515.3874726, 2515.2588325, 2515.4247500, 2516.1809818, respectively. Cs symmetry)
HF 6-31111G pp
MP2 ˆ full 6-31111G pp
QCISD 6-31111G pp
MP2 ˆ full cc-pVTZ
DFT/B3LYP AUG-cc-pVTZ
Bond length (pm)
C1yC2
132.07
C1±S3
175.84
C4±S3
180.49
C1±H7
107.69
C2±H5
107.42
C2±H6
107.54
C4±H8
108.21
C4±H9,10
108.24
134.49
174.43
180.49
108.74
108.30
108.41
109.08
109.18
134.38
175.76
180.69
108.86
108.51
108.61
109.30
109.38
133.42
173.91
179.25
107.73
107.44
107.56
108.29
108.42
133.11
175.43
181.36
108.35
108.01
108.10
108.74
108.35
Bond angles (8)
C2C1S3
129.13
C1S3C4
103.23
C1C2H5
123.33
C1C2H6
119.79
C2C1H7
120.10
S3C4H8
106.17
S3C4H9,10 111.06
H8C4H9,10 109.09
H9C4H10
110.24
128.57
100.91
123.10
119.40
119.85
106.71
111.12
108.93
109.94
128.73
101.30
123.18
119.66
119.99
106.58
111.15
108.94
109.98
128.23
100.69
122.78
119.45
120.01
106.68
110.69
109.35
110.01
129.19
102.71
123.18
119.66
120.40
106.58
110.90
109.29
110.12
indirectly in Tables 1±4 the corresponding differences
between structure parameters of the syn and skew
conformations are remarkable constant both with
respect to the level of theory and size of the basis
set. This ®nding has been used to obtain an accurate
torsional angle of the skew rotamer (see below).
In Table 6 are listed different parameters connected
to the potential energy distribution for rotation about
the C1±S3 bond. There are several striking features in
Table 6 which should be emphasized when going
from the 6-31111G pp basis set to the cc-pVTZ
basis set. The barrier separating the syn and the
skew rotamers increases by 6±8 kJ mol 21 to about
20 kJ mol 21 while the anti barrier decreases by
several kJ/mol to 0.3±0.6 kJ mol 21. Moreover, the
torsional angle increases by more than 138, and the
energy difference between the syn and the skew
conformations increases to about 6±9 kJ mol 21. The
results obtained using the cc-pVTZ basis sets are in
good agreement with the MW results, as shown
below.
Comparison of the calculated rotational constants
and the dipole moment components of the syn form
with the experimental ones are shown in Table 7. The
A rotational constant is excellently predicted by the
cc-pVTZ basis set, while the B and C rotational
constants are relatively poorly predicted. The
observed rotational constants seem to be the average
of the MP2 and DFT calculations indicating that the
experimental structure parameters would be something in between those found in the two calculations.
It is important to note that the m a dipole component of
the syn rotamer is grossly overstimated at all levels of
theory.
The calculated unscaled vibrational frequencies
(B3LYP/AUG-cc-pVTZ level) are given in Table 8
together with observed and assigned vibrational
frequencies of the gas phase [10]. There is quite
good agreement for the syn form.
3.2. MW spectrum of the syn conformation
The ground state and two vibrationally excited
states, viz the C±S torsion and the methyl torsion,
were assigned by Penn and Curl [2]. Later some
more vibrational excited states were assigned by
Samdal et al. [7]. In order to assign the second conformation of MVS, our strategy was to assign as many
syn transitions in the spectrum as possible hoping that
some of the unassigned transitions could be assigned
44
K.-M. Marstokk et al. / Journal of Molecular Structure 567±568 (2001) 41±57
Table 2
Molecular structures of the skew conformation of methyl vinyl sul®de from quantum chemical calculations (energies (hartree) for column 1±5
are: 2514.6338418, 2515.3855112, 2515.2575761, 2515.4212990, 2516.1787892, respectively. No symmetry constraints were used in these
calculations)
HF 6-31111G pp
MP2 ˆ full 6-31111G pp
QCISD 6-31111G pp
MP2 ˆ full cc-pVTZ
DFT/B3LYP AUG-cc-pVTZ
Bond length (pm)
C1yC2
131.88
C1±S3
176.89
C4±S3
181.26
C1±H7
107.78
C2±H5
107.59
C2±H6
107.61
C4±H8
108.24
C4±H9
108.29
C4±H10
108.18
134.19
175.55
180.66
108.90
108.50
108.48
109.14
109.21
109.07
134.15
176.76
181.52
108.99
108.70
108.68
109.35
109.43
109.30
133.17
174.83
180.21
107.86
107.62
107.58
108.33
108.39
108.28
132.93
176.31
182.21
108.42
108.18
108.10
108.76
108.84
108.75
Bond angles (8)
C2C1S3
123.51
C1S3C4
100.45
C1C2H5
121.98
C1C2H6
120.55
H5C2H6
117.47
C2C1H7
115.62
S3C4H8
106.79
S3C4H9
110.78
S3C4H10
111.00
H8C4H9
108.80
H8C4H10
109.28
H9C4H10
110.10
123.27
98.79
121.45
120.41
118.14
115.87
107.24
110.76
111.37
108.33
109.02
110.01
123.36
98.95
121.66
120.59
118.34
115.66
107.22
110.76
111.31
108.44
109.05
109.96
123.71
98.94
121.42
120.23
117.36
115.47
106.99
110.47
111.03
108.82
109.19
110.25
124.20
100.70
122.25
120.39
117.75
114.85
106.48
110.68
111.13
108.90
109.25
110.29
Torsional angles (8)
C2C1S3C4
137.68
H5C2C1S3
24.03
H5C2C1H7
179.67
H6C2C1S3
175.16
H6C2C1H7
21.14
H7C1S3C4
245.84
C1S3C4H8
176.29
C1S3C4H9
57.95
C1S3C4H10 264.68
139.89
25.10
179.64
174.12
21.13
244.65
174.01
55.98
266.79
139.66
24.71
179.66
174.46
21.17
244.50
174.37
56.23
266.44
152.58
24.34
179.44
174.91
21.31
231.02
176.00
57.70
264.94
157.62
24.00
179.56
175.21
21.23
225.74
175.07
56.86
266.05
to the skew conformation. In this process four more
vibrationally excited states were found. The ground
states of the four isotopic species of the heavy atoms
of the skeleton of the syn form in their natural
abundance were assigned too.
All rotational constants and centrifugal distortion
coef®cients are much more precisely determined in
this work than in previous studies [2,7]. The improved
spectroscopic constants are given in Tables 9±12. All
vibrational excited states lower than approximately
500 cm 21 have been assigned.
The three combined v1 1 v2, v1 1 v3 and v2 1 v3
vibrationally excited states were readily assigned
using:
Xvi1vj ˆ Xvi 1 Xvj 2 X0 ;
where X is a rotational constant. The estimated rotational constants (A, B, C) are: (10613.4, 4719.0,
3353.2), (10685.1, 4750.4, 3352.4) and (10707.7,
4724.3, 3336.4 MHz) for v1 1 v2, v1 1 v3 and
v2 1 v3, respectively. These values are close to the
observed ones given in Table 11. This con®rms the
assignment of these combination excited states.
The assignments of the isotopic species were made
K.-M. Marstokk et al. / Journal of Molecular Structure 567±568 (2001) 41±57
45
Table 3
Molecular structures of the transition state between syn and skew rotamers of methyl vinyl sul®de from quantum chemical calculations (energies
(hartree) for column 1±5 are: 2514.6300685, 2515.3819834, 2515.2458824, 2515.4170344, 2516.1734780, respectively. No symmetry
constraints were used in these calculations)
HF 6-31111G pp
Bond length (pm)
C1yC2
131.81
C1±S3
178.31
C4±S3
181.65
C1±H7
107.63
C2±H5
107.61
C2±H6
107.66
C4±H8
108.29
C4±H9
108.16
C4±H10
108.09
MP2 ˆ full 6-31111G pp
134.07
177.45
181.07
108.71
108.57
108.58
109.21
109.09
109.02
Bond angles (8)
C2C1S3
125.02123.43123.73123.48124.31
C1S3C4
102.01
98.96
C1C2H5
122.35
121.46
C1C2H6
120.52
120.59
H5C2H6
117.13
117.95
C2C1H7
120.45
120.44
S3C1H7
114.52
116.13
S3C4H8
106.25
106.98
S3C4H9
110.63
110.91
S3C4H10
111.37
111.32
H8C4H9
109.30
108.84
H8C4H10
109.04
108.83
H9C4H10
110.15
109.87
Torsional angles (8)
C2C1S3C4
62.14
H5C2C1S3
0.18
H5C2C1H7
181.56
H6C2C1S3
180.48
H6C2C1H7
1.86
H7C1S3C4
2119.17
C1S3C4H8
176.13
C1S3C4H9
57.60
C1S3C4H10
265.26
70.36
1.17
181.47
181.39
1.69
2109.92
176.86
58.29
264.36
QCISD 6-31111G pp
MP2 ˆ full cc-pVTZ
DFT/B3LYP AUG-cc-pVTZ
134.05
178.55
181.92
108.83
108.76
108.78
109.41
109.31
109.24
133.04
176.89
180.84
107.76
107.69
107.72
108.42
108.29
108.23
132.61
178.81
183.03
108.35
108.22
108.30
108.84
108.74
108.67
99.46
121.73
120.75
117.52
120.58
115.69
106.87
110.88
111.37
108.91
109.85
109.87
99.32
121.21
120.58
118.21
120.38
116.14
106.61
110.54
110.85
109.30
109.27
110.19
101.33
122.04
120.71
117.27
120.67
115.00
106.01
110.71
111.05
109.32
109.31
110.32
69.15
0.87
181.41
181.20
1.75
2111.37
176.80
58.23
264.45
68.73
0.96
181.55
181.39
1.98
2111.84
178.36
59.67
262.82
71.91
0.14
181.67
181.44
1.87
2107.37
178.62
60.16
262.75
in the following manner. The differences between the
rotational constants of the parent and the isotopic
molecule were calculated from the equilibrium
geometry of high-level quantum chemical calculations
and added to the observed rotational constants. This
approach gave a very good prediction of the rotational
constants for all the isotopic species. The assignments of
the transitions of the three 13C species in their natural
abundance were possible even if the intensity of the
transitions were small. The results are given in Table 12.
3.3. Vibrational frequencies
The calculated frequencies (B3LYP/AUG-ccpVTZ) for the two conformations are given in Table
8 together with their assignment and observed
frequencies [10]. The calculated frequencies are not
scaled. There is good agreement between the observed
and calculated frequencies that support the proposed
assignments. However, the calculated frequency for
n 1 of the skew conformer is considerably smaller
46
K.-M. Marstokk et al. / Journal of Molecular Structure 567±568 (2001) 41±57
Table 4
Molecular structures of the anti transition state of methyl vinyl sul®de from quantum chemical calculations (energies (hartree) for column 1±5
are: 2514.6328740, 2515.3839652, 2515.2561690, 2515.4210527, 2516.1786508, respectively, Cs symmetry)
HF 6-31111G pp
MP2 ˆ full 6-31111G pp
QCISD 6-31111G pp
MP2 ˆ full cc-pVTZ
DFT/B3LYP AUG-cc-pVTZ
Bond length (pm)
C1yC2
131.88
C1±S3
176.71
C4±S3
181.04
C1±H7
107.69
C2±H5
107.63
C2±H6
107.51
C4±H8
108.21
C4±H9,10
108.23
134.26
175.33
180.43
108.80
108.52
108.36
109.14
109.11
134.20
176.56
181.34
108.89
108.73
108.57
109.35
109.34
133.19
174.80
180.09
107.78
107.63
107.53
108.32
108.31
132.96
176.34
182.09
108.36
108.20
108.07
108.77
108.79
Bond angles (8)
C2C1S3
124.17
C1S3C4
100.75
C1C2H5
122.32
C1C2H6
120.37
C2C1H7
120.65
S3C4H8
106.46
S3C4H9,10 111.10
H8C4H9,10 108.89
H9C4H10
110.30
124.36
99.43
122.00
120.05
120.36
106.71
111.36
108.44
110.39
124.28
99.51
122.11
120.33
120.56
106.73
111.29
108.55
110.30
124.28
99.14
121.76
120.03
120.58
106.85
110.83
109.92
110.40
124.59
100.86
122.48
120.26
120.71
106.30
111.02
108.99
110.40
than the proposed experimental assignment (69 and
106 cm 21, respectively).
Vibrational frequencies can be obtained from MW
spectroscopy by means of relative intensity measurements which have been carried out as described in
Ref. [21]. The large uncertainties of this method are
mainly due to the dif®culties of estimating the
position of the base line. For the syn rotamer relative
intensity measurements gave n 1 ˆ 120 ^ 30 cm 21 for
the C±S torsional frequency, n 2 ˆ 190 ^ 40 cm 21 for
the methyl torsion, n 3 ˆ 190 ^ 40 cm 21 for the CSC
bending and n 4 ˆ 370 ^ 50 cm 21 for the CCS
bending. These estimated frequencies are all smaller
than the observed frequencies [10] in the gas phase.
Hanyu et al. [22] have obtained a formula for the
calculation of the torsional frequency from the difference between the inertial defect of two consecutive
torsionally excited states:
pVTZ gives 181.4 cm 21 which is not in agreement
with our results.
The ®rst and second excited state of the torsional
motion of the skew conformer are estimated from relative intensity measurements to be as low as 28 (10)
and 81 (15) cm 21. The torsional frequency of 28 (10)
cm 21 is considerably smaller than both the calculated,
69 cm 21 and proposed experimental value of
Dvt11 2 Dvt ˆ 267=nt ;
where D vt11 and D vt are the inertia defects of the two
states and n t is the torsional frequency. This give a
frequency of 125 cm 21 for n 1. Durig et al. [10] give
170 cm 21 for this frequency and B3LYP/AUG-cc-
Fig. 1. The numbering of the atoms for the syn conformations for
methyl vinyl sul®de. The torsional angle about the C1±S3 bond is
de®ned as 08 for this conformation.
K.-M. Marstokk et al. / Journal of Molecular Structure 567±568 (2001) 41±57
47
Table 5
Molecular structures differences between syn and skew rotamers for methyl vinyl sul®de from quantum chemical calculations (D ˆ skew2syn.
Distances in pm and angles in degrees)
D (C1yC2)
D (C1±S3)
D (S3±C4)
D (C2C1S3)
D (C1S3C4)
HF 6-31111G pp
MP2 ˆ full 6-31111G pp
QCISD 6-31111G pp
MP2 ˆ full cc-pVTZ
DFT/B3LYP AUG-cc-pVTZ
20.19
1.05
0.77
25.62
22.78
20.30
1.12
0.98
25.30
22.12
20.23
1.00
0.83
25.37
22.35
20.25
0.92
0.96
24.52
21.75
20.18
0.88
0.85
24.99
22.01
106 cm21 (Table 8). The proposed observed value [10]
of 106 cm 21 is not consistent with our measurements.
3.4. Assignment of the skew conformer
The assignments for the syn conformer given above
include almost 2000 transitions, most of which have
been assigned for the ®rst time. All strong transitions
and all lines of medium intensity as well as a large
number of weak ones had at this point been assigned.
The spectrum of methyl vinyl sul®de is a rich one, and
numerous relatively weak absorptions still remained
unassigned. If the skew form indeed existed, it had
plenty of `hiding room' amongst these many weak
remaining transitions.
There was an additional complication. The theoretical results given in Table 7 indicate that m b of
skew is much larger than the other two dipole moment
components. A b-type spectrum will always depend
very much on the A rotational constant. It is seen in
the same table that the theoretical predictions of A
vary by about 2 GHz. The spectrum of the skew
conformer was thus not just weak, but the frequencies
of its b-type transitions were uncertain by at least
^5 GHz. The possibility that some of them were
overlapped by the much stronger transitions
belonging to the syn rotamer was also quite likely.
One thing could be helpful in assigning the spectrum. The theoretical calculations predict that a low,
double-minimum barrier exists at the anti position.
This barrier could lead to a splitting of the transitions
into a symmetrical (1) state and an antisymmetrical
(2) state. A pair of lines of equal intensity would then
result. Such a spectrum has previously been observed
for the skew forms of the related compounds
H2CyCHSCuN [23,24] and H2CyCHSH [25].
With this in mind, searches were made for bQbranch K21 ˆ 2 à K21 ˆ 1 series of lines which
are the strongest ones present in the 30±40 GHz spectral region. Success in assigning this Q-branch series
was obtained after only a few trials using transitions
that appeared to be split into closely spaced doublets
of equal intensity. Representative examples are shown
in Table 14 for what is assumed to be the ground
vibrational state, since it had the strongest spectrum
of all the states that were subsequently assigned. The
b
Q-branch series of two additional states appearing as
doublets were assigned next. These transitions, not
given in Table 13, are assumed to be vibrationally
excited states of the skew form.
Attempts were then made to assign the bR-branch
transitions. However, these lines are considerably
weaker than the Q-branch lines. Our ®rst attempts to
®nd them were therefore unsuccessful.
Table 6
Energy differences for methyl vinyl sul®de from quantum chemical calculations (energy differences, DE, in kJ/mol in relation to the syn
conformation. The torsion angle, w , in degree is 08 for syn)
DE (tr. st.)
DE (skew)
DE (anti)
w (tr. st.)
w (skew)
HF 6-31111G pp
MP2 ˆ full 6-31111G pp
QCISD 6-31111G pp
MP2 ˆ full cc-pVTZ
DFT/B3LYP AUG-cc-pVTZ
12.3
2.3
4.9
62.1
137.7
14.4
5.2
9.2
70.4
139.9
12.3
3.3
7.0
69.2
139.7
20.3
9.1
9.7
68.7
152.6
19.7
5.8
6.1
72.0
157.6
HF 6-31111G pp
0.07 b
1.13 b
1.14 b
c
b
a
See Table 9.
See Ref. [2].
See Table 14.
Dipole moments
ma
mb
mc
mt
Skew conformation
Rotational constants
A
17503 c
B
3520 c
C
3051 c
Dipole moments
ma
mb
mt
Syn conformation
Rotational constants
A
10606.6 a
B
4784.3 a
C
3366.2 a
Observed
0.25
1.64
0.20
1.67
16281.0
3541.3
3117.0
0.83
1.11
1.38
10763.7
4676.6
3328.4
0.30
1.58
0.18
1.62
16086.5
3579.7
3130.6
0.80
1.02
1.30
10537.2
4834.4
3385.5
MP2 ˆ full 6-31111G pp
0.29
1.60
0.18
1.63
15967.1
3544.9
3100.8
0.81
1.06
1.33
10485.9
4755.5
3341.7
QCISD 6-31111G pp
0.49
1.53
0.11
1.61
17236.8
3574.7
3089.6
0.75
0.98
1.24
10606.3
4904.6
3426.1
MP2 ˆ full cc-pVTZ
0.72
1.36
0.09
1.54
17758.2
3463.7
3000.5
0.87
0.74
1.14
10610.3
4695.3
3323.7
DFT/B3LYP AUG-cc-pVTZ
Table 7
Rotational constants (MHz) and dipole moments (Debye) for the syn and skew conformation of methyl vinyl sul®de from quantum chemical calculations
48
K.-M. Marstokk et al. / Journal of Molecular Structure 567±568 (2001) 41±57
K.-M. Marstokk et al. / Journal of Molecular Structure 567±568 (2001) 41±57
49
Table 8
Calculated frequencies for the syn and skew conformation of methyl vinyl sul®de (Cs symmetry only for the syn conformation)
Sym Cs
Assignment
No.
n1
n2
n3
n4
n5
n6
n7
n8
n9
n 10
n 11
n 12
n 13
n 14
n 15
n 16
n 17
n 18
n 19
n 20
n 21
n 22
n 23
n 24
A 00
A 00
A0
A0
A 00
A0
A0
A 00
A 00
A0
A 00
A0
A0
A0
A0
A 00
A0
A0
A0
A 00
A0
A0
A0
A0
Asym. tors
CH3 tors
CSC bend
CCS def
CH bend
CSC sym str
CSC asym str
CH2 rock
CH3 asym def
CH3 rock
CH3 rock
CH2 wag
CH bend
CH3 sym def
CH2 scissor
CH3 asym def
CH3 asym def
CyC stretch
CH3 sym str
CH3 asym str
CH3 asym str
CH2 sym str
CH stretch
CH2 asym str
Syn
Skew
Observed
B3LYP AUG-cc-Pvtz
170
240
250
455
592
678
741
862
953
953
963
1040
1277
1315
1390
1432
1442
1585
2922
2981
2994
3014
3033
3095
181.4
217.5
234.6
456.2
608.5
672.5
735.2
885.7
969.2
976.1
994.0
1061.1
1309.3
1348.4
1430.9
1467.9
1486.2
1638.9
3039.0
3114.3
3133.3
3147.6
3160.4
3230.1
Observed
106
401
698
735
1268
1378
B3LYP AUG-cc-pVTZ
69.0
156.4
221.4
402.0
605.0
691.0
726.0
913.3
969.6
979.4
992.1
1060.0
1300.0
1355.2
1422.9
1470.6
1483.8
1644.1
3041.9
3119.0
3132.3
3136.4
3146.8
Table 9
Spectroscopic constants for the ground state and three excited states of the C1±S3 torsion
Av (MHz)
Bv (MHz)
Cv (MHz)
D J (kHz)
D JK (kHz)
D K (kHz)
d J (kHz)
d K (kHz)
F J (Hz)
F JK (Hz)
F KJ (Hz)
F K (Hz)
f J (Hz)
f JK (Hz)
f K (Hz)
IA 1 IB2IC/10 220 (u m 2)
No
Rms
Ground state
1.excited tors
2.excited tors.
3.excited tors.
10606.5838 (21)
4784.3241 (10)
3366.2351 (7)
2.3052 (17)
0.631 (8)
9.5178 (33)
0.75605 (29)
5.554 (8)
0.2398 (6)
0.572 (11)
3.051 (32)
20.488 (21)
20.02450 (23)
2.177 (7)
21.67 (5)
23.148117 (10)
468
0.059
10598.7399 (31)
4764.7320 (13)
3367.7055 (14)
2.230 (10)
20.09 (6)
9.683 (24)
0.770 (4)
5.11 (8)
20.124 (33)
20.87 (34)
22.0 (6)
0.45 (24)
0.011 (13)
20.4 (4)
1.0 (7)
23.68328 (4)
350
0.050
10593.022 (5)
4744.4398 (20)
3369.3158 (20)
2.311 (17)
20.34 (9)
10.18 (4)
0.767 (7
5.34 (14)
0.05 (4)
21.0 (4)
20.8 (4)
0.44 (4)
20.043 (22)
1.6 (5)
1.0
24.23439 (6)
238
0.066
10590.05 (12)
4723.30 (11)
3371.00 (11)
2.6 (15)
21.7 (11)
10.18
0.28 (6)
5.34
0.05
21.0
20.8
0.44
20.043
1.6
1.0
24.7993 (20)
27
0.859
50
K.-M. Marstokk et al. / Journal of Molecular Structure 567±568 (2001) 41±57
Table 10
Spectroscopic constants for the ®rst and second excited state of the methyl torsion and the excited states of the C1±S3±C4 and C2yC1±S3
bending motion
Av (MHz)
Bv (MHz)
Cv (MHz)
D J (kHz)
D JK (kHz)
D K (kHz)
d J (kHz)
d K (kHz)
F J (Hz)
F JK (Hz)
F KJ (Hz)
F K (Hz)
f J (Hz)
f JK (Hz)
f K (Hz)
IA 1 IB2IC/10 220 (u m 2)
No
Rms
1.excited methyl
2.excited methyl
1.excited C1±S3±C4
1.excited C2yC1±S3
10621.275 (13)
4738.618 (6)
3351.722 (4)
2.015 (21)
3.84 (18)
5.18 (9)
0.780 (13)
1.01 (27)
0.330 (7)
3.86 (21)
3.051
20.488
20.0245
2.177
21.67
23.45100 (12)
112
0.187
10647.741 (16)
4710.056 (7)
3340.410 (7)
2.79 (6)
0.93 (32)
13.3 (10)
0.965 (24)
7.15 (4)
0.33
3.86
3.051
20.488
20.0245
2.177
21.67
23.46886 (22)
46
0.135
10693.048 (8)
4770.0065 (34)
3350.8913 (33)
2.379 (15)
22.03 (13)
14.15 (6)
0.820 (10)
8.86 (20)
0.2222 (32)
0.63 (10)
3.051
20.55 (7)
20.0245
2.177
21.67
22.39244 (10)
174
0.110
10600.824 (9)
4770.463 (4)
3359.440 (4)
1.72 (4)
2.37 (21)
6.5 (4)
0.492 (16)
2.75 (27)
0.2398
0.572
3.051
20.488
20.0245
2.177
21.67
23.17725 (13)
41
0.080
The theoretical predictions (Table 7) indicate that
the skew form should possess a sizeable m a dipole
component. It was then decided to use the MWRFDR
method [14] in an attempt to ®nd the aR-transitions
because this technique is so speci®c. These experiments met with immediate success. Six very weak
series of lines belonging to the ground and to ®ve
vibrationally excited states were observed. This gave
Table 11
Spectroscopic constants for the combination band of methyl vinyl sul®de
Av (MHz)
Bv (MHz)
Cv (MHz)
D J (kHz)
D JK (kHz)
D K (kHz)
d J (kHz)
d K (kHz)
F J (Hz)
F JK (Hz)
F KJ (Hz)
F K (Hz)
f J (Hz)
f JK (Hz)
f K (Hz)
IA 1 IB2IC/10 220 (u m 2)
No
Rms
1.excited methyl
11.excited tors.
1.excited C1±S3±C4
11. excited tors.
1.excited C1±S3±C4
11.excited methyl
10616.020 (13)
4721.943 (5)
3353.177 (5)
2.277 (25)
0.31 (19)
8.69 (9)
0.575 (17)
5.21 (30)
0.046 (5)
20.29 (5)
0.53
20.02
20.007
0.89
20.34
23.91659 (18)
94
0.143
10682.754 (11)
4748.374 (5)
3352.680 (4)
2.416 (20)
21.92 (18)
13.70 (7)
0.752 (14)
8.06 (26)
0.063 (12)
20.99 (20)
0.53
20.05
20.007
0.89
20.34
23.00115 (13)
95
0.127
10697.02 (4)
4722.33 (4)
3337.21 (4)
1.2 (9)
20.1 (8)
15 (4)
0.59 (5)
3.5 (8)
0.276
2.25
3.051
20.52
20.0245
2.177
21.67
22.8261 (8)
29
0.172
K.-M. Marstokk et al. / Journal of Molecular Structure 567±568 (2001) 41±57
51
Table 12
Spectroscopic constants for the ground state of the isotopic species for methyl vinyl sul®de
13
13
10550.153 (11)
4736.075 (11)
3336.646 (10)
1.98 (22)
0.29 (11)
9.5189
0.742 (6)
5.554
0.2398
0.572
3.051
20.488
20.0245
2.177
21.67
23.14776 (22)
29
0.091
10543.159 (13)
4635.526 (14)
3285.965 (13)
2.01 (16)
0.31 (15)
9.518
0.679 (19)
5.554
0.2398
0.572
3.051
20.488
20.0245
2.177
21.67
23.15806 (31)
32
0.120
C1
Av (MHz)
Bv (MHz)
Cv (MHz)
D J (kHz)
D JK (kHz)
D K (kHz)
d J (kHz)
d K (kHz)
F J (Hz)
F JK (Hz)
F KJ (Hz)
F K (Hz)
f J (Hz)
f JK (Hz)
f K (Hz)
IA 1 IB2IC/10 220 (u m 2)
No
Rms
three additional series of excited states in addition to
the previously assigned bQ-branch series just
described. The several excited states found in our
case parallel the ®ndings made for skew
H2CyCHSCuN [23,24] where many excited states
were observed. These new aR transitions were
not split into resolved doublets, presumably
because they are not very sensitive to the A
rotational constant.
The a-type R-branch transitions were then used
with the bQ-branch lines mentioned above to predict
b-type R-branch transitions. The assignments of bRbranch transition were essential in order to con®rm the
assignments. The bR-branch lines were in all cases
found to be split into doublets, as was expected. The
resulting spectroscopic constants of the three states
that were fully assigned are listed in Table 14. Only
rough values of B and C are available for the additional three vibrationally excited states identi®ed in
the MWRFDR experiments. These values are as
follows: B ˆ 3498 2† and C ˆ 3031 2† for the
fourth, B ˆ 3525 3† and C ˆ 3039 4† for the ®fth,
B ˆ 3533 3† MHz and C ˆ 3062 3† MHz for the
sixth excited state, respectively. No quantitative relative intensity measurements could be made for these
three states because they are so weak. In fact they
`drowned' in the background of other transitions and
34
C2
S3
10453.155 (7)
4741.3211 (31)
3329.4435 (25)
2.186 (21)
0.93 (13)
8.48 (5)
0.676 (9)
5.80 (17)
0.158 (27)
20.9 (5)
1.7(5)
20.61 (5)
20.0275 (20)
2.177
21.67
23.14653 (7)
140
0.075
13
C4
10338.935 (16)
4723.048 (15)
3308.772 (15)
2.5 (4)
20.14 (7)
9.518
0.768 (9)
5.554
0.2398
0.572
3.051
20.488
20.0245
2.177
21.67
23.14475 (30)
28
0.087
Stark components when we tried to observed them
using Stark spectroscopy.
The assignment of the vibrationally excited states
of the skew rotamer is not straightforward. The strongest lines are of course assigned to the ground vibrational state. The second strongest excited state is
assigned to the ®rst excited state of the C1±S torsional
mode. This state is about 80% as intense as the ground
vibrational state at dry ice temperature. Relative
intensity measurements [21] yielded 28 (15) cm 21
for this fundamental vibration.
The correct vibrational assignment of the next
excited state, which has a frequency of 86 (20)
cm 21, is uncertain. It could be the second excited
state of the C1±S torsional vibration, or some other
mode involving heavy atoms, since the changes of the
rotational constants upon excitation are relatively
large. No speci®c assignments are offered for the
remaining three excited states, whose rough B and C
constants are given in the text above.
Our assignments of the (1) and (2) states (Tables
13 and 14) are somewhat arbitrary. The (1) state of
the ground vibrational state has been chosen because
Ia 1 Ib 2 Ic is larger than for the (2) state. This is an
indication that the (1) state is less planar than the (2)
state, as would be expected in this case. The
remaining (1) and (2) assignments are arbitrary.
52
K.-M. Marstokk et al. / Journal of Molecular Structure 567±568 (2001) 41±57
Table 13
Transitions (MHz) for the 0 1 and the 02 ground state of the skew conformation
Transition
01
Observed2calculated
31,2 Ã 30,3
31,3 Ã 20,2
40,4 Ã 30,3
41,3 Ã 31,2
42,2 Ã 32,1
42,3 Ã 32,2
51,5 Ã 41,4
60,6 Ã 50,5
50,5 Ã 41,4
51,4 Ã 50,5
51,5 Ã 40,4
60,6 Ã 51,5
61,5 Ã 51,4
61,5 Ã 60,6
61,6 Ã 50,5
61,6 Ã 51,5
62,4 Ã 52,3
62,5 Ã 52,4
63,3 Ã 53,2
63,4 Ã 53,3
64,2 Ã 54,1
64,3 Ã 54,2
65,1 Ã 55,0
65,2 Ã 55,1
70,7 Ã 61,6
71,6 Ã 70,7
71,7 Ã 60,6
72,5 Ã 71,6
72,5 Ã 62,4
75,2 Ã 65,1
75,3 Ã 65,2
76,1 Ã 66,0
76,2 Ã 66,1
80,8 Ã 71,7
81,7 Ã 80,8
82,6 Ã 81,7
83,6 Ã 73,5
85,3 Ã 75,2
85,4 Ã 75,3
91,8 Ã 82,7
91,8 Ã 90,9
92,7 Ã 82,6
93,6 Ã 83,5
94,5 Ã 84,4
94,6 Ã 84,5
95,4 Ã 85,3
95,5 Ã 85,4
96,3 Ã 86,2
96,4 Ã 86,3
100,10 Ã 91,9
101,9 Ã 92,8
101,9 Ã 100,10
102,8 Ã 101,9
111,10 Ã 110,11
112,9 Ã 111,10
122,10 Ã 121,11
132,11 Ã 131,12
142,12 Ã 141,13
15674.91
32528.38
0.07
20.21
26392.49
26276.98
0.04
0.12
20610.78
18054.74
43650.82
28004.58
0.18
20.09
20.21
0.20
19762.41
48957.05
20.16
20.14
39498.24
39488.29
39470.11
39470.11
39461.65
39461.65
35442.07
21867.96
54143.96
37681.42
0.07
20.14
20.15
20.07
20.04
20.04
20.19
20.14
0.11
0.33
46045.36
46045.36
46039.77
46039.77
42875.47
24409.21
36838.93
52688.56
52632.39
52632.39
20.14
20.14
0.22
0.22
0.12
20.03
0.23
20.02
20.08
20.08
27416.25
20.06
59256.50
59254.58
59222.88
59222.88
59208.30
59208.30
57551.98
0.06
20.04
20.19
20.17
0.19
0.19
20.02
30906.87
35748.42
34881.59
35640.94
35907.20
0.06
0.05
0.23
20.05
20.08
37760.12
20.12
02
Observed2calculated
26169.92
27199.97
26392.49
26276.98
31635.13
39029.07
0.13
0.06
20.04
0.06
0.00
0.08
28003.96
40736.74
20.04
20.14
48957.75
37932.78
39774.33
39376.20
39498.24
39488.29
39470.11
39470.11
39461.65
39461.65
35442.07
21869.48
54143.96
0.13
0.15
20.14
0.09
0.02
0.09
20.14
20.06
0.04
0.04
0.10
20.03
20.23
46536.27
46045.36
46045.36
20.11
20.03
20.03
24410.81
20.03
52688.56
52632.39
52632.39
26379.82
27418.12
60204.12
59377.63
59256.50
59254.58
59222.88
59222.88
20.08
0.05
0.05
20.30
0.01
0.14
0.06
0.10
20.01
20.04
20.02
34966.10
30909.01
35750.84
34883.87
35643.44
35909.68
36601.23
37762.60
0.11
0.18
0.19
0.30
0.12
20.02
20.16
20.33
K.-M. Marstokk et al. / Journal of Molecular Structure 567±568 (2001) 41±57
53
Table 13 (continued)
Transition
01
Observed2calculated
02
Observed2calculated
152,13 Ã 151,14
162,14 Ã 161,15
163,13 Ã 162,14
173,14 Ã 172,15
182,16 Ã 181,17
183,15 Ã 182,16
192,17 Ã 191,18
202,18 Ã 201,19
203,17 Ã 202,18
223,19 Ã 222,20
233,20 Ã 232,21
243,21 Ã 242,22
39429.74
41638.21
58185.57
56720.67
47753.02
55513.63
51667.40
56131.27
54200.82
0.14
20.06
0.19
20.22
20.01
20.05
0.14
20.23
20.10
39432.13
20.33
58190.13
56725.43
0.03
20.15
56134.71
20.12
55976.51
57759.06
0.00
0.12
54819.64
55981.36
57764.17
0.18
20.15
0.22
3.5. Energy difference between syn and skew form
Relative intensity measurements were carried out
as described in Ref. [21] to determine the energy
difference. This difference depends on the ratio of
the dipole moments. Ratios of dipole moments from
several of the calculations in Table 7 were tested to
derive a value of 5.0 (3) kJ mol 21, which should be
compared with the calculated values of 5.8 and
9.1 kJ mol 21 as shown in Table 6. The estimated
standard deviation of ^0.3 kJ mol 21 is assumed to
take both systematic and random errors into account.
Previous determined experimental values for the
energy difference are: 4.2 (13) using a static model
and 7.9 (8) kJ mol 21 using a dynamic model [7],
5.9 kJ mol 21 from IR [3] and 8.9 (8) kJ mol 21 [10].
The ED results [7] could be further improved by using
a better force ®eld and including relaxation of all the
structure parameters from high level quantum
chemical calculations in the dynamic model.
3.6. Structure
The substitution coordinates [26] of the heavy
atoms are given in Table 15. The a- and b-axis coordinates were derived from the rotational constants
shown in Table 12 using Kraitchman's equations
[27]. The c-axis coordinates were assumed to be
zero for symmetry reasons. The errors, s (x), of the
a- and b-coordinates were obtained from the formula
s (x) ˆ K/uxu where K was taken from van Eijck's
tabulation [28]. The rs-structure of the heavy atoms
was then calculated from these coordinates. The
uncertainties have been calculated from the s (x)
values using the formula for propagation of errors.
Owing to the fact that all the coordinates are far
Table 14
Spectroscopic constants for the skew conformation methyl vinyl sul®de
Ground state
01
Av
Bv
Cv
DJ
D JK
DK
dJ
dK
IC2IA2IB
No
Rms
1st tors. excited state
02
11
Excited state
12
21
22
17502.951 (32)
17503.67 (4)
17786.58 (6)
17787.66 (6)
17657.189 (26)
17658.417 (25)
3520.238 (6)
3520.267 (6)
3512.239 (6)
3512.281 (7)
3515.538 (6)
3515.560 (6)
3051.174 (6)
3051.168 (5)
3035.119 (6)
3035.122 (6)
3041.201 (6)
3041.188 (6)
1.35 (5)
1.35 (4)
1.60 (4)
1.58 (4)
0.97 (5)
0.97 (5)
218.06 (15)
217.25 (20)
220.06 (9)
219.47 (10)
23.71 (9)
23.61 (10)
322 (5)
277 (5)
360 (13)
338 (14)
92 (4)
95 (4)
0.0918 (26)
0.1123 (25)
20.003 (4)
0.025 (4)
0.1886 (23)
0.1900 (19)
8.24 (30)
5.69 (30)
9.24 (33)
7.6 (4)
21.59 (22)
21.41 (21)
26.80350 (12)
26.80080 (12)
25.79392 (19)
25.79057 (20)
26.20008 (10)
26.19650 (10)
55
51
40
43
48
47
0.154
0.147
0.145
0.156
0.105
0.103
54
K.-M. Marstokk et al. / Journal of Molecular Structure 567±568 (2001) 41±57
Table 15
Kraitchmans coordinates for the skeleton atoms of the syn conformation in the principal axes system and the structure (coordinates and distances
in pm and bond angles in degrees)
Atom
A
B
C
C1
C2
S3
C4
104.04 (8)
184.48 (4)
269.82 (4)
2116.47 (7)
51.23 (16)
254.87 (15)
60.52 (5)
2113.02 (7)
0
0
0
0
Structure
Bond distances
C1±C2 ˆ 133.15 (18)
C1±S3 ˆ 174.11 (0)
S3±C4 ˆ 179.70 (9)
Bond angles
C2±C1±S3 ˆ 130.23 (5)
C1±S3±C4 ˆ 101.99 (5)
from the principal axes an accurate geometry of the
heavy atom skeleton of the syn conformation could be
derived which is also given in Table 15. The agreement between these experimental determined
structure parameters and those calculated using the
cc-pVTZ basis sets (Table 1) is very good.
The torsional angle of skew has been estimated in
the following way. To the experimental determined
geometry of the heavy atom skeleton of the syn
conformer (Table 15) the calculated changes
between the syn and skew conformers have been
added using B3LYP/Aug-cc-pVTZ values (Table
5). All the calculated structure parameters determining the H-atoms for the skew conformer (Table
2) have been added to this skeleton frame. The
torsional angle has been ®tted to reproduce the
observed rotational constants for the ground state
of the skew conformer. A torsional angle of 1548
gives the following rotational constants 17525,
3503 and 3038 MHz for A, B and C, respectively.
This should be compared with the corresponding
values in Table 14. The A rotational constant is
very dependent on the torsional angle while the B
and C are less sensitive. It is therefore felt that the
torsional angle is rather accurately determined using
this method. Exactly the same torsional angle is
found for the second conformer of vinyl mercaptan
[25] while a small basis set ab initio calculations
gives 1408 [29].
3.7. The barrier height at the anti form
MVS most likely has a barrier at the anti position. It
is possible to estimate this barrier height using the
information obtained from the ground and the vibrationally excited states of the skew rotamer. Gwinn et
al. [30] have given a quantitative treatment of this
problem, see also Ref. [31]. They have shown that it
Table 16
Comparison of calculated and observed rotational constants (MHz) for the skew conformer using B ˆ 211.48 (observed rotational constants are
listed in Table 13)
vT
Calculated2observed (MHz)
A
B
C
01
02
12
11
Calculated rotational constants (MHz)
Av ˆ 20798 (39)2594.4 (22) kz 2lv20.4 (11) kz 4lv
Bv ˆ 3428.1 (13)116.73 (7) kz 2lv20.006 (34) kz 4lv
Cv ˆ 2865.51 (32)133.601 (18) kz 2lv10.003 (9) kz 4lv
20.53
0.53
20.46
0.46
20.017
0.017
20.015
0.015
0.004
20.004
0.004
20.004
K.-M. Marstokk et al. / Journal of Molecular Structure 567±568 (2001) 41±57
55
Table 17
Internal rotation splittings of the methyl torsional second excited state (uncertainty is one least square standard deviation)
Transition
vA(obs)
vA2vE (observed)
vA2vE (calculated)
52,3 Ã 51,4
62,4 Ã 61,5
51,4 Ã 50,5
22,1 Ã 21,2
72,5 Ã 71,6
73,4 Ã 72,5
63,3 Ã 62,4
82,6 Ã 81,7
61,5 Ã 60,6
53,2 Ã 52,3
52,4 Ã 51,5
43,1 Ã 42,2
33,0 Ã 32,1
50,5 Ã 51,4
62,5 Ã 61,6
63,4 Ã 62,5
73,5 Ã 72,6
31,3 Ã 20,2
21,2 Ã 10,1
41,4 Ã 30,3
Calculated rotational constants (MHz)
A ˆ 10646.97 (7), B ˆ 4709.06 (5),
C ˆ 3340.41 (5)
Direction cosine: 0.293 (9)
Methyl barrier (kJ mol 21): V3 ˆ 13.61 (5)
16848.29
18719.67
21240.77
21921.57
22220.09
25495.01
27166.27
27396.31
27851.60
29211.79
30730.29
31024.64
32232.62
34842.79
35115.25
36378.19
38697.36
26752.53
20668.70
32508.76
1.79
5.56
13.22
5.99
11.77
24.00
25.46
18.76
18.53
26.91
8.99
214.27
233.86
3.08
12.31
6.04
8.19
0.00
0.24
0.00
1.47
6.25
14.16
4.81
12.48
25.25
27.23
20.78
19.34
28.16
9.15
214.23
233.58
3.68
12.66
5.25
7.83
20.07
0.45
20.48
is possible to de®ne a potential function for the torsion
as
V ˆ A kz 4 l 1 Bkz2 l†;
where z is a dimensionless coordinate. If B is positive,
the heavy atom skeleton has a symmetry plane, if B is
negative, the equilibrium conformation will be
nonplanar. According to this theory the rotational
constants can be expanded in a power series of the
expectation values kz 2l and kz 4l, where bn is the An, Bn
or Cn rotational constants in the nth excited state of the
torsion. The empirical parameters b0, b2 and b4 are
adjusted to give the best ®t to the data. The values
kz 2ln and kz 4ln depend only on the B constant in the
equation given above. The rotational constants of
successively excited states of the torsional vibration
were least-squares ®tted to the equation
Bn ˆ b0 1 b2 kz2 ln 1 b4 kz4 ln
employing a program described in Ref. [32] for a series
of B-values. Using two rotational constants for the
ground state and two for the ®rst excited state of the
torsion, it was found that the value B ˆ 211.48 yields
the best overall ®t (Table 16). This negative B-value
indeed shows that the heavy atom skeleton is nonplanar.
The A constant was then adjusted to reproduce the
torsional fundamental frequency of 28 cm 21. This was
achieved with A ˆ 3 cm 21, which gives a barrier of
100 cm 21 or 1.2 kJ mol 21. This barrier is considerably
larger than the barrier found for vinyl mercaptan which
is 19 cm21[25].
3.8. Methyl barrier
Internal rotation A±E splittings for the v ˆ 1 methyl
torsional state have been measured for four transitions
by Penn and Curl [2], and they determined the V3
methyl barrier to be 13.5 ^ 0.4 kJ mol 21. The
measured splittings were between 0.5 and 1.0 MHz.
We have assigned the v ˆ 2 methyl torsional state,
and used 20 transitions with splittings as large as
about 34 MHz to determine the methyl barrier using
56
K.-M. Marstokk et al. / Journal of Molecular Structure 567±568 (2001) 41±57
Fig. 2. The potential energy functions derived from the (O) MP2 ˆ full/cc-pVTZ and (X) B3LYP/AUG-cc-pVTZ results (see text).
the mb15 computer program [33] which is based on
the principal axis method [34]. The transitions
together with the observed and calculated vA ±vE splittings are given in Table 17. The methyl barrier was
determined to be 13.61 (15) kJ mol 21, which is in
excellent agreement with the value of Penn and Curl
[2]. Durig et al. [10] have determined the barrier to be
16.28 ^ 0.04 kJ mol 21 for the -d3 compound which is
not in agreement with the results from MW spectroscopy. This strongly indicates that it must be something in the proposed assignment [10] for the
observed bands which is wrong. The calculated
methyl torsional frequency from the methyl barrier
is 222 cm 21 which is in very good agreement with
the B3LYP/AUG-cc-pVTZ frequency of 217.5 cm 21
(Table 8).
3.9. Potential function
The potential function, V(w ), for internal rotation
about the C1±S bond can be expand as a Fourier series
X
1 2 cos nw†:
V w† ˆ
nˆ1
From the values of the extremal points, i.e. 2V=2w ˆ
0; given in Table 6 can ®ve Fourier coef®cients Vn be
determined by solving the set of linear equations:
V wts † ˆ Vts ;
2V=2w†wts ˆ 0;
V wskew † ˆ Vskew ;
2V=2w†wskew ˆ 0 and V 180† ˆ V180 : The Fourier series
always satisfy the additional requirements V 0† ˆ
2V=2w†wˆ0 ˆ 2V=2w†wˆ180 ˆ 0: The Fourier coef®cients Vn derived from the MP2/cc-pVTZ and B3LYP/
Aug-cc-pVTZ are: V1 ˆ 0.22 and 21.10, V2 ˆ 6.78 and
7.50, V3 ˆ 4.02 and 3.68, V4 ˆ 0.81 and 0.34, and
V5 ˆ 0.61 and 0.47 kJ mol 21, respectively, and these
two potential functions are shown in Fig. 2.
The torsional frequencies can be calculated from the
relation: 22 V=2w2 † ˆ 4p2 n2 G21
which give nsyn ˆ
tt
190 cm21 and nskew ˆ 67 cm21 for the B3LYP/Augcc-pVTZ calculation and nsyn ˆ 204 cm21 and nskew ˆ
74 cm21 for the MP2/cc-pVTZ. Compared to the
observed torsional frequencies nsyn ˆ 170 cm21 [10]
and n syn < 130 cm 21 (MW) and nskew ˆ 106 cm21
[10] and n skew < 28 cm 21 (MW). These results indicate
that the syn/skew barrier is too high and that the nskew ˆ
106 cm21 assignment [10] must be wrong.
Some features of the experimental potential function
can be estimated from MW results using the same set of
K.-M. Marstokk et al. / Journal of Molecular Structure 567±568 (2001) 41±57
equations as given above. The energy difference, which
corresponds to the difference between the ground states,
is 5.0 (3) kJ mol 21 from MW. The differences in the zero
point energies is 1.4 kJ mol 21, which gives an energy
difference between the skew and syn minimum, Vskew,
equal to 6.4 kJ mol 21. The anti barrier is determined to
be 1.3 kJ mol 21 which should give V180 equal to
7.7 kJ mol 21. Since the torsional vibrational frequencies
are calculated to be too large this strongly indicates that
the calculated syn/skewbarrier is too large.
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
4. Conclusions
Gaseous methyl vinyl sul®de exists in syn and skew
conformations. The syn form (Fig. 1) is the preferred
conformer being 5.0 (3) kJ mol 21 more stable than skew.
The syn rotamer has a planar heavy atom skeleton. Ten
vibrationally excited states were determined for this
rotamer. Less information is available for the skew
rotamer which is obtained from the syn form by rotating
approximately 1548 around the C±S bond.
Elaborate quantum chemical calculations have been
carried out for the two forms, and it became evident that
a 6-31111G pp basis set or smaller basis sets were not
suf®cient to reproduce the torsional angle of the skew
conformer. The best agreement with experimental
values is found using the cc-pVTZ basis set.
5. Uncited reference
[8].
Acknowledgements
We are grateful to The Research Council of
Norway (Programme for Supercomputing) for a
grant of computer time at the IBM RS6000 cluster
at the University of Oslo.
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