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Microwave spectrum and conformation of vinyl mercaptan
JOURNAL
OF
MOLECULAR
Microwave
SPECTROSCOPY
Spectrum
78, 106- 119 (1979)
and Conformation The anti
of Vinyl Mercaptan
Rotamer
MITSUTOSHI TANIMOTO Sagami Chemical Research
Center,
Nishi-Ohnuma
4-4-1, Sagamihara,
Kanagawa
229, Japan
AND
J.N.
MACDONALD
School of Physical and Molecular Sciences, University College of North Wales, Bangor, LL57 2UW, Gwynedd, United Kingdom
Microwave spectra of the anti rotamer of vinyl mercaptan and its SD isotopic species have been studied in the frequency range 12-60 GHz. For the normal species rotational and centrifugal distortion constants have been obtained for the ground and first three excited states of the SH torsional mode, the ground state values being A = 49 422.75(5) MHz, B = 5 897.215(9) MHz, C = 5 279.436(9) MHz, D, = 3.12(11) kHz, DJ, = -38.5q1.71) kHz, and & = 0.498(51) kHz. An approximate potential function for the SH torsion in the vicinity of the anti conformation, derived using the observed variation of rotational constants with vibrational quantum number, reveals the presence of a small potential barrier of 19 cm-l at the planar conformation. The u = 0 state lies above this barrier so the molecule is essentially planar in the ground state in spite of the observed negative value for the inertia defect (-0.1976(2) a.m.u.&). The anti rotamer is found to be 50 2 25 cm-’ less stable than the syn rotamer. The dipole moment has the ground state values IL. = 0.425(10), pLb= 1.033(10), and kotal = 1.117(14) D and is shown to vary considerably with vibrational quantum number. Evidence for significant structural changes in going from the syn rotamer to the anti rotamer is also presented. 1. INTRODUCTION
In a preceding paper we reported an analysis of the microwave spectrum of syn-vinyl mercaptan (I ) in which it was shown that this rotamer behaves in every respect like a conventional planar molecule. In addition the dipole moment was measured and some conclusions drawn concerning the molecular structure. Recent ab initio calculations (2, 3) have predicted the existence of a second rotameric form of the molecule, less stable than the syn rotamer and with a nonplanar gauche or skew conformation ((I), Fig. 1). The equilibrium SH torsional angle is computed to lie in the region of -130-150”. In this paper we present an analysis of the microwave spectrum of the second rotamer in the ground and excited vibrational states of the SH torsional mode. The dipole moment of the molecule in each vibrational state has also been measured. Considerations concerning the potential function governing the torsional 0022-2852/79/110106-14$02.00/O Copyright
0 1979 by Academic
AU rights of reproduction
Press,
Inc.
in any form reserved.
106
MICROWAVE
SPECTRUM
OF VINYL
MERCAPTAN
107
mode suggest that this form of the molecule is best described as a “quasi-planar” anti conformation. 2. EXPERIMENTAL
DETAILS
As the experimental details in this investigation were largely the same as in the case of the study of the syn rotamer (1) only certain points are made here. The vinyl mercaptan was again prepared by two different methods, photolysis of a mixture of hydrogen sulphide and acetylene and the thermal decomposition of either 1,Zethanedithiol or thiirane. However, for this work, thiirane was mainly used as the starting material in the pyrolysis experiments since it was found to give by far the best yields of vinyl mercaptan, maximizing at a pyrolysis temperature of 900-950°C. This procedure produces thioacetaldehyde and vinyl mercaptan in comparable quantities, however, the spectrum of the former was readily distinguished from that of the mercaptan by making use of the known rotational constants (4). All spectra were studied at room temperature and dipole moment measurements were carried out in the usual manner (I, 5). 3. THE MICROWAVE
SPECTRUM
OF ANTI-VINYL
MERCAPTAN
Assuming that a suitable model for the structure of any second rotamer of vinyl mercaptan can be generated simply by rotation of the thiol hydrogen atom around the C-S bond of the syn rotamer with all other structural parameters held constant, any reasonable gauche or anti conformation remains a near-prolate rotor with K - -0.97. The pattern of lowJ, a-type transitions characteristic of such a molecule is predicted to lie to low frequency of the corresponding pattern for the syn rotamer for all likely values of the torsional angle. For the planar anti rotamer, bond moment calculations show that the b component of the dipole moment (- 1 D) is considerably larger than the a component (-0.5 D) in contrast to the syn rotamer. The effect of nonplanarity on the dipole moment components is to reduce the b component while increasing the a component relative to their values in the anti rotamer. A search of the spectrum revealed that, while no suitabis low-J transitions could be located in the vicinity of predicted transition frequencies based on the above model, sets of comparatively weak lines could be identified a few hundred megahertz to high frequency of each of the u-type transitions of the syn rotamer. Each set comprised two lines of virtually equal intensities accompanied by several weaker lines (see Fig. la). Sets of b-type Q-branch transitions of the series Jm-l + Jo,./,together with certain R-branch transitions, were also identified, each consisting of a ground state line, strong in comparison to the u-type lines, accompanied by definitely weaker vibrational satellites (see Fig. lb). None of these lines could be attributed to any likely impurity or to the syn rotamer. Consequently, on the basis of well-resolved Stark effects, the relative intensities of the b-type to u-type lines and the fit to a rigid rotor model, the two strongest members of each of the a- and b-type line patterns were assigned as the u = 0 and u = 1 transitions of the lowest vibrational mode, the SH torsion, of a second
108
TANIMOTO AND MACDONALD V:l
v=o
v:4 I 22380
22360
22340
MHz
v=2 v=3 I 20020
?,
I
A
CL 20623
20345
21178
MHz
tb, FIG. 1. Observed vibrational satellite patterns for (a) the & 6 l,,, a-type transition and (b) the 2,, +- 11, b-type transition of anti-vinyl mercaptan.
rotamer of vinyl mercaptan. It is shown later that the conformation of this second rotamer is nearly that of the anti rotamer so hereafter we refer to it by this title for clarity. The large deviation of the observed transitions from the predictions of the simple model can be rationalized in terms of structural relaxation during rotation around the C-S bond of the type noted in the case of cis- and trunsthioformic acid (6). The observed transition frequencies for the isotopic species CH,=CHSH and CH,=CHSD of this rotamer are listed in Tables I and II. The appropriate rotational constants, centrifugal distortion constants, and inertia defects are given in Table III. As in the case of the syn rotamer the observed transitions permit only limited accuracy in the determination of centrifugal distortion data, DK and SK being indeterminate. The negative inertia defect in the ground vibrational state (see Table III) together with the presence of an intense vibrational satellite accompanying each ground state line, appears to exclude the possibility that the rotamer giving rise to the spectra is the planar anti rotamer in any strict sense of the term. However, it is now well known, particularly for certain ring compounds (7) and amides (8),
109
MICROWAVE SPECTRUM OF VINYL MERCAPTAN TABLE I in the u = 0
Rotational Transition Frequencies (MHz) of anti-CH,=CHSH and o = 1 States of the SH Torsional Vibratio@ VZO
Transition
obs.
202-101 212-111 211-110
v= ohs.
22
346.
21
735.55
72
22
971.07
33 503.52
303-202
32
313-212 312-211
599.
11
34 452.
35
414-313 413-312 423-322 422-321 505 -404 515-414 514-413
1
lo-‘01
211-202 312-303 413-404 514-505
22
374.33
-0.07
0.04
21
787.75
-0.08
0.05
22
973.
817-*08 lll-“oo 2
12-101
202-111 bOb-5l5 707-b16 808-717 909-818
(4
0. 06
32 677.
-0.24
0.01
34 455.84
0. 10
.0.23*
33 571.36
0.15*
-o.zo*
33 595.29
-0.01
44
699.24
0.03
43 457.81
-0.05
43
562.76
0.01
-0.01
0. 01
45
928.
45
933.66
44
702.00
-0.04
44
757.10
44
767.21
-0.01
44
817.
37
55
751.
38
-0.02
55 827.99
54
309.
78
-0.01
54 441.88
57
397.
38
55
999.00
44
143.40
44
767.
79
45
716.
75
47
004.46
48
650.39
0.11*
0.07 -0.02 0. 04 -0.06 0. 02
96
114.81
-0.17
0.57* 38
-0.02 0.04 -0.03
57 404.90
-0.12
55 937.54
-0.04
56
058.94
43
590.63
44
189.
45
099.83
0.19*
46
334.01
0. 02
0.10 0. 00 75
-0.06
0.00
47
910.79
-0.01
0. 01
49
851.88
-0.04
52
-0.06
182.99
0.01
55 991.57
0.02
54 933.13
0.01
54
702.24
0.01
54
187.99
0.01
65 261.21
-0.01
64
785.48
-0.02
21
178.87
-0.07
20
623.43
-0.10
27
443.69
27
906.28
0. 04
40
159.21
-0.01
40
581.59
0. 00
53
035.64
-0.02
53 415.24
66
034.93
Accuracy
Lines
0. 05 32
556.45
better
frequencies *
0.03
640.57
53
716-‘07
32
33 545.94
33
50 677.
%5-‘06
-talc.
-0.05
524-423 523-422
1 obs.
44
321-220 404-303
obs.
-0.02
33 530.77
322-221
-talc.
not
use used
0.07
-0.01
0.02
than the
0. 10 MHz. rotational
in least
squaren
The constants analysis.
calculated of
Table
III.
110
TANIMOTO AND MACDONALD TABLE II Rotational Transition Frequencies (MHz) of anti-CH,=CHSD and u = 1 States of the SH Torsional VibratioP) v=o
Transition
lo1 2 2 2 3 3 3 4 4 4
obs.
-1
10
03-‘02 13-‘12 12-211 04-303 14-313 -3
12
5 5 5 5 5 1
22-321 05-404
14-413 24-423 23-422 lo-lo1
12-303
413-404 5 6
14-505 15-606
‘16-‘07
1 2
11
-0
00
12-‘01
‘07~%6 *08-‘17
(a)
**
638.65
-0.03
20
980.77
-0.01
21
049.04
-0.03
22
242.05
0.17**
22
242.
05
0.12**
32
386.
0.00
32 440.
54
0.04
31 466.28
0.01
31 569.09
0.01
33
0.05
33
0.01
39
357.81
43
146.29
-0.02
43 221.75
0.00
946.10
-0.01
42
083.96
0.00
44
467.81
44
469.86
-0.01
43 285.86
-0.02
0.01 -0.02
43 292.42
0.00
43
353.67
53 875.81
0.00
53
975.
37
0.00
-0.92* 32
-0.01
52 591.96
0.00
55
569.69
-0.06
55
54
011.02
-0.02
54 098.22
54
162.01
0. 06
54 235.52
0.01
39 698.25
0. 04
39 094.19
0. 03
573.60
-0.01 0.01
40
356.53
41
307.99
42
629.
44
323.38
46
416.39
-0.19*
45
435.24
48
940.66
0.02
47
812.03
-0.06
0.01
50 624.23
-0.02
49
-0.09
-0.01
55
39 697.74
0.04
40
615.96
0. 06
41
864.04
0.02
43 462.29
49
873.42
60
048.45
0. 04
59 546.59
42
034.67
0.01
42
54
513.66
better
not
Overlapped
358.74
41
Accuracy
Lines
-0.01
21
frequencies *
79
-talc.
0.01
51 930.59
*17-*08
1 obs.
603.64
52 418.
15-414
211-202 3
obs.
21
43 216.80
423-322 4
-talc.
10 822.
12-ill
13
v= obs.
-Ooo
oz-lo1
11
in the u = 0
me used with
-0.05
-0.01
than the
0.10
in least each
MHz.
rotational squares other.
0.03 -0.02 0. 00 -0.03 0.07
320.50
0.05
461.43
-0.02
54 886.70
0.02
The constants analysis.
calculated of
Table
III.
111
MICROWAVE SPECTRUM OF VINYL MERCAPTAN TABLE III
Rotational Constants (MHz), Centrifugal Distortion Constants (kHz), and Inertia Defect (a.m.u. A*) for anti-Vinyl Mercaptan in the Ground and Excited Vibrational State@) VZO
“Z
1
v=2
v=3
-anti-CH2=CHSH A
49 422.75(5)
48 592.28(
48 889.25(6)
13)
48 245.44(e)
B
5 897.215(9)
5 891. 532(15)
5 880.469(43)
5 868.029(21)
C
5 279.436(9)
5 298.692(13)
5 305. 592(45)
5 313. 306(16)
D D
3.12(11)
J
3. 07( 17)
-38. 50(1. 71)
JK
d
-36.28(2.
0.498(51)
A:b)
13)
0.450(74)
-0. 1976(2)
-0.7397(3)
3.20(46)
3. 13(22)
-33.65(5.38)
-33. 36(3.45)
0.521(54)
0.472(90)
- 1. 0883( 10)
-1.4836(4)
anti-CH2=CHSD A
44 207.27(5)
44 785.79(4)
B
5 718. 114(11)
5 709.692(14)
C
5 087.542(7)
5 113. lOl(10)
2. 78(11)
DJ D
JK
6 A:b)
2.69(15)
-23. 51(1. 16)
-22. 37(1. 70)
0.415(43)
0.349(58) -1.1045(3)
-0. 3299(Z)
(a)
The numbers deviations
b)
in parenthesis
are 2.5
times
the standard
in all caaee.
A = I=-la-$,
that the question of planarity in the presence of large amplitude, low-frequency vibrations can be a subtle one. As the observed patterns of lines, typified by Fig. 1, implied some degree of anharmonicity of the SH torsional mode, we measured the weak lines arising from the u = 2 and v = 3 excited states of this mode with a view to obtaining some insight into the shape of the potential function governing the torsional motion and the extent of any nonplanarity of the rotamer. A tentative assignment has also been obtained for several a-type lines of the u = 4 state but ambiguity exists in the assignment of the b-type lines so the A rotational constant is not determined for this case. The assignments were based on relative intensities (see below), frequency fits, and behavior of rotational constants
112
TANIMOTO AND MACDONALD
with vibrational quantum number. The inertia defect was especially helpful in distinguishing the weakest lines from any which might belong to excited states of the CCS bending vibration (I). The observed transition frequencies are listed in Table IV and the rotational constants, centrifugal distortion constants, and inertia defects in Table III. 4. DIPOLE MOMENT AND VIBRATIONAL
ENERGY DIFFERENCES
It has been implied above, and is evident from Fig. 1, that a determination of energy differences between the vibrational levels of the SH torsional mode through relative intensity measurements of the a- and b-type lines will yield anomalous results if the usual assumption of only a minor change in dipole moment components with vibrational quantum number is retained. Consequently we have carried out detailed Stark-effect measurements, using the methods already described (I, 5), on several transitions of the normal species of the anti rotamer in the vibrational states ZJ= 0, 1, 2, and 3. In all cases the observed frequency shifts with the applied field obeyed second-order theory satisfactorily and the absorption cell was calibrated using the J = 2 + 1 transition of OCS assuming p = 0.71521(20) D for that molecule. The results of the dipole moment determination are summarized in Tables V and VI. Any CL,contribution to the Stark effect has been neglected here. Since this component of the dipole moment is antisymmetric with respect to the torsion, it soon became apparent that the magnitude of the u = 1 c 0 energy difference (-75 cm-‘) is such that the contribution from the transition moment to the Stark effects of the observed transitions would be too small to be reliably detected. The relative magnitudes of the a and b components of the dipole moment in the ground torsional state are in accord with the simple model predictions for the anti rotamer of vinyl mercaptan. The large variations in the components with torsional state are almost certainly the main factor responsible for the apparently anomalous relative intensities of the excited state lines mentioned above. The changes in the components are in the same sense as those predicted by a model assuming a curvilinear path for the thiol hydrogen atom with a constant SH bond length and no appreciable variation of bond moments during the vibration. Relative intensity measurements, carried out on the various vibrational satellites by the method of Esbitt and Wilson (9) and taking due account of the variation in dipole moment components with vibrational quantum number, yield the energy differences between the torsional states given in Table VII. Similar measurements comparing the lines of the syn rotamer with those of the anti rotamer show an energy difference of 50 + 25 cm-l between their respective ground vibrational states in favor of the syn rotamer. 5. THE POTENTIAL FUNCTION FOR THE TORSIONAL VIBRATION OF THE ANTI ROTAMER
The experimentally determined rotational constants when plotted against the quantum number of the SH torsional vibration exhibit a peculiar zigzagging variation (see Fig. 2). Such variations are known to arise as a result of double
MICROWAVE
113
SPECTRUM OF VINYL MERCAPTAN TABLE IV
Rotational Transition Frequencies of anri-CH,=CHSH in the D = 2 and o = 3 States of the SH Torsional Vibration’“)
_ v=2
v=3
obs.
2
02-101
212-111 2
11
3
10
22
337.13
797.22
0.07
21
808.11
22
946.99
0.03
22
917.45
33 534.94
33
04-303
4
13-312 23-322 22-321
5
606.85
43
582.
45
881.76
44
740.02
44
797.64
54 467.
515-414 5 5
14-413 24-423
5 1
23-422 lo-‘01
211-202 312-303 4 5
13-404 14-505
615-606
1 2 8
11 -Ooo 12-101
02
-1
11
(4
-0.33*
33 565.77
-0.23
1.02*
44
670.44
0.04
0. 07
43
604.75
0. 12
-0.08
45
823.25
-0.02
44
721.53
-0.03
44
775.
-2.52*
55
797.03
-0.07
54 495.52
0.28* 0.02
55
56
032.23
0.08
56 002.34
43 286.81
-0.05
42
932.22
867.57
-0.05
43
492.47
44
749.26
45
944.26
47
473.42
49
351.42
53
898.05
64
509.09
66
395.25
20
345.63
not
use used
-0.04
1.34*
46
966.55
0.01
1.07*
48
775.20
0.05
1.05*
50
944.65
0.01
1.35*
53
500.69
64
0. 10 MHz.
squares
-0.01
185.
-0. 35
08
0. 12 -0.02
53 572.09
0.00
rotational
0.01
33 558.88
0. 04
least
0.03 -0.03
342.91
-0.01
in
0.01 -0.10
495.69
0. 08
than
0. 03 -0.02
45
-0.11
the
0.05
44
-1.05*
better
0. 14 -0.29
894.42
43
0. 15
-0.04
18
917.23
18
0.13 -0.02
340.56
Accuracy
Lines
37
55
frequencies *
372.
33 544.49
57
53 483.46
08-‘17
909-818 2
34
54 264.69
l?-808
34
-0.02
57 268.16
51 606.
‘16-?O? 8
59
55 811.10
05 -404
33 521.64
0.10
558.87
44
16
-talc.
-0.04
33 581.37
414-313
4
15
34 416.58
21-220
4
-0:
32 692.
22-221
4
-0.06
obs.
366.32
12-211
3
obs.
21
13-‘12
3
-talc.
22
03-‘02
3
3
-1
obs.
66 433.08
0.03
20
0.02
The constants analysis.
020.34
calculated of
Table
III.
114
TANIMOTO AND MACDONALD TABLE V Determination
of the Dipole Moment of anti-CH2=CHSH in the Ground and Excited States of the SH Torsional Vibration ,(a)
Transition
”
A./E
M obs.
2
02-111
0
0 1
1. 528 -8.500
1
2
02-lo1
0
12
0
0.484
0.591
0.585
0.684
0.672
0.761
0.900 15.47
0 1
211-110
2 3
1.033 19.64
-18.40 1
-23.
0.624 -22.55
0.500
10.31
2
0.903
0.625 -23.
12-111
1. 162 -14.12
0.900
3
2
1.531 -8.503
1.158 -14.12
2
talc.
0.758 10.30 0.883 15.49 0.985 19.63
-18.42 92
-23.
91
minimum character in the potential function (7). As can be seen from Fig. 2 the deviations of the constants for the u = 1,2, and 3 states from the smooth curve expected for a slightly anharmonic vibration are, on the whole, quite small, however, in the case of the u = 0 state the measured constants are very clearly out of line with the values extrapolated from those of the higher states. The implication is that only the ground torsional state is severely affected by any barrier which may exist at the planar conformation. Assuming that the SH torsional vibration may be treated separately from all other vibrational modes, the vibrational kinetic energy may be written in the
MICROWAVE
115
SPECTRUM OF VINYL MERCAF’TAN TABLE VI
Dipole Moment (D) of an&CH,=CHSH in the Ground and Excited States of the SH Torsional Vibration’“’
Y
Y
*I
a
,+ total
b
0
0.425
(10)
1.033
(10)
1.117
(14)
1
0.523
(10)
0.964
(10)
1.097
(14)
2
0.561
(10)
0.903
(15)
1.063
(18)
3
0.630
(30)
0.854
(30)
1.061
(42)
The
numbers
(a)
standard
in parenthesis
deviations
in all
are
2.5
times
the
cases.
form, (IO), 2T = M,( 1 + Aq2 + Bcp4 + . - .)e2 = Md2
(1)
where cp represents the angle of internal rotation around the C-S bond relative to a zero at the planar anti conformation, and MO is the reduced mass for the torsional vibration. The transformation of cpinto a reduced coordinate 4 is defined by the equation, 4 =
(1 + A$
+ Bcp4 + -.)1’2&
I
(2)
Clearly the potential function for the SH torsional vibration of the anti rotamer constitutes a part of the overall potential function for the internal rotation around the C-S bond. However, for the present we have replaced this portion of the overall potential function by a nonperiodic potential which has the following form in terms of the reduced coordinate, V(q) = -v2q2
+ v4q4.
This is expected to be a reasonable representation function in the vicinity of the anti conformation. sionless coordinate 5 defined by,
(3)
of the lowest part of the potential Transforming q into the dimen-
5 = (4”;MO)“‘q the Hamiltonian
for the torsional vibration takes the form, H = %4hvo(pz - 4’ + v4cf4)
(4)
where v. = (Vo/27?Mo)1’Z and v4 = (2/~/16tii@u$)V,. Here v. may be considered to be a scale factor, irrelevant in the analysis of the rotational constant variation
116
TANIMOTO AND MACDONALD TABLE VII Observed and Calculated Energy Differences (cm-‘) between the Energy Levels of the SH Torsional Vibration of anti-CH,=CHSH
Energy
Difference
obs.
1
-
0
2-l
talc.
74
f.
30
74
112
+
20
123
3
-
2
114
+
30
143
3
-
0
318
i
20
340
-
O(E)
50
+
25
O(anti)
(a)
Calculated Hamiltonian
from
the
(4)
with
eigenvalues h v.
= 90.
(a)
(scaled)
of the 6 cm
-1
.
with vibrational quantum number but determined later using the energy difference between the ground and first excited vibrational state. The eigenvalues of (4) were determined in a harmonic oscillator basis, 30 basis functions being sufficient for our present purpose. The effective rotational constants were determined from the average values of 5” in the appropriate vibrational state (7) through the expansion, (5) (P% = P8” - PP(P”)C - PY(%)L’ where g = a, b, and c. Expectation values of odd powers of 6 are excluded by the symmetry of the vibration. The observed rotational constants were numerically fitted to the above expression enabling the potential function to be determined in terms of 5. The data for the u = 0, 1, and 2 states were used to determine the expansion coefficients and the 2, = 3 data to test the quality of the resulting potential function. Good agreement was obtained using V(5) = ?
[ -,$* + 0.6054].
(6)
The expansion coefficients of the rotational constants are given in Table VIII. The shape of the potential function and the disposition of the lowest energy levels are shown in Fig. 3. The scale factor hv,, is determined to be 90.6 cm-l from the observed energy difference between the ground and first excited state, 74 cm-’ (Table VII). As anticipated the potential hump at the planar configuration is not large and it is seen that the r = 0 vibrational state in fact lies slightly above the top of the barrier.
117
MICROWAVE SPECTRUM OF VINYL MERCAPTAN
h-4
I
\
-1200 L MHZ FIG.
b
2. The variation of rotational constants with vibrational quantum number
in anti-vinyl
mercaptan. 6. DISCUSSION
It has been established above that the SH torsional vibration of the anti rotamer of vinyl mercaptan is subject to a double minimum potential function. This result is striking since the otherwise apparently comparable thioformic acid (6) and also formic acid (II ) have been found to exist in two planar rotameric forms. The difference between the species is particularly evident when the inertia defects of the u = 0 states of the appropriate rotamers of these molecules are compared. In the cases of cis- and truns-thioformic acid and cis- and truns-formic acid the small positive inertia defects imply planarity, whereas for anti-vinyl mercaptan the inertia defect is clearly negative. Correspondingly any attempt.to apply Kraitchman’s equations to determine the substitution coordinates of the thiol hydrogen atom in anti-vinyl mercaptan yields a nonzero out of plane coordinate for this atom in the z, = 0 state. This latter conclusion clearly conflicts with the implications of the potential function of Fig. 3 and serves to emphasize the probTABLE VIII Expansion Coefficients of the Rotational Constants (MHz) of anti-Vinyl Mercaptan
A
49 842.37
-630. 71
-49.99
B
5 897.78
10.51
-7.76
C
5 262.41
31.09
-1.69
118
TANIMOTO AND MACDONALD -1
ENERGY/CM
FIG. 3. Potential function for the SH torsional vibration in the vicinity of the nnti conformation of vinyl mercaptan. The barrier height is 19 cm-l and the minima occur at 26” relative to a zero at the planar conformation.
lems of applying Kraitchman’s equations in the presence of a large amplitude vibration. The rotational constants for the hypothetical planar molecule (see Table VIII) yield a small positive value for the inertia defect (0.21 a.m.u. A’) while the corresponding values for the ground and excited states of the SH torsional mode are negative. The contribution of the SH torsional motion to the inertia defect of the ground state is close to -0.40 a.m.u. A2. Since the ground vibrational state lies above the small barrier at the planar conformation, antivinyl mercaptan can be regarded as a quasi-planar form of the molecule. Comparison between the experimental results given above and the theoretical predictions (2, 3) reveals general agreement in the sense that the latter predict the existence of two rotamers, a planar syn form and a nonplanar less stable form. It is interesting that the minima of the potential function shown in Fig. 3 approximately correspond to 26” of torsional displacement from the exactly planar position in agreement with the prediction of the theoretical computation mentioned in the introduction (2). For the present, extensive isotopic substitution data are not available, however, some conclusions can be drawn regarding the structure of the anti rotamer. The disposition of the observed transitions relative to those of the syn rotamer imply considerable structural changes on rotation of the SH bond from the syn to anti conformations. If the same structure is assumed for the vinyl framework as for the syn rotamer (I ), it is found that a reduction of the CCS angle by some 5 to 122” and an accompanying increase in the CSH angle by 2 to 98” yields a model with rotational constants acceptably close to those for the anti rotamer. These changes parallel closely those found for thioformic acid (6) where a detailed structural study was undertaken. Comparison with the relevant predictions
MICROWAVE SPECTRUM OF VINYL MERCAPTAN
119
of both sets of ab initio calculations mentioned above is again favorable, particularly as regards the magnitude of the CCS angle variation in going from the syn to the anti conformation. ACKNOWLEDGMENTS The authors are grateful to Professors Yonezo Morino and John Sheridan for encouragement and valuable discussions throughout the present study. Thanks are also due to Drs. Noel Owen and Stuart Charles for their continued interest and many suggestions, and to Veronica Almond for the preparation of samples. RECEIVED:
November 13, 1978 REFERENCES
I. M. TANIMOTO, V. ALMOND, S. W. CHARLES, J. N. MACDONALD,AND N. L. OWEN, J. Mol. Spectrosc. 78, 95 105 (1979). 2. S. SAMDALAND H. M. SEIP, J. Mol. Struct. 28, 193-203 (1975). 3. D. G. LISTER AND P. PALMIERI,f. Mol. Strucr. 48, 133-138 (1978). 4. H. W. KROTO AND B. M. LANDSBERG,J. Mol. Specrrosc. 62, 346-363 (1976). 5. W. GORDY AND R. L. COOK, “Microwave Molecular Spectra,” Wiley Interscience, New York, 1970. 6. W. H. HOCKING AND G. WINNEWISSER,2. Narurforsch. A 31,438-453 (1976). 7. W. D. GWINN AND A. S. GAYLORD, “International Review of Science, Physical Chemistry Series 2,” Vol. 3 (D. A. Ramsay, Ed.), Chapter 5, Butterworths, London, 1976. 8. E. HIROTA, R. SUGISAKI, C. J. NIELSEN, AND G. 0. SBRENSEN, J. Mol. Spectrosc. 49, 251-267 (1974). 9. A. S. ESBITT AND E. B. WILSON, JR., Rev. Sci. Instr. 34, 981-907 (1963). 10. D. 0. HARRIS, H. W. HARRINGTON,A. C. LUNTZ, AND W. D. GWINN, J. C/tern. Whys. 44, 3467-3480 (1966). 11. W. H. HOCKING,~. Naturforsch. A 31, 1113-1121 (1976).
OF
MOLECULAR
Microwave
SPECTROSCOPY
Spectrum
78, 106- 119 (1979)
and Conformation The anti
of Vinyl Mercaptan
Rotamer
MITSUTOSHI TANIMOTO Sagami Chemical Research
Center,
Nishi-Ohnuma
4-4-1, Sagamihara,
Kanagawa
229, Japan
AND
J.N.
MACDONALD
School of Physical and Molecular Sciences, University College of North Wales, Bangor, LL57 2UW, Gwynedd, United Kingdom
Microwave spectra of the anti rotamer of vinyl mercaptan and its SD isotopic species have been studied in the frequency range 12-60 GHz. For the normal species rotational and centrifugal distortion constants have been obtained for the ground and first three excited states of the SH torsional mode, the ground state values being A = 49 422.75(5) MHz, B = 5 897.215(9) MHz, C = 5 279.436(9) MHz, D, = 3.12(11) kHz, DJ, = -38.5q1.71) kHz, and & = 0.498(51) kHz. An approximate potential function for the SH torsion in the vicinity of the anti conformation, derived using the observed variation of rotational constants with vibrational quantum number, reveals the presence of a small potential barrier of 19 cm-l at the planar conformation. The u = 0 state lies above this barrier so the molecule is essentially planar in the ground state in spite of the observed negative value for the inertia defect (-0.1976(2) a.m.u.&). The anti rotamer is found to be 50 2 25 cm-’ less stable than the syn rotamer. The dipole moment has the ground state values IL. = 0.425(10), pLb= 1.033(10), and kotal = 1.117(14) D and is shown to vary considerably with vibrational quantum number. Evidence for significant structural changes in going from the syn rotamer to the anti rotamer is also presented. 1. INTRODUCTION
In a preceding paper we reported an analysis of the microwave spectrum of syn-vinyl mercaptan (I ) in which it was shown that this rotamer behaves in every respect like a conventional planar molecule. In addition the dipole moment was measured and some conclusions drawn concerning the molecular structure. Recent ab initio calculations (2, 3) have predicted the existence of a second rotameric form of the molecule, less stable than the syn rotamer and with a nonplanar gauche or skew conformation ((I), Fig. 1). The equilibrium SH torsional angle is computed to lie in the region of -130-150”. In this paper we present an analysis of the microwave spectrum of the second rotamer in the ground and excited vibrational states of the SH torsional mode. The dipole moment of the molecule in each vibrational state has also been measured. Considerations concerning the potential function governing the torsional 0022-2852/79/110106-14$02.00/O Copyright
0 1979 by Academic
AU rights of reproduction
Press,
Inc.
in any form reserved.
106
MICROWAVE
SPECTRUM
OF VINYL
MERCAPTAN
107
mode suggest that this form of the molecule is best described as a “quasi-planar” anti conformation. 2. EXPERIMENTAL
DETAILS
As the experimental details in this investigation were largely the same as in the case of the study of the syn rotamer (1) only certain points are made here. The vinyl mercaptan was again prepared by two different methods, photolysis of a mixture of hydrogen sulphide and acetylene and the thermal decomposition of either 1,Zethanedithiol or thiirane. However, for this work, thiirane was mainly used as the starting material in the pyrolysis experiments since it was found to give by far the best yields of vinyl mercaptan, maximizing at a pyrolysis temperature of 900-950°C. This procedure produces thioacetaldehyde and vinyl mercaptan in comparable quantities, however, the spectrum of the former was readily distinguished from that of the mercaptan by making use of the known rotational constants (4). All spectra were studied at room temperature and dipole moment measurements were carried out in the usual manner (I, 5). 3. THE MICROWAVE
SPECTRUM
OF ANTI-VINYL
MERCAPTAN
Assuming that a suitable model for the structure of any second rotamer of vinyl mercaptan can be generated simply by rotation of the thiol hydrogen atom around the C-S bond of the syn rotamer with all other structural parameters held constant, any reasonable gauche or anti conformation remains a near-prolate rotor with K - -0.97. The pattern of lowJ, a-type transitions characteristic of such a molecule is predicted to lie to low frequency of the corresponding pattern for the syn rotamer for all likely values of the torsional angle. For the planar anti rotamer, bond moment calculations show that the b component of the dipole moment (- 1 D) is considerably larger than the a component (-0.5 D) in contrast to the syn rotamer. The effect of nonplanarity on the dipole moment components is to reduce the b component while increasing the a component relative to their values in the anti rotamer. A search of the spectrum revealed that, while no suitabis low-J transitions could be located in the vicinity of predicted transition frequencies based on the above model, sets of comparatively weak lines could be identified a few hundred megahertz to high frequency of each of the u-type transitions of the syn rotamer. Each set comprised two lines of virtually equal intensities accompanied by several weaker lines (see Fig. la). Sets of b-type Q-branch transitions of the series Jm-l + Jo,./,together with certain R-branch transitions, were also identified, each consisting of a ground state line, strong in comparison to the u-type lines, accompanied by definitely weaker vibrational satellites (see Fig. lb). None of these lines could be attributed to any likely impurity or to the syn rotamer. Consequently, on the basis of well-resolved Stark effects, the relative intensities of the b-type to u-type lines and the fit to a rigid rotor model, the two strongest members of each of the a- and b-type line patterns were assigned as the u = 0 and u = 1 transitions of the lowest vibrational mode, the SH torsion, of a second
108
TANIMOTO AND MACDONALD V:l
v=o
v:4 I 22380
22360
22340
MHz
v=2 v=3 I 20020
?,
I
A
CL 20623
20345
21178
MHz
tb, FIG. 1. Observed vibrational satellite patterns for (a) the & 6 l,,, a-type transition and (b) the 2,, +- 11, b-type transition of anti-vinyl mercaptan.
rotamer of vinyl mercaptan. It is shown later that the conformation of this second rotamer is nearly that of the anti rotamer so hereafter we refer to it by this title for clarity. The large deviation of the observed transitions from the predictions of the simple model can be rationalized in terms of structural relaxation during rotation around the C-S bond of the type noted in the case of cis- and trunsthioformic acid (6). The observed transition frequencies for the isotopic species CH,=CHSH and CH,=CHSD of this rotamer are listed in Tables I and II. The appropriate rotational constants, centrifugal distortion constants, and inertia defects are given in Table III. As in the case of the syn rotamer the observed transitions permit only limited accuracy in the determination of centrifugal distortion data, DK and SK being indeterminate. The negative inertia defect in the ground vibrational state (see Table III) together with the presence of an intense vibrational satellite accompanying each ground state line, appears to exclude the possibility that the rotamer giving rise to the spectra is the planar anti rotamer in any strict sense of the term. However, it is now well known, particularly for certain ring compounds (7) and amides (8),
109
MICROWAVE SPECTRUM OF VINYL MERCAPTAN TABLE I in the u = 0
Rotational Transition Frequencies (MHz) of anti-CH,=CHSH and o = 1 States of the SH Torsional Vibratio@ VZO
Transition
obs.
202-101 212-111 211-110
v= ohs.
22
346.
21
735.55
72
22
971.07
33 503.52
303-202
32
313-212 312-211
599.
11
34 452.
35
414-313 413-312 423-322 422-321 505 -404 515-414 514-413
1
lo-‘01
211-202 312-303 413-404 514-505
22
374.33
-0.07
0.04
21
787.75
-0.08
0.05
22
973.
817-*08 lll-“oo 2
12-101
202-111 bOb-5l5 707-b16 808-717 909-818
(4
0. 06
32 677.
-0.24
0.01
34 455.84
0. 10
.0.23*
33 571.36
0.15*
-o.zo*
33 595.29
-0.01
44
699.24
0.03
43 457.81
-0.05
43
562.76
0.01
-0.01
0. 01
45
928.
45
933.66
44
702.00
-0.04
44
757.10
44
767.21
-0.01
44
817.
37
55
751.
38
-0.02
55 827.99
54
309.
78
-0.01
54 441.88
57
397.
38
55
999.00
44
143.40
44
767.
79
45
716.
75
47
004.46
48
650.39
0.11*
0.07 -0.02 0. 04 -0.06 0. 02
96
114.81
-0.17
0.57* 38
-0.02 0.04 -0.03
57 404.90
-0.12
55 937.54
-0.04
56
058.94
43
590.63
44
189.
45
099.83
0.19*
46
334.01
0. 02
0.10 0. 00 75
-0.06
0.00
47
910.79
-0.01
0. 01
49
851.88
-0.04
52
-0.06
182.99
0.01
55 991.57
0.02
54 933.13
0.01
54
702.24
0.01
54
187.99
0.01
65 261.21
-0.01
64
785.48
-0.02
21
178.87
-0.07
20
623.43
-0.10
27
443.69
27
906.28
0. 04
40
159.21
-0.01
40
581.59
0. 00
53
035.64
-0.02
53 415.24
66
034.93
Accuracy
Lines
0. 05 32
556.45
better
frequencies *
0.03
640.57
53
716-‘07
32
33 545.94
33
50 677.
%5-‘06
-talc.
-0.05
524-423 523-422
1 obs.
44
321-220 404-303
obs.
-0.02
33 530.77
322-221
-talc.
not
use used
0.07
-0.01
0.02
than the
0. 10 MHz. rotational
in least
squaren
The constants analysis.
calculated of
Table
III.
110
TANIMOTO AND MACDONALD TABLE II Rotational Transition Frequencies (MHz) of anti-CH,=CHSD and u = 1 States of the SH Torsional VibratioP) v=o
Transition
lo1 2 2 2 3 3 3 4 4 4
obs.
-1
10
03-‘02 13-‘12 12-211 04-303 14-313 -3
12
5 5 5 5 5 1
22-321 05-404
14-413 24-423 23-422 lo-lo1
12-303
413-404 5 6
14-505 15-606
‘16-‘07
1 2
11
-0
00
12-‘01
‘07~%6 *08-‘17
(a)
**
638.65
-0.03
20
980.77
-0.01
21
049.04
-0.03
22
242.05
0.17**
22
242.
05
0.12**
32
386.
0.00
32 440.
54
0.04
31 466.28
0.01
31 569.09
0.01
33
0.05
33
0.01
39
357.81
43
146.29
-0.02
43 221.75
0.00
946.10
-0.01
42
083.96
0.00
44
467.81
44
469.86
-0.01
43 285.86
-0.02
0.01 -0.02
43 292.42
0.00
43
353.67
53 875.81
0.00
53
975.
37
0.00
-0.92* 32
-0.01
52 591.96
0.00
55
569.69
-0.06
55
54
011.02
-0.02
54 098.22
54
162.01
0. 06
54 235.52
0.01
39 698.25
0. 04
39 094.19
0. 03
573.60
-0.01 0.01
40
356.53
41
307.99
42
629.
44
323.38
46
416.39
-0.19*
45
435.24
48
940.66
0.02
47
812.03
-0.06
0.01
50 624.23
-0.02
49
-0.09
-0.01
55
39 697.74
0.04
40
615.96
0. 06
41
864.04
0.02
43 462.29
49
873.42
60
048.45
0. 04
59 546.59
42
034.67
0.01
42
54
513.66
better
not
Overlapped
358.74
41
Accuracy
Lines
-0.01
21
frequencies *
79
-talc.
0.01
51 930.59
*17-*08
1 obs.
603.64
52 418.
15-414
211-202 3
obs.
21
43 216.80
423-322 4
-talc.
10 822.
12-ill
13
v= obs.
-Ooo
oz-lo1
11
in the u = 0
me used with
-0.05
-0.01
than the
0.10
in least each
MHz.
rotational squares other.
0.03 -0.02 0. 00 -0.03 0.07
320.50
0.05
461.43
-0.02
54 886.70
0.02
The constants analysis.
calculated of
Table
III.
111
MICROWAVE SPECTRUM OF VINYL MERCAPTAN TABLE III
Rotational Constants (MHz), Centrifugal Distortion Constants (kHz), and Inertia Defect (a.m.u. A*) for anti-Vinyl Mercaptan in the Ground and Excited Vibrational State@) VZO
“Z
1
v=2
v=3
-anti-CH2=CHSH A
49 422.75(5)
48 592.28(
48 889.25(6)
13)
48 245.44(e)
B
5 897.215(9)
5 891. 532(15)
5 880.469(43)
5 868.029(21)
C
5 279.436(9)
5 298.692(13)
5 305. 592(45)
5 313. 306(16)
D D
3.12(11)
J
3. 07( 17)
-38. 50(1. 71)
JK
d
-36.28(2.
0.498(51)
A:b)
13)
0.450(74)
-0. 1976(2)
-0.7397(3)
3.20(46)
3. 13(22)
-33.65(5.38)
-33. 36(3.45)
0.521(54)
0.472(90)
- 1. 0883( 10)
-1.4836(4)
anti-CH2=CHSD A
44 207.27(5)
44 785.79(4)
B
5 718. 114(11)
5 709.692(14)
C
5 087.542(7)
5 113. lOl(10)
2. 78(11)
DJ D
JK
6 A:b)
2.69(15)
-23. 51(1. 16)
-22. 37(1. 70)
0.415(43)
0.349(58) -1.1045(3)
-0. 3299(Z)
(a)
The numbers deviations
b)
in parenthesis
are 2.5
times
the standard
in all caaee.
A = I=-la-$,
that the question of planarity in the presence of large amplitude, low-frequency vibrations can be a subtle one. As the observed patterns of lines, typified by Fig. 1, implied some degree of anharmonicity of the SH torsional mode, we measured the weak lines arising from the u = 2 and v = 3 excited states of this mode with a view to obtaining some insight into the shape of the potential function governing the torsional motion and the extent of any nonplanarity of the rotamer. A tentative assignment has also been obtained for several a-type lines of the u = 4 state but ambiguity exists in the assignment of the b-type lines so the A rotational constant is not determined for this case. The assignments were based on relative intensities (see below), frequency fits, and behavior of rotational constants
112
TANIMOTO AND MACDONALD
with vibrational quantum number. The inertia defect was especially helpful in distinguishing the weakest lines from any which might belong to excited states of the CCS bending vibration (I). The observed transition frequencies are listed in Table IV and the rotational constants, centrifugal distortion constants, and inertia defects in Table III. 4. DIPOLE MOMENT AND VIBRATIONAL
ENERGY DIFFERENCES
It has been implied above, and is evident from Fig. 1, that a determination of energy differences between the vibrational levels of the SH torsional mode through relative intensity measurements of the a- and b-type lines will yield anomalous results if the usual assumption of only a minor change in dipole moment components with vibrational quantum number is retained. Consequently we have carried out detailed Stark-effect measurements, using the methods already described (I, 5), on several transitions of the normal species of the anti rotamer in the vibrational states ZJ= 0, 1, 2, and 3. In all cases the observed frequency shifts with the applied field obeyed second-order theory satisfactorily and the absorption cell was calibrated using the J = 2 + 1 transition of OCS assuming p = 0.71521(20) D for that molecule. The results of the dipole moment determination are summarized in Tables V and VI. Any CL,contribution to the Stark effect has been neglected here. Since this component of the dipole moment is antisymmetric with respect to the torsion, it soon became apparent that the magnitude of the u = 1 c 0 energy difference (-75 cm-‘) is such that the contribution from the transition moment to the Stark effects of the observed transitions would be too small to be reliably detected. The relative magnitudes of the a and b components of the dipole moment in the ground torsional state are in accord with the simple model predictions for the anti rotamer of vinyl mercaptan. The large variations in the components with torsional state are almost certainly the main factor responsible for the apparently anomalous relative intensities of the excited state lines mentioned above. The changes in the components are in the same sense as those predicted by a model assuming a curvilinear path for the thiol hydrogen atom with a constant SH bond length and no appreciable variation of bond moments during the vibration. Relative intensity measurements, carried out on the various vibrational satellites by the method of Esbitt and Wilson (9) and taking due account of the variation in dipole moment components with vibrational quantum number, yield the energy differences between the torsional states given in Table VII. Similar measurements comparing the lines of the syn rotamer with those of the anti rotamer show an energy difference of 50 + 25 cm-l between their respective ground vibrational states in favor of the syn rotamer. 5. THE POTENTIAL FUNCTION FOR THE TORSIONAL VIBRATION OF THE ANTI ROTAMER
The experimentally determined rotational constants when plotted against the quantum number of the SH torsional vibration exhibit a peculiar zigzagging variation (see Fig. 2). Such variations are known to arise as a result of double
MICROWAVE
113
SPECTRUM OF VINYL MERCAPTAN TABLE IV
Rotational Transition Frequencies of anri-CH,=CHSH in the D = 2 and o = 3 States of the SH Torsional Vibration’“)
_ v=2
v=3
obs.
2
02-101
212-111 2
11
3
10
22
337.13
797.22
0.07
21
808.11
22
946.99
0.03
22
917.45
33 534.94
33
04-303
4
13-312 23-322 22-321
5
606.85
43
582.
45
881.76
44
740.02
44
797.64
54 467.
515-414 5 5
14-413 24-423
5 1
23-422 lo-‘01
211-202 312-303 4 5
13-404 14-505
615-606
1 2 8
11 -Ooo 12-101
02
-1
11
(4
-0.33*
33 565.77
-0.23
1.02*
44
670.44
0.04
0. 07
43
604.75
0. 12
-0.08
45
823.25
-0.02
44
721.53
-0.03
44
775.
-2.52*
55
797.03
-0.07
54 495.52
0.28* 0.02
55
56
032.23
0.08
56 002.34
43 286.81
-0.05
42
932.22
867.57
-0.05
43
492.47
44
749.26
45
944.26
47
473.42
49
351.42
53
898.05
64
509.09
66
395.25
20
345.63
not
use used
-0.04
1.34*
46
966.55
0.01
1.07*
48
775.20
0.05
1.05*
50
944.65
0.01
1.35*
53
500.69
64
0. 10 MHz.
squares
-0.01
185.
-0. 35
08
0. 12 -0.02
53 572.09
0.00
rotational
0.01
33 558.88
0. 04
least
0.03 -0.03
342.91
-0.01
in
0.01 -0.10
495.69
0. 08
than
0. 03 -0.02
45
-0.11
the
0.05
44
-1.05*
better
0. 14 -0.29
894.42
43
0. 15
-0.04
18
917.23
18
0.13 -0.02
340.56
Accuracy
Lines
37
55
frequencies *
372.
33 544.49
57
53 483.46
08-‘17
909-818 2
34
54 264.69
l?-808
34
-0.02
57 268.16
51 606.
‘16-?O? 8
59
55 811.10
05 -404
33 521.64
0.10
558.87
44
16
-talc.
-0.04
33 581.37
414-313
4
15
34 416.58
21-220
4
-0:
32 692.
22-221
4
-0.06
obs.
366.32
12-211
3
obs.
21
13-‘12
3
-talc.
22
03-‘02
3
3
-1
obs.
66 433.08
0.03
20
0.02
The constants analysis.
020.34
calculated of
Table
III.
114
TANIMOTO AND MACDONALD TABLE V Determination
of the Dipole Moment of anti-CH2=CHSH in the Ground and Excited States of the SH Torsional Vibration ,(a)
Transition
”
A./E
M obs.
2
02-111
0
0 1
1. 528 -8.500
1
2
02-lo1
0
12
0
0.484
0.591
0.585
0.684
0.672
0.761
0.900 15.47
0 1
211-110
2 3
1.033 19.64
-18.40 1
-23.
0.624 -22.55
0.500
10.31
2
0.903
0.625 -23.
12-111
1. 162 -14.12
0.900
3
2
1.531 -8.503
1.158 -14.12
2
talc.
0.758 10.30 0.883 15.49 0.985 19.63
-18.42 92
-23.
91
minimum character in the potential function (7). As can be seen from Fig. 2 the deviations of the constants for the u = 1,2, and 3 states from the smooth curve expected for a slightly anharmonic vibration are, on the whole, quite small, however, in the case of the u = 0 state the measured constants are very clearly out of line with the values extrapolated from those of the higher states. The implication is that only the ground torsional state is severely affected by any barrier which may exist at the planar conformation. Assuming that the SH torsional vibration may be treated separately from all other vibrational modes, the vibrational kinetic energy may be written in the
MICROWAVE
115
SPECTRUM OF VINYL MERCAF’TAN TABLE VI
Dipole Moment (D) of an&CH,=CHSH in the Ground and Excited States of the SH Torsional Vibration’“’
Y
Y
*I
a
,+ total
b
0
0.425
(10)
1.033
(10)
1.117
(14)
1
0.523
(10)
0.964
(10)
1.097
(14)
2
0.561
(10)
0.903
(15)
1.063
(18)
3
0.630
(30)
0.854
(30)
1.061
(42)
The
numbers
(a)
standard
in parenthesis
deviations
in all
are
2.5
times
the
cases.
form, (IO), 2T = M,( 1 + Aq2 + Bcp4 + . - .)e2 = Md2
(1)
where cp represents the angle of internal rotation around the C-S bond relative to a zero at the planar anti conformation, and MO is the reduced mass for the torsional vibration. The transformation of cpinto a reduced coordinate 4 is defined by the equation, 4 =
(1 + A$
+ Bcp4 + -.)1’2&
I
(2)
Clearly the potential function for the SH torsional vibration of the anti rotamer constitutes a part of the overall potential function for the internal rotation around the C-S bond. However, for the present we have replaced this portion of the overall potential function by a nonperiodic potential which has the following form in terms of the reduced coordinate, V(q) = -v2q2
+ v4q4.
This is expected to be a reasonable representation function in the vicinity of the anti conformation. sionless coordinate 5 defined by,
(3)
of the lowest part of the potential Transforming q into the dimen-
5 = (4”;MO)“‘q the Hamiltonian
for the torsional vibration takes the form, H = %4hvo(pz - 4’ + v4cf4)
(4)
where v. = (Vo/27?Mo)1’Z and v4 = (2/~/16tii@u$)V,. Here v. may be considered to be a scale factor, irrelevant in the analysis of the rotational constant variation
116
TANIMOTO AND MACDONALD TABLE VII Observed and Calculated Energy Differences (cm-‘) between the Energy Levels of the SH Torsional Vibration of anti-CH,=CHSH
Energy
Difference
obs.
1
-
0
2-l
talc.
74
f.
30
74
112
+
20
123
3
-
2
114
+
30
143
3
-
0
318
i
20
340
-
O(E)
50
+
25
O(anti)
(a)
Calculated Hamiltonian
from
the
(4)
with
eigenvalues h v.
= 90.
(a)
(scaled)
of the 6 cm
-1
.
with vibrational quantum number but determined later using the energy difference between the ground and first excited vibrational state. The eigenvalues of (4) were determined in a harmonic oscillator basis, 30 basis functions being sufficient for our present purpose. The effective rotational constants were determined from the average values of 5” in the appropriate vibrational state (7) through the expansion, (5) (P% = P8” - PP(P”)C - PY(%)L’ where g = a, b, and c. Expectation values of odd powers of 6 are excluded by the symmetry of the vibration. The observed rotational constants were numerically fitted to the above expression enabling the potential function to be determined in terms of 5. The data for the u = 0, 1, and 2 states were used to determine the expansion coefficients and the 2, = 3 data to test the quality of the resulting potential function. Good agreement was obtained using V(5) = ?
[ -,$* + 0.6054].
(6)
The expansion coefficients of the rotational constants are given in Table VIII. The shape of the potential function and the disposition of the lowest energy levels are shown in Fig. 3. The scale factor hv,, is determined to be 90.6 cm-l from the observed energy difference between the ground and first excited state, 74 cm-’ (Table VII). As anticipated the potential hump at the planar configuration is not large and it is seen that the r = 0 vibrational state in fact lies slightly above the top of the barrier.
117
MICROWAVE SPECTRUM OF VINYL MERCAPTAN
h-4
I
\
-1200 L MHZ FIG.
b
2. The variation of rotational constants with vibrational quantum number
in anti-vinyl
mercaptan. 6. DISCUSSION
It has been established above that the SH torsional vibration of the anti rotamer of vinyl mercaptan is subject to a double minimum potential function. This result is striking since the otherwise apparently comparable thioformic acid (6) and also formic acid (II ) have been found to exist in two planar rotameric forms. The difference between the species is particularly evident when the inertia defects of the u = 0 states of the appropriate rotamers of these molecules are compared. In the cases of cis- and truns-thioformic acid and cis- and truns-formic acid the small positive inertia defects imply planarity, whereas for anti-vinyl mercaptan the inertia defect is clearly negative. Correspondingly any attempt.to apply Kraitchman’s equations to determine the substitution coordinates of the thiol hydrogen atom in anti-vinyl mercaptan yields a nonzero out of plane coordinate for this atom in the z, = 0 state. This latter conclusion clearly conflicts with the implications of the potential function of Fig. 3 and serves to emphasize the probTABLE VIII Expansion Coefficients of the Rotational Constants (MHz) of anti-Vinyl Mercaptan
A
49 842.37
-630. 71
-49.99
B
5 897.78
10.51
-7.76
C
5 262.41
31.09
-1.69
118
TANIMOTO AND MACDONALD -1
ENERGY/CM
FIG. 3. Potential function for the SH torsional vibration in the vicinity of the nnti conformation of vinyl mercaptan. The barrier height is 19 cm-l and the minima occur at 26” relative to a zero at the planar conformation.
lems of applying Kraitchman’s equations in the presence of a large amplitude vibration. The rotational constants for the hypothetical planar molecule (see Table VIII) yield a small positive value for the inertia defect (0.21 a.m.u. A’) while the corresponding values for the ground and excited states of the SH torsional mode are negative. The contribution of the SH torsional motion to the inertia defect of the ground state is close to -0.40 a.m.u. A2. Since the ground vibrational state lies above the small barrier at the planar conformation, antivinyl mercaptan can be regarded as a quasi-planar form of the molecule. Comparison between the experimental results given above and the theoretical predictions (2, 3) reveals general agreement in the sense that the latter predict the existence of two rotamers, a planar syn form and a nonplanar less stable form. It is interesting that the minima of the potential function shown in Fig. 3 approximately correspond to 26” of torsional displacement from the exactly planar position in agreement with the prediction of the theoretical computation mentioned in the introduction (2). For the present, extensive isotopic substitution data are not available, however, some conclusions can be drawn regarding the structure of the anti rotamer. The disposition of the observed transitions relative to those of the syn rotamer imply considerable structural changes on rotation of the SH bond from the syn to anti conformations. If the same structure is assumed for the vinyl framework as for the syn rotamer (I ), it is found that a reduction of the CCS angle by some 5 to 122” and an accompanying increase in the CSH angle by 2 to 98” yields a model with rotational constants acceptably close to those for the anti rotamer. These changes parallel closely those found for thioformic acid (6) where a detailed structural study was undertaken. Comparison with the relevant predictions
MICROWAVE SPECTRUM OF VINYL MERCAPTAN
119
of both sets of ab initio calculations mentioned above is again favorable, particularly as regards the magnitude of the CCS angle variation in going from the syn to the anti conformation. ACKNOWLEDGMENTS The authors are grateful to Professors Yonezo Morino and John Sheridan for encouragement and valuable discussions throughout the present study. Thanks are also due to Drs. Noel Owen and Stuart Charles for their continued interest and many suggestions, and to Veronica Almond for the preparation of samples. RECEIVED:
November 13, 1978 REFERENCES
I. M. TANIMOTO, V. ALMOND, S. W. CHARLES, J. N. MACDONALD,AND N. L. OWEN, J. Mol. Spectrosc. 78, 95 105 (1979). 2. S. SAMDALAND H. M. SEIP, J. Mol. Struct. 28, 193-203 (1975). 3. D. G. LISTER AND P. PALMIERI,f. Mol. Strucr. 48, 133-138 (1978). 4. H. W. KROTO AND B. M. LANDSBERG,J. Mol. Specrrosc. 62, 346-363 (1976). 5. W. GORDY AND R. L. COOK, “Microwave Molecular Spectra,” Wiley Interscience, New York, 1970. 6. W. H. HOCKING AND G. WINNEWISSER,2. Narurforsch. A 31,438-453 (1976). 7. W. D. GWINN AND A. S. GAYLORD, “International Review of Science, Physical Chemistry Series 2,” Vol. 3 (D. A. Ramsay, Ed.), Chapter 5, Butterworths, London, 1976. 8. E. HIROTA, R. SUGISAKI, C. J. NIELSEN, AND G. 0. SBRENSEN, J. Mol. Spectrosc. 49, 251-267 (1974). 9. A. S. ESBITT AND E. B. WILSON, JR., Rev. Sci. Instr. 34, 981-907 (1963). 10. D. 0. HARRIS, H. W. HARRINGTON,A. C. LUNTZ, AND W. D. GWINN, J. C/tern. Whys. 44, 3467-3480 (1966). 11. W. H. HOCKING,~. Naturforsch. A 31, 1113-1121 (1976).