1. Synchrotron Spectroscopy and Torsional Structure of the
CSH-Bending and CH3-Rocking Bands of Methyl Mercaptan
Ronald M. Lees, Li-Hong Xu, Elias M. Reid
Centre for Laser, Atomic and Molecular Sciences (CLAMS),
Dept. of Physics, University of New Brunswick, Saint John, NB
Brant E. Billinghurst
Canadian Light Source, University of Saskatchewan, Saskatoon, SK

2. Background and Motivation
The spectra of the lower frequency modes of methyl mercaptan are quite different in appearance from those of methanol. Thus, there is interest in exploring the vibrational manifold at high resolution, since there is almost no previous study of the torsion-rotation structure.
The vt = 4 and 5 excited torsional levels of the ground state cross the lower vibrational modes, hence one expects to see perturbations due to torsion-vibration coupling that may be significant in terms of intramolecular energy redistribution and the internal dynamics.
Interesting theoretical questions have arisen concerning torsional inversion and conical intersections in the potential energy surface involving vibrational modes of E parentage, prompting a detailed experimental investigation of the methyl-rocking modes.
CH3SH is a significant sulfur-bearing species in astrophysical and atmospheric environments, hence understanding of its vibrational behavior and spectroscopy is of interest and importance.

3. Vibrational Modes of CH332SH
Description nobs / cm-1
A' n1 CH asym stretch 3015
n2 CH sym stretch 2948
n3 SH stretch 2605
n4 CH3 asym bend 1453
n5 CH3 sym bend 1332
n6 CH3 in-plane rock 1072
n7 SH bend 802
n8 CS stretch 710
A" n9 CH asym stretch 3015
n10 CH3 o-o-p bend 1444
n11 CH3 o-o-p rock 956
n12 CH3 torsion ~200
This is the table of vibration modes
Vibrational Coupling
to Torsional Ladder
Wavenumbers from I. W. May and E. L. Page, Spectrochim. Acta. 24A (1968)

4. Spectral Comparison with C-13 Methanol
CH3SH
CS Stretch
SH Bend
Out-of-plane
CH3 Rock
In-plane
CH3 Rock
13CH3OH
CO Stretch
In-plane
CH3 Rock
Out-of-plane
CH3 Rock
OH Bend

5. CH3SH In-Plane Methyl-Rocking Band Origin
K’¬ K” TS
b-Type
0¬ 1 A
1¬ 0 E2
1¬ 0 A
0¬ 1 E1
a-Type 5A 4A 3A 7A 6A 6E

6. CH3SH Out-of-Plane Methyl-Rocking Band Origin
c-Type
0¬ 1 A
1¬ 0 E1
1¬ 0 A
0¬ 1 E2
1¬ 0 E2
0¬ 1 E1

7. K-Reduced Torsion-Vibration Energies
Out-of-Plane A" CH3 Rock
A' CSH Bend
A' Ground State

9. 2a1 Fourier Series « Barrier Height s = 1 (E levels) s = -1 a = 4V3/F
Hindered Rotor Energy
E = F<Pg2> + V3/2 <1- cos3g>
Basis functions: Y ~ ei(m+rK)g
E is periodic in rK with period 3
E(rK) = E0 – a1cos[(rK + s)2p/3]
+ a2cos[(rK + s)4p/3]
\ a1 = [E(1.5) – E(0)]/2
s = 1
(E levels)
2a1
This is a function only of a reduced barrier parameter
a = 4V3/F
a1 Þ a Þ V3
(if F is assumed to be fixed)
s = -1
= 0
(A levels)
0.5
1.0
1.5

10. a1 ® a ® V3 Conversion Curve for the
Fourier Model of the vt = 0 Substate Origins
V3 = 4aF

11. Fourier Fits to the Torsion-Vibration vt = 0 Substate Origins
E0(K,s) = E0 – a1cos[(rK + s)2p/3] + a2cos[(rK + s)4p/3] + (A – B)K2 –DKK4
Parameter
Ground
C-S Stretch
CSH Bend
CH3 Rock A"
CH3 Rock A' E0
0.6737
1074.0 a1 – ? a2
A-B
3.045 DK
2.03E-05
1.44E-05
1.549E-05
2.702E-05 r
S.D.
0.016
0.0241
0.0311
0.0310
# Origins 35 30 32
# Fitted 25 27
nvib
1073.3
“V3” [1-D]
443.93
419.95
515.45
363.82

12. Fermi Perturbations to the C-S Stretch vt = 0 Levels
The curves are calculated Fourier-series fits to the K-reduced substate origins. Those origins with large deviations highlighted by arrows are perturbed by anharmonic Fermi resonances with vt = 4 excited torsional levels of the ground state, and were excluded from the fit.
K-Reduced Torsion-Vibration Energy (cm-1) A
E (K>0)
E (K<0)
A calc
E (K>0) calc
E (K<0) calc K
Energies are referenced to the bottom of the torsional barrier, cm-1 below the 00 A nt = 0 level.

13. Fermi Resonances between vt = 0 C-S Stretch and vt = 4 Ground Levels
Where the vt = 4 curves pass through the C-S stretch vt = 0 state, anharmonic
resonances can occur between close levels of the same torsional symmetry.

14. Fermi-Induced Forbidden Sub-band for the 5A Resonance
(0 A 5)gd 12 J 11 13
(4 A 5)gd
(0 A 5)cs d
Fermi Interaction at J = 12
d(5A, 12) = 1.97
DE(12) = 8.20
DEo(12) = 4.26
Interaction Parameter W
DE = Ö(DEo2 + W2)
W = 0.5*Ö(DE2 – DEo2)
W = cm-1
|b/a|2 = (DE-DEo)/(DE+DEo)
= 0.32

15. Summary
The lower vibrational bands in the FTIR synchrotron spectrum of CH332SH have been analyzed at high resolution, and term values and substate origins have been determined.
Both the C-S stretch and CSH bend show the normal torsional energy pattern while the out-of-plane CH3 rock is inverted, and all have been fitted to a Fourier model to characterize the torsional structure. The in-plane rock does not display the expected oscillatory behaviour so does not conform to the traditional Hamiltonian model.
Substantial variation in the Fourier amplitudes suggests significant differences in effective torsional barrier height among the modes.
Interactions between the vibrations and high-vt torsional states lead to numerous perturbations and associated forbidden sub-bands in the spectrum, with coupling constants of up to several cm-1.