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

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.

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) 1605-1615.

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

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

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

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

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

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

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 710.9807 801.5503 957.0436 1074.0 a1 0.65287 0.80125 0.36219 –1.33263 ? a2 0.00167 -0.0583 -0.004023 -0.0188 A-B 3.004472 3.004592 3.020647 3.000452 3.045 DK 2.03E-05 1.44E-05 1.549E-05 2.702E-05 r 0.651753 0.653632 0.641697 0.648306 S.D. 0.016 0.0241 0.0311 0.0310 # Origins 35 30 32 # Fitted 25 27 nvib 710.3070 800.8766 956.3699 1073.3 “V3” [1-D] 443.93 419.95 515.45 363.82

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, 112.17587 cm-1 below the 00 A nt = 0 level.

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.

Fermi-Induced Forbidden Sub-band for the 5A Resonance (0 A 5)gd 12 J 700.68750 711.64888 721.76742 692.52806 703.48932 713.60797 11 13 (4 A 5)gd (0 A 5)cs 151.94945 140.98815 130.86960 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 = 3.50 cm-1 |b/a|2 = (DE-DEo)/(DE+DEo) = 0.32

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.