High-resolution Fourier transform synchrotron spectroscopy of the C–S stretching band of methyl mercaptan, CH332SH

Abstract

The C–S stretching fundamental band of 12CH332SH, the principal isotopologue of methyl mercaptan, has been investigated by Fourier transform infrared spectroscopy on the Far-Infrared beamline at the Canadian Light Source synchrotron. The band is centered around 710cm−1 and shows well-resolved a-type parallel structure. Most of the A and E spectral sub-bands have been assigned up to K=12 for the vt=0 torsional state and K=9 for the vt=1 state, along with a smaller variety of sub-bands for vt=2. C–S stretching energy term values have been determined employing known ground-state energies, and have been fitted to series expansions in powers of J(J+1) to determine the substate origins. The origins have in turn been fitted to a Fourier model to characterize the oscillatory torsional energy structure of the C–S stretching state. The amplitude of oscillation of the vt=0 torsional curves is significantly larger for the C–S stretch state compared to the ground state. A strategy devised to relate this amplitude to an effective torsional barrier height indicates a decrease of about 7% in the effective V3 for the C–S stretch. The vibrational frequency determined for the stretching fundamental from the Fourier fit is 710.3cm−1. The C–S stretching manifold is crossed by excited vt=4 torsional levels of the ground state, and strong torsion–vibrational resonant coupling is observed via perturbations in the spectrum together with forbidden sub-bands induced by mixing and intensity borrowing.