FTIR synchrotron spectroscopy of the S–H stretching fundamental of the 12CH332SH species of methyl mercaptan

The infrared Fourier transform spectrum of the S–H stretching fundamental band of 12CH3 32SH has been recorded using synchrotron radiation at the far-infrared beamline of the Canadian Light Source in Saskatoon. The S–H stretch is a hybrid band predominantly of perpendicular b-type with a small parallel a-component. With both ΔK = +1 and ΔK = −1 sub-bands present, the assignments are well determined from ground-state combination difference relations. The identified sub-bands access S–H stretching ground torsional substates from K′ = 0–14 for A and E torsional species. The substate origins have been obtained by expanding the term values in J(J + 1) power series, and have been fitted to a simple 6-parameter Fourier Hamiltonian to deduce the torsional energies. The oscillation amplitude of the S–H stretching torsional curves is 0.547 cm− 1 compared to 0.653 cm− 1 for the ground state, implying an increase on the order of 6.9% in the torsional barrier height. The vibrational wavenumber for the S–H stretch mode is found to be 2603.5 cm− 1.