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 <i>b</i>-type with a small parallel <i>a</i>-component. With both Δ<i>K</i> = +1 and Δ<i>K</i> = −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 <i>K′</i> = 0–14 for <i>A</i> and <i>E</i> torsional species. The substate origins have been obtained by expanding the term values in <i>J</i>(<i>J</i> + 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.