Ftir synchrotron spectroscopy of the asymmetric C-H stretching bands of methyl mercaptan (CH3SH) – a perplexity of perturbations

Abstract

The infrared Fourier transform spectrum of the asymmetric C-H stretching bands of CH$_{3}$SH has been recorded in the 2950-3100 cm$^{-1}$ region at Doppler limited resolution using synchrotron radiation at the FIR beamline of the Canadian Light Source in Saskatoon. Assignment of numerous torsion-rotation sub-bands for the asymmetric stretches has revealed a surprising pseudo-symmetric behavior, in which each band is seen in only one of the two possible $\Delta$K selection rules. The upper states of the two asymmetric stretching vibrational bands thus appear to behave more like $\it{l}$ = $\pm$ 1 components of a degenerate E state of a symmetric top rather than distinct vibrational states. The two components are separated by about 1.5 cm$^{-1}$ at K = 0, and then diverge linearly at higher K with torsional oscillation amplitude similar to that of the ground state of about 1.3 cm$^{-1}$. The divergence is consistent with an a-type Coriolis splitting picture with an effective Coriolis constant $\zeta$ $\approx$ 0.075.