Far-infrared synchrotron-based spectroscopy of CH3SH from 50 to 560 cm−1: Near-free-rotor torsion-rotation structure and torsion-vibration interactions

Abstract

Analysis of torsion-rotation structure in the far-infrared Fourier transform synchrotron spectrum of CH332SH recorded from 50 to 560 cm−1 at the Far-Infrared Beamline of the Canadian Light Source has been extended to higher near-free rotor torsional states reaching up into the region of the small-amplitude vibrations. A variety of sub-bands has been assigned for torsional quantum number up to vt = 5, expanding the database of ground-state energy term values to higher K and vt levels to provide a broad foundation for associated investigations of the vibrational spectrum. The origins of the high-vt sub-states fit quite well to a pattern of free-rotor parabolic curves as a function of K, but with a lower value of the torsional constant F than obtained from previous global analysis of lower-vt data. Numerous resonances perturbing and mixing coupled near-degenerate sub-states have been observed, arising from interactions both within the ground state and between ground and excited vibrational levels.