Torsion-rotation structure and quasi-symmetric-rotor behaviour for the CH₃SH asymmetric CH₃-bending and C-H stretching bands of E parentage

High-resolution Fourier transform spectra of the asymmetric methyl-bending and methyl-stretching bands of CH₃SH have been recorded employing synchrotron radiation at the FIR beamline of the Canadian Light Source. Analysis of the torsion-rotation structure and relative intensities has revealed the novel feature that for both bend and stretch the in-plane and out-of-plane modes behave much like a Coriolis-coupled l-doublet pair originating from degenerate E modes of a symmetric top. As the axial angular momentum K increases, the energies of the coupled “l = ±1” modes diverge linearly, with effective Coriolis ζ constants typical for symmetric tops. For the methyl-stretching states, separated at K = 0 by only about 1 cm⁻¹, the assigned sub-bands follow a symmetric top Δ(K − l) = 0 selection rule, with only ΔK = −1 transitions observed to the upper l = −1 in-plane A′ component and only ΔK = +1 transitions to the lower l = +1 out-of-plane A″ component. The K = 0 separation of the CH₃-bending states is larger at 9.1 cm⁻¹ with the l-ordering reversed. Here, both ΔK = +1 and ΔK = −1 transitions are seen for each l-component but with a large difference in relative intensity. Term values for the excited state levels have been fitted to J(J + 1) power-series expansions to obtain substate origins. These have then been fitted to a Fourier model to characterize the torsion-K-rotation energy patterns. For both pairs of vibrational states, the torsional energies display the customary oscillatory behaviour as a function of K and have inverted torsional splittings relative to the ground state. The spectra show numerous perturbations, indicating local resonances with the underlying bath of high torsional levels and vibrational combination and overtone states. The overall structure of the two pairs of bands represents a new regime in which the vibrational energy separations, torsional splittings and shifts due to molecular asymmetry are all of the same order, creating a challenging and complex vibration-torsion-rotation coupling environment.