Rotation, Vibration, Torsion Coupling Effects in the Spectrum of Dimethyl Sulfide

The room temperature, high-resolution (8-16 MHz) rovibrational absorption spectrum of low-pressure dimethyl sulfide [ J. Quant. Spectrosc. Radiat. Transfer 2025, 37, 109690] shows spectral congestion at transition energies as low as ∼1000 cm^-1. The origin of this congestion is investigated theoretically by developing a rotation/torsion/vibration Hamiltonian and dipole moment surface that allow us to model the excitation of the methyl rocking vibrations that are excited in the experiment. The vibrations that are described by this Hamiltonian include the two torsions and the seven additional low-frequency vibrations with harmonic frequencies below 1100 cm^-1. The effect of Coriolis coupling is minimized by defining a body-fixed embedding that satisfies the Eckart condition at the nine equivalent minima on the potential surface. The couplings between the torsions and vibrations, especially for those states excited in hot-band transitions, lead to substantial state mixing. Not only does this state mixing lead to spectral congestion due to intensity borrowing, it also leads to the quartet of transitions, which are associated with the torsional tunneling of the two methyl groups, becoming spectrally distinct at the experimental resolution. These combined effects lead to a model spectrum with similar levels of congestion as are observed experimentally, allowing us to deconvolute the various contributions to this low-energy spectral congestion. This analysis identifies torsion-induced vibrational mode mixing as the most important contributor to the observed spectral congestion.