Theoretical Spectroscopic Characterization at Low Temperatures of Dimethyl Sulfoxide: The Role of Anharmonicity

The structural and spectroscopic parameters of dimethyl sulfoxide (DMSO) are predicted from CCSD(T)-F12 calculations that can help to resolve the outstanding problem of the rovibrational spectroscopy. DMSO is a near oblate top that presents a trigonal pyramidal geometry. Rotational parameters are determined at the equilibrium and in selected vibrational states. For the ground state, the rotational constants were calculated to be A(0) = 7031.7237 MHz, B-0 = 6920.1221 MHz, and C-0 = 4223.3389 MHz, at few megahertz from the previous experimental measurements. Ab initio calculations allow us to assert that DMSO rotational constants are strongly dependent on anharmonic effects. Asymmetry increases with the vibrational energy. Harmonic frequencies, torsional parameters, and a two-dimensional potential energy surface (2D-PES) focused to describe the internal rotation of the two methyl groups are determined at the CCSD(T)-F12 level of theory. For the medium and small amplitude motions, anharmonic effects are estimated with MP2 theory getting an excellent agreement with experimental data for the v(11) and v(23) fundamentals. Torsional energies and transitions are computed variationally form the 2D-PES that denotes strong interactions between both internal tops. The vibrationally corrected V-3 torsional barrier is evaluated to be 965.32 cm(-1). The torsional splitting of the ground vibrational state has been estimated to be lower than 0.01 cm(-1). Although the v(13) torsional fundamental is found at 229.837 cm(-1) in good agreement with previous assessment, there is not accord for the low intense transition v(24). knew assignment predicting v(24) to lie between 190 and 195 cm(-1) is proposed.