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

Abstract

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 A0 = 7031.7237 MHz, B0 = 6920.1221 MHz, and C0 = 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 ν11 and ν23 fundamentals. Torsional energies and transitions are computed variationally form the 2D-PES that denotes strong interactions between both internal tops. The vibrationally corrected V3 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 ν13 torsional fundamental is found at 229.837 cm(-1) in good agreement with previous assessment, there is not accord for the low intense transition ν24. A new assignment predicting ν24 to lie between 190 and 195 cm(-1) is proposed.

Structural and spectroscopic characterization of methyl isocyanate, methyl cyanate, methyl fulminate, and acetonitrile N-oxide using highly correlated ab initio methods

“…9 For the remaining structures, the angles between the heavy atoms are the following: C3N1C2 = 136. The CCSD(T)-F12/AVTZ-F12 equilibrium rotational constants of Table II were employed to determine the corresponding ground state parameters with the straightforward formula [26][27][28][29] B 0 = B e + ∆B core e + ∆B vib ,…”

“…4 A better agreement is obtained with the parameters fitted by Koput 18 and Halfen et al, 3 although the differences |A 0 ab initio − A 0 exp | = 2315 MHz 18 and |A 0 ab initio − A 0 exp | = 2231 MHz 3 are too large if we compare them with the usual accuracy found when CCSD(T)-F12 theory is applied. [26][27][28][29] This method usually provides a precision of few MHz.…”

“…In this paper, we use explicitly correlated ab initio methods that usually provide very accurate rotational and torsional parameters [26][27][28][29] for a theoretical characterization of the C 2 H 3 NO isomers at low temperatures. Equilibrium structures, vibrational frequencies, and rovibrational parameters are determined.…”

An ab initio study of the ground and excited electronic states of the methyl radical

“…In order to compare them with experimental data and to estimate the quality of the geometry, the last set was used to obtain observable ground vibrational state rotational constants according to the previously tested formula [58–59]: B0=Be(RCCSD(T)-F12)+ΔBeCORE(RCCSD(T))+ΔBVIB(MP2)…”

“…In this formula, that usually provides very accurate parameters [58–59], Δ B VIB represents the vibrational contribution to the rotational constants derived from the VPT2 ari vibration-rotation ΔBeCORE=Be(CV)−Be(V) interaction parameters determined using the MP2 cubic force field (see below), and ΔBeCORE is computed from B e (CV) and B e (V), which were calculated correlating both core and valence electrons (CV) or just the valence electrons (V) in the post-SCF process:…”