The force field and molecular structure of dimethylsulfoxide from spectroscopic and gas electron diffraction data and ab initio calculations

Abstract

Scale factors on the ab initio force field of dimethyl sulfoxide (DMSO) produced by the HF/6-31G∗∗ method have been fitted using vibrational frequencies of the isotopomers DMSO-h6 and DMSO-d6 as well as centrifugal distortion constants of a total of nine isotopomers. As indicated by the ab initio calculated sequences of the methyl deformation and methyl rocking vibrations, the attribution to the symmetry species A′ and A″ have been reversed compared to previous work. Then fitting 12 scale factors, the experimental vibrational frequencies are reproduced to 0.6%.Subsequently, the molecular structure of gaseous DMSO has been investigated by a combined analysis of electron diffraction data, rotational constants and ab initio calculations. The fitted quadratic force field was used to calculate the mean amplitudes of vibration, to obtain the necessary harmonic corrections for the conversion of B0 rotational constants to Bz, and to determine the isotopic differences in bond lengths. The rav structural parameters (uncertainties in parenthesis are 3σ) we obtained from the present analysis are: SO=1.4837(11)Å,C–S=1.8071(8)Å,C–H=1.0934(23)Å, ∠C–S–C=96.56(6)°, ∠(C–S–C)plane and SO bond=115.43(5)°. The geometry of DMSO has been optimized using the methods: HF/6-31G(p,d), DFT/B3LYP/6-311++G(3df,3pd), DFT/B3PW91/6-311++G(3df,3pd), MP2/6-31G(d,p), MP2/6-311++G(3df,3pd) and QCISD(T)/6-311++G(2d). The structural parameters predicted at these levels of theory have been discussed and compared with the experimental results. The values for the geometrical parameters provided by the DFT/B3PW91/6-311++G(3df,3pd) method are in best agreement with the experimental results. The importance of the inclusion of the d-polarization functions on sulfur has also been investigated. Our results support earlier observations emphasizing the needlessness of the hypervalency concept for describing the bonding properties in DMSO because the SO bond is ionic and thus the octet rule is not violated.