Theoretical investigation of the gas phase oxidation mechanism of dimethyl sulfoxide by OH radical

Gas phase reactions of dimethyl sulfoxide (DMSO) with OH radical were studied using DFT and conventional ab initio method PMP2. Using a factorial design analysis of two levels and three factors (23), the effect of polarization and diffuse functions and the effect of hybrid functionals were analyzed. For the addition channel, the free energy of activation is affected by all the analyzed factors, whereas, for the abstraction channel, the main factor is the DFT method chosen. The kinetics of the gas phase reactions was studied by evaluating the reaction paths and the rate coefficients of the overall oxidation process. The rate coefficients were obtained by conventional transition state theory (TST) and variational transition state theory (vTST). Since the quantitative description provided by the density functionals used was not satisfactory, the kinetics study was conduted using PMP2 level energy surface. The kinetics is mainly determined by the formation of a van der Waals complex in the first channel and the proton abstraction in the second channel of oxidation. According to this methodology the CH3S(O)OH + CH3 product channel is likely to be the dominant pathway, in good agreement with the experimental results.