COMPUTATIONAL STUDY OF ADDITION AND ABSTRACTION REACTIONS BETWEEN OH RADICAL AND DIMETHYL SULFIDE - A DIFFICULT CASE

The addition complex between OH radical and dimethyl sulfide (DMS) could not be located unless a level of theory higher than MP2/6-31G(d) was used. At the MP2/6-31+G(2d) level, a complex was found which was bound by 9.3 kcal/mol. At the estimated QCISD(T)/6-31+G(2d,p) level the complex is 8.7 kcal/mol more stable than DMS plus OH. This is reduced to 6.0 kcal/mol with zero-point correction. The calculated binding is underestimated with respect to experiment (13 +/- 3 kcal/mol). A transition state for abstraction of a hydrogen from DMS by OH was located at the MP2/6-31+G(2d) level 1.8 kcal/mol higher than reactants. Complexes of Cl and Br atoms with DMS and with dimethyl sulfoxide (DMSO) could be located at much lower levels of theory. Chlorine and bromine are much more polarizable than OH, and the interaction can be viewed as a soft acid (halogen radical) interacting with a soft base (DMS or DMSO) which appears to be described adequately at lower levels of theory. On the other hand, the orbital energies of OH and DMS are similar, and binding is due to a two-center, three-electron (2c-3e) interaction.