THEORETICAL INVESTIGATION OF ISOTOPIC SCRAMBLING MECHANISMS IN 2-CHLOROETHYL METHYL SULFIDE

Ab initio and semiempirical molecular orbital calculations have been a pplied to study the concerted and stepwise isotopic scrambling mechani sms of 2-chloroethyl methyl sulfide in the gas phase and in aqueous so lution. The calculations reveal the structural details of the reactant s, transition structures, and intermediates involved in this reaction and provide relative energy estimates. The concerted mechanism is foun d to be competitive with the stepwise mechanism in the gas phase, but the stepwise mechanism is favored in aqueous solution as no true trans ition structure for the concerted mechanism could be found using the s olvation models. A combined approach of evaluating solvation energies with the generalized-Born-plus-surface-tensions SMx solvation models o f Cramer and Truhlar at ab initio optimized geometries is found to del iver the best agreement with experimentally determined reaction barrie rs. Together with the recent experimental results of McManus and co-wo rkers, the present study provides insights into the controlling factor s involved in the elementary reaction steps of sulfur mustards and a s olid foundation for investigations into more complex reactions of rela ted compounds.