Ab initio GB study of chemical intermediates in solution; Ethylenesulfonium ion in hydrolysis of 2-chloroethyl methyl sulfide

Abstract

Ab initio MO theory including solvent effects has been applied to the structure and reactivity of methyl ethylenesulfonium ion, 1, in aqueous solution as a model of the three-membered cyclic sulfonium intermediate expected in the toxic action of sulfur mustard. The 6-31 + G* geometry optimization of the cyclic sulfonium ion 1 suggested that the ring size of 1 is expanded slightly by solvation. The contour lines map of the interaction energy between 1 and Cl− has a very shallow and wide well at 5–6 Å distance from 1. This is the solvent-separated ion pair, and the contact ion pair was not found between 1 and Cl−. The calculated energy diagrams for the SN2-type reactions of 1 with Cl−, H2O, and OH− that give ring-opened compounds indicated the following: (1) The energy of the 1 + Cl− system is similar to that of chloroethyl methyl sulfide (CEMS, 2), and the interconversion between 1 + Cl− and 2 occurs easily in aqueous solution. The 3-21 + G(*) and 6-31 + G* activation energies for the 2 → 1 + Cl− reaction, 20–22 kcal/mol, agree well with the experimental enthalpy of activation for the hydrolysis of 2. (2) The reaction of 1 with OH− gives a very stable hydroxyl compound, 4, and no transition state was found. (3) The reaction of 1 with H2O gives an unstable addition product that is expected to be converted to 4 with the assistance of another H2O molecule. This mechanism is consistent with that proposed by Bartlett and Swain in their pioneering work on the hydrolysis of sulfur mustard. © 1998 John Wiley & Sons, Inc. Heteroatom Chem 9:503–510, 1998