Ab Initio Molecular Orbital and Density Functional Studies on the Solvolysis of Sarin and O,S-Dimethylmethylphosphonothioate, a VX-like Compound

Seckute, J.; Menke, J. L.; Emnett, R. J.; Patterson, E. V.; Cramer, C. J.
J. Org. Chem. 2005, 70, 8649.

Potential energy surfaces for the alkaline hydrolysis of sarin and O,S-dimethyl methylphosphonothiolate, a VX model compound, and the perhydrolysis of the latter have been computed at the MP2/6-31+G(d)//mPW1K/MIDI! level of theory. The effect of aqueous solvation was accounted for via the integral equation formalism polarizable continuum model (IEF-PCM), at the HF/6-31+G*(d) level. Excellent agreement with the experimental enthalpy of activation for alkaline hydrolysis of sarin is found. For the alkaline hydrolysis of O,S-dimethyl methylphosphonothiolate, it is found that the P-O and P-S bond cleavage processes are kinetically competitive, but that the products of P-S bond cleavage are thermodynamically favored. For the perhydrolysis of O,S-dimethyl methylphosphonothiolate, it is found that P-O bond cleavage cannot compete with P-S bond cleavage. In both cases, the data support initial formation of trigonal bipyramidal intermediates, and demonstrate kinetic selectivity for nucleophilic attack on the face opposite the more apicophilic methoxide ligand.

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