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Molecular Orbital Calculations on the P-S Bond Cleavage Step in the Hydroperoxidolysis of Nerve Agent VX

Patterson, E. V.; Cramer, C. J.
J. Phys. Org. Chem. 1998, 11, 232.

The P-S bond cleavage process in the hydroperoxidolysis of a model system for the nerve agent VX is studied using ab initio and semiempirical molecular orbital methods. Aqueous solvation effects are included through single-point calculations using the semiempirical SM5.2PD/A continuum solvation model and geometries optimized at the HF/MIDI! level of theory. The predominant pathway for P-S bond cleavage involves pseudorotation of a low-energy trigonal bipyramidal intermediate followed by apical ligand ejection. In aqueous solution, the free energy barriers for these processes are found to be 14.3 kcal/mol and 4.6 kcal/mol, respectively, with electronic energies calculated at the MP2/cc-pVDZ//HF/MIDI! level of theory. By comparison to another continuum model of solvation (PCM), we conclude that the SM5.2PD/A model performs well even for hypervalent phosphorus species, in spite of not having included any such molecules in the model's parameterization set.

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