CONFORMATIONAL-ANALYSIS OF SULFUR MUSTARD FROM MOLECULAR MECHANICS, SEMIEMPIRICAL, AND AB-INITIO METHODS

We report the results: of a comprehensive computational investigation on the energetics of sulfur mustard (S(CH2CH2Cl)(2)) conformations. Mo lecular mechanics (MM2, MM3*, AMBER*, and OPLS*), semiempirical (MNDO , AM1, and PM3), and ab initio (HF/3-21G, HF/6-311G*, MP2/6-31G*, and MP2/6-311G*/ /HF/6-311G**) methods were applied to 12 low-energy str uctures obtained from a Monte Carlo conformational search using force fields contained in MacroModel 3.5a. In general, there is reasonable a greement between molecular mechanics and ab initio for geometrical pro perties, but significant differences in the energy predictions. There was more scatter from the semiempirical computations, with the AM 1 mo del most successfully reproducing the ab initio results. The ab initio calculations identify at least three conformations of sulfur mustard lower in energy than the all-anti structure, depending on the level of theory employed. Vibrational infrared spectra were computed for the f our lowest energy structures at the HF/6-311G* level and compared to experimental data, producing a better match than obtained from conside ration of a single conformer, but at greater computational cost.