Thorough conformational analyses have been performed on representative sulfur, oxygen, and nitrogen mustards. A total of 23, 18, and 38 unique conformers have been located for SM, OM, and NM, respectively, at the MP2/aug-cc-pVDZ level of theory. Despite the fact that these molecules differ only in the identity of the central heteroatom, comparison of their low energy conformations reveals that the shapes they adopt are distinctive to each molecule. Potential energy surfaces for CH_2-X (X = S, O, and N-CH_3) and CH_2-CH_2 bond rotations are presented and, where possible, compared with dihedral angle distributions observed in crystal structure data. These results were used to benchmark and improve the performance of the MM3 and MMFF94 force fields.
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