Abstract
Two diaryl disulfides, bis(2‐pyrimidyl)disulfide and diphenyldisulfide, have been characterized by molecular mechanics and molecular orbital calculations. When complexed with Cu(I), the pyrimidyl disulfide is known to have a unique 180° CSSC dihedral angle and to be coplanar. Our analysis of the conformational energies of the rotamers suggests that copper–ring interactions are primarily responsible for altering the molecular and electronic structure of the disulfide such that the trans barrier to rotation about the SS bond is effectively overcome. In contrast, the phenyl and uncomplexed pyrimidyl disulfides prefer the usual CSSC torsional angle near 85° and have trans barriers greater than 7 kcal/mol. The barrier to rotation about SC bonds (2–3 kcal/mol) is low and consistent with ab initio calculations and experiments on other molecules. The force field calculations were done using MMP1. A serious problem was encountered in the use of the MMP1 computer program on molecules with N‐type atoms that contribute only one electron to the π‐electron system. A solution is to modify the variable electronegativity self‐consistent field (VESCF) part of the MMP1 calculation to use another set of parameters in the program that pertain to a type of nitrogen with one π electron. CNDO/2 was used for most of the MO calculations, although INDO and extended Hückel were also used.