Force field and self‐consistent field calculations on a trans coplanar disulfide: Bis(2‐pyrimidyl)disulfide

Abstract

Two diary1 disulfides, bis(2-pyrimidy1)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' C-S-S-C 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 S-S bond is effectively overcome. In contrast, the phenyl and uncomplexed pyrimidyl disulfides prefer the usual C-S-S-C torsional angle near 8 5 O and have trans barriers greater than 7 kcal/mol. The barrier to rotation about S-Cspz 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 M M P~. A serious problem was encountered in the use of the M M P l computer program on molecules with N,,a-type atoms that contribute only one electron to the a-electron system. A solution is to modify the variable electronegativity self-consistent field (VESCF) part of the M M P l calculation to use another set of parameters in the program that pertain to a type of nitrogen with one x electron. CND0/2 was used for most of the MO calculations, although INDO and extended Hiickel were also used.

Molecular mechanics: The method and its underlying philosophy

“…The tables of existing parameters in the program are excellent starting points for guessing needed parameters. However, any new applications of the parameters should be tested if at all possible on some related molecules whose structures are known (see, e.g., (30)).…”