Conformational analysis—CXVII : Methyl ethyl disulfide. Molecular mechanics and molecular orbital calculations

Abstract

The conformation of methyl ethyl disulfide was investigated by molecular mechanics calculations using a recently developed force field for sulfur-containing alkanes. The results indicate that in the gas phase the molecule exists predominantly in two conformations, both with the CSSC dihedral angle gauche (84°), and the SSCC dihedral angle either gauche (72°) or trans (179°), and the methyl protons staggered. Ab initio molecular orbital calculations using an STO-3G basis set were employed to corroborate that these two conformations are of roughly equal stability, and that the next most stable conformation (by 0.6 kcal/mole) has the SSCC dihedral angle gauche (295°) with the terminal methyls proximal. In contrast to earlier CNDO/2 (spd) predictions, the SSCC cis conformer is the least stable, and no sizable attractive S⋯HC nonbonded interactions are discerned. Reasons for this are traced to a failure of the CNDO/2 method, which is especially serious when d orbitals are included in the basis set (spd) and the rigid rotor approximation is used. The present results are found to be consistent with recent electron diffraction, IR, Raman spectroscopic and X-ray diffraction data. The conformation of diethyl disulfide was also investigated by molecular mechanics calculations, and again gauche and trans SSCC arrangements are predicted to be preferred.