Ab Initio Molecular Orbital Studies of the Rotational Barriers and the ³³S and ¹³C Chemical Shieldings for Dimethyl Disulfide

A series of ab initio molecular orbital calculations were carried out for dimethyl disulfide as a model for the disulfide bridges in proteins and peptides. The potential energy profile for rotation around the S-S bond was obtained at the HF/6-31G* level with full geometry optimization. Cis- and trans-barrier heights were estimated to be 11.40 and 6.27 kcal/mol, respectively, on the basis of fourth-order Moller-Plesset perturbation theory and 6-311G** basis sets. Calculations of the torsion angle dependence of the isotropic ³³S and ¹³C NMR shieldings were based on the method of individual gauge for localized orbitals (iglo). These are of interest for NMR studies of the disulfide bond in peptides and proteins. The minimum in the plot of ¹³C shielding as a function of torsion angle occurs for a C1-S1-S2-C2 angle close to 110°, which is an optimum arrangement for lone pair back-bonding. An analysis of the paramagnetic bond contributions to the ¹³C shielding at C1, for example, shows that the conformational dependence is dominated by the paramagnetic contributions to the C1-H1 bond, which points away from a lone pair on S2.