A theoretical MO ab initio approach to the conformational properties and homolytic bond cleavage in aryl disulphides

The structural and conformational properties of disulphides R-1-S-S-R-2, with R-1 = phenil ring (Ph) and R-2 = H,Me and Ph and, for a comparison, those of the disulphides with R-1,R-2 = H,Me were determined with MO ab initio calculations. The level of theory chosen was the 3-21G* with full geometry relaxation and electron correlation corrections at a second order Moller-Plesset perturbation scheme (MP2/3-21G*/MP2/3-21G*). This choice allowed a comparison of the properties calculated for the molecules containing alkyl and phenyl groups at the same level of theory. Ail the disulphides examined showed patterns of the potential energy for internal rotation with a minimum conformation of skew type and two maxima, one of cis and one of trans type, with the former representing the higher transition state for internal rotation. The effect of the R group is higher on the cis barrier and the effect of Me and Ph is respectively to increase and decrease the barrier, with respect to R=H. The bond energy required for homolitically breaking the bonds (BDE) in these molecules and in the radicals R-S-S was estimated both from calculated total molecular energies, and from heats of formation of the species involved in the dissociation processes. The BDE´s from the ab initio energies were found to have been largely underestimated, which applies to a different extent to the S-S, S-H and C-S bonds. For the S-S bond, the BDE´s calculated at the level of theory adopted here are proportional to values from experiments or from higher levels of theoretical approaches. For the disulphides with R-1,R-2 = H,Me, calculations were performed with the sophisticated Gaussian-2 (G2) scheme and the BDE´s obtained are very close to the most accurate determinations, showing that a high level of theory is necessary to obtain these quantities at a convenient level of confidence. The origin of the barriers for internal rotation and of the different bond strengths in the molecules and radicals containing the S-S bond was analyzed within the natural bond orbital (NBO) description of donor-acceptor interactions.