A THEORETICAL MO AB-INITIO APPROACH TO THE CONFORMATIONAL PROPERTIES AND HOMOLYTIC BOND-CLEAVAGE IN ARYL DISULFIDES

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 compar ison, those of the disulphides with R-1,R-2 = H,Me were determined wit h 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/MP2/3-2 1G). This choice allowed a comparison of the properties calculated fo r the molecules containing alkyl and phenyl groups at the same level o f theory. Ail the disulphides examined showed patterns of the potentia l energy for internal rotation with a minimum conformation of skew typ e and two maxima, one of cis and one of trans type, with the former re presenting the higher transition state for internal rotation. The effe ct of the R group is higher on the cis barrier and the effect of Me an d Ph is respectively to increase and decrease the barrier, with respec t to R=H. The bond energy required for homolitically breaking the bond s (BDE) in these molecules and in the radicals R-S-S was estimated bot h from calculated total molecular energies, and from heats of formatio n of the species involved in the dissociation processes. The BDE's fro m the ab initio energies were found to have been largely underestimate d, 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, c alculations were performed with the sophisticated Gaussian-2 (G2) sche me and the BDE's obtained are very close to the most accurate determin ations, showing that a high level of theory is necessary to obtain the se quantities at a convenient level of confidence. The origin of the b arriers for internal rotation and of the different bond strengths in t he molecules and radicals containing the S-S bond was analyzed within the natural bond orbital (NBO) description of donor-acceptor interacti ons. (C) 1997 Elsevier Science B.V.