A THEORETICAL-STUDY ON THE MODES OF HOMOLYTIC BOND FRAGMENTATION IN HNX-YHM, H-S-X-Y AND H-S(O)-X-Y SYSTEMS

Recent studies in the authors' laboratories have involved studies on t he formation and reactions of substituted alkoxy radicals derived by t he photo-induced homolytic cleavage of the S-O bond of substituted alk yl 4-nitrobenzenesulfenates (1), and the alkyl radicals formed by the beta-scission of the alkoxy radicals. An interesting extension of thes e studies would be consideration of the homolytic cleavage of the S-X and X-Y bonds in Ar-S-X-Y and Ar-S(O)-X-Y systems where X and Y are va rious combinations of nitrogen, oxygen and sulfur. Ab initio theoretic al calculations have been carried out on various parent H-S-X-Y-H and H-S(O)-X-Y-H systems and on the radical species formed by the homolyti c dissociation of the S-X and X-Y bonds using the G2MP2 method in orde r to determine the relative magnitudes of the homolytic bond dissociat ion energies (taken as the difference in the sum of the total energies of the product radicals minus the total energy of the parent system, E(rex)s). For calibration purposes, similar calculations have been car ried out on hydrogen peroxide and hydrazine for which experimental hom olytic bond dissociation energies have been measured. It is found that , in the latter calculations, the results obtained at the HF/UHF level give very poor correspondence with the experimental values, but the r esults obtained at the UMP2 and QCISD(T) levels nicely bracket the exp erimental values. The results of the calculations have given an indica tion of whether the S-X or X-Y bond prefers to undergo homolytic cleav age. The results of these studies could very well be of significant im portance in the areas of both physical and synthetic organic chemistry . In these homolytic cleavage reactions some very interesting radical species are formed, only a few of which have been studied previously e xperimentally or theoretically. The results of these studies indicate that the preferred mode of homolytic cleavage of the S-X and X-Y bonds is essentially dependent only on the relative stability of the radica ls formed in the two modes of homolytic bond cleavage. (C) 1998 Elsevi er Science B.V.