A theoretical study on the modes of homolytic bond fragmentation in H nXYH m, HSXY and HS(O)XY systems

Abstract

Recent studies in the authors' laboratories have involved studies on the formation and reactions of substituted alkoxy radicals derived by the photo-induced homolytic cleavage of the SO bond of substituted alkyl 4-nitrobenzenesulfenates ( 1), and the alkyl radicals formed by the β-scission of the alkoxy radicals. An interesting extension of these 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 various combinations of nitrogen, oxygen and sulfur. Ab initio theoretical 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 homolytic dissociation of the SX and XY bonds using the G2MP2 method in order to determine the relative magnitudes of the homolytic bond dissociation 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 rexs). For calibration purposes, similar calculations have been carried out on hydrogen peroxide and hydrazine for which experimental homolytic 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 results obtained at the UMP2 and QCISD(T) levels nicely bracket the experimental values. The results of the calculations have given an indication of whether the SX or XY bond prefers to undergo homolytic cleavage. The results of these studies could very well be of significant importance 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 experimentally 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 radicals formed in the two modes of homolytic bond cleavage.