CONFORMATIONAL PROPERTIES AND HOMOLYTIC BOND-CLEAVAGE OF ORGANIC PEROXIDES .1. AN EMPIRICAL-APPROACH BASED UPON MOLECULAR MECHANICS AND ABINITIO CALCULATIONS

The conformational features of a large number of hydroperoxides ROOH and peroxides ROOR', where R and R' are alkyl groups of different and increasing size and phenyl rings, including ortho substituted derivatives, were obtained from molecular mechanics calculations by employing a standard package. For the molecules of small molecular size, comparison was carried out with the results of ab initio calculations. Heats of formation were also obtained from molecular mechanics for hydroperoxides and peroxides: The values are, in general, overestimated. For the molecules containing the CF3 group, the calculated values are subject to large errors and heats of formation were obtained from ab initio total energies in the ''atom equivalents'' scheme. To estimate the homolytic dissociation energies of the different bonds in the peroxide molecules, heats of formation of R., .OR, and .OOR radicals were employed and several of them had to be calculated. Different approaches were employed-molecular mechanics calculations, ab initio energies within the atom equivalent and isodesmic reaction schemes, and Benson's group additivity rule; values consistent within the different calculation methods were chosen for estimating dissociation energies. The bond dissociation energies indicate different trends in these molecules as a function of the nature of the R and R' groups and the possible electronic effects operating in these molecules are discussed.