Theoretical MO ab initio investigation of the reductive C–Cl bond cleavage in benzyl chloride, benzotrichloride† and in the analogous 4-pyridine derivatives

Abstract

Reductive electron transfer on the title compounds has been studied theoretically with MO ab initio methods, using the 6-31G* basis set and second-order Møller–Plesset perturbation theory, in order to verify the eventual stability of their radical anion and to analyze the C–Cl bond breaking process both in the neutral molecules and in the anions. The effect of the basis set has been tested at single point energies with the 6-311G** basis set. The geometries of the neutral molecules and radicals formed after bond dissociation are fully relaxed. The energy profiles of the radical anions as a function of the C–Cl bond distance have been found to be dissociative. The energy of activation and the structure of the activated complex have been studied in the forbidden crossing of the energy profiles of the neutral molecule and radical anion. The results show that the activation energy of the process is affected both by the number of chlorine atoms on the methyl group and by the different aromatic ring, yet the energy of reaction is significantly affected only by the number of chlorine atoms. When compared with reduction potentials determined experimentally in a previous work, these activation energies show an excellent linear relationship. The results are discussed in the frame of the Marcus–Hush model of electron transfer and it appears that chloromethyl derivatives of pyridine and benzene do not strictly follow the same reaction mechanism at the end of applying this model.