Electronic reduction of haloaromatic compounds. A theoretical study

Abstract

A series of haloaromatic molecules with a wide range of standard reduction potentials (E°) and bond-cleavage rate constants (k) has been studied using the MNDO semiempirical procedure. It has been found that the standard potential correlates better with the difference in energy between the radical anion and the neutral geometry when the geometry of the former is kept unchanged, in agreement with the Franck–Condon principle. A reaction co-ordinate study of the bond cleavage of the radical anion for the chloro derivatives shows the existence of two radical anions (π and σ) that differ in the orbital occupied by the unpaired electron. Whereas the π-radical anion is the most stable, the bond-breaking process takes place through the σ-structure. The study of the energies as well as of different molecular parameters related to the C–Cl bond cleavage allows us to explain qualitatively the experimental ordering of rate constants when different substituent groups are considered. However, the theoretical predictions do not account for the kinetic differences when only small structural changes are present. The significance of the solvent effect in these cases is pointed out.