Ab Initio Evaluation of Intramolecular Electron Transfer Reactions in Halobenzenes and Stabilized Derivatives

The potential energy surfaces for the fragmentation of the radical anions of p-nitrochlorobenzene
and p- and m-chloroacetophenones were explored using first principle methods. The behavior of
these compounds, stabilized by π acceptors, is compared to that shown by the unsubstituted
halobenzenes (PhX, X = F, Cl, Br, I). The presence of π and σ radical anions was inspected as well
as the intramolecular electron transfer (intra-ET) from the π to the σ surface, responsible for the
dissociation of these intermediates. The profiles obtained with the B3LYP functional in the gas
phase and in the presence of a polar solvent are in agreement with the spectroscopic evidence and
with the experimentally observed reactivity of the compounds under study. The stability of the
radical anion of p-nitrochlorobenzene and the adiabatic and endothermic nature of its dissociation
are explained. The order of the rate constants for dissociation m-chloroacetophenone < p-chloroacetophenone is interpreted on the basis of the differences in the adiabatic character of the
intra-ET of both isomers which is ascribed to the nodal properties of their SOMOs. In the
halobenzene family, the electronic factors responsible for the intra-ET are analyzed. The stabilization
of the σ surface exerted by the different halogens and its effect on the rate constants for dissociation
are explained.