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

Abstract

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 pi acceptors, is compared to that shown by the unsubstituted halobenzenes (PhX, X = F, Cl, Br, I). The presence of pi and sigma radical anions was inspected as well as the intramolecular electron transfer (intra-ET) from the pi to the sigma 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 sigma surface exerted by the different halogens and its effect on the rate constants for dissociation are explained.

Electroreduction of Organic Compounds, 36 [1]. Electroreduction of Chlorinated Methyl Benzoates

“…Furthermore, prior to the bond breaking the π-SOMO is transformed into a σ -SOMO, which is mainly localized at the C-Cl bond, cf. phase as well as in polar solvents, which is obviously important at least for the final formation of the free chloride anion [24]. Their results agree with the experimentally observed reactivities.…”

Electrocatalysis and electron transfer mechanisms in the reduction of organic halides at Ag

“…The role of the π* orbital energy becomes well evident by a close examination of both the chloride and bromide series. The trends of π* energies are 5a < 6a for the chlorides and 8 < 5b < 6b for the bromides [39,40]. This trend is exactly the opposite of that found for the α values.…”

The solvent effect on the electrocatalytic cleavage of carbon-halogen bonds on Ag and Au

“…In the same context, detailed studies have been carried out on the role of single factors modulating the process, such as the reactant molecular structure (organic moiety and halide leaving group), the nature, morphology and state of the electrode surface [29], the adsorption of the reagents and products [30,31], and the role of the supporting electrolyte [32], while few preliminary studies have been reported concerning the role of the solvent [33][34][35], and a more systematic investigation is still overdue.…”

“…This modulation can be huge and is typically asymmetric for DET processes, since a polar solvent, like acetonitrile, will preferentially stabilize the species having the highest charge density, such as the halide anions resulting from the process; thus solvation affects both the process thermodynamics and kinetics [35]. At the same time the solvent also affects the rate of mass transfer (through its viscosity) and the extent and mode of specific adsorption processes.…”