DISSOCIATIVE ELECTRON-TRANSFER - ABINITIO STUDY OF THE CARBON HALOGEN BOND REDUCTIVE CLEAVAGE IN METHYL AND PERFLUOROMETHYL HALIDES - ROLE OF THE SOLVENT

MP3 level calculations using pseudo-potentials for the halogens and semidiffuse functions for the heavy atoms indicate that in the series CH3X (X = F, Cl, Br, I), the reaction CH3X + e- --> CH3. + X- is a concerted electron transfer-bond breaking process in accord with previous experimental findings (gas phase, solid matrixes, electrochemistry in polar solvents). The activation barriers and transition-state geometries thus found are in good agreement with the predictions of a recently developed empirical Morse curve based model leading to a quadratic activation-driving force relationship and an intrinsic barrier equal to one-fourth of the bond dissociation energy. The empirical model thus validated is of general applicability to the kinetic reactivity of the vast class of organic and inorganic molecules undergoing dissociative electron transfer upon reaction with homogeneous and heterogeneous outer-sphere electron donors. The comparison of the energy profiles obtained by means of full basis set calculations for CH3Cl and CF3Cl reveals the existence of an energy minimum in the anionic profile at large C-Cl distances with a much larger stabilization energy for CF3Cl.-. Analysis of charge densities maps indicates that the bond between carbon and chlorine is of the electrostatic type and that the large stabilization in CF3Cl.- is due to strong polarization of CF3.. Simulation of the effect of polar solvents shows the disappearance of the CF3Cl.- minimum. The reductive cleavage thus becomes a concerted electron transfer-bond breaking process in accord with previous conclusions derived from the electrochemistry of perfluoroalkyl halides in polar solvents. This result emphasizes and rationalizes the role of polar solvents among the parameter that govern the occurrence of concerted versus stepwise reductive cleavage mechanisms.