Dependence of nonadiabatic intramolecular dissociative electron transfers on stereochemistry and driving force

Abstract: The intramolecular dissociative electron transfer (ET) across donor-bridge-acceptor (D-B-A) systems consisting in a series of trans ring-substituted 4-benzoyloxy-1-methylcyclohexyl bromides in N,N-dimethylformamide has been studied by cyclic voltammetry. X-Ray diffraction crystallography and 1H NMR spectroscopy showed that the investigated D-B-A molecules have the trans(cyclohexane) axial(benzoyloxy)-axial(bromide) conformation and the same D/A orientation. As previously found with the corresponding cis equatorial,axial conformers (S. Antonello, F. Maran, J. Am. Chem. Soc. 120 (1998) 5713), electroreduction entails initial formation of a benzoate radical anion (donor D) followed by intramolecular dissociative ET to the C-Br bond (acceptor A) through the 1,4-cyclohexanediyl spacer (bridge B). The intramolecular ETs are exergonic, with driving force in the range from 0.5 to 1.2 eV. The electrode process follows the same mechanism previously established for the cis series of isomers, but the ET rate constants obtained with the trans axial,axial isomers are larger by 1.1 order of magnitude. Molecular models and X-ray crystallography structures show that the rate increase cannot be ascribed to a decrease of the distance between the electron-exchanging centers and, therefore, the results witness a remarkable stereochemical effect on the ET rate. Application of the German-Kuznetsov theory of nonadiabatic dissociative ET (E. D. German, A. M. Kuznetsov, J. Phys. Chem. 99 (1995) 9095) shows that the rate increase is caused by a more favorable coupling between the electronic wave functions describing the reagent and product states. The data and trends are discussed in comparison with other nonadiabatic intramolecular dissociative ET processes.