The intramolecular dissociative electron transfer (ET) across donorbridgeacceptor (DBA) systems consisting of a series of trans ring-substituted 4-benzoyloxy-1-methylcyclohexyl bromides in N,Ndimethylformamide has been studied by cyclic voltammetry. X-ray diffraction crystallography and 1H NMR spectroscopy showed that the investigated DBA molecules have the trans(cyclohexane) axial(benzoyloxy)-axial(bromide) conformation. As previously found with the corresponding cis equatorial, axial conformers (Antonello and Maran, 1998 [25]), electroreduction entails initial formation of a benzoate radical anion (donor D) followed by intramolecular dissociative ET to the CBr 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 orders of magnitude. X-ray crystallography structures show that the rate increase cannot be ascribed to a simple decrease of the geometrical distance between the electron-exchanging centers and, therefore, the results witness a remarkable stereochemical effect on the ET rate. Application of the GermanKuznetsov theory of nonadiabatic dissociative ET (German and Kuznetsov, 1994 [35]) shows that the rate increase is caused by a more favorable coupling etween 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
Dependence of nonadiabatic intramolecular dissociative electron transfers on stereochemistry and driving force
Venzo A;
2011
Abstract
The intramolecular dissociative electron transfer (ET) across donorbridgeacceptor (DBA) systems consisting of a series of trans ring-substituted 4-benzoyloxy-1-methylcyclohexyl bromides in N,Ndimethylformamide has been studied by cyclic voltammetry. X-ray diffraction crystallography and 1H NMR spectroscopy showed that the investigated DBA molecules have the trans(cyclohexane) axial(benzoyloxy)-axial(bromide) conformation. As previously found with the corresponding cis equatorial, axial conformers (Antonello and Maran, 1998 [25]), electroreduction entails initial formation of a benzoate radical anion (donor D) followed by intramolecular dissociative ET to the CBr 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 orders of magnitude. X-ray crystallography structures show that the rate increase cannot be ascribed to a simple decrease of the geometrical distance between the electron-exchanging centers and, therefore, the results witness a remarkable stereochemical effect on the ET rate. Application of the GermanKuznetsov theory of nonadiabatic dissociative ET (German and Kuznetsov, 1994 [35]) shows that the rate increase is caused by a more favorable coupling etween the electronic wave functions describing the reagent and product states. The data and trends are discussed in comparison with other nonadiabatic intramolecular dissociative ET processesI documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
