| Résumé | A simple experiment is described (photomodulation voltammetry) that allows the electrochemical oxidation and reduction potentials of short lived intermediates to be measured directly. These data are used in thermochemical cycles to determine a number of radical cation a-bond energies, DO(R-R+'), in acetonitrile solution. By comparison with gas phase ionization potentials it is shown these bond energies are generally similar in the gas phase implying that the solvation energies of the radical cations and the product cations are the same. Only in those cases where a there is a significant change in the charge distribution of R+ compared to R-R+' are large differences between gas phase and solution observed. A comparison of hydrazine and diphosphine radical cation bond energies also is made. In contrast to hydrazines which form strong two-center three electron bonds upon removal of one electron, there appears to be very little three electron bonding interaction between the phosphorous atoms of tetraaryldiphosphine radical cations. This results in a modest weakening of the P-P bond relative to the neutral species. The standard potentials, EO, for the oxidation of the tributyl- and triphenylstannyl radicals were estimated from a combination of electrochemical and kinetic measurements. These data were combined with reduction potentials of nitroalkanes to estimate the thermochemistry for electron transfer between these species. The free energy change for the electron transfer reaction (outer sphere) must be > 12 kcal mol-I. The rates of these reactions, measured by laser flash photolysis, were on the order of 108 M-Is-I showing, unequivocally, that mechanism of the reaction of stannyl radicals with nitroalkanes does not proceed via outer sphere electron transfer. |
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