Kinetics of ferncyanide-mediated oxidation of various 1-(Csubstituted phenyl)-I ,4dihydronicotinamides were studied assuming either a rate determining initial electron loss or a rate determining proton loss following a rapid pre-equilibrium step. Values obtained using both methods were similar. In addition, vertical ionization potentials generated using the AM1 molecular orbital approximation were found to be highly correlated with log(oxidation rates) while calculation on the proton loss from the dihydropyridine radical cations showed much less dependence. Measured kinetic deuterium isotope effects were small. These data suggest that the initial electron loss from the dihydronicotinamides is the most important event determining the reaction rates. lntroductlon Various models of electron transfer have been studied In order to gain insight into the biologically important NAD(P)HwNAD(P)' interconversion 3c. In many instances, system involving dihydronicotinamldes have been applied to this purpose". Interestingly, the manner in which electrons are shuttled between the redox couples strongly depend upon the reaction conditions. If a good hydrtde acceptor, such as the lmethylacrldinium ion, serves as the oxidant, the oxidation most likely proceeds via concerted hydride migration, i.e., the simultaneous transfer of a proton and two electrons from the dihydropyridine to the quaternary salta'. If, on the other hand, a good kinetic and thermodynamic one electron oxidant is present in the reaction media, a sequential electron, proton, electron "hydride-like" transfer occ~rs'~". Species which are suitable to induce this type of chemical behavior Include the ferrlcyanlde anion and the fenicenium cation. Miller has characterized the reactions of ferricenlum derivatives with l-substftuted-1,4dihydronlcotinamides as being first order with respect to each spedes'0'2. The rates of these reactions were unaffected by pH or by substitution of the 4hydrogen(s) on the dihydronicotinamide moiety with deuterium atoms. These data are fully consistent with an initial, rate-determining electron transfer. The reaction of the ferricyanide ion with dihydropyridines does not give as unambiguous a picture. Various studies have shown that this redox interaction is first order with respect to each species and that a small to negligible kinetic isotope effect is observed for oxidation of the 4,4-dideuterio analogs'a's. These data suggest that a simple