STERIC AND KINETIC ISOTOPE EFFECTS IN THE DEPROTONATION OF CATION RADICALS OF NADH SYNTHETIC ANALOGS

The deprotonation rate constants and kinetic isotope effects of the cation radicals have been determined by combined use of direct electrochemical techniques at micro- and ultramicroelectrodes, redox catalysis, and laser flash photolysis, over a extended series of opposing bases. Significant steric hindrance to deprotonation results from encumbering of the opposing base and of the functional carbon in the cation radical by alkyl groups. Kinetic isotope effects, ranging from 2 to 12 in terms of k(H)/k(D), appear upon substituting H to D at the functional carbon of the cation radical. The modest magnitude of the kinetic isotope effects and the fact that they are insensitive to steric hindrance show that proton (or H-atom) tunneling does not interfere significantly in the deprotonation reaction. All the cation radicals in the methylacridan series are strong acids, with pK(a)'s ranging from 0.8 to 1.7, as determined from thermodynamic cycles involving measured standard potentials and hydride-transfer equilibrium constants.