The role of homolytic bond dissociation energy in the deprotonation of cation radicals.: Examples in the NADH analogues series

The deprotonation of the cation radical of 9-cyanomethylacridane by a series of normal bases is investigated and its pK(a) and homolytic bond dissociation energy determined experimentally. The latter parameter has the largest value in the NADH analogue series, thanks to the strong destabilization of the corresponding cation by the cyano group. It thus allows a significant extension of the attempted correlation between the intrinsic barriers and homolytic bond dissociation energies (D). Aside from members of the series where bulky substituents cause a decelerating steric effect, the correlation is close to a proportionality to D/4. The same correlation applies for all the other cation radicals where the rate constants of deprotonation by normal bases are available. The respective contributions of the homolytic and ionic states in the dissociation of the two types of acid, cation radicals and the conjugate acid of the normal base, are such that a simple model can be developed which regards the deprotonation reaction as a concerted H atom/one-electron transfer. It explains why, for each cation radical, the deprotonation by normal bases gives rise to a single Bronsted plot and why the intrinsic barriers are proportional to D/4. In the NADH analogue series, the deviations from proportionality observed with bulky substituents, and to a lesser extent, upon changing the extent of charge delocalization over the cation radical molecule are accounted for by product and reactant work terms, respectively.