FREE-ENERGY HYDRIDE AFFINITIES OF QUINONES IN DIMETHYL-SULFOXIDE SOLUTION

Free energy hydride affinities in solution defined by reaction i [(i) Q(s) + H-s reversible (QH-)s, DELTAG(hydride)(Q)s] were evaluated for a number of quinones (Q) in dimethyl sulfoxide solution using reaction ii [(ii) -DELTAG(hydride)(Q)s = 2.303RTpK(a)(QH-)s + FDELTAE-degrees(NHE)[(Q.-/Q2-)s + (Q/Q.-)s] + C] derived from a thermochemical cycle. The constant C corresponds to -FE-degrees(NHE)[(H+/H.)s + (H./H-)s] which is equal to 69.9 kcal/mol in dimethyl sulfoxide. The pK(a) of hydroquinone monoanions (QH-) were determined by the overlapping indicator method. Reversible electrode potentials for both the first and second charge transfers to Q were determined by cyclic voltammetry. Values of DELTAG(hydride)(Q)DMSO ranging from -58 to -101 kcal/mol were observed. Hydride affinities of substituted benzoquinones (BQ) were found to be linearly correlated with the corresponding electron affinities in solution. Equilibrium constants for the reactions of BQ with the NADH model compound 10-methylacridan were estimated, and the thermochemical results are discussed in terms of the one- and the two-step mechanisms of hydride-transfer reactions.