Thermodynamics of disproportionation and heats of hydration of 1,4-benzosemiquinone and 1,4-naphthosemiquinone at pH 7.1

The thermodynamics of disproportionation of 1,4-benzosemiquinone (BQ{sup {sm bullet}{minus}}) and 1,4-naphthosemiquinone (NQ{sup {sm bullet}{minus}}) in cacodylic-cacodylate buffer at pH 7.1 was studied by electron spin resonance (ESR) spectroscopy and solution calorimetry. The disproportionation of BQ{sup {sm bullet}{minus}} was found to be more exergonic and exothermic than that corresponding to NQ{sup {sm bullet}{minus}} in both aqueous and gas phases. These semiquinones disproportionate about 40 times more exothermically in the gaseous than in the aqueous phase. The differences between the enthalpies of disproportionation corresponding to BQ{sup {sm bullet}{minus}} and NQ{sup {sm bullet}{minus}} are 51 and 14 kJ/mol in the gaseous and aqueous phases, respectively. The heats of hydration of BQ{sup {sm bullet}{minus}} and NQ{sup {sm bullet}{minus}} were determined with a convenient thermochemical cycle based on the enthalpies of disproportionation. The single-ion heats of hydration of BQ{sup {sm bullet}{minus}} and NQ{sup {sm bullet}{minus}} were found to be {minus}360 and {minus}377 kJ/mol, respectively. The heats of hydration of the net negative charge in BQ{sup {sm bullet}{minus}} and NQ{sup {sm bullet}{minus}} were found to be the same.