Temperature effects on the one-electron reduction potentials of nitroaryl compounds.

Pulse radiolysis was used to establish one-electron transfer equilibria between radical cations of methyl or benzyl viologens (V2+) and nitroaryl compounds (ArNO2): a nitroimidazole (misonidazole or metronidazole), 4-nitrobenzoate or nitrofurazone. The equilibrium constants in water at pH 8 were estimated over the temperature range approximately 5 to 75 degrees C. The difference delta E in mid-point one-electron reduction potentials between the nitro compounds and the viologens varied with temperature T; increasing temperature made the nitro compounds apparently less electron-affinic compared to the effects of temperature on the viologen potential. Values of delta(delta E)/delta T were in the range -0.7 to -1.1 mV K-1 at 25 degrees C. If delta[E(V2+/V.+)]/delta T = -0.9 mV K-1 for methyl viologen then delta[E(ArNO2/ArNO2.-)]/delta T is about -2 mV K-1 for these compounds.