Electron Transfer in Supercritical Carbon Dioxide: Ultraexothermic Charge Recombination at the End of the ldquoInverted Regionrdquo

Abstract

Charge-recombination rates in contact radical-ion pairs, formed between aromatic hydrocarbons and nitriles in supercritical CO2 and heptane, decrease with the exothermicity of the reactions until they reach -70 kcal mol-1, but from there on an increase is observed. The first decrease in rate is typical of the ldquoinverted regionrdquo of electron-transfer reactions. The change to an increase in the rate for ultra-exothermic electron transfer indicates a new free-energy relationship. We show that the resulting ldquodouble-inverted regionrdquo is not due to a change in mechanism. It is an intrinsic property of electron-transfer reactions, and it is due to the increase of the reorganisation energy with the reaction exothermicity.