Temperature Dependence of the Rate of Intramolecular Electron Transfer

Quantum mechanical/molecular mechanical simulations are used to explore the temperature dependence of intramolecular electron-transfer rates in systems that represent both the "normal" and the "inverted" regions of the Marcus curve. The treatment uses an approach that includes effects of vibrational relaxations and dephasing and is largely free of adjustable parameters. Effects of temperature on the distribution of the energy gap between the reactant and product ( P( x_o)), the electronic-interaction matrix element, and the rates of dephasing and vibrational relaxations are considered. The simulations reproduce the measured rate constant and temperature dependence well for photochemical charge separation in a porphyrin-benzoquinone cyclophane and for a ground-state charge-shift reaction in a biphenylyl-androstane-naphthylyl radical. They overestimate the rate of the charge-shift reaction in a biphenylyl-androstane-benzoquinone adduct but are in accordance with the observation that this reaction is almost independent of temperature. Arrhenius plots of rate constants calculated with various P( x_o) distributions show that the apparent activation enthalpy depends on whether or not P( x_o) shifts with temperature.