Estimation of the rate constant for electron transfer in room temperature ionic liquids

The kinetic parameters for simple electron transfer are estimated in seven room temperature ionic liquids containing a 1-alkyl-3-methyl-imidazolium cation and an inorganic anion, namely, tetrafluoroborate or hexafluorophosphate. The electron transfer system considered is the oxidation of ferrocene. The calculations make use of dielectric relaxation data obtained recently in these ionic liquids. The parameters estimated include the outer sphere Gibbs activation energy for electron transfer, the longitudinal relaxation time, and the standard rate constant. It is shown that the outer sphere Gibbs activation energy is virtually independent of the length of the alkyl chain in the cation. On the other hand, the longitudinal relaxation time increases with increase in the length of the alkyl chain. As a result the standard rate constant for oxidation of ferrocene decreases as the cation in the ionic liquid is made longer.