Determination of heterogeneous electron-transfer kinetics of decamethylferrocene at low temperatures (120 K < T < 200 K) by a.c. impedance

Low-temperature heterogeneous electron-transfer kinetics of decamethylferrocene obtained from a.c. impedance analysis are reported. The experiments have been performed for 120 K < T < 200 K with a gold working electrode in two different electrolyte solution mixtures containing equal amounts of decamethylferrocene and decamethylferricinium tetrafluoroborate: 7:1:16 (v:v:v) tetrahydrofuran/2-methyltetrahydrofuran/ethylchloride with lithium tetrafluoroborate, and 1:2 (v:v) butyronitrile/ethylchloride with tetrabutylammonium perchlorate. Values of standard rate constants for each temperature have been evaluated by fitting the experimental a.c. impedance spectra with a simple RC parallel circuit in the middle frequency part of complex-plane plots. The analysis of Arrhenius plots, corrected from solvent dynamics (ln k0T1/2 vs T−1), for the range of temperatures studied yields the free activation energy ΔG∗ and the pre-exponential factor A′ for the two solvents mixtures (respectively 18.22 kJ mol−1 and 1140 cm s−1 in the tetrahydrofuran-based solvent system, 22.53 kJ mol−1 and 2618 cm s−1 in the butyronitrile-based solvent system). The very slight discrepancies between the experimental results and theoretical values calculated from the solvent dielectric continuum model are discussed.