Effect of the Second Coordination Sphere on the Standard Rate Constants of Charge Transfer for the Cr(III)/Cr(II) Redox Couple in Chloride Melts

The cyclic voltammetry method was used to determine the standard rate constants of charge transfer (k(s)) on a glassy carbon electrode for the Cr(III)/Cr(II) redox couple in the NaCl-KCl-CrCl3, KCl-CrCl3, and CsCl-CrCl3 melts in the temperature range of 973-1173 K. It was shown that ks increases with an increase in temperature and decrease as sodium cations are replaced by potassium and cesium cations in the second coordination sphere of chromium complexes. Optimized geometrical structures and energies of such model particles as (M+)(n)[Cr(III)Cl-6](3-) and (M+)(n)[Cr(II)Cl-6](4-) (M - Na, K, Cs; n=1 - 6) were obtained by quantum-chemical calculations. The most stable compositions of these particles are determined. Based on the calculated reorganization energies the activation energies of charge transfer have been computed. These values change monotonously in the Na-K-Cs series in accordance with the ratio of reorganization energies: E-act (Na) <E-act (K) <E-act (Cs). In its turn, this leads to monotonous variation of the charge transfer rate constants, which is observed in experiments.