Electrochemical and quantum-chemical studies of chromium(III,II) fluoride complexes in alkali chloride melts

Abstract

815Previously, we studied the effect of the second coordination sphere on k s in chloride melts [17,18]. The NaCl-KCl-CrCl 3 , KCl-CrCl 3 , and CsCl-CrCl 3 systems were studied. It was shown that the standard rate constants of charge transfer increased at elevated Abstract-The standard rate constants (k s ) of charge transfer on a glass carbon electrode were determined for the Cr(III)/Cr(II) redox pair in the NaCl-KCl-K 3 CrF 6 , KCl-K 3 CrF 6 , and CsCl-K 3 CrF 6 systems at 973-1173 K by cyclic voltammetry. The k s constant was found to increase at elevated temperatures and the follow ing nonmonotonic dependence of k s on the nature of the outer spheric cation was found: k s (CsCl) > k s (NaCl-KCl) > k s (KCl). On the basis of quantum chemical data for the M 3 CrF 6 + 18MCl (M = Na, K) model systems, it was shown that the complex chromium particles with four or five outer spheric sodium or potassium cations had maximum thermodynamic stability. Quantum chemical calculations were performed to interpret the experimental data on the effect of the second coordination sphere of the complexes on the standard charge transfer rate constants.

Kinetic and Thermodynamic Properties of Ytterbium Chloride and Fluoride Complexes in Chloride Melts

“…In our previous studies 53,54 for the Nb(V)/Nb(IV) redox couple in chloride-fluoride melts the nonmonotonic change of the standard rate constants was established as follows: k s (KCl) < k s (CsCl) < k s (NaCl-KCl). The same nonmonotonic behavior of k s was found also for the Cr(III)/Cr(II) 55 and Sm(III)/Sm(II) 22 redox couples. Ab initio calculations performed using the quantum-chemical PC Gamess/ Firefly program showed that the charge transfer activation energy can actually vary nonmonotonically in the Na + -K + -Cs + series in accordance with the reorganization energy ratio.…”

“…Ab initio calculations performed using the quantum-chemical PC Gamess/ Firefly program showed that the charge transfer activation energy can actually vary nonmonotonically in the Na + -K + -Cs + series in accordance with the reorganization energy ratio. 53,55 In turn, this leads to a nonmonotonic change in the charge transfer rate constant.…”

Electrochemical Behavior of SmF3 in Alkali Chloride Melts

“…However, it seems impossible to predict the influence of the second coordination sphere on the standard rate constants of charge transfer a priori. In our previous studies for the Nb(V)/Nb(IV) redox couple 58,59 and for the Cr (III)/Cr(II) redox couple 60,61 the nonmonotonic changes of the standard rate constants were established too. Ab initio calculations performed using the quantum-chemical PC Gamess/Firefly program showed that the charge transfer activation energy can vary nonmonotonically in the Na + -K + -Cs + series in accordance with the reorganization energy ratio.…”

Comparison of Model Systems (M+)n·[CrX63−] and M3CrX6+ 18MX Based on Quantum-Chemical Calculations (X: F, Cl)

“…Journal of Chemistry 5 Furthermore, according to calculations, in some cases a higher rate of ET can occur for particles with less stability [13].…”

“…Some results for the CrF 6 -containing systems I and II were given previously [11][12][13]; for CrCl 6 -containing systems I and II calculations were made in the nonrelativistic basis sets for M: Na and K in the main [2,4,14,15]. Here we provide data for the systems (M + ) ⋅[CrCl 6 ] and M 3 CrCl 6 + 18MCl in a quasi-relativistic ECP basis for M: Na, K, and Cs and an additional analysis of all these systems.…”