Experimental determination of the reorganization energy of the NO2+/NO2 redox couple. Comparison with theory

Abstract

The reaction between the heteropolyanion Co^IIW₁₂O₄₀^6-, in the form of its (Bu₄N)₆ (2) or K₂(Bu₄N)₄ salt (3), and NO₂⁺BF₄^- has been studied kinetically in acetonitrile or nitromethane at 26 °C. In acetonitrile, the rate constants of 2 and 3 are almost identical, 0.097 and 0.073 dm³ mol^-1 s^-1, whereas in nitromethane 2 reacts >10² times faster, k = 13 dm³ mol^-1 s^-1 (3 has not been studied in nitromethane for solubility reasons). A second inorganic species, (Ph₄P)₂Os^IVCl₆, has also been similarly investigated, for which k = 1.4 × 10³ dm³ mol^-1 s^-1 in acetonitrile.The rate constants of the reaction of 2 (3) with NO₂⁺ could be correctly estimated by the Marcus theory of outer-sphere electron transfer, using the exceptionally high value of 140 kcal mol^-1 of the reorganization energy of the NO₂⁺/NO₂ homoexchange reaction which earlier had been estimated theoretically. Thus it seems that this value is supported by experimental evidence.The reversible potential of NO₂⁺/NO₂ in nitromethane has been determined to be 0.95 V vs. the ferricinium/ferrocene couple, thus confirming previously reported values in the vicinity of 1.0 V, but in disagreement with a higher value, 1.62 V.