Small Molecule Tunnelling Systems: Variation of Isotope Effects

Abstract

Variations of the kinetic isotope effects and tunnelling regime in two small molecule tunnelling systems, the reaction of ascorbate monoanion with hexacyanoferrate(III) ions in water-acetonitrile (1:1 v/v), water-dioxane (1:1 v/v) and the oxidation of ascorbate with ferricinium ions in water were investigated. The kinetic isotope effects and tunnelling regime undergo to changes due to presence of cations in the reactions. The evidence comprise: 1.) the spectroscopic and kinetic evidences for the interaction of ascorbate monoanion with hexacyanoferrate(III) ions in water-acetonitrile (1:1 v/v), water-dioxane (1:1 v/v) and for the oxidation of ascorbate with ferricinium ions in water; 2.) for the interaction of ascorbate monoanion with hexacyanoferrate(III) ions: a) the observation of change of KIE from k_H/k_D=8.25(0.09) in water-acetonitrile (1:1 v/v) to 5.86(0.17) in presence of 0.1 M K⁺ in the same solvent; b) the observation of change of KIE from k_H/k_D=8.37(0.16) in water-dioxane (1:1 v/v) to 5.75(0.1) in presence of 0.3 M Na⁺ in the same solvent; c) the observation of change of isotope effect on the Arrhenius pre-factors, from A_H/A_D=0.49(0.09) in the reaction in water-acetonitrile (1:1 v/v) to A_H/A_D=0.14(0.02) in presence of 0.1 M K⁺ in the same solvent; d) the observation of change of isotope effect on the Arrhenius pre-factors, from A_H/A_D=0.046(0.008) in the reaction in water-dioxane (1:1 v/v) to A_H/A_D=0.07(0.01) in presence of 0.3 M Na⁺ in the same solvent; 3.) for the oxidation of ascorbate with ferricinium ions: a) the observation of change of KIE from k_H/k_D=1.91(0.02) in water to k_H/k_D=2.01(0.03) in presence of 0.5 M Na⁺; b) the observation of change of isotope effect on the Arrhenius pre-factors, from A_H/A_D=0.80(0.09) in the reaction in water to A_H/A_D=0.52(0.10) in presence of 0.5 M Na⁺; c) the observation of change of KIE from k_H/k_D=1.91(0.02) in water to k_H/k_D=1.72(0.02) in presence of 0.1 M of acetate ion; d) the observation of catalysis by acetate ions in the reaction. These results were discussed tentatively following a framework of Marcus-like tunnelling model, taking into account differences between the transition structures in the reactions and dynamical features of the systems.