Modulating Hydrogen Tunnelling in Ascorbate Proton-Coupled Electron Transfers

Abstract

The oxidation of ascorbate with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radical in water and in water-acetonitrile (1:1 v/v) mixed solvent has been demonstrated to be a proton-coupled electron transfer (PCET) process, involving hydrogen tunnelling at room temperature. The observations of significant changes in isotope effect on the Arrhenius pre-factor, A_H/A_D caused by the presence of tetraethylammonium ions in the reaction clearly suggest changes in tunnelling regime in the reaction. The evidence comprises: a) the spectroscopic and kinetic evidence for the interaction of ascorbate and TEMPO; b) the observation of KIEs k_H/k_D of 25.4(0.3) in water-acetonitrile (1:1 v/v) without tetraethylammonium ions and 23.3(0.1) in presence of teraethylammonium ions; c) the observation of k_H/k_D of 21.9(0.2) in water in presence of tetraethylammonium ions; d) the observation of isotope effect on the Arrhenius pre-factor, A_H/A_D of 0.82(0.10) in the reaction in water-acetonitrile (1:1 v/v) without tetraethylammonium ions; e) the observation of isotope effect on the Arrhenius pre-factor, A_H/A_D of 0.22(0.03) in the reaction in water-acetonitrile (1:1 v/v) in the presence of tetraethylammonium ions; f) the observation of isotope effect on the Arrhenius pre-factor, A_H/A_D of 1.34(0.15) in the reaction in water in the presence of tetraethylammonium ions; g) the observation of isotope differences in the enthalpies of activation in water and D₂O, in presence of tetraethylammonium ions Δ_rH^‡ (in H₂O) = 33.7(0.4) kJ/mol, Δ _rH^‡ (in D₂O) = 40.7(0.1) kJ/mol; h) the observation of isotope differences in the enthalpies of activation in water-acetonitrile (1:1 v/v) and D₂O-acetonitrile (1:1 v/v) in absence of tetraethylammonium ions, Δ _rH^‡ (in H₂O-acetonitrile) = 31.1(0.1) kJ/mol, Δ _rH^‡ (in D₂O-acetonitrile) = 39.5(0.4) kJ/mol; i) the observation of isotope differences in the enthalpies of activation in water-acetonitrile (1:1 v/v) and D₂O-acetonitrile (1:1 v/v) in presence of tetraethylammonium ions, Δ _rH^‡ (in H₂O-acetonitrile) = 29.4(0.3) kJ/mol, Δ _rH^‡ (in D₂O-acetonitrile) = 41.0(0.4) kJ/mol. These results were discussed following a framework of Marcus-like tunnelling model, taking into account dynamical features of the systems.