Solvent Dependence of the Kinetic Isotope Effect in the Reaction of Ascorbate with the 2,2,6,6-Tetramethylpiperidine-1-oxyl Radical: Tunnelling in a Small Molecule Reaction

The oxidation of ascorbate with the 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radical in water and water-dioxane mixed solvent has been demonstrated to be a proton-coupled electron transfer (PCET) process, involving hydrogen tunnelling at room temperature. The magnitude of the kinetic isotope effect (KIE) k(H)/k(D) in the reaction increases with decrease of the solvent polarity. The evidence comprise: (a) the spectroscopic and kinetic evidence for the interaction of ascorbate and TEMPO; (b) the observation of KIEs k(H)/k(D) of 24.2(0.6) in water and 31.1(1.1) in 1:1 v/v water-diox. (diox = dioxane), at 298 K; (c) the observation of isotope effect on the Arrhenius prefactor, A(H)/A(D) of 0.6(0.2) in the reaction in water and 1.2(0.2) in 1:1 v/v water-diox solvent; (d) the observation of isotope differences in the enthalpies of activation in water and D(2)O, Delta(r)H(double dagger) (in H(2)O) = 31.0(0.4) kJ/mol, Delta(r)H(double dagger) (in D(2)O) = 40.0 (0.5) kJ/mol; in 1:1 v/v water-diox and 1:1 v/v D(2)O-diox, Delta(r)H(double dagger) (in H(2)O/diox) = 23.9(0.2) kJ/mol, Delta(r)H(double dagger) (in D(2)O/diox) = 32.1(0.3) kJ/mol; (e) the temperature dependence of the KIEs in water and 1:1 v/v water-dioxane; these KIEs range from 27.3 at 285.4 K to 19.1 at 317.4 K in water and from 34.3 to 24.6 at the corresponding temperatures in 1:1 v/v water-diox, respectively; (f) the observation of an increase of the KIE in 10-40% v/v dioxane-water solvents relative to the KIE in water alone. There is a weak solvent dependence of the rate constant on going from water to 1:1 v/v water-diox. solvent, from 2.20(0.03) mol(-1) dm(3) s(-1) to 5.50(0.14) mol(-1) dm(3) s(-1), respectively, which originates from the mutual compensation of the enthalpy and entropy of activation.