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.

The effects of charged species on proton-coupled electron-transfer (PCET) reaction should be of significance for understanding/application of important chemical and biological PCET systems. Such species can be found in proximity of activated complex in a PCET reaction, although they are not involved in the charge transfer process. Reported here is the first study of the above-mentioned effects. Here, the effects of Na , 5.5 (0.9) (at 0.001m Ca 2þ ), and 7.9 (2.8) (at 0.001m Mg 2þ ) kJ mol À1 ; e) nonlinear proton inventory in reaction. In the H 2 O/dioxane 1 : 1, the observed KIE is 7.8 and 4.4 in the absence and in the presence of 0.1m K þ , respectively, and A H /A D ¼ 0.14 (0.03). The changes when cations are present in the reaction are explained in terms of termolecular encounter complex consisting of redox partners, and the cation where the cation can be found in a near proximity of the reaction-activated complex thus influencing the proton/electron double tunneling event in the PCET process. A molecule of H 2 O is involved in the transition state. The resulting configuration is more rigid and more appropriate for efficient tunneling with Na þ or K þ (extensive tunneling observed), i.e., there is more precise organized H transfer coordinate than in the case of Ca 2þ and Mg 2þ (moderate tunneling observed) in the reaction.Introduction. -Proton-coupled electron-transfer (PCET) reactions are of crucial importance in a wide range of biological, biochemical, and chemical systems [1 -16], including those related to artificial photosynthesis and solar fuels [4 -8], and nanostructures and interfaces [14] [15]. It is recognized now that many processes in biology would not be possible without the coupling of proton and electron motion [3] [5]. Within a unified theoretical framework, both sequential electron transfer, followed by proton transfer (ET/PT) or vice versa (PT/ET), and the concerted transfer of these particles with conspicuous quantum-mechanical character could be viewed as PCET reactions [16]. However, the concerted transfer of a proton and an electron should be of central importance in the issue; here, the single chemical reaction step and

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