Naslov
Solvent dependence of the kinetic isotope effect in the reaction of ascorbate with TEMPO radical. Tunnelling in a small molecule reaction

Autori
Sajenko, Ivana ; Pilepić, Viktor ; Jakobušić Brala, Cvijeta ; Uršić, Stanko

Izvornik
Journal of Physical Chemistry A (1089-5639) 114 (2010); 3423-3430

Vrsta, podvrsta i kategorija rada
Radovi u časopisima, članak, znanstveni

Ključne riječi
proton-coupled electron transfer; kinetic isotope effect; hydrogen bonding; hydrogen tunnelling; ascorbate; TEMPO; kinetics

Sažetak
The oxidation of ascorbate with 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) kH/kD 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 kH/kD 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 pre-factor, AH/AD 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 D2O, ΔrH‡(in H2O) = 31.0(0.4) kJ/mol, ΔrH‡(in D2O) = 40.0 (0.5) kJ/mol ; in 1:1 v/v water-diox. and 1:1 v/v D2O-diox., ΔrH‡(in H2O/diox.) = 23.9(0.2) kJ/mol, ΔrH‡(in D2O/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-1dm3s-1 to 5.50(0.14) mol-1dm3s-1, respectively which originates from the mutual compensation of the enthalpy and entropy of activation. Keywords: Proton-coupled electron transfer, Kinetic isotope effect, Hydrogen bonding, Hydrogen tunnelling, Ascorbate, TEMPO, Kinetics.

Izvorni jezik
Engleski

Znanstvena područja
Kemija

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