Naslov
Modulating hydrogen tunnelling in ascorbate proton-coupled electron transfers

Autori
Sajenko, Ivana ; Pilepić, Viktor ; Uršić, Stanko

Izvornik
Zeitschrift für physikalische Chemie (München. 1991) (0942-9352) 225 (2011), 8; 805-820

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

Ključne riječi
proton-coupled electron transfer; kinetic isotope effect; hydrogen tunnelling; dynamics; ascorbate

Sažetak
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, AH/AD caused by the presence of tetraethylammonium ions in the reaction clearly suggest changes in tunnelling regime in the reaction. The evidence comprise: a) the spectroscopic and kinetic evidence for the interaction of ascorbate and TEMPO ; b) the observation of KIEs kH/kD 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 ; 1:1 b) the observation of kH/kD of 21.9(0.2) in water in presence of tetraethylammonium ions ; c) the observation of isotope effect on the Arrhenius pre-factor, AH/AD of 0.82(0.10) in the reaction in water-acetonitrile (1:1 v/v) without tetraethylammonium ions ; d) the observation of isotope effect on the Arrhenius pre-factor, AH/AD of 0.22(0.03) in the reaction in water-acetonitrile (1:1 v/v) in the presence of tetraethylammonium ions ; e) the observation of isotope effect on the Arrhenius pre-factor, AH/AD of 1.34(0.15) in the reaction in water in the presence of tetraethylammonium ions ; f) the observation of isotope differences in the enthalpies of activation in water and D2O, in presence of tetraethylammonium ions ΔrH‡(in H2O) = 33.7(0.4) kJ/mol, ΔrH‡(in D2O) = 40.7(0.1) kJ/mol ; g) 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, ΔrH‡(in H2O-acetonitrile) = 31.1(0.1) kJ/mol, ΔrH‡(in D2O-acetonitrile) = 39.5(0.4) 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 presence of tetraethylammonium ions, ΔrH‡(in H2O-acetonitrile) = 29.4(0.3) kJ/mol, ΔrH‡(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.

Izvorni jezik
Engleski

Znanstvena područja
Kemija

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