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Solvent dependence of the kinetic isotope effect in the reaction of ascorbate with TEMPO radical. Tunnelling in a small molecule reaction (CROSBI ID 562683)

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Sajenko, Ivana ; Pilepić, Viktor ; Jakobušić Brala, Cvijeta ; Uršić, Stanko Solvent dependence of the kinetic isotope effect in the reaction of ascorbate with TEMPO radical. Tunnelling in a small molecule reaction. 2009

Podaci o odgovornosti

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

engleski

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

The oxydation 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)1-6 process, involving hydrogen tunneling at room temperature and the change of magnitude of the kinetic isotope effect (KIE) kH/kD in the reaction with change of the solvent polarity. The facts that support the above description comprise: i) the spectroscopic and kinetic evidence for the interaction of ascorbate and TEMPO and the formation of its corresponding hydroxylamine TEMPOH and ascorbyl radical anion ; ii) the observation of the kinetic isotope effects (KIEs) kH/kD of 21.62(0.51) in water and 24.45(0.77) in 50/50 dioxane-water, at 298K ; iii) the observation of isotope efect on the Arrhenius preexponential factor, AH/AD of 0.8 in the reaction in water and 1.6 in 50/50 dioxane-water solvent ; iv) the observation of isotope differences in the enthalpies and entropies of activation i.e. (diox.= dioxane): ΔH ‡(H2O) = 31.00(0.38) kJ/mol, ΔH ‡(D2O) = 39.07(0.39) kJ/mol , ΔS ‡ (H2O) = -134.3(2.70) J/Kmol, ΔS ‡ (D2O) = -132.8(2.64) J/Kmol ; ΔH ‡(H2O/diox.) = 23.86(0.22) kJ/mol, ΔH ‡(D2O/diox.) = 30.42 (0.26) kJ/mol, ΔS ‡ (H2O/diox.) = -150.9(2.40) J/Kmol, ΔS ‡ (D2O/diox.) = -155.2(3.21) J/Kmol ; v) the temperature dependence of the KIEs in water and 50/50 v/v water-dioxane ; these KIEs range from 24.02 at 285.4K to 17.50 at 317.4K in water and from 26.47 to 20.30, at the corresponding temperatures in 50/50 v/v water-dioxane, respectively ; vi) an increase of the KIE in 10, 20, 30 and 40% v/v dioxane-water solvents relative to the KIE in water alone. In addition, the analysis of thermochemistry of reaction using the “square scheme”3 suggests the PCET reaction. There is a weak solvent dependence of the rate constant on going from water to 50/50 water-dioxane solvent, from 2.20(0.03) mol-1dm3s-1 to 5.50(0.14) mol-1dm3s-1, respectively. The observed change of the KIE in the reaction with the decrease of solvent polarity on going from water to water-dioxane is relatively weak, although occured in the direction predicted by theory involving a model for the PCET between two oxygen atoms6, 7.Taken together, obtained evidence suggests an important role of a hydrogen bonding preformed in the precursor complex1, 6-8 between 2-OH of ascorbate and the N-oxyl group in the reaction. The obtained results are of importance for a better understanding of fundamental features of PCET reactions, especially with regard to a relative scarcity of experimental evidence related to solvent influences on PCET reactions in general and on KIEs in the processes particularly. The results also provide a comparison of theoretical predictions with experimental results. REFERENCES 1 Huynh, H. V. ; Meyer, T. J. Chem. Rev. 2007, 107, 5004 and references therein. 2 Reece, S.Y. ; Hodgkiss, J. M. ; Stubbe, J. ; Nocera, D.G. Phil. Trans. R. Soc. B 2006, 361, 1351. 3 Mayer, J. M. Annu. Rev. Phys. Chem. 2004, 55, 363. 4 Rosenthal, J. ; Nocera, D. G. Acc. Chem. Res. 2007, 40, 543. 5 Itoh, S. ; Fukuzumi, S. Acc. Chem. Res. 2007, 40, 592. 6 Hammes-Schiffer, S. ; Soudackov, A. V. J. Phys. Chem. B 2008, 112, 14108. 7 Decornez, H. ; Hammes-Schiffer, S. J. Phys. Chem. A 2000, 104, 9370. 8 Ludlow, M. K. ; Soudackov, A. V. ; Hammes-Schiffer, S. J. Am. Chem. Soc. 2009, 131, 7094.

PCET; Kinetic isotope effect; TEMPO; ascorbate; solvent

nije evidentirano

nije evidentirano

nije evidentirano

nije evidentirano

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nije evidentirano

Podaci o prilogu

2009.

objavljeno

Podaci o matičnoj publikaciji

Podaci o skupu

Faraday Discussion 145: Frontiers in Physical Organic Chemistry

poster

02.09.2009-04.09.2009

Cardiff, Ujedinjeno Kraljevstvo

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Kemija