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Ions Can Move a Proton‐Coupled Electron‐Transfer Reaction into Tunneling Regime
Author(s) -
Brala Cvijeta Jakobušić,
Pilepić Viktor,
Sajenko Ivana,
Karković Ana,
Uršić Stanko
Publication year - 2011
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.201100035
Subject(s) - chemistry , kinetic isotope effect , electron transfer , reaction rate constant , ion , arrhenius equation , proton , proton coupled electron transfer , quantum tunnelling , reaction rate , analytical chemistry (journal) , kinetics , atomic physics , activation energy , deuterium , biochemistry , physics , optoelectronics , organic chemistry , quantum mechanics , chromatography , catalysis
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 + , K + , Li + , Ca 2+ , Mg 2+ , and Me 4 N + observed in PCET reaction of ascorbate monoanions with hexacyanoferrate(III) ions in H 2 O reveal that, in presence of ions, this over‐the‐barrier reaction entered into tunneling regime. The observations are: a ) dependence of the rate constant on the cation concentration, where the rate constant is 71 (at I = 0.0023), and 821 (at 0.5 M K + ), 847 (at 1.0 M Na + ), and 438 M −1 s −1 (at 0.011 M Ca 2+ ); b ) changes of kinetic isotope effect (KIE) in the presence of ions, where k H / k D =4.6 (at I = 0.0023), and 3.4 (in the presence of 0.5 M K + ), 3.3 (at 1.0 M Na + ), 3.9 (at 0.001 M Ca 2+ ), and 3.9 (at 0.001 M Mg 2+ ), respectively; c ) the isotope effects on Arrhenius pre‐factor where A H / A D =0.97 (0.15) in absence of ions, and 2.29 (0.60) (at 0.5 M Na + ), 1.77 (0.29) (at 1.0 M Na + ), 1.61 (0.25) (at 0.5 M K + ), 0.42 (0.16) (at 0.001 M Ca 2+ ) and 0.16 (0.19) (at 0.001 M Mg 2+ ); d ) isotope differences in the enthalpies of activation in H 2 O and in D 2 O, where ΔΔ H ‡ (D,H)=3.9 (0.4) kJ mol −1 in the absence of cations, 1.3 (0.6) at 0.5 M Na + , 1.8 (0.4) at 0.5 M K + , 1.5 (0.4) at 1.0 M Na + , 5.5 (0.9) (at 0.001 M Ca 2+ ), and 7.9 (2.8) (at 0.001 M Mg 2+ ) kJ mol −1 ; 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 0.1 M K + , respectively, and A H / A D =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 + or K + (extensive tunneling observed), i.e. , there is more precise organized H transfer coordinate than in the case of Ca 2+ and Mg 2+ (moderate tunneling observed) in the reaction.