Abstract
Our study of tunnelling in proton-coupled electron transfer (PCET) oxidation of ascorbate with hexacyanoferrate(III) follows the insights obtained from ultrafast 2D IR spectroscopy and theoretical studies of the vibrational water dynamics that led to the proposal of the involvement of collective intermolecular excitonic vibrational water dynamics in aqueous chemistry. To test the proposal, the hydrogen tunnelling modulation observed in the PCET reaction studied in the presence of low concentrations of various partial hydrophobic solutes in the water reaction system has been analyzed in terms of the proposed involvement of the collective intermolecular vibrational water dynamics in activation process in the case. The strongly linear correlation between common tunnelling signatures, isotopic values of Arrhenius prefactor ratios ln AH/AD and isotopic differences in activation enthalpies ΔΔH‡ (H,D) observed in the process in fairly diluted water solutions containing various partial hydrophobic solutes (such as dioxane, acetonitrile, ethanol, and quaternary ammonium ions) points to the common physical origin of the phenomenon in all the cases. It is suggested that the phenomenon can be rooted in an interplay of delocalized collective intermolecular vibrational dynamics of water correlated with vibrations of the coupled transition configuration, where the donor-acceptor oscillations, the motions being to some degree along the reaction coordinate, lead to modulation of hydrogen tunnelling in the reaction.
Highlights
We report a study of kinetic isotope effects (KIE) and hydrogen tunneling in proton-coupled electron transfer (PCET) reaction of ascorbate with hexacyanoferrate(III) that follows recent Tokmakoff and coworker’s studies [1–3]
In this study we have examined the temperature dependence of KIE and the modulation of hydrogen tunnelling induced by the partially hydrophobic solutes in the PCET [11–17] interaction of ascorbate monoanion with hexacyanoferrate(III) ions in order to gain information related to the assumed involvement of the collective vibrational water dynamics to aqueous chemistry
We have performed our experiments in the fairly diluted water solutions of the partially hydrophobic substrates in order to avoid the effects of presence of substantial concentrations of the substrates on bulk solvent properties and the vibrational excitonic dynamics of water, and KIE and hydrogen tunneling in the reaction
Summary
Where they have suggested that traditional way of conceiving solutes in water and aqueous chemical reactivity may be revised; the notion follows the insights obtained from ultrafast 2D IR spectroscopy and theoretical studies of the vibrational water dynamics [1–10]. These studies revealed the highly collective and intermolecular excitonic nature of coupled water vibrations; there are the observations of coherent vibrational character of coupled solute and solvent when there is strong hydrogen bonding between solute and the water molecule [1,3]. In aqueous chemical reactions, these vibrational motions would aqueous chemical these motions processes
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