Hydrogen Tunnelling as a Probe of the Involvement of Water Vibrational Dynamics in Aqueous Chemistry?

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.


The Reaction of Ascorbate with Hexacyanoferrate(III) Ions
The investigated process is the known* interaction of monoascorbate anion with hexacyanoferrate(III) (Scheme S1).

Scheme S1
Ascorbate reduces hexacyanoferrate(III), giving hexacyanoferrate(II) and dehydroascorbic acid. The overall stoichiometry of the reaction is 1:2 (ascorbate:hexacyanoferrate(III)). The ascorbyl radical anion formed in the relatively slow first step (Scheme S1) will reduce another [Fe ( The rate parameters for the reaction of ascorbate with hexacyanoferrate(III) (Scheme S1) have been obtained from the pseudo-first order rate constants (kobs). Pseudo-first order rate constants have been determined spectrophotometrically by monitoring the decrease Optic Spectroscopy was used throughout to collect spectral and absorbance-time data.
Kinetic measurements were performed under carefully maintained temperature conditions (within the limits of ± 0.1C). In a typical kinetic run, at least 200 pairs of S3 absorbance-time data were collected and fitted to the common least-square algorithm.
At least three to four observed pseudo-first rate constants were always used to calculate the corresponding rate parameters under the specified reaction conditions. The measurements were performed under the pseudo-first order conditions, taking ascorbate concentration to be typically 15 or 20-fold in excess, (taking into regard the 2:1 reaction stoichiometry) over the concentration of hexacyanoferrate(III) ion. Very good pseudo-first order kinetics were obtained throughout. The second-order rate constants for the reaction have been calculated from the observed pseudo-first order rate constant using the concentration of ascorbate determined spectroscopically (see below) in the calculation. Furthermore, essentially the same rate constants (within the limits of the experimental error) were obtained when the ascorbate excess was 12-fold (experiments with benzyltrimethylammonium chloride and acetylcholine chloride) as in the case of 20-fold excess of ascorbate.
The conditions and procedure of the kinetic measurements involved also the following: the pseudo-first order rate constants were determined typically at neutral pH, 5.6 -6.6, in the presence of EDTA disodium salt (5·10 -4 M). from the UV-Vis spectra of the reaction mixture before each kinetic run and UV-Vis spectra of ascorbate and ascorbic acid of known concentrations in water/heavy water as calibration standards in the appropriate wavelength range, usually from 220 -330 nm.
The UV-Vis spectra of ascorbate and ascorbic acid used as calibration standards have been measured in conditions where the only relevant species is ascorbate monoanion or ascorbic acid, respectively. Using this procedure, a painstaking determination of relatively small changes of pKa values due to variation in ionic strength was avoided.              Table   S1.  Table S2.    Table S6.   Table S8.   Table S10.  Table S11.   Table   S1.  Table S2.   Table S5.   Table S7.   Table S9.   Table S11.    Table S1. Figure S2B. Dependence of lnKIE on 1/T in the reaction of ascorbate with hexacyanoferrate(III) ion in 1,4-dioxane-water solvent mixture (0.1 : 0.9 v/v) (•). Data from Table S2. (1/T) / K -1 S27 Figure S3B. Dependence of lnKIE on 1/T in the reaction of ascorbate with hexacyanoferrate(III) ion in MeCN-water solvent mixture (0.1 : 0.9 v/v) (•). Data from Table S3. Figure S4B. Dependence of lnKIE on 1/T in the reaction of ascorbate with hexacyanoferrate(III) ion in ethanol-water solvent mixture (0.1 : 0.9 v/v) (•). Data from Table S4.    Data from Table S10.

Thermochemistry
Thermochemical analysis for the reaction of ascorbate and hexacyanoferrate (

Ion Pairing in the Presence of Quaternary Ions
The slope of the straight line of log (k/ko) vs. I 1/2 , (the square root of ionic strength, Brønsted-Debye-Hückel equation) for the reaction in water in the presence of Na + or K + is 3.0, revealing the interaction between [Fe(CN)6] 3-and HAscˉ ion.* However, in the presence of quaternary ammonium ions, markedly curved dependences of log (k/ko) vs I 1/2 were obtained with certain of ions. The observation should be consistent with the increasing formation of ion pairs between the [Fe(CN)6] 3-and the quaternary ions added (an example is presented in Figure S14)