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Sizeable Increase of Kinetic Isotope Effects and Tunnelling in Coupled Electron–Proton Transfers in Presence of the Quaternary Ions. PCET Processes and Hydrogen Tunnelling as a “Probe” for Structuring and Dynamical Phenomena in Water Solution

The presence of quaternary ammonium ions unexpectedly leads to a sizable increase of the kinetic isotope effects in the coupled electron-proton transfer (PCET) reaction of an ascorbate monoanion with the hexacyanoferrate(III) ions in water and this, in “neat” water over-the-barrier coupled electron-proton transfer interaction, entered into tunnelling regime in the presence of the quaternary ions. The kinetic isotope effect between ascorbate monoanion and its 2-OD derivative in the investigated reaction with hexacyanoferrate(III) increased from kH/kD = 4.40(0.08) in the reaction in water (in the presence of 8×10-3 M NaCl) without the added quaternary ions, to kH/kD = 10.08(0.07) in the presence of 1.0 M tetraethylammonium ion, to kH/kD = 8.01(0.19) in the presence of 1.0 M of benzyltrimethylammonium ion and to kH/kD = 7.25(0.02) in the presence of only 0.1 M of tetraethylammonium ion. In contrast, kH/kD = 4.06(0.15) has been observed in presence of 0.1 M NaCl. The isotopic ratio of Arrhenius pre-factors AH/AD = 0.16(0.01) has been obtained in the presence of only 0.1 M of tetraethylammonium ions and AH/AD = 0.10 (0.02) in the presence of 0.5 M of the ions. The corresponding observed value is AH/AD = 0.23 (0.02) in the presence of 0.5 M of benzyltrimethylammonium ions and AH/AD = 0.35 (0.06) in the presence of 0.5 M tetramethylammonium ions. The differences in the enthalpies of activation H‡ between D2O and H2O all are well beyond the semiclassical value of 5.1 kJ/mol for the difference between zero-point energies EoDEoH for dissociation of an OH bond. The observed tunnelling phenomena point to a role of dynamics of the transition configuration of the PCET process, coupled with dynamics of hydrogen-bonded structures related to the solvent shell of the reactive configuration and its environment including the nearby quaternary ammonium ions.

proton-coupled electron transfer; kinetic isotope effect; hydrogen tunnelling; dynamics; quaternary ammonium ion; ascorbate

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