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

Abstract

Abstract 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 k H/k D=4.40(0.08) in the reaction in water (in the presence of 8 × 103 M NaCl) without the added quaternary ions, to k H/k D=10.08(0.07) in the presence of 1.0 M tetraethylammonium ion, to k H/k D=8.01(0.19) in the presence of 1.0 M of benzyltrimethyl ammonium ion and to k H/k D=7.25(0.02) in the presence of only 0.1 M of tetraethylammonium ion. In contrast, k H/k D=4.06(0.15) has been observed in presence of 0.1 M NaCl. The isotopic ratio of Arrhenius pre-factors A H/A D=0.16(0.01) has been obtained in the presence of only 0.1 M of tetraethylammonium ions and A H/A D=0.10(0.02) in the presence of 0.5 M of the ions. The corresponding observed value is A H/A D=0.23(0.02) in the presence of 0.5 M of benzyltrimethylammonium ions and A H/A D=0.35(0.06) in the presence of 0.5 M tetramethylammonium ions. The differences in the enthalpies of activation Δ Δ H‡ between D2O and H2O all are well beyond the semiclassical value of 5 .1 kJ/mol for the difference between zero-point energies E o D–E o H 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.