Reaction pathway and H/D kinetic isotope effects of the triple proton transfer in a 7‐hydroxyquinoline‐methanol complex in the ground state: A computational approach

Multiple proton transfer (PT) is an essential process in long‐range proton transport such as proton relay in enzymes. 7‐Hydroxyquinoline (7HQ) undergoes alcohol‐mediated PT in both the excited and ground states, and this system has been investigated as a biomimetic model. In the present study, the reaction pathway of triple PT in a 7HQ‐methanol cluster, 7HQ·(MeOH)₂, in the ground state has been investigated using density functional theory calculations to clarify the reaction mechanism and the origin of the experimentally observed kinetic isotope effect (KIE). The PT takes place in an asynchronous concerted fashion, in which the oxygen atom in 7HQ first accepts a proton from the directly hydrogen‐bonded MeOH. The rate constants and primary H/D KIEs have been estimated with canonical variational transition state theory in combination with the small curvature tunneling approximation. The tunneling effect on the PT rate is significant, and the KIE is much greater than 1 at room temperature. The rule of the geometric mean for the KIEs breaks down because of the asynchronicity in the motions of 3 protons and tunneling effect. In addition, the rate constant is smaller, the KIE is larger, and the activation energy is higher compared with the experimental values in heptane solution, suggesting that the PT dynamics in solution is governed by not only the intrinsic PT process but also thermal fluctuation of the solute and solvent molecules, which plays an important role in the configurational change of the 7HQ·(MeOH)₂ complex.