Proton dissociation and transfer in a phosphoric acid doped imidazole system

Abstract

The dynamics and mechanisms of proton exchange in a phosphoric acid (H₃PO₄) doped imidazole (Im) system were studied using a quantum chemical method at the B3LYP/TZVP level and Born–Oppenheimer molecular dynamics (BOMD) simulations. The theoretical studies began with selecting the appropriate presolvation models for proton dissociation and transfer, which were represented by embedded and terminal hydrogen bond (H-bond) structures of H⁺(H₃PO₄)(Im)_n(n = 1–4), respectively. B3LYP/TZVP calculations confirmed that excess proton conditions are required to promote proton exchange, and the intermediate complexes are preferentially formed in a low local-dielectric environment. In contrast, a high local-dielectric environment is required to stabilize the positive charge and prevent the proton from returning to the original Im molecule. The static results also revealed that the embedded structure with n = 2 represents the smallest, most active intermediate complex for proton dissociation, whereas the terminal structure with n = 3 favors proton transfer in the Im H-bond chain. BOMD simulations confirmed the static results and further suggested that the fluctuations of the H-bond chain lengths and the local-dielectric environment must be included in the proton dissociation and transfer mechanisms. Analyses of the time evolutions of torsional angles in the Im H-bond chain showed characteristic solvent structure reorganization, regarded as helical-rotational motion, which drives the proton away from the H₃PO₄ dopant. The current theoretical results showed in detail for the first time the interplay among the “key molecular motions” that fundamentally underlie the dynamics and mechanisms of proton exchange in the H₃PO₄ doped Im H-bond system.