Effect of Quantum Delocalization on Temperature Dependent Double Proton Transfer in Molecular Crystals of Terephthalic Acid

Double proton transfers (DPTs) are important for severalphysicalprocesses, both in molecules and in the condensed phase. While thesehave been widely studied in biological systems, their study in crystallineenvironments is rare. In this work, using path integral moleculardynamics simulations, we have studied temperature dependent DPT inmolecular crystals of terephthalic acid (TPA). In accordance withexperimental reports, we find evidence for a double proton transferinduced order-to-disorder transition that is sensitive to the inclusionof nuclear quantum effects. Our simulations show that in additionto the presence of L and R tautomers of terepthalic acid, there area small but non-negligible concentration of positive and negativelycharged pairs of TPA molecules. At the onset of the transition atlow temperatures, DPT likely occurs through a tunneling mechanismwhile at room temperature, likely involving the dominance of activatedhopping. Through an analysis of the electronic structure of the systemusing Wannier functions, we show that the H atom shuttling betweenthe donor and acceptor O atoms involves a proton.