Protic ionic liquids (PILs) have attracted significant attention due to their promising properties and potential use in various applications. Recent studies revealed exceptionally strong decoupling of proton conductivity from structure relaxation in a mixture of lidocaine with phosphoric acid.¹ Proton conductivity in this mixture even exceeds that of phosphoric acid at the same viscosity. Both lidocaine and phosphoric acid have proton donor and acceptor sites that facilitates the formation of a well-connected hydrogen bond network contributing to their high proton diffusivity. Thus, it is necessary to study the effects of the molecular geometry of both the acid and base, the number of the proton donating and accepting sites on the acid and base, and the solvation on the proton dissociation and transfer.

In this study, fully optimized structures of pairs and clusters of the phosphoric acids (PAs) with different bases (lidocaine, imidazole, creatinine, and trimethylammonium) were determined through ab initio electronic structure calculations at the B3LYP/6-311G^** level of theory in both the gas phase and the SMD model. The molecular binding energies of PA and the base molecules were calculated from both the uncorrected and zero point energies (ZPEs) corrected total electronic energies based on the same level of theory. Subsequently, potential energy surface (PES) scans for the transfer of a proton for pairs and clusters of the acids with various bases were performed at the same level of theory to study the effects of different bases and the number of PAs on the dissociation of PA and the proton transfer energetics. The results reveal that dissociated protons are generally more favorable in the SMD model since the continuum solvation model stabilizes the charge separation.

Reference:

1. Z. Wojnarowska, Y. Wang, K. J. Paluch, A. P. Sokolov, M. Paluch, Observation of highly decoupled conductivity in protic ionic conductors. Phys.Chem.Chem.Phys 2014, 16, 9123-9127.