Quantum Properties of the Excess Proton in Liquid Water

Abstract

AbstractThe classical and quantum equilibrium properties of an excess proton in bulk phase water are examined computationally with a special emphasis on the influence of an explicit quantum dynamical treatment of the nuclei on the calculated observables. The potential model used, our recently developed multistate empirical valence bond (MS‐EVB) approach is described. The MS‐EVB model takes into account the interaction of an exchange charge distribution of the charge‐transfer complex with the polar solvent, which qualitatively changes the nature of the solvated complex. The impact and importance of the exchange term on the stability of the solvated H5O2+ (Zundel) cation relative to the H9O4+ (Eigen) cation in the liquid phase is demonstrated. Classical and quantum path‐integral molecular dynamics (PIMD) simulations of an excess proton in bulk phase water reveal that quantization of the nuclear degrees of freedom results in an increased stabilization of the solvated Zundel cation relative to the Eigen cation, and that species intermediate between the two are also probable. Quantum effects lead to a significant broadening of the probability distributions used to characterize the two species, and a definite differentiation and sharp characterization of the species connected to the excess proton in liquid water is found to be difficult.