Quantum properties of the excess proton in liquid water

The classical and quantum equilibrium properties of an excess proton in bul k phase water are examined computationally with a special emphasis on the i nfluence of an explicit quantum dynamical treatment of the nuclei on the ca lculated observables. The potential model used, our recently developed mult istate empirical valence bond (MS-EVB) approach is described. The MS-EVE mo del takes into account the interaction of an exchange charge distribution o f the charge-transfer complex with the polar solvent, which qualitatively c hanges the nature of the solvated complex. The impact and importance of the exchange term on the stability of the solvated H5O2+ (Zundel) cation relat ive to the H9O4+ (Eigen) cation in the liquid phase is demonstrated. Classi cal and quantum path-integral molecular dynamics (PIMD) simulations of an e xcess proton in bulk phase water reveal that quantization of the nuclear de grees of freedom results in an increased stabilization of the solvated Zund el cation relative to the Eigen cation: and that species intermediate betwe en the two are also probable. Quantum effects lead to a significant broaden ing of the probability distributions used to characterize the two species, and a definite differentiation and sharp characterization of the species co nnected to the excess proton in liquid water is found to be difficult.