JPC Abstract

Abstract

The structure of linear water wires with an excess proton was studied at room temperature using ab initio path integral molecular dynamics. The ab initio Car-Parrinello (CP) methodology employed the density functional theory (DFT) description of the electronic structure, and the Feynman path integral approach allowed for quantization of the nuclear degrees of freedom. Thus, the influence of proton tunneling and zero point nuclear vibrations were automatically included. Four or five water molecules were linearly arranged, with an excess proton (H^*), to form tetramer and pentamer complexes, respectively. In classical studies of the tetramer complex, the excess proton H^*, centered within the wire, formed H₃ ⁺ and H₅O₂⁺ ions with the two inner water molecules. In the pentamer complex, the H^* was found attached to the inner water molecule, forming a stable H₃O ⁺ ion with the two covalent, hyperextended bonds that were hydrogen bonded to neighboring water molecules on both opposite sides. Although the addition of nuclear quantization via path integrals broadened the calculated distribution functions for both complexes, the overall features were unaltered, which suggests that nuclear quantum effects are minimal in these small, linear clusters. However, instantaneous path integral configurations revealed the formation of an extended H₇O₃⁺ complex predominantly in the pentamer wire where the excess proton H^* was delocalized over three adjacent water molecules simultaneously. Since the computational demands of CP make long simulations cost prohibitive, angular distribution functions, requiring much longer simulation times, were obtained using an MP2-based empirical valence bond (EVB) model [Sagnella, D.E. and Tuckerman, M.E. J. Chem. Phys. 1998, 108, 2073]. Additional classical CP calculations, where the water wire ends were solvated with additional capping waters, were also performed. In these studies, the proton was observed to be much more mobile; proton transfer occurred along the full water wire and occasionally into the water solvation caps.