Mechanism of fast proton transfer in ice:Potential enertgy surface and reaction coordinate analyses

摘要

The mechanism of proton transfer in ice is invetigated theoretically by examining the potential energy surfaces and determining the reaction coordinates. It is found to be quite different from that in liquid water. As shown by many authors, proton transfer in liquid water is promoted by the structure fluctuation, creating three-coordinated water molecules in the hydrogen bond network rearrangement, and the excess proton makes transtions among these three-coordinated water molecules as forming a so-called Zundel structure,(H_5O_2)~+. This kind of large structuralrearrangement cannot take place in ice. Nevertheless, the proton transfer in ice can be very fast. It is found that the srtrong constraint on the molecular beometry in ie is the source of the facile proton transfer. This constraint reduces tha stabilization of the excess proton state in two ways:(1) as O-O cannot shrink freely, it canot form a stable Zundel structure in which two water molecules share the excess proton lo ating at the center of the shortened O-O bond, and (2) as the existence of the repulsive force, an Eigen structure cannot be cmuch stabilized. This repulsive force slso contributes to partially shorten the O-O distance and thus facilitating a proton transfer. As the result, the excess proton is not trapped in a deep energy minimum but makes the transfers on small energy barriers. The molecular geometry rela ation along the proton transfer is analyzed; it is found that O-O stretchings/shrinkages at the excess proton moiety are mutually coupled to assist the sequential proton transfers in a concerted fashionl. The energetics and geometrical changes along these reaction coordinates are analyzed .The potential energies are found to be fairly flat for different locations of the excess proton. The nature of the excess proton solvation from the surrounding water molecules are analyzed; it is shown that the solveation by even distant shells yields a significant contribution to the potential energy surface of the proton transfer