Explicit Outer Bonding Transformations in Liquid Water. The Key to its Understanding.
SPQR Labs, Texas Tech University
Dept. of Chemistry and Biochemistry, P.O. BOX 41061
Lubbock, Texas, 79409-1061
Conference Dates - November 2 to November 30
Jacob Urquidi, G. Wilse Robinson, Chul Hee Cho, Bo Xiao, Surjit Singh
Currently being developed for the liquid state of water are new molecular-level descriptions and computational methodologies. Unlike previous methodologies, these are designed to be consistent with the presence of rapidly interconverting low-density outer neighbor ice-Ih-like 4.5Å O···O bonding and high-density ice-II-like 3.5 Å bonding in the liquid. Experimental diffraction studies have now conclusively confirmed the presence of these outer structural features as well as the continual transformation from the low-density form with increasing temperature or pressure, without significantly affecting the inner tetrahedral bonding. This picture transparently leads to a molecular-level description of all the anomalies of water and suggests methods for improving computational studies of this substance. For instance, the density maxima near 4.0°C (H2O), 11.2°C (D2O), and 13.4°C (T2O) arise because of thermal transformations from the low-density ice-Ih-like second neighbor structure to the more dense ice-II-like structure in competition with the normal thermal expansion. The minimum in the isothermal compressibility near 47°C (H2O) is caused by the fact that, besides the normally behaving compressibilities of the component structures, the open to dense transformation creates a new contribution to the compressibility, which, at low temperatures, decreases very rapidly with increasing temperature. The decrease in frequency of intermolecular modes with rising temperature or pressure is because the dense structure corresponds to more fragile intermolecular potential surfaces. The decrease in viscosity with increasing pressure, the anomalous heat capacity and the strong non-Arrhenius dynamics result from this same type of effect. Finally, the H-D-T isotope effect on properties such as the density and the viscosity do not posses the expected mass dependencies, mass or square-root-mass, respectively, because of a zero-point induced "thermal lag" (7.2°C for D2O, 9.4°C for T2O) of their temperature-dependent structural properties compared with H2O. Following these leads, the simple point charge (SPC) model for water has been successfully modified to give temperature-dependent radial distribution functions and isochoric temperature differentials that agree well with the results from experimental diffraction measurements.