(123d) Hydrogen Bonding in Ultra-High Pressure Water: From Open HB Networks to Closed-Packing Coordination and Their Link to Induced Charge Asymmetry

Water is usually described as a fluid having a tetrahedral liquid structure based on the number of nearest neighbors in its first coordination shell given by the volume integral over the oxygen-oxygen pair radial distribution function, gOO(r). The tetrahedral distribution of water molecules is the direct manifestation of an underlying hydrogen bonding network, characterized at ambient conditions by the first two peaks of the gOO(r) at ~2.8Å and ~4.5Å, respectively, as well as the first peak of the corresponding gOH(r) at ~1.8Å, and obeying the intermolecular angle O-H?O ~ 104.5 degrees. Because water intrinsic polarizability, the approaching pair of molecules experience an additional attractive force beyond that from purely Coulombic interactions, i.e., a multi-body induced polarization resulting from the surrounding environment. This polarization manifests as a net increase of the total dipole moment of the water molecule from that of the original permanent dipole moment of the isolated molecules, as well as an enhancement of the charge asymmetry between the two hydrogen sites. While much of what we know to date about water's HB behavior has been (and still being) gained by studying non-polarizable models, they are basically unable to account for charge asymmetry. In contrast, polarizable water models are inherently charge-asymmetric, i.e., charge asymmetry arising as the direct response to the inhomogeneous nature of the local electric field around the water's interactions sites, a fluctuating quantity whose average behavior cannot be reproduced by pairwise potential models.

The tetrahedral character of the HB network in water is currently analyzed in terms of the so-called ?tetrahedral parameter' qT, a measure of the tendency of the four nearest water-oxygens to a central water-oxygen to arrange in a tetrahedral network (ERRINGTON and DEBENEDETTI, 2001). While a useful framework, a tetrahedral arrangement of water-oxygen does not necessarily guarantee a tetrahedral HB network, i.e., this scenario suggests that in order to gain better understanding on the water's HB behavior, we must also characterize the corresponding angular H?O-H distributions and the evolution of the second-nearest neighbors population including the inconspicuous participation of interstitial (i.e., so-called fifth) neighbors around the central water molecule.

Moreover, one disturbing fact in this context is the lack of an unambiguous HB definition, a condition that complicates the interpretation of experimental rawdata, because the (temporal and spatial) sensitivity of the probe depends on what particular feature of the HB interaction the technique is targeting. In other words, oftentimes we find that the links between the experimental signal and the microscopic picture are unsupported conjectures, and consequently, inducing controversial claims. Under these circumstances, molecular simulation can provide valuable microscopic information about the HB behavior, its connection with particular features of the interaction model, and its correspondence with experimental probes.

Toward that end, in this presentation: (a) we address explicitly the induced charge asymmetry behavior as predicted by our Gaussian Charge Polarizable water model (CHIALVO and CUMMINGS, 1998; PARICAUD et al., 2005), (b) we discuss its significance and implications in understanding the HB structure under the light of the recent controversial interpretation of XAS data (WERNET et al., 2004), and (c) we characterize the dramatic change of water coordination from the open tetrahedral network to the closed-packed twelve-coordination under extreme aqueous environments such as T>500C and P>5GPa.

REFERENCES:

Chialvo, A. A. and Cummings, P. T., 1998. Simple Transferable Intermolecular Potential for the Molecular Simulation of Water over Wide Ranges of State Conditions. Fluid Phase Equilibria 150-151, 73-81.

Errington, J. R. and Debenedetti, P. G., 2001. Relationship between structural order and the anomalies of liquid water. Nature 409, 318-321.

Paricaud, P., Predota, M., Chialvo, A. A., and Cummings, P. T., 2005. From dimer to condensed phases at extreme conditions: Accurate predictions of the properties of water by a Gaussian charge polarizable model. Journal Of Chemical Physics 122.

Wernet, P., Nordlund, D., Bergmann, U., Cavalleri, M., Odelius, M., Ogasawara, H., Naslund, L. A., Hirsch, T. K., Ojamae, L., Glatzel, P., Pettersson, L. G. M., and Nilsson, A., 2004. The structure of the first coordination shell in liquid water. Science 304, 995-999