Unpredicted density dependence of hydrogen bonding in water found by neutron diffraction

Abstract

The oxygen-hydrogen pair correlation functions for water are determined at five density states along the Tϭ573 K isotherm, using the hydrogen isotope substitution technique in a neutron-diffraction experiment. The results are sufficiently accurate to reveal significant discrepancies with computer simulations of water, which use the traditional site-site charge interactions to generate hydrogen bonds. They imply that these models of the interatomic potential overemphasize the strength of the hydrogen bonding interaction in water. ͓S0163-1829͑96͒08541-4͔The phenomenon of hydrogen bonding plays a fundamental role in the structure, function and properties of many liquids of chemical and biological importance. The interaction manifests itself as a pronounced, directional correlation between specific sites on neighboring molecules. It also produces a highly characteristic vibrational density of states in hydrogen bonded molecules, and dramatically affects the thermodynamic properties of a fluid compared to what they would be if the molecules interacted simply by van der Waals interactions. 1 Arguments have persisted over many decades over the true cause of the hydrogen bond interaction. Conventional analyses, particularly with regard to understanding the solid forms of water, 2 treat the hydrogen bond as if it were a weakly covalent effect associated with the so-called ''lone pair'' electrons on a hydrogen-accepting site: hence the use of the term ''bond.'' Moreover, simple ''sticky hard sphere'' ͑that is short-range͒ models of the hydrogen bond are able to reproduce the essential features of the atom-atom correlation functions of water under ambient conditions and successfully describe the solvation of simple ions in solution. 3 Yet computer simulations of liquid water and other hydrogen bonded liquids reproduce both thermodynamic and structural properties with some success 4 using an empirical intermolecular potential energy function which consists of long-range Coulomb's law interactions between a few strategically placed charges on the molecules, plus weak attractive dispersion and short-range repulsive forces. 5 The hydrogen bond interaction appears as the strong attractive interaction between a positively charged site ͑usually a hydrogen atom͒ on one molecule and a negatively charged site, usually close to an oxygen atom, on another. This kind of potential model predicts that hydrogen bonding in water is retained even at temperatures and pressures well above the critical point, 6,7 a feature which is not unexpected, given that the intermolecular interactions are being modelled by static electrical charges.However, recent neutron-diffraction experiments on superheated water 8,9 have cast doubt on these charge models for the water potential. They suggest that as the critical temperature of water is approached the hydrogen bond interaction, as manifested by the height and position of the first peak near 1.9 Å in the OH site-site distribution function, diminishes more rapidly than predicted by computer simulation wit...

Computer simulation of structural properties of dilute aqueous solutions of argon at supercritical conditions

“…16,17,18 Due to the interest in supercritical water several polarizable potentials have been recently developed with the aim of reproducing the properties of water around and above the critical point. 19,20,21 According to a recent detailed analysis 22 performed by comparing the simulation with the more recent neutron diffraction data 23,24,25 the polarizable models give pair correlation functions which agree with the experimental data in the supercritical region much better than the nonpolarizable models. On the other hand the nonpolarizable models reproduce better the temperature dependence of some thermodynamical quantities 22 and the SPC/E potential for instance predicts a critical point for water in agreement with the experiments 26 , while some polarizable models do not reproduce well the coexistence curve.…”

Comparison of different water models from ambient to supercritical conditions: A Monte Carlo simulation and molecular Ornstein–Zernike study

“…Experimental evidences show that the height of this peak decreases and its position shifts towards larger distances with increasing temperature and decreasing density. 18,19 These facts indicate the decrease of the number of hydrogen bonds and the increase of the bond length. However, the existence of such an elongation of the hydrogen bonds can only be reproduced by polarizable models.…”

“…The critical point of the simple point charge ͑SPC͒ water has also been determined, 1 its critical temperature and density have been found to be about 60 K and 0.05 g/cm 3 lower than the experimental value. Water around the critical point has also been the subject of an ab initio molecular-dynamics simulation 9 as well as several experimental studies, such as infrared ͑IR͒ 10 and nuclear magnetic resonance ͑NMR͒ 11,12 spectroscopic as well as x-ray, 13,14 and neutron diffraction [15][16][17][18][19][20][21][22] measurements.…”

“…Firstly, after some criticism of the quality of the neutron diffraction data 23,24 the NDIS experiments have been repeated. 18,19 In this measurement both the experimental setup and the data reduction process have considerably been improved. The obtained partial pair-correlation functions have also been reproduced by reverse Monte Carlo modeling, and thus configurations fully consistent with the results of the new NDIS experiment have been generated and analyzed in detail.…”

Molecular Simulations of Liquid and Supercritical Water: Thermodynamics, Structure, and Hydrogen Bonding

“…Direct experimental investigations of the water structure at high temperatures and pressures represent a very challenging undertaking, and any new set of structural or spectroscopic information obtained under such conditions is extremely valuable. Recent introduction into this field of the powerful technique known as neutron diffraction with isotope substitution (NDIS) (Postorino et al 1993;Bruni et al 1996;Soper et al 1997), signified a very important step forward, since this method allows one to experimentally probe all three atom-atom structural correlations in water (OO, OH, and HH) simultaneously. However, it was quite surprising when the very first results of such neutron diffraction measurements were interpreted as the direct evidence of the complete absence of H-bonds in water at near-critical temperatures (Postorino et al 1993).…”