Computer simulation as a tool to analyze neutron scattering experiments: Water at supercritical temperatures

Löffler, Gerald; Schreiber, Hellfried; Steinhauser, Othmar

doi:10.1002/bbpc.19940981211

Cited by 60 publications

(41 citation statements)

References 8 publications

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“…These data are combined with a method of analyzing the neutron experiment which serves to circumvent some of the systematic artifacts which have been postulated. 7,11,12 A full account of this work is being published elsewhere; 13 here we highlight the principle conclusion, that is density dependent effects on the hydrogen bonding in water are certainly discernible in the neutron experiment. It is worth pointing out that although the structure of superheated water has been investigated in several previous diffraction experiments, both x ray 14,15 and neutron, [8][9][10] the present data represent an indepth study of the site-site radial distribution functions for water for a range of densities at fixed temperature.…”

Section: ͓S0163-1829͑96͒08541-4͔mentioning

confidence: 71%

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

1996

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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...

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Section: ͓S0163-1829͑96͒08541-4͔mentioning

confidence: 71%

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

1996

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“…He showed, using the TIP4P model, that the hydrogen bonding persists in super-and subcritical water over a wide density range 0.1-1 g͞cm 3 . This observation is supported by most of the later simulation studies [4][5][6][7][8][9][10] with an exception of an ab initio molecular dynamics simulation by Fois et al [11] A markedly different conclusion was obtained, however, from neutron scattering studies by Postorino et al [12][13][14]. They showed with their O-H pair correlation function that at a supercritical temperature 400 ± C, the hydrogen bonding does not persist apparently even at a liquidlike density (0.66 g͞cm 3 ).…”

mentioning

confidence: 89%

“…They showed with their O-H pair correlation function that at a supercritical temperature 400 ± C, the hydrogen bonding does not persist apparently even at a liquidlike density (0.66 g͞cm 3 ). Although there seems no established conclusion concerning the hydrogen bonding between a neighboring pair of water molecules in a supercritical state, it is concluded from all of the computer simulations [3][4][5][6][7][8][9][10][11], the neutron scattering studies [12][13][14], and the x-ray diffractometry [15] that the hydrogen bond network involving more than two water molecules is destroyed at high temperatures. Thus, it is desirable to study supercritical water with an experimental method which sensitively probes the short-range order of water.…”

Section: Nmr Study Of Water Structure In Super-and Subcritical Conditmentioning

confidence: 99%

NMR Study of Water Structure in Super- and Subcritical Conditions

1997

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The proton chemical shift of water is measured at temperatures up to 400 ± C and densities of 0.19, 0.41, 0.49, and 0.60 g͞cm 3 . The magnetic susceptibility correction is made in order to express the chemical shift relative to an isolated water molecule in dilute gas. Comparison of the chemical shifts of water in neat fluids at high temperatures to those in organic solvents at ambient conditions shows that the hydrogen bonding persists in supercritical water. At each density, the strength of the hydrogen bonding is found to reach a plateau value at high temperatures. [S0031-9007(97)02757-9] PACS numbers: 61.25. Em, 76.60.Cq, 82.30.Nr Water, which is an unusual solvent in ambient conditions, has recently been revealed to be a unique medium for chemical processes in super-and subcritical conditions [1]. In these extreme conditions, water loses its characteristics in ambient conditions and the solvation properties change drastically [1,2]. In order to understand and control the solvation properties on the molecular level, it is indispensable to characterize the microscopic structure of water in super-and subcritical conditions.Typically, the microscopic structure of water is described in terms of the correlation functions between the intermolecular pairs of atoms O-H, O-O, and H-H. In particular, the O-H pair correlation function provides information on the hydrogen bonding between a pair of water molecules. An important observation on the hydrogen bonding of water in super-and subcritical conditions was presented by Mountain [3] in his computer simulation studies. He showed, using the TIP4P model, that the hydrogen bonding persists in super-and subcritical water over a wide density range 0.1-1 g͞cm 3 . This observation is supported by most of the later simulation studies [4][5][6][7][8][9][10] with an exception of an ab initio molecular dynamics simulation by Fois et al. [11] A markedly different conclusion was obtained, however, from neutron scattering studies by Postorino et al. [12][13][14]. They showed with their O-H pair correlation function that at a supercritical temperature 400 ± C, the hydrogen bonding does not persist apparently even at a liquidlike density (0.66 g͞cm 3 ). Although there seems no established conclusion concerning the hydrogen bonding between a neighboring pair of water molecules in a supercritical state, it is concluded from all of the computer simulations [3][4][5][6][7][8][9][10][11], the neutron scattering studies [12][13][14], and the x-ray diffractometry [15] that the hydrogen bond network involving more than two water molecules is destroyed at high temperatures. Thus, it is desirable to study supercritical water with an experimental method which sensitively probes the short-range order of water.In this work, we study water in super-and subcritical conditions using proton NMR spectroscopy. The proton chemical shift is known to be sensitive to the hydrogen bonding of the observed proton with its environment, and we measure the proton chemical shifts of water up to a supercritical ...

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“…These studies have followed two different ways. 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.…”

Section: Introductionmentioning

confidence: 99%

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

Jedlovszky

Richardi

1999

The Journal of Chemical Physics

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A molecular Ornstein-Zernike study of popular models for water and methanol Analysis of the hydrogen-bonded structure of water from ambient to supercritical conditions Gibbs ensemble Monte Carlo simulation of the properties of water with a fluctuating charges model Structural, thermodynamic, and dielectric properties of three polarizable and two nonpolarizable water models are compared with experimental data at four different thermodynamic states from ambient to supercritical conditions. Pair-correlation functions and thermodynamic data are obtained from Monte Carlo simulations, performed both on the (N,V,T) and ͑N,p,T͒ ensembles. The dielectric constants are determined with the molecular Ornstein-Zernike theory. It is found that the polarizable models can reproduce the experimental structure considerably better than the nonpolarizable ones at the high-temperature states. In particular, the elongation of the hydrogen bonds with increasing temperature, which is observed by neutron diffraction measurements, in only seen in the simulations with the polarizable potential models. On the other hand, the polarizable models fail to describe the correct temperature dependence of the thermodynamic properties. Although at ambient conditions they overestimate both the density and the dielectric constant of the system, around the critical temperature they result in 10%-50% lower densities than the experimental values. The obtained magnitude of the internal energies as well as the dielectric constants are also considerably smaller than their experimental values at these thermodynamic state points. The results of this study point out the need of new polarizable water models which, besides the reasonable reproduction of the experimental pair-correlation functions, are also able to describe the dependence of the thermodynamic properties on the temperature and pressure.

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Computer simulation as a tool to analyze neutron scattering experiments: Water at supercritical temperatures

Cited by 60 publications

References 8 publications

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

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

NMR Study of Water Structure in Super- and Subcritical Conditions

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

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