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Abstract
Thinking about water is inextricably linked to hydrogen bonds, which are highly directional in character and determine the unique structure of water, in particular its tetrahedral H‐bond network. Here, we assess if this common connotation also holds for supercritical water. We employ extensive ab initio molecular dynamics simulations to systematically monitor the evolution of the H‐bond network mode of water from room temperature, where it is the hallmark of its fluctuating three‐dimensional network structure, to supercritical conditions. Our simulations reveal that the oscillation period required for H‐bond vibrations to occur exceeds the lifetime of H‐bonds in supercritical water by far. Instead, the corresponding low‐frequency intermolecular vibrations of water pairs as seen in supercritical water are found to be well represented by isotropic van‐der‐Waals interactions only. Based on these findings, we conclude that water in its supercritical phase is not a H‐bonded fluid.
Highlights
Hydrogen-bonding and the resulting three-dimensional network topology certainly is the hallmark of water[1,2,3] and provides much of the mechanistic underpinnings of its many so-called anomalies.[4,5] thinking about water implies thinking about H-bonding
We unveil that the H-bond lifetime in supercritical water is on average shorter than a single oscillation period of an intermolecular vibration between two adjacent water molecules
Our ab initio simulation results are shown to nicely agree with long existing experimental data in the supercritical phase of water, such as reorientational relaxation times obtained from nuclear magnetic resonance (NMR) relaxometry or orientationally and time-averaged radial distribution functions from XRD or ND experiments
Summary
Hydrogen-bonding and the resulting three-dimensional network topology certainly is the hallmark of water[1,2,3] and provides much of the mechanistic underpinnings of its many so-called anomalies.[4,5] thinking about water implies thinking about H-bonding. It could be shown that this pronounced resonance is sensitive to local perturbations of the H-bond network induced by for example, simple ions[46,47,48,49] or small molecules.[50] Recently, the network mode has been shown to respond very sensitively to increasing hydrostatic pressure.[51] At supercritical conditions, preliminary FFMD simulations[18] yielded qualitatively different THz spectra compared to RTW, without being able to disclose the underlying molecular mechanism due to methodological shortcomings of the simulation method Based on all this evidence accumulated in recent years only, it is suggestive that the H-bond THz mode should provide a most sensitive probe to monitor H-bonding in the supercritical state of water. In this Research Article, we go back to square one and ask, in a fresh effort, the question if supercritical water is a Hbonded fluid by using advanced simulation and spectral analyses techniques
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