Assessing the properties of supercritical water in terms of structural dynamics and electronic polarization effects

Abstract

Evolution of water's structural dynamics from ambient liquid to supercritical dense liquid-like and dilute gas-like conditions.

Supercritical Water is not Hydrogen Bonded

“…Overall, numerous computational studies which use vastly different H‐bond criteria, sampling protocols and water models predict H‐bond lifetimes of about 100 fs in SCW . Moreover, our computed reorientational relaxation times, which do not depend on any H‐bond definition, excellently agree with available experiments and yield values somewhat smaller than 100 fs.…”

“…Turning now to the data in Figure (a), both time scales, τ HB and τ 2R , are found to dramatically decrease with respect to RTW when the temperature is increased. This qualitative behavior is not unexpected since the H‐bond lifetime follows an Arrhenius‐type behavior as indeed explicitly confirmed by us for RPBE‐D3 water . Given that τ HB is sensitive to the H‐bond criterion we used our RTW and SCW criteria throughout as explained in the SI.…”

“…These dynamical properties complement the analyses of intermolecular H‐bond vibrations and have been proven valuable to investigate SCW by FFMD and AIMD simulations, see for example, refs. . Taking into account the well‐known ambiguities in the selected H‐bond criterion, existing studies nevertheless broadly agree that the continuous H‐bond lifetime τ HB is more than one order of magnitude smaller compared to RTW and amounts to about 100 fs in SCW including long‐time tails.…”

“…Moreover, the extent of H‐bonding, as quantified by the number of H‐bonds per water molecule, systematically and significantly decreases as a function of increasing temperature . Last but not least, the famous cooperative H‐bonding effect, which is responsible for the unusually large molecular dipole moment of H 2 O molecules in RTW, is drastically reduced in SCW . These experimental and computational findings indicate that H‐bonds are present in SCW, but are strongly weakened, and that the properties of the H‐bond network are significantly modified compared to RTW.…”

“…In this vein, advanced experimental and computational studies tacitly assume the existence of H‐bonds when analyzing and interpreting the data and, thus, conclude that “H‐bonding is drastically reduced”, see for example, refs. .…”