Intramolecular structure and energetics in supercooled water down to 255 K.

Felix Lehmkühler,Thomas Buslaps,Mikko Hakala,Thomas Büning,Yury Forov,Ingo Steinke,Metin Tolan,Christoph J Sahle,Karin Julius,Christian Sternemann

doi:10.1039/c5cp07721d

Abstract

We studied the structure and energetics of supercooled water by means of X-ray Raman and Compton scattering. Under supercooled conditions down to 255 K, the oxygen K-edge measured by X-ray Raman scattering suggests an increase of tetrahedral order similar to the conventional temperature effect observed in non-supercooled water. Compton profile differences indicate contributions beyond the theoretically predicted temperature effect and provide a deeper insight into local structural changes. These contributions suggest a decrease of the electron mean kinetic energy by 3.3 ± 0.7 kJ (mol K)(-1) that cannot be modeled within established water models. Our surprising results emphasize the need for water models that capture in detail the intramolecular structural changes and quantum effects to explain this complex liquid.

Highlights

We study the structure of slightly supercooled water by means of X-ray Raman scattering (XRS) and X-ray Compton scattering
XRS spectra were taken at various temperatures between 293 K and 255 K and analyzed following the scheme discussed in ref
Under supercooled conditions a strengthening of the hydrogen bond network of liquid water is expected. This is typically expressed by shortening of the hydrogen bond length between neighboring water molecules that is accompanied by a stretching and a narrowing of the distribution of the effective intramolecular OH bond length, as discussed e.g. from the neutron scattering and infrared spectroscopy results.[17,19,20]

Summary

Introduction

We study the structure of slightly supercooled water by means of X-ray Raman scattering (XRS) and X-ray Compton scattering. Additional contributions to the experimental spectra are found that cannot be modeled by an expected temperature effect on hydrogen bonds for non-supercooled conditions. This is accompanied by a strong increase of the mean kinetic energy of the electrons. These results demonstrate the need for water models and simulations with special attention on the supercooled state that consider both intramolecular structural changes and quantum effects

Methods

Results

Conclusion