Network topology of deeply supercooled water

Abstract

Empirical potential structure refinements have been made to recent high-energy x-ray diffraction data, providing molecular models of deeply supercooled water. The average O-O coordination number is found to drop from 5.13 at 293 K to 4.85 at 244 K, within 3.5 Å. Triplet O-O-O bond angle distributions reveal a broad peak centred at 96.4° at 293 K which shifts to 100.0° at 244 K, indicative of the local geometry becoming increasingly tetrahedral with decreasing temperature. However, although the number of non-bonded interstitial molecules between the first and second shells is depleted upon cooling, the number of interstitial molecules forming triplets that are embedded within the hydrogen bonded tetrahedral network at θ_OOO = 53°, remains constant. This is consistent with previous observations of an invariant O-O coordination number with temperature (4.24 out to 3.3 Å) and corresponds to non-bonded molecules positioned at close to half the ideal tetrahedral angle. Both -O-O-O- and hydrogen-bonded -O-H-O- ring length distributions show increases in 6 and 7-membered rings upon supercooling. This is concomitant with a shift and increase in intensity of peaks at r₄ ∼8.7 Å and r₅ ∼10.8 Å in the oxygen-oxygen pair distribution function, which in the models correspond to correlations between adjacent and next-nearest-neighbour hydrogen-bonded rings.

GRAPHICAL ABSTRACT

KEYWORDS:

• Water
• supercooled
• liquid
• modelling
• pair distribution function

ORCID

O. L. G. Alderman http://orcid.org/0000-0002-2342-811X

C. J. Benmore http://orcid.org/0000-0001-7007-7749

Additional information

Funding

This work was supported by the Advanced Photon Source, Argonne National Laboratory, which is funded under US Department of Energy, BES, [contract number DE-AC02-06CH11357]. CS gratefully acknowledges financial support from the China Scholarship Council [CSC No. 201706890053].