I will discuss recent experimental and simulation data of liquid water and the picture of fluctuations between high-density (HDL) and low-density (LDL) liquid this has led to. The HDL would be a more close-packed form, favored by entropy and dominates at high temperature. Below about 50°C correlated fluctuations into tetrahedral (LDL) structures, favored by hydrogen-bonding (enthalpy), begin to appear and become increasingly important upon further cooling. These are evidenced through specific peaks in the O-O radial distribution function at 4.5, ~8.5 and ~9.7 Å, which become increasingly sharp upon supercooling. In addition, Maréchal has shown that the temperature-dependent IR spectrum of liquid water (0-80 °C) can be decomposed as a linear combination of two temperature-independent, complete (0-4000 cm<sup>-1</sup>) spectra that can be associated with LDL and HDL environments. Here I will show computed IR spectra for two water models, chains and fused dodecahedra, which for the OH-stretch region remarkably well reproduces the extracted experimental HDL and LDL spectrum, respectively. I will show results from MD simulations of the dodecahedral model both in gas phase and as a seed in simulations of liquid water and discuss implications of such hypothetical structures for our understanding of water properties.

Proposal for the structure of high- and low-density fluctuations in liquid water

2019 Fall National Meeting

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