Are pentameric rings dominant transient structures in liquid water? In her Journal of Molecular Structure article “From the trimer, through the pentamer, to liquid water,” Clinical Professor (and alumna) Margaret Mandziuk provides arguments strengthening this hypothesis. Her model, developed initially for the water trimer, and now applied to liquid water, accounts surprisingly well for the 2-D vibrational spectra of the liquid, observed by the Tokmakoff group. The model assumes a quantum mechanical, wave-like nature for protons delocalized between oxygen neighbors in a hydrogen bond. Mandziuk also demonstrates that the thermally excited states of a pentamer, or its isomers, contribute to the changing properties of the liquid at higher temperatures.
Abstract: High resolution spectra, obtained for small water clusters in the Vibrational-Rotational-Tunneling spectroscopy experiments in Saykally's group, provide invaluable data that should guide us in untangling the quite elusive properties of liquid water [PNAS 98 (2001) 10533]. Recently, we suggested that the bifurcation splittings in cyclic water trimers originate from the tunneling of hydrogen bonded protons in a mean field, double well potential between two oxygen neighbors [Chem.Phys.Lett. 661 (2016) 263]. Based on the similar/corresponding pattern of splittings in cyclic water pentamers, the model developed for the trimer should be also applicable to pentamers. Assuming that pentamers are major transient components in the fluctuating hydrogen bonded network of water, the model is applied to liquid water. In this paper, we show that our 1-D model gives surprisingly good agreement with experimental 2-D spectra in the OH stretch region of liquid water, reported by Tokmakoff's group [JCP 145(2016) 094501]. Our results strengthen the hypothesis that cyclic pentamers should be considered as transiently present in the liquid, and that their fused structures form low density liquid. We also postulate that the higher energy isomers of the pentamer contribute to high density liquid water.