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Minimal Ice
Water clusters comprising fewer than 100 molecules have long been studied in gas phase to model the more complex structures of ice and liquid water. At some stage, as clusters grow larger, they effectively become tiny crystals of ice, but it has been hard to pinpoint precisely where in the range between 100 and 1000 molecules the formal transition takes place. Pradzynski et al. (p. 1529) used vibrational spectroscopy to show that the onset of an icelike structure, indicated by a characteristically distinct absorption band in the infrared, occurs at a cluster size of approximately 275 molecules.
Abstract
The number of water molecules needed to form the smallest ice crystals has proven challenging to pinpoint experimentally. This information would help to better understand the hydrogen-bonding interactions that account for the macroscopic properties of water. Here, we report infrared (IR) spectra of precisely size-selected (H2O)n clusters, with n ranging from 85 to 475; sodium doping and associated IR excitation–modulated photoionization spectroscopy allowed the study of this previously intractable size domain. Spectral features indicating the onset of crystallization are first observed for n = 275 ± 25; for n = 475 ± 25, the well-known band of crystalline ice around 3200 cm−1 dominates the OH-stretching region. The applied method has the potential to push size-resolved IR spectroscopy of neutral clusters more broadly to the 100- to 1000-molecule range, in which many solvents start to manifest condensed phase properties.
