Deconstructing water’s diffuse OH stretching vibrational spectrum with cold clusters

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Science  19 Apr 2019:
Vol. 364, Issue 6437, pp. 275-278
DOI: 10.1126/science.aaw4086

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Wet surface sightings in clusters

In principle, the surface structure of water (H2O) should be discernable from the O–H vibrations. In practice, however, so many configurations rapidly interconvert that the bands are bewilderingly broad. Yang et al. studied a cluster of 20 H2O surrounding a cesium ion, using isotopomers that vary in the position of one H2O amid 19 heavy water (D2O) molecules. Precisely assigned spectral features from contributing configurations mapped well onto a bulk surface spectrum.

Science, this issue p. 275


The diffuse vibrational envelope displayed by water precludes direct observation of how different hydrogen-bond topologies dictate the spectral response of individual hydroxy group (OH) oscillators. Using cold, isotopically labeled cluster ions, we report the spectral signatures of a single, intact water (H2O) molecule embedded at various sites in the clathrate-like cage structure adopted by the Cs+·(D2O)20 ion. These patterns reveal the site-dependent correlation between the frequencies of the two OH groups on the same water molecule and establish that the bound OH companion of the free OH group exclusively accounts for bands in the lower-energy region of the spectrum. The observed multiplet structures reveal the homogeneous linewidths of the fundamentals and quantify the anharmonic contributions arising from coupling to both the intramolecular bending and intermolecular soft modes.

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