Isomer-Specific Spectroscopy of Benzene–(H<sub>2</sub>O)<sub><i>n</i></sub>, <i>n</i> = 6,7: Benzene’s Role in Reshaping Water’s Three-Dimensional Networks

Abstract

The water hexamer and heptamer are the smallest sized water clusters that support three-dimensional hydrogen-bonded networks, with several competing structures that could be altered by interactions with a solute. Using infrared–ultraviolet double resonance spectroscopy, we record isomer-specific OH stretch infrared spectra of gas-phase benzene-(H<sub>2</sub>O)<sub>6,7</sub> clusters that demonstrate benzene’s surprising role in reshaping (H<sub>2</sub>O)<sub>6,7</sub>. The single observed isomer of benzene-(H<sub>2</sub>O)<sub>6</sub> incorporates an inverted book structure rather than the cage or prism. The main conformer of benzene-(H<sub>2</sub>O)<sub>7</sub> is an inserted-cubic structure in which benzene replaces one water molecule in the <i>S</i><sub>4</sub>-symmetry cube of the water octamer, inserting itself into the water cluster by engaging as a π H-bond acceptor with one water and via CH···O donor interactions with two others. The corresponding <i>D</i><sub>2d</sub>-symmetry inserted-cube structure is not observed, consistent with the calculated energetic preference for the <i>S</i><sub>4</sub> over the <i>D</i><sub>2d</sub> inserted cube. A reduced-dimension model that incorporates stretch–bend Fermi resonance accounts for the spectra in detail and sheds light on the hydrogen-bonding networks themselves and on the perturbations imposed on them by benzene

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