Isomer-Specific Spectroscopy of Benzene–(H₂O)_<i>n</i>, <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-(H₂O)_6,7 clusters that demonstrate benzene’s surprising role in reshaping (H₂O)_6,7. The single observed isomer of benzene-(H₂O)₆ incorporates an inverted book structure rather than the cage or prism. The main conformer of benzene-(H₂O)₇ is an inserted-cubic structure in which benzene replaces one water molecule in the <i>S</i>₄-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>_2d-symmetry inserted-cube structure is not observed, consistent with the calculated energetic preference for the <i>S</i>₄ over the <i>D</i>_2d 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