Isomer-Specific Spectroscopy of Benzene-(H2O)n, n=6,7: Benzene's Role in Reshaping Water's Three-Dimensional Networks

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 S-4-symmetry cube of the water octamer, inserting itself into the water cluster by engaging as a pi H-bond acceptor with one water and via C-H center dot center dot center dot O donor interactions with two others. The corresponding D-2d-symmetry inserted-cube structure is not observed, consistent with the calculated energetic preference for the S-4 over the D-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.