Resonant Ion-Dip Infrared Spectroscopy of Ternary Benzene−(Water)n(Methanol)m Hydrogen-Bonded Clusters

Resonant two-photon ionization, IR−UV hole-burning, and resonant ion-dip infrared (RIDIR) spectroscopies have been employed along with density functional theory (DFT) calculations to assign and characterize the hydrogen-bonded topologies and structures of eight benzene−(H 2 O) n (CH 3 OH) m, cluster isomers (hereafter shortened to BW n M m ) with n + m ≤ 4. The O−H stretch infrared fundamentals are used to determine the H-bonding topology of the clusters. However, in several cases, the O−H stretch spectrum leaves an ambiguity regarding the position of the methanols within the structure. In these cases, the methyl CH stretch region serves as a secondary probe capable of distinguishing among the various possibilities. For n + m = 2, a single BWM isomer is observed with OH stretch fundamentals at 3508, 3606, and 3718 cm -1 . A comparison of the methyl CH stretch transitions of BWM and BM 2 reveals that the methanol in BWM accepts a H-bond from water and forms a π H-bond with benzene. The n + m = 3 results show the subtle effects that solvent composition can have on the lowest-energy structure of the cluster. Two isomers of BW 2 M are observed but just one of BWM 2 . Both water-rich BW 2 M isomers possess transitions characteristic of cyclic W 2 M subclusters in which water is π H-bonded to benzene. Each isomer shows a set of three single-donor OH stretch transitions, a π H-bonded OH stretch near 3650 cm -1 and a free OH stretch (∼3716 cm -1 ). The two BW 2 M isomers differ in the position of the methanol in the H-bonded cycles. Conversely, the methanol-rich BWM 2 cluster OH stretch transitions are those of a WM 2 chain, with a signature π H-bonded OH stretch located at about 3590 cm -1 . The methyl CH stretch absorptions are used to deduce that the water molecule in BWM 2 is in the donor position in the chain, furthest from benzene's π cloud. Finally, the spectra for the n + m = 4 series show systematic changes in the spectrum with changing methanol content in the cluster. In all cases, both OH stretch and CH stretch transitions point firmly to cyclic W n M m subclusters in which one of the free OH groups on a water molecule in the cycle is used to π H-bond to benzene.