Resonant ion-dip infrared spectroscopy of the S4 and D2d water octamers in benzene-(water)8 and benzene2-(water)8

Abstract

The techniques of resonant two-photon ionization (R2PI), UV–UV (ultraviolet) 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-bonding topologies of two isomers each of the benzene-(water)8 and (benzene)2(water)8 gas-phase clusters. The BW8 isomers (B=benzene, W=water) have R2PI spectra which are nearly identical to one another, but shifted by about 5 cm−1 from one another. This difference is sufficient to enable interference-free RIDIR spectra to be recorded. As with smaller BWn clusters, the BW8 clusters fragment following photoionization by loss of either one or two water molecules. The OH stretch IR spectra of the two BW8 isomers bear a close resemblance to one another, but differ most noticeably in the double-donor OH stretch transitions near 3550 cm−1. Comparison to DFT calculated minimum energy structures, vibrational frequencies, and infrared intensities leads to an assignment of the H-bonding topology of the BW8 isomers as nominally cubic water octamers of S4 and D2d symmetry surface attached to benzene through a π H-bond. A series of arguments based on the R2PI and hole-burning spectra leads to an assignment of additional features in the R2PI spectra to two isomers of B2W8. The OH stretch RIDIR spectra of these isomers show them to be the corresponding S4 and D2d analogs of B2W8 in which the benzene molecules each form a π H-bond with a different dangling OH group on the W8 sub-cluster.