RESONANT ION-DIP INFRARED-SPECTROSCOPY OF THE S-4 AND D-2D WAFER OCTAMERS IN BENZENE-(WATER)(8) AND BENZENE(2)-(WATER)(8)

The techniques of resonant two-photon ionization (R2PI), W-UV (ultravi olet) hole-burning, and resonant ion-dip infrared (RIDIR) spectroscopi es have been employed along with density functional theory (DFT) calcu lations to assign and characterize the hydrogen-bonding topologies of two isomers each of the benzene-(water)(8) and (benzene)(2)(water)8 ga s-phase clusters. The BW8 isomers (B = benzene, W = water) have R2PI s pectra which are nearly identical to one another, but shifted by about 5 cm(-1) from one another. This difference is sufficient to enable in terference-free RIDIR spectra to be recorded. As with smaller BWn clus ters, the BW8 clusters fragment following photoionization by loss of e ither 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 S-4 and D-2d symmetry surface attached to benzene through a pi H-bo nd, A series of arguments based on the R2PI and hole-burning spectra l eads to an assignment of additional features in the R2PI spectra to tw o isomers of B2W8. The OH stretch RIDIR spectra of these isomers show them to be the corresponding S-4 and D-2d analogs of B2W8 in which the benzene molecules each form a pi H-bond with a different dangling OH group on the W-8 sub-cluster. (C) 1998 American Institute of Physics. [S0021-9606(98)00140-8].