Resonant ion-dip infrared spectroscopy of benzene-(water)n-(methanol)m clusters with n+m=4,5

Resonant two-photon ionization and resonant ion-dip infrared (RIDIR) spectra of benzene-(water)(n)-(methanol)(m) clusters thereafter shortened to BWnMm) have been recorded for a total of seven clusters with n + m = 4 and 5. The infrared spectra in the OH and CH stretch regions show absorptions characteristic of H-bonded WnMm clusters which are bound to benzene by a pi H-bond involving a dangling OH on the WnMm sub-unit. Density functional theory (DFT) calculations identify a number of conformational isomers in the n + m = 4 series which meet the general criteria imposed by the experimental spectra. The structures, binding energies, harmonic vibrational frequencies, and infrared intensities for these isomers have been calculated for comparison with experiment. Based on the calculations, tentative assignments of several of the observed species are given. The calculations uncover the fact that complexation of benzene to the cyclic water tetramer imposes much the same perturbations on the cycle as substitution of methanol for water. In particular, the single-donor OH stretch spectra of WnMm and BWn+1Mm-1 are calculated to be virtually identical to one another. The comparison of experiment and theory for this series of cyclic structures is used to assess the strengths and limitations of the calculations at the DFT Becke3LYP/6-31 + G * level of theory. (C) 1998 Published by Elsevier Science B.V. All rights reserved.