MULTIPHOTON IONIZATION STUDIES OF C6H6-(CH3OH)N CLUSTERS .1. COMPARISONS WITH C6H6-(H2O)N CLUSTERS

Resonant two-photon ionization (R2PI) scans of the S0-S1 spectra of C6H6-(CH3OH)n clusters with n = 1-5 have been recorded. These scans provide an interesting comparison with earlier spectra from our laboratory on C6H6-(H2O)n clusters. A variety of vibronic level arguments are used to constrain the geometries of the C6H6-(CH3OH)n clusters. The 1:1 and 1:2 clusters possess vibronic level features which are very similar to their aqueous counterparts. The 1:1 cluster places the methanol molecule in a pi-hydrogen-bonded configuration on or near the sixfold axis of benzene. The spectral characteristics of the 1:2 cluster are consistent with both methanol molecules residing on the same side of the benzene ring as a methanol dimer. Higher C6H6-(CH3OH)n clusters show distinct differences from the corresponding C6H6-(H2O)n clusters. Vibronic level arguments lead to the following conclusions: the methanol molecules in the 1:3 cluster show the strongest hydrogen bonding to the pi-cloud of any of the clusters and attach to benzene in such a way as to strongly break the sixfold symmetry of its pi-cloud. The 1:4 clusters are at most only very weakly hydrogen bonded to the pi-cloud, break benzene's sixfold symmetry moderately well, and possess strong activity in a very low frequency intermolecular mode. The methanol molecules in the 1:5 cluster show no hydrogen-bonding interaction with benzene's pi-cloud, induce remarkably little asymmetry in the pi-electron density, and produce very little van der Waals' activity. Monte Carlo simulations using intermolecular potentials developed for liquid simulations serve as a guide to the possible minimum-energy structures for the clusters. The experimental results are used to distinguish between the possible structures. In all cases, the lowest energy structures produced by the calculations satisfactorily fit the vibronic level constraints placed on the structures by our data.