Identification of CH3OH2+ and H3O+-centered cluster isomers from fragment-dependent vibrational predissociation spectra of H+(CH3OH)(4)H2O

Cluster isomers of H+(CH3OH)(4)H2O have been identified by vibrational pred issociation spectroscopy in combination with mass-selected detection of pho tofragments. Ab initio calculations indicate that the cluster ion can exist in either CH3OH2+(CH3OH)(3)H2O or H3O+(CH3OH)(4) isomeric forms. They can dissociate via a methanol loss or water loss channel, depending on the stru cture of the isomers. While water loss is the dominant channel of the disso ciation, the methanol loss channel is readily accessible by the H3O+-center ed cluster isomer. We demonstrate in this study that mass-selected detectio n of photofragments produced by vibrational excitation is an effective way of identifying cluster isomers in the gas phase. (C) 2000 American Institut e of Physics. [S0021-9606(00)01917-6].