Infrared spectroscopy of cluster ions: High energy conformer trapping and multiple photon absorption
Infrared predissociation (IRPD) spectra of several cluster ions are presented, resulting in the emergence of two major themes. Trapped, high energy conformers are observed in argon tagged clusters. In non-argon tagged hydrated alkali metal ion clusters, multiple photon absorption processes can occur. The cluster ions discussed in this dissertation are generated by impacting neutral clusters with ions. If there is an energetic barrier between the neutral cluster configuration and the global minimum cluster ion configuration that is larger than the binding energy of the most labile ligand, then high energy conformers can be trapped. Thus, the trapped conformers retain some of the bonds of their neutral precursors. For example, neutral methanol dimer contains a hydrogen bond and a trapped, high energy Cl (CH3OH)2Ar conformer was observed which also contains a methanol•••methanol hydrogen bond. Similar results are reported for M+(C6H6)2-4(H2O)2Ar and M+(benzyl alcohol)1(H2O)1-2Ar where IRPD spectra reveal hydrogen bonds resulting from the configurations of the neutral precursors. For most of the clusters studied, spectral features from high energy conformers and global minimum energy conformers were observed in the same spectra. However, a special case is reported in chapter 5 in which spectral features from high energy conformers and global minimum energy conformers of Li+(C6H6)2-4(H2O)2Ar were observed in different spectra corresponding to the fragmentation channel monitored. This conformer specific fragmentation greatly simplified the interpretation of the spectra and provided useful information on the cluster energetics. Conformer specific fragmentation was also observed for M+(H2O)5-7, but for a different reason. In the hydrogen bonded O-H stretching region, IRPD spectra obtained by monitoring the loss of two waters only contained features from linear hydrogen bonds. This mode dependent fragmentation is believed to be the result of multiple photon absorption. This conclusion is supported by laser-fluence dependence studies and by evaporative ensemble calculations.
Infrared spectroscopy, Cluster ions, multiple photon absorption, chloride, benzyl alcohol, benzene, methanol, gas phase, conformer trapping
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign