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Gas phase infrared photodissociation spectroscopy of mass-selected ionic clusters: metal oxides and microhydrated anions


Wende,  Torsten
Molecular Physics, Fritz Haber Institute, Max Planck Society;

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Wende, T. (2012). Gas phase infrared photodissociation spectroscopy of mass-selected ionic clusters: metal oxides and microhydrated anions. PhD Thesis, Freie Universität, Berlin.

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The central goal of the present thesis is the structural characterization of metal oxide clusters in the framework of the Dedicated Research Center “Structure, Dynamics and Reactivity of Transition Metal Oxide Aggregates” (SFB 546), funded by the German Research Foundation. The SFB 546 aimed at developing a detailed understanding of the relationships between structure and reactivity in catalytic systems. A second goal of this thesis is the characterization of the solvation behavior of anions in water clusters at the molecular level, one water molecule at a time. In order to gain insights into the cluster structure, vibrational spectra of mass-selected gas phase cluster ions are measured by way of infrared photodissociation (IR-PD) spectroscopy. The use of an ion trap-tandem mass spectrometer combined with the Free Electron Laser for Infrared eXperiments FELIX (The Netherlands), allows for the measurement of vibrational spectra of cryogenically-cooled ions over a wide spectral range (100-4000 cm-1). The subsequent structural assignment is based on a comparison between experimental and simulated spectra of different isomers derived from quantum chemical calculations. The structures of metal oxide clusters are investigated using the messenger atom technique, where a weakly bound rare gas atom is used as a probe to measure a vibrational spectrum of the cluster ion. Although rare gas atoms are chemically inert, they can have an influence on the structure of the cluster ion. The study of vanadium oxide clusters demonstrates that the interaction between a rare gas atom and the ion can change the energetic ordering of the isomers, depending on which rare gas atom is used. It is also shown that the contributions from multiple isomers to the vibrational spectrum can be isolated by systematically varying the number of rare gas atoms attached to the cluster. The first vibrational spectra of gas phase cerium-containing clusters are presented. The spectra represent ideal benchmarks for assessing the applicability of different density functionals, which differ in the description of localized Ce-4f states. The study reveals that even small clusters with 2-5 cerium atoms have properties similar to the bulk. The geometric structures of cerium oxide clusters represent only “slightly modified” fragments of bulk ceria making them interesting systems for reactivity studies to mimic catalytic processes on ceria surfaces. In the mixed metal oxide clusters the cerium atoms are preferentially reduced. This key finding supports that the high reducibility of Ce(+4) accounts for the promoting role of ceria in supported vanadium oxide catalysts. The combination of IR-PD spectroscopy with molecular dynamics simulations allows for the systematic study of the relationships between conformational changes of dicarboxylate dianions in water clusters and the corresponding spectroscopic features. The relative intensities of the symmetric versus antisymmetric carboxylate stretching modes represent a sensitive probe for the conformations of the dianions. The addition of water molecules to the dianion, one at a time, leads to a conformational transition from a linear to a folded structure, once a critical solvent number is reached. The conformational change is reflected in a significant drop of the intensity of the symmetric stretching modes. The conformational transition depends on the solvent number as well as on the chain length of the dianions. The study shows that the stability of the folded conformation is attributed to the formation of additional solute-solvent (rather than solvent-solvent) hydrogen bonds in the cluster.