Selective oxidation catalysis with metal clusters in an ion trap


Prof. Dr. Thorsten M. Bernhardt

Gas phase reaction kinetics measurements in a radio frequency ion trap setup under multi-collision conditions are applied to reveal the detailed mechanisms of the catalytic conversions as a function of the metal cluster size and composition.1,2 The experiments aim at one hand at providing conceptual mechanistic insight into well known heterogeneously catalyzed reactions. As an example, the low temperature CO combustion on small palladium clusters will be discussed.3,4 On the other hand, employing this gas phase technique, we recently discovered new catalytic routes that tackle the activation and selective oxidation of methane by employing free gold cluster cations.5-7 In both respects, small isolated metal clusters with accurately defined number of atoms can serve as effective and experimentally tractable model systems for such heterogeneous metal particle catalysts. One particularly important issue is the variable but limited size of isolated cluster complexes that also renders them ideally suited for detailed theoretical treatment, which in turn perfectly corroborates the intention to gain molecular level insight into catalytic processes.8,9

1 T. M. Bernhardt, Int. J. Mass Spectrom., 2005, 243, 1.
2 T. M. Bernhardt, J. Hagen, S. M. Lang, D. M. Popolan, L. Socaciu-Siebert, and L. Wöste, J. Phys. Chem. A, 2009, 113, 2724.
3 S. M. Lang, T. Schnabel, and T. M. Bernhardt, Phys. Chem. Chem. Phys., 2012, 14, 9364.
4 S. Lang, I. Fleischer, T. M. Bernhardt, R. N. Barnett, and U. Landman, J. Am. Chem. Soc., 2012, 134, 20654.
5 S. M. Lang, T. M. Bernhardt, R. N. Barnett, and U. Landman, Angew. Chem. Int. Ed., 2010, 49, 980.
6 S. M. Lang, T. M. Bernhardt, R. N. Barnett, and U. Landman, J. Phys. Chem. C, 2011, 115, 6788.
7 S. M. Lang, T. M. Bernhardt, R. N. Barnett, B. Yoon, and U. Landman, J. Am. Chem. Soc., 2009, 131, 8939.
8 V. Bonačić-Koutecký and T. M. Bernhardt, Phys. Chem. Chem. Phys., 2012, 14, 9252.
9 S. M. Lang and T. M. Bernhardt, Phys. Chem. Chem. Phys., 2012, 14, 9255.

Differential Ion Mobility Spectrometry (FAIMS): Principles and Selected Applications


Alexandre A. Shvartsburg


Differential or field asymmetric waveform ion mobility spectrometry (FAIMS) separates and identifies gas-phase ions by the derivative of mobility with respect to electric field strength. While FAIMS was known since 1990-s, its appeal was limited by low resolving power (typically R ~ 10). Employing new planar-geometry devices with high-definition electronics, elevated fields, extended separation times, and gas mixtures with dominant He or H2 fractions, we have raised R to >100 in general and up to ~500 for multiply-charged peptides. This performance enables many new biological analyses, in both global separations (for example, of proteolytic digests) and targeted applications such as resolution of sequence inversions and localization of post-translational modifications, including the smallest ones like methylation and acetylation in histone analyses. In another variation, FAIMS microchips with extreme electric fields filter ions in <100 us and cleanly separate large proteins from other species using the dipole-locking effect. We shall also discuss metabolomic applications, comprising lipid classification and regioisomer separations, and the new FAIMS approach involving oligomers.

Two color enhanced IR‐MPD spectroscopy of organic acids and of oligo‐nuclear metal complexes

Gereon Niedner‐Schatteburg
Department of Chemistry, TU Kaiserslautern, Germany

The Infra‐Red‐Multi‐Photon‐Dissociation (IR‐MPD) technique is well established for the characterization of isolated ionic molecules, complexes and clusters. It fails somewhat short, however, to provide for reliable intensities. This limits the direct comparison with IR absorption spectra and with ab initio spectra. Occasionally, well known IR absorption lines are totally missing in the corresponding IR‐MPD spectra.

We devise a two color pumping scheme that enables to utilize resonant 1+1 absorptions for IR‐MPD studies. Based on our prior spectroscopy of hydrated ions [1,2] we start to characterize protonated and deprotonated dicarboxylic acid ions,HOOC‐(CH2)n‐C(OH)2+ and HOOC‐(CH2)n‐COO‐, as stabilize in cyclic form through ionic hydrogen bonding. Other than e.g. in hydrogen bonded clusters [1] the alkyl back bone causes steric constraints which are expected to vary by the alkyl chain length. Ab initio calculations [3] predict a possible coexistence of symmetric (Zundel‐ type) and asymmetric (Eigen‐type) hydrogen bonds in the case of adipic acid (n=4). A multitude of transition states define a stepwise proton transfer reaction path in concert with an equally stepwise twisting of the alkyl chain. We present Infra‐Red‐Multi‐Photon‐Dissociation (IR‐MPD) spectra of dicarboxyilic acid cations and anions n=1,.., 6 and discuss fingerprints of proton localization and / or delocalization within the hydrogen bond [4].

Most recently we started to extend our two color IR‐MPD investigations towards oligo‐nuclear transition metal complexes that stabilize by either dipeptides [5] or nucleobases [6]. We will report on evidence for non‐statistical Internal Vibrational Relaxation (IVR) and on our efforts of structural characterization. These investigations are of paramount importance for our trans‐regional collaborative research center that focusses on homo and hetero metallic complexes [7].

[1] Jyh‐Chiang Jiang, Yi‐Sheng Wang, Hai‐Chou Chang, Sheng H. Lin, Yuan T. Lee, GNS, and Huan‐Cheng Chang, J. Am. Chem. Soc. (2000), 122(7), 1398‐1410
[2] Tobias Pankewitz, Anita Lagutschenkov, GNS, Sotiris S. Xantheas, and Yuan T. Lee, J. Chem. Phys. (2007), 126, 074307
[3] S.K.Min, M. Park, N. J. Singh, H. M. Lee, E. C. Lee, K. S. Kim, A. Lagutschenkov, and GNS, Chemistry ‐ a European Journal (2010), 16(34), 10373‐10379
[4] Christine Merkert, Anita Lagutschenkov, Fabian Menges, Philippe Maitre, and GNS, manuscript in preparation
[5] Fabian Menges, Lars Barzen, Christine Merkert, Yevgeniy Nosenko, and GNS, manuscript in preparation
[6] Yevgeniy Nosenko, Fabian
[7] http://3met.de

Recent Advances in the Vibrational Spectroscopy of Mass-Selected Cluster Ions

Knut R. Asmis

Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin, Germany

I will give an overview of infrared photodissociation (IRPD) experiments performed in our group, focusing on the structural characterization of mass-selected cluster ions and how these results can help in the fundamental understanding of (i) heterogeneous catalysis and (ii) proton hydration.
In many supported catalysts neither the size and distribution of the active metal oxide particles on the support surface, nor their structure, is sufficiently known. Therefore it proves helpful to study the size-dependent properties of such particles under well-controlled conditions in the gas phase. Our ongoing IRPD experiments on metal oxide clusters are aimed at characterizing their geometric and electronic structure, delivering benchmark data for testing electronic structure calculations, as well as gaining a molecular level understanding of their structure-reactivity relationship.
Understanding how protons are hydrated remains an important and challenging research area. The anomalously high proton mobility of water can be explained by a periodic isomerization between the Eigen and Zundel binding motifs, H3O+(aq) and H5O2+(aq), respectively, even though the detailed mechanism is still under debate. These rapidly interconverting structures from the condensed phase can be stabilized, isolated and studied in the gas phase in the form of protonated water clusters. Here, I present first results on IR/IR double resonance experiments, which allow to spectrally separate the IR signatures of individual isomers, on H+(H₂O)n・H2 clusters with n=5-10.