Book of Abstracts Book of Abstracts - Universität Konstanz

B - 17
Probing Strong Hydrogen Bonds and Cold Metal Oxide Clusters with
Infrared Photodissociation Spectroscopy
Knut R. Asmis
Abteilung Molekülphysik, Fritz-Haber-Institut der Max-Planck-Gesellschaft
Faradayweg 4-6, D 14195 Berlin
In recent years much effort has been aimed at improving the sensitivity and selectivity of
experimental methods to study the vibrational spectroscopy of gas phase ions. Due to the lack of
widely tunable infrared (IR) light sources most of these studies had been limited to the region
above 2400 cm -1 , i.e., the spectral region of hydrogen-stretching motions and of IR-active
combination bands. The application of the infrared free-electron laser FELIX, which generates
tunable radiation in the 40 – 2200 cm -1 region, to molecular spectroscopy by Meijer, von Helden
and coworkers has bridged this gap [1]. We have recently extended their technique to study the
vibrational spectroscopy of mass-selected gas-phase cluster cations and anions at variable
temperature. Application of this technique to two important compound classes will be discussed
and compared to the results of complimentary experiments on mass-selected neutral clusters
using femtosecond pump-probe spectroscopy.
The hydrogen bond interaction is key to understanding the structure and properties of water
and biomolecules. However, our understanding of strong, low-barrier hydrogen bonds and their
central role in enzyme catalysis, biomolecular recognition, proton transfer across biomembranes
and proton transport in aqueous media remains sketchy. We recently measured the first IR
spectra of several prototypical systems, directly probing the shared proton region of the
potential energy surface [2,3]. Our experiments demonstrate that the theoretical description of
the vibrations of strong hydrogen bonds is considerably more complex than originally
anticipated and not yet solved satisfactorily, even for small systems and at the highest levels of
theory currently available.
Transition metal oxides play an increasingly important role in heterogeneous catalysis.
Recent gas-phase reactivity studies on vanadium oxide cluster ions were aimed at understanding
the nature of the reactive sites. The interpretation of the reactivity data requires information on
the structure of the cluster ions. We employ IR photodissociation spectroscopy in combination
with electronic structure calculations to identify the geometric structure as well as structural
trends as a function of cluster size in vanadium oxide cluster cations and anions [4]. For the
cluster anions we present the first experimental evidence for the formation of polyhedral
vanadium oxide cages in the gas phase. The measured IR spectra are sensitive probes for charge
localization of unpaired d-electrons and show a strong resemblance with the IR spectrum of a
V2O5 surface already at medium cluster sizes.
References
[1] G. von Helden, I. Holleman, G. M. H. Knippels, A. F. G. van der Meer, and G. Meijer, Phys. Rev.
Lett. 79, 5234 (1997).
[2] K. R. Asmis, N. L. Pivonka, G. Santambrogio, M. Brümmer, C. Kaposta, D. M. Neumark, and L.
Wöste, Science 299, 1375 (2003).
[3] M. Nee, C. Kaposta, A. Osterwalder, C. Cibrian Uhalte, T. Xie, A. Kaledin, S. Carter, J. M.
Bowman, G. Meijer, D. M. Neumark, and K. R. Asmis, J. Chem. Phys. 121, 7259 (2004).
[4] K. R. Asmis, G. Meijer, M. Brümmer, C. Kaposta, G. Santambrogio, L. Wöste, and J. Sauer, J.
Chem. Phys. 120, 6461 (2004).
55