Our methods

• FRITZ
• GERTI
• External ion sources: LVAP, ESI, LILBID, aggregate
• Cold ICR cell
• ab initio calculations
• IR-OPO
• XMCD @ BESSY
• Femtosecond spectroscopy

Former projects:

• EPITOPES @ CLIO U Paris Sud

Reactivity of isolated transition metal clusters

In the heterogeneously catalyzed functionalization of saturated hydrocarbons, the initial adsorption or association or the kinetic inhibition of the subsequently required C-H activation often limits the conversion. We use transition metal clusters as model systems to investigate the corresponding elementary reactions.

Clusters generated from laser evaporation in combination with pulsed jet expansion are transferred to an ion trap and reacted with KWS substrates in the gas phase and under single impact conditions. High-resolution mass analysis according to the Fourier transform ion cyclotron resonance (FT-ICR) principle identifies reactants and products unambiguously and thus also according to cluster size and charge, sometimes with surprising results that cannot be predicted and still represent a challenge for ab initio theoretical interpretation today. So far, the elements of the 5th and 9th subgroups have been investigated as model systems, with selected other metals being added.

It has been shown that aromatics coordinate via their delocalized π electron system, heteroaromatics via the free electron pair on the heteroatom. Highly reactive transition metal cluster "surfaces" such as those of niobium or vanadium should in any case be capable of subsequent C-H activation and subsequent dehydrogenation. This makes the non-reactive stabilization of some complexes such as ^Nb+/-(_C6H6) or ^Nb+/-(naphthalene), which provide strong evidence for a highly symmetric cluster geometry, all the more remarkable.

The Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer shown here has been expanded to include an ion switch and two additional external ion sources, so that metal clusters can now be imaged with a laser evaporation source (left), metal organics with an electrospray source (center) or large biomolecular complexes with a LILBID source (right, under construction), transferred to the actual spectrometer and further characterized there.

Further investigations of the reactions of selected transition metal clusters (niobium, vanadium, tantalum and cobalt) with nitriles provide additional details of the activation process and complete our picture of the successive elementary reactions. Further exceptions with regard to unusual behavior of individual cluster sizes are added.

The task of the coming months and years will be to integrate all these special features into a systematic overview of size-selected clusters of transition metals, to supplement them with further measurements and to prepare them for an ab initio theoretically supported interpretation. In order to put this overview on as broad a data basis as possible, we are also in the process of expanding our range of methods beyond the investigation of bimolecular reactivity in the direction of optical spectroscopy:

Infrared spectroscopy of molecular clusters and isolated complexes

Infrared-induced multiphoton dissociation of size-selected cluster ions makes it possible to indirectly record the absorption spectra of these isolated ions in suitable mass spectrometers. The combination with ab intio calculations brings these "fingerprints" together with the associated structures and thus establishes a structural analysis in the gas phase that can be as powerful as X-ray structure and NMR analysis in condensed phases. When and why vibration-mode-specific deviations occur - we were recently able to show this in collaboration with the working group of Y.T. Lee at the Academia Sinica in Taiwan. In order to extend the scannable wavelength range in the red, we have also started a strategic partnership with the Université de Paris-Sud in Orsay and set up a new experiment at the CLIO free-electron laser there, with which IR MPD spectra of a wide range of isolated (cluster) ions can be recorded in the future, down to ^600cm-1, a range that is still not accessible for more conventional laser techniques. Even the initial feasibility studies could be easily extended and showed how nitrile ligands link reductively C-C within a metal complex.

Infrared-induced multiphoton dissociation (IR-MPD) spectra of the gas phase complexes of the niobium cation with four acetonitrile ligands agree well with theoretically modeled spectra for a square-planar structure (top left). With five ligands, experiment and theory agree on reductive nitrile coupling (bottom right). Simple MO considerations and explicitly calculated activation barriers explain the difference conclusively.

In the meantime, we have installed a new, powerful FT-ICR spectrometer at CLIO and obtained first, very promising results on intramolecular proton transfer in dicarboxylic acid cations and anions. With a new ion storage cell developed by us, which operates at 20 Kelvin, we will be able to obtain well-resolved IR spectra of cold ions in the foreseeable future.

(Bio)analytics with high-resolution mass spectrometry

We also regularly use the existing mass spectrometry equipment(electrospray with FT-ICR, MALDI with tandem-TOF) for analytical investigations. Among other things, this has enabled us to reach a decision on the disputed sequence of a plant protein (gelonin). In order to be able to visualize even larger non-covalently specifically bound complexes largely without fragmentation in the future, we have set up an innovative droplet-thrower ion source based on the LILBID principle (coop. Prof. B. Brutschy, Frankfurt). Resonant absorption of a nanosecond infrared laser causes the droplets to explode and "gently" releases the previously solvated macromolecules without destroying them. The first high-resolution mass spectra will be obtained shortly.

Spin and orbital moments of isolated transition metal clusters

As coordinators of the GAMBIT project, we are setting up another FT-ICR spectrometer at the Berlin synchrotron BESSY II, which will be permanently coupled to a beamline for polarized X-ray light. The aim is to utilize the so-called X-ray-induced magnetic circular dichroism (XMCD effect) to determine the spin and orbital contributions to the magnetic moments of size-selected transition metal clusters as isolated ions in the gas phase. These values are urgently needed to be compared with corresponding values of clusters deposited on surfaces on the one hand, and to serve as reference values for the theoretical modeling of the electronic structure of open-shell systems on the other hand

Coordinated research on dinuclear and trinuclear transition metal complexes

Together with colleagues from the chemistry department, some colleagues from the physics department and other colleagues from the chemistry department at TU Karlsruhe, we are pursuing an initiative for coordinated research in the field of oligonuclear transition metal complexes. In addition to standard characterization, specifically synthesized complexes are to be further investigated using optical and mass spectrometric methods of physical chemistry and physics in order to elucidate their electronic structure and spin dynamics. These results will provide important starting points for accompanying ab initio theoretical modeling. In the medium term, such fundamental work with these challenging classes of materials will open up new possibilities in the fields of catalytic, magnetic and optical applications.