Multiconfigurational Approach to X-ray Spectroscopy of Transition Metal Complexes

Close correlation between theoretical modeling and experimental spectroscopy allows for identification of the electronic and geometric structure of a system through its spectral fingerprint. This is can be used to verify mechanistic proposals and is a valuable complement to calculations of reaction mechanisms using the total energy as the main criterion. For transition metal systems, X-ray spectroscopy offers a unique probe because the core-excitation energies are element specific, which makes it possible to focus on the catalytic metal. The core hole is atom-centered and sensitive to the local changes in the electronic structure, making it useful for redox active catalysts. The possibility to do time-resolved experiments also allows for rapid detection of metastable intermediates. Reliable fingerprinting requires a theoretical model that is accurate enough to distinguish between different species and multiconfigurational wavefunction approaches have recently been extended to model a number of X-ray processes of transition metal complexes. Compared to ground-state calculations, modeling of X-ray spectra is complicated by the presence of the core hole, which typically leads to multiple open shells and large effects of spin–orbit coupling. This chapter describes how these effects can be accounted for with a multiconfigurational approach and outline the basic principles and performance. It is also shown how a detailed analysis of experimental spectra can be used to extract additional information about the electronic structure

Resonant Inelastic X-ray Scattering (RIXS) Studies in Chemistry : Present and Future

This chapter illustrates how resonant inelastic x-ray scattering (RIXS) is used to address questions in chemistry, with special focus on the electronic structure and catalytic activity of first row transition metals. RIXS is a two-photon process that is the x-ray equivalent of resonance Raman spectroscopy. The final states correspond to vibrational, valence electronic or even core excitations. In addition to the advantages of a local element-selective x-ray spectroscopic probe, RIXS gives new information compared to single-photon x-ray absorption and x-ray emission experiments. Metal L-edge RIXS shows intense metal-centered ligand- field transitions, even in cases where they are spin or parity forbidden in optical absorption spectroscopy. By selecting different resonances by appropriately tuning the incident energy, it is possible to isolate different ligand-field and charge-transfer transitions. The observation of a large number of electronic states that can be properly assigned, sometimes with the help of theoretical methods, gives novel opportunities to quantify metal-ligand interactions and their contributions to reactivity. RIXS in the K pre-edge can be used to obtain L- and M-edge like spectra including insight into charge-transfer excitations all with the advantages of a hard x-ray probe. Finally, it is shown how time-resolved RIXS down to the femtosecond timescale probes the orbitals of transient reaction intermediates. The usefulness of RIXS in chemistry is shown for a diverse set of systems, including coordination complexes, metal enzymes, and nanoparticles