Abstract
The Frenkel exciton model was adapted to describe X-ray absorption and resonant inelastic scattering spectra of polynuclear transition metal complexes by means of the restricted active space self-consistent field method. The proposed approach allows to substantially decrease the requirements on computational resources if compared to a full supermolecular quantum chemical treatment. This holds true, in particular, in cases where the dipole approximation to the electronic transition charge density can be applied. The computational protocol was applied to the calculation of X-ray spectra of the hemin complex, which forms dimers in aqueous solution. The aggregation effects were found to be comparable to the spectral alterations due to the replacement of the axial ligand by solvent molecules.
Highlights
Soft X-ray L-edge spectroscopy has become a standard technique to investigate the intricate details of the electronic structure of transition metal compounds
Before addressing effects of the excitonic coupling in the hemin dimer, the change of the X-ray Absorption (XAS), Partial Fluorescence Yield (PFY), and RIXS spectra due to the different solvents need to be quantified for the case of the monomer
Spectra have been calculated for various axial ligands: Cl, H2O, and dimethyl sulfoxide (DMSO), which have been used as solvents for experimental measurements,18,19 and OH– that can be formed in an aqueous solution due to hydrolysis (Fe3þ þ H2O ! FeOH2þ þ Hþ)
Summary
Soft X-ray L-edge spectroscopy has become a standard technique to investigate the intricate details of the electronic structure of transition metal compounds. For metal complexes, where the spectroscopically active region is rather localized, the prediction of L-edge spectra requires an active space including the 2p and all orbitals with notable metal 3d-contributions Such a choice corresponds to account for the most important correlation terms as well as dipole allowed transitions. The effect of aggregation was recently addressed by means of soft X-ray Fe L-edge absorption spectroscopy in transmission (XAS) and partial fluorescence (PFY) modes as well as by off-resonant X-ray emission (XES) and RIXS on the examples of DMSO and aqueous solutions.. The general shape of the spectra for both cases was quite similar, and the pronounced difference in broadenings for RIXS as well as a 1.3 eV energy shift in off-resonant XES was taken as an indication of aggregation These effects were, solely attributed to the p – p-stacking, ligand p-orbitals are barely influencing local Fe 2p !
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