X-ray absorption spectroscopy, in particular X-ray absorption near-edge structure (XANES) and related techniques, probe the relative energies between the ground state and core-excited states and the associated transition moments. In transition metal and f-element chemistry, such experiments have been used to great effect to gain information about the extent of covalent metal-ligand bonding. The interpretation of the spectra, however, often rests on using the same molecular orbital diagram for the ground state and the excited states, which may not always be appropriate. Since 2018, we have been conducting a series of calculations of XANES spectra of a variety of f-element complexes, using multi-reference wavefunction theory, with accompanying analyses of the chemical bonding in the various states involved in the transitions. The results from these calculations shed light on differences in donation bonding in the ground vs. excited states, and they are able to tell, for instance, to what extent Ce L3-edge spectra reveal multi-configurational character of the ground state of cerium complexes. In this context, I will discuss XANES spectra for [AnO2](2+) with An = U-Pu, [AmCl6](3-), [AnCl6](2-) with An = Th-Pu and transition metal analogs, and Ce L-edge spectra for selected cerium complexes.

Chemical bonding in the ground and excited electronic states probed in X-ray absorption experiments

ACS Spring 2022

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