Use of Molecular Orbital Theory to Interpret X-Ray <i>K</i>-Absorption Spectral Data

This paper demonstrates how molecular orbital theory can be useful in obtaining chemical information from x-ray absorption spectra. Basic molecular orbital theory concepts such as symmetry and overlap of wavefunctions are used to explain the K-absorption spectra of first-row transition metal complexes. In x-ray K-absorption spectroscopy, 1s electrons are excited to high-energy unoccupied orbitals; thus, the main absorption maxima (for first row transition metals) are due to excitations to the triply degenerate 4p antibonding orbitals. The shifts in these peaks are due to changes in the orbital energy levels, which, of course, are a function of chemical bonding. A splitting or a broadening of a peak is an indication of a splitting of the degenerate energy levels, which in turn is an indication of unsymmetrical metal to ligand bonding. It is also shown how x-ray absorption spectra can be useful in determining the lattice sites of atoms of nearly equal x-ray scattering power.