Interpreting the Cu-O₂ Antibonding Nature in Two Cu-O₂ Complexes from Cu L-Edge X-ray Absorption Spectra

Cu-O₂ structures play important roles in bioinorganic chemistry and enzyme catalysis, where the bonding between the Cu and O₂ parts serves as a fundamental research concern. Here, we performed a multiconfigurational study on the copper L_2,3-edge X-ray absorption spectra (XAS) of two copper enzyme model complexes to gain a better understanding of the antibonding nature from the clearly interpreted structure-spectroscopy relation. We obtained spectra in good agreement with the experiments by using the restricted active space second-order perturbation theory (RASPT2) method, which facilitated reliable chemical analysis. Spectral feature interpretations were supported by computing the spin-orbit natural transition orbitals. All major features were assigned to be mainly from Cu 2p to antibonding orbitals between Cu 3d and O₂ π*, Cu 3d-π_O-O* (type A), and a few also to mixed antibonding/bonding orbitals between Cu 3d and O₂ π, Cu 3d ± π_O-O (type M). Our calculations provided a clear illustration of the interactions between Cu 3d and O₂ π*/π orbitals that are carried in the metal L-edge XAS.