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

Cu-O-2 structures play important roles in bioinorganic chemistry and enzyme catalysis, where the bonding between the Cu and O-2 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 O-2 pi*, Cu 3d-pi(O-O)* (type A), and a few also to mixed antibonding/bonding orbitals between Cu 3d and O-2 pi, Cu 3d +/- pi(O-O) (type M). Our calculations provided a clear illustration of the interactions between Cu 3d and O-2 pi*/pi orbitals that are carried in the metal L-edge XAS.