Revealing the electronic properties of the B-B bond: the bis-catecholato diboron molecule

The electronic properties of a diboron molecule, namely bis(catecholato)diboron (2-(1,3,2-benzodioxaborol-2-yl)-1,3,2-benzodioxaborole) (B₂Cat₂), have been studied by comparing the results of photoemission (XPS) and near edge X-ray absorption spectroscopy (NEXAFS) experiments with the outcome of DFT calculations. The B 1s, C 1s and O 1s K-edges have been investigated for both the isolated gas phase molecule and the adsorbed one on the Au(111) surface. The main features of the polarized NEXAFS spectra at each of the three edges considered are not significantly affected by the presence of the substrate, with respect to the isolated molecule, indicating that the molecule-gold interaction is weak. Moreover, the comparison between the observed dichroism in the NEXAFS spectra of the adsorbed B₂Cat₂ and that in the NEXAFS spectra of the isolated molecule has confirmed the orbital symmetry assigned in the gas phase absorption spectra. The transitions to π(B-B) bonding and π*(B-B) anti-bonding final states represent the most relevant probe of the chemistry of the B₂Cat₂ molecule. We show that their theoretical description requires that the treatment of the relaxation changes among different excited state configurations, which we successfully implemented by using ΔSCF-DFT (ΔSCF) calculations.