Chemical bonding in carborane/aromatic co-polymers: a first-principles analysis of experimental photoemission spectra

First-principles density functional theory calculations have been used to study the relative stability and analyse the chemical bonding of novel cross-linked carborane polymers. Atomic charges with several population analysis methods based on fully relaxed structures were calculated to interpret the chemical binding energy shifts of XPS spectra of these boron carbide polymers. The results indicate that a base structure with one aromatic linking unit with carborane is energetically favoured. The linear relationship between experimental core-level photoemission binding energies and computational partial atomic charges from four population analysis methods (Mulliken, Hirshfeld, atoms-in-molecules (AIM) and natural bond order (NBO)) were analysed and the results indicate that cross-linking occurs at icosahedral B sites non-adjacent to icosahedral carbon sites, in agreement with recently reported experimental results. The role of basis set size in determining partial atomic charges was found to vary with population analysis method. Best linear correlations were identified with the more robust population analysis methods (Hirshfeld, AIM and NBO) with the AIM methods noted as being particularly sensitive to basis set size.