Carbon Core Electron Spectra of Polycyclic Aromatic Hydrocarbons

Aromaticity profoundly affects molecular orbitals in polycyclic aromatic hydrocarbons. X-ray core electron spectroscopy has observed that carbon 1s-π* transitions can be broadened or even split in some polycyclic systems, although the origin of the effect has remained obscure. The π electrons in polycyclic systems are typically classified in the Clar model as belonging to either true aromatic sextets (similar to benzene) or isolated double bonds (similar to olefins). Here, bulk-sensitive carbon core excitation spectra are presented for a series of polycyclic systems and show that the magnitude of the 1s-π* splitting is determined primarily by the ratio of true aromatic sextets to isolated double bonds. The observed splitting can be rationalized in terms of ground state energetics as described by Hückel, driven by the π electron structure described by Clar. This simple model including only ground state energetics is shown to explain the basics physics behind the spectral evolution for a broad set of polycyclic aromatic hydrocarbons, although some residual deviations between this model and experiment can likely be improved by including a more detailed electronic structure and the core hole effect.