Carbon Core Electron Spectra of Polycyclic Aromatic\nHydrocarbons

Abstract

Aromaticity\nprofoundly affects molecular orbitals in polycyclic\naromatic hydrocarbons. X-ray core electron spectroscopy has observed\nthat carbon 1s−π* transitions can be broadened or even\nsplit in some polycyclic systems, although the origin of the effect\nhas remained obscure. The π electrons in polycyclic systems\nare typically classified in the Clar model as belonging to either\ntrue aromatic sextets (similar to benzene) or isolated double bonds\n(similar to olefins). Here, bulk-sensitive carbon core excitation\nspectra are presented for a series of polycyclic systems and show\nthat the magnitude of the 1s−π* splitting is determined\nprimarily by the ratio of true aromatic sextets to isolated double\nbonds. The observed splitting can be rationalized in terms of ground\nstate energetics as described by Hückel, driven by the π\nelectron structure described by Clar. This simple model including\nonly ground state energetics is shown to explain the basics physics\nbehind the spectral evolution for a broad set of polycyclic aromatic\nhydrocarbons, although some residual deviations between this model\nand experiment can likely be improved by including a more detailed\nelectronic structure and the core hole effect.