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, carbon X-ray Raman spectra are presented for a series of polycyclic systems and show 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 Huckel, driven by the π electron structure described by Clar. Additionally, carbon X-ray Raman spectra are presented for of a wide variety of forms of naturally occurring aromatic carbon: kerogen (insoluble organic carbon in sedimentary rocks, which is the most abundant form of naturally occurring organic carbon in the earth’s crust) over a range of types and thermal maturities, fresh materials of a variety of forms, and coal and petroleum asphaltenes (toluene soluble and heptane insoluble materials, which represent organic carbon resulting from extensive thermal processing). Using this interpretation of the 1s−π* splitting, it is observed that all these materials are dominated by aromatic sextets over isolated double bonds. Based on the diversity of materials analyzed, it is concluded that naturally occurring organic carbon is generally dominated by aromatic sextets over isolated double bonds. This conclusion is rationalized in terms of statistical and thermodynamic effects.

Electronic structure of naturally occurring aromatic carbon

2019 Fall National Meeting

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.