The Electronic Structure of Naturally Occurring Aromatic Carbon

Aromatic carbon in fused ring systems can be classified into two forms of electronic structure: aromatic sextets which have large HOMO-LUMO gaps and resemble benzene, and isolated double bonds which have small HOMO-LUMO gaps and resemble olefins. The relative abundance of aromatic sextets vs isolated double bonds in mixtures can be probed by carbon X-ray Raman spectroscopy. Here we report the carbon X-ray Raman spectra 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). It is observed that all these materials are dominated by aromatic sextets over isolated double bonds. Based on the diversity of materia...