Polycyclic aromatic hydrocarbons of asphaltenes analyzed by molecular orbital calculations with optical spectroscopy

The number and geometry of the rings in polycyclic aromatic hydrocarbons (PAHs) in petroleum asphaltene has remained unresolved for many years. Many sophisticated imaging and spectroscopic methods have been utilized to narrow the list of candidate structures for asphaltene PAHs. Here, we exploit a canonical property of petroleum asphaltenes, their color, along with their fluorescence emission properties. These universal spectral properties are analyzed through the lens of molecular orbital (MO) calculations, thereby providing quantitative bounds on asphaltene PAH systems. Energetic considerations mandate that these fused aromatic ring systems are predominantly aromatic sextet carbon (within the Clar representation) but not entirely sextet carbon. Matching the ubiquitous asphaltene spectral data with MO calculations shows that asphaltene ring systems predominantly consist of 4-10 rings. PAHs with 6-8 rings are most predominant in petroleum asphaltenes. Not surprisingly, polydispersity is implied in this analysis. These results are very consistent with all direct imaging of asphaltene PAHs. When these results are coupled with known molecular weights of asphaltenes, we find that asphaltene molecules possess primarily one fused ring system per molecule. This finding is independently obtained when combining MO results with spectral dispersion measured for asphaltene diffusion constants. The monomeric molecular structure of asphaltenes is supported, and the archipelago model is refuted.