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 an...