Determination of 13C NMR Chemical Shift Structural Ranges for Polycyclic Aromatic Hydrocarbons (PAHs) and PAHs in Asphaltenes: An Experimental and Theoretical Density Functional Theory Study

Nowadays, 13C nuclear magnetic resonance (NMR) is used in the structural study of asphaltenes. However, the different carbon types in the polycyclic aromatic hydrocarbon (PAH) core of asphaltenes, to be able to establish their main backbone, have been reported to have different 13C NMR chemical shift ranges by different authors in the literature. Therefore, in the present study, the 13C NMR chemical shift ranges for the structural carbon types(a) Y-carbons (CY) or internal triple bridgehead aromatic carbons; (b) external peripheral aromatic carbon atoms at the junction of two fused rings, or external double bridgehead aromatic carbons (CAP3); (c) aromatic carbon atoms bonded to hydrogen atoms (CAH); and (d) carbon atoms bonded to heteroatoms (CAX) and carbon atoms bonded to hydrogen in the β position to a heteroatom (CAHβX)have been determined by combining calculated 13C NMR chemical shifts, at the gauge-independent atomic orbital-density functional theory level (GIAO-DFT), with measured experimental 13C NMR chemical shifts for PAHs that have been identified as being asphaltene-PAH cores and for PAHs that are not examples of asphaltene-PAH cores. The determined 13C NMR chemical shift ranges are reported.