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

Abstract

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.

Experimental and Theoretical Approach To Determine the Average Asphaltene Structure of a Crude Oil from the Golden Lane (Faja de Oro) of Mexico

“…26 Not using 13 C DEPT NMR leads to an underestimation of bridgehead aromatic carbon content, which, in turn, leads to a lower aromatic ring size in asphaltene determination. 26,60 Andrews et al 26 mention that such is the case in the studies performed by Michon et al, 41 Japanwala et al, 42 and Sheremata et al 48 Zheng et al 56−58 have characterized the asphaltene structures of samples extracted from oil sand and vacuum residues from petroleum refining using NMR, 13 C DEPT-135°, and matrix-assisted laser desorption/ionization−time of flight (MALDI− TOF) mass spectrometry. These authors followed the method proposed by Andrews et al 26 With regard to the 13 C NMR chemical shift ranges defined for the different structural carbon atoms in the PAH core of asphaltenes (see Figure 1), there is no consensus in the literature, and in some cases, as a result of overlapping of signals, the chemical shift ranges are assigned taking into account some assumptions.…”

Hazardous Creosote Wood Valorization via Fractionation and Enzymatic Saccharification Coupled with Simultaneous Extraction of the Embedded Polycyclic Aromatic Hydrocarbons Using Protic Ionic Liquid Media

“…Chemical shifts in this region are typically are characteristics of a wide range of carbon and hydrogen atoms that make up the core of PAH compounds, including those investigated in this study (naphthalene, anthracene, and dibenzofuran). 41,42 42 The strong intensity of these chemical shifts in the lignin structure extracted from creosote wood along with the FT-IR and GC-FID findings confirms the coprecipitation of the PAH compounds with lignin upon water addition. The coprecipitation of PAHs with lignin provides a self-cleaning route in the process, as a special processing unit is not required to clean up and recover the IL for recycling.…”

“…However, we note the appearance of new peaks in the creosote wood lignin with chemical shifts of 13 C 124–130 ppm and 1 H 6.9–8.9 ppm. Chemical shifts in this region are typically are characteristics of a wide range of carbon and hydrogen atoms that make up the core of PAH compounds, including those investigated in this study (naphthalene, anthracene, and dibenzofuran). , Ruiz-Morales et al established the 13 C NMR chemical shifts of three key structural carbons creating the core of PAH compounds, which are (a) C Y internal aromatic carbon atom at the junction of three fused rings (120.07–130.05 ppm), (b) C AP3 external aromatic carbon atom at the junction of two rings (127.60–135.09 ppm), and (c) C AH aromatic carbon atom bonded to hydrogen (116.53–130.95 ppm) . The strong intensity of these chemical shifts in the lignin structure extracted from creosote wood along with the FT-IR and GC-FID findings confirms the coprecipitation of the PAH compounds with lignin upon water addition.…”

Base Promoted Intumescence of Phenols

“…It could be seen that many new aromatic carbons appeared at 113-136, 141-144, and 159-169 ppm as illustrated in Figure 4. Considering the chemical shifts and 1 H NMR and IR results, new peaks at 129-135 ppm can be assigned to fused aromatic ring carbons [32,36], those at 141-144 ppm probably arose from aryl substituted carbons of polyarenes [37], and peaks at 159-169 ppm are due to oxygen substituted aromatic carbons [32]. Consistent with 1 H NMR, no significant signals associated with aliphatic carbons could be identified in 13 C NMR.…”

“…It could be seen that many new aromatic carbons appeared at 113-136, 141-144, and 159-169 ppm as illustrated in Figure 4. Considering the chemical shifts and 1 H NMR and IR results, new peaks at 129-135 ppm can be assigned to fused aromatic ring carbons [32,36], those at 141-144 ppm probably arose Second, highly useful information concerning structural changes was also obtained from the 1 H NMR analysis of aromatic protons. For all heated samples, many new doublet and triplet peaks appeared in the aromatic region.…”