Density functional theory study of the effects of substituents on the carbon-13 nuclear magnetic resonance chemical shifts of asphaltene model compounds

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DOIResolve DOI: http://doi.org/10.1021/ef502078n
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TypeArticle
Journal titleEnergy and Fuels
ISSN0887-0624
Volume29
Issue5
Pages28432852; # of pages: 10
SubjectAromatic compounds; Asphaltenes; Carbon; Chemical analysis; Chemical compounds; Chemical shift; Density functional theory; Molecules; Nuclear magnetic resonance spectroscopy; Transition metals; Correlation coefficient; Experimental values; Nuclear magnetic resonance chemical shifts; Petroleum asphaltenes; Reference compounds; Spectroscopic technique; Nuclear magnetic resonance
AbstractPetroleum asphaltenes are a complex mixture of organic molecules containing mainly fused polyaromatic and naphthenic systems and pendant chains, polar moieties with heteroatoms (S, N, and O), and transition metals. A variety of spectroscopic techniques has been employed to characterize asphaltenes, but their structures remain largely elusive because of the complexity, variety of samples, and assignment limitations. Carbon-13 nuclear magnetic resonance (13C NMR) spectroscopy has contributed extensively to asphaltene characterization. However, proper assignment of 13C NMR spectra is very challenging because spectra of natural asphaltenes feature a large number of peaks in unusual environments, which may be hard to assign and interpret. We employ the dispersion-corrected ωB97X-D density functional with 6-31G(d,p) basis set to rationalize common trends in the 13C NMR chemical shifts of asphaltene model compounds. The calculated 13C NMR chemical shifts for a calibration series of 14 aromatic and heterocyclic reference compounds containing C atoms of types similar to those in the asphaltene model compounds are found to correlate linearly with the respective experimental values. The linear fitting yields a correlation coefficient of R2 = 0.99 and absolute errors of less than 10 ppm. Moreover, we calculate and calibrate the 13C chemical shifts of asphaltenes extracted from Brazilian vacuum residues to analyze and correlate the C atom types with those of the reference compounds. It is found that the presence of heteroatoms as well as environments with a high aromatic condensation index can significantly affect the chemical shifts. The effect of heteroatoms on the chemical shift, a situation that has scarcely been addressed in the literature, is evaluated here in detail. The results are intended to help interpret 13C NMR spectra and allow for a more complete characterization of asphaltene molecules.
Publication date
PublisherACS Publications
LanguageEnglish
AffiliationNational Research Council Canada (NRC-CNRC); National Institute for Nanotechnology
Peer reviewedYes
NPARC number21275738
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Record identifier4b978d3c-01d1-4d16-957e-83c61babe7c6
Record created2015-07-14
Record modified2016-05-09
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