科研之友
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摘要
Petroleum 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 (C-13 NMR) spectroscopy has contributed extensively to asphaltene characterization. However, proper assignment of C-13 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 omega B97X-D density functional with 6-31G(d,p). basis set to rationalize common trends in the C-13 NMR chemical shifts of asphaltene model compounds. The calculated C-13 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 R-2 = 0.99 and absolute errors of less than 10 ppm. Moreover, we calculate and calibrate the C-13 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 C-13 NMR spectra and allow for a more complete characterization of asphaltene molecules.
- 出版日期2015-5
