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Issue 7, 2018
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A convenient and accurate method for predicting 13C chemical shifts in organic molecules

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Abstract

The 13C NMR shifts of a variety of organic compounds including pinanes and steroids were predicted using combined molecular mechanics (Pcmod 9.1/MMFF94) and ab initio GIAO (B3LYP/DFT,6-31G(d)) calculations. The accuracy of the ab initio calculations was no different when using the 6-31G(d) basis set than more complex basis sets but the calculations were much faster. The MM calculations for steroids took a few seconds and the Gaussian calculations ca. 45 minutes on a desktop computer. For the data set of alkanes, alkenes and alkynes the 13C shifts were obtained from the GIAO isotropic shieldings (Ciso) by (a) empirically scaled shieldings (δc = AB·Ciso) and (b) separate references for sp3, sp2 and sp carbons (δc = δrefCiso). The second method gave better agreement and was used henceforth. For the data set of 34 alkanes, alkenes and aromatics with 96 shifts the mean absolute error (MAE) was excellent (0.90) ppm but the alkyne data set (7 compounds with 27 shifts) did not give as good agreement (MAE 1.7 ppm). For the more complex substituted compounds, pinanes and steroids the MAE < 2 ppm but with large errors mainly from the C–X carbons (X = F, Cl, Br, O, [double bond, length as m-dash]O). The agreement for fluorocyclohexane (axial and equatorial) was, apart from C1 so good that the fluorine substituent effect is completely explained by this model. The calculations for the pinanes gave good agreement with two exceptions, C6 in the cyclobutane ring and the C[double bond, length as m-dash]C–CO carbon fragment. In both cases the use of different geometries (MM3 and 6-31G(d) res.) gave good results (apart from the CO carbon) suggesting that the errors are due to the molecular geometries in these strained molecules. In contrast for the seven steroids examined the MAE was 1.4 ppm and the assignments of the calculated and experimental shifts were almost identical. Thus this method could be very useful in assigning the 13C spectra and determining the conformations of relatively large molecules.

Graphical abstract: A convenient and accurate method for predicting 13C chemical shifts in organic molecules

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Publication details

The article was received on 18 Jan 2018, accepted on 20 Feb 2018 and first published on 23 Feb 2018


Article type: Paper
DOI: 10.1039/C8NJ00312B
Citation: New J. Chem., 2018,42, 5024-5036
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    A convenient and accurate method for predicting 13C chemical shifts in organic molecules

    R. J. Abraham and M. A. Cooper, New J. Chem., 2018, 42, 5024
    DOI: 10.1039/C8NJ00312B

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