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. 2023 Oct 30;13(45):31811-31819.
doi: 10.1039/d3ra04688e. eCollection 2023 Oct 26.

DFT approach towards accurate prediction of 1H/13C NMR chemical shifts for dipterocarpol oxime

Phong Q Le  1 , Nhu Q Nguyen  1 , Thien T Nguyen  2   3
Affiliations

DFT approach towards accurate prediction of 1H/13C NMR chemical shifts for dipterocarpol oxime

Phong Q Le et al. RSC Adv. .

Abstract

A computational NMR approach for accurate predicting the 1H/13C chemical shifts of triterpenoid oximes featuring the screening of 144 DFT methods was demonstrated. Efficiently synthesized dipterocarpol oxime was employed as a model compound. The six highest accurate methods from the screening generated root-mean-square-error (RMSE) values in the range of 0.84 ppm (0.55%) to 1.14 ppm (0.75%) for calculated 13C shifts. For 1H results, simple, economical 6-31G basis set unexpectedly outperformed other more expensive basic sets; and the couple of it with selected functionals provided high accuracy shifts (0.0617 ppm (1.49%) ≤ RMSE ≤ 0.0870 ppm (2.04%)). These computational results strongly supported the proton and carbon assignments of the oxime including the difficult ones of diastereotopic methyl groups, the methyl groups attached to an internal olefin, and diastereotopic α-protons.

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Conflict of interest statement

There is no conflict to declare.

Figures

Fig. 1
Fig. 1. (a) Structures of oxime 1 and (b) synthetic potentials of the oxime functional group.
Fig. 2
Fig. 2. DFT approach toward accurate 1H/13C NMR chemical shift computation.
Fig. 3
Fig. 3. (a) Synthesis of oxime 1 and (b) its global minimum geometry optimized at IEFPCM(CHCl3)/B3LYP/6-31G(d,p).
Fig. 4
Fig. 4. Relative RMSE values and NMR calculation times of the selected functionals for (a) 13C and (b) 1H.
Fig. 5
Fig. 5. Averages of absolute deviations for 13C chemical shifts using selected combinations.
Fig. 6
Fig. 6. Averages of absolute deviations for 1H chemical shifts using selected combinations.
See this image and copyright information in PMC

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