Comparison of different theory models and basis sets in the calculation of13C NMR chemical shifts of natural products

Abstract

The influence of the calculation method in mimicking experimental (13)C NMR chemical shifts of 15 low-polarity natural products singularly containing 10-20 carbon atoms was investigated by employing different quantum chemistry approaches and basis sets, both in the preliminary geometry optimizations and in the following single-point (13)C GIAO calculations of the NMR chemical shifts. The geometries of the involved species were optimized at the PM3, HF, B3LYP and mPW1PW91 levels whereas the (13)C NMR parameters were determined at the HF, B3LYP and mPW1PW91 levels. Different combinations of basis sets were also tested. The consistency and efficiency of the considered combinations of geometry optimizations and GIAO (13)C NMR calculations were thoroughly checked by the analysis of statistical parameters concerning computed and experimental (13)C NMR chemical shift values.

Toward the Complete Prediction of the 1H and 13C NMR Spectra of Complex Organic Molecules by DFT Methods: Application to Natural Substances

“…As will be seen later, most calculations concern chemical shifts rather than couplings, and these have been analyzed in greater detail. For each system we present the parameters a and b of the linear regression d calcd = a + bd exptl ; the correlation coefficient, r 2 ; the mean absolute error (MAE) defined as AE n j d calcd Àd exptl j / n; the corrected mean absolute error, CMAE, [18] defined as AE n j d corr Àd exptl j /n, where d corr = (d calcd Àa)/b and therefore corrects for systematic errors. For coupling constants the above formulae may be used by replacing d with J.…”

“…Quite naturally, organic biomolecules such as proteins, nucleic acids, [11] and carbohydrates [12] have been the object of such investigations. Particular emphasis was placed by ourselves [13][14][15] and by Bifulco and co-workers, [16][17][18][19][20] as well as others, [21] on the modeling of organic and naturally occurring molecules through the calculation of 1 H and 13 C NMR spectra. For natural products with challenging NMR spectra we face four major problems: a) These molecules possess a The question is whether the accuracy of quantum chemistry is adequate for this task.…”

Experimental and ab initio Computational Studies on Dimethyl-(4-{4-{3-methyl-3-phenyl-cyclobutyl)-thiazol-2-yl]-hydrazonomethyl}-phenyl)-amine

“…In this regard, DFT methods have been preferred in the study of large organic molecules, metal complexes and organometallic compounds in all those cases in which the electron correlation contributions were not negligible. [30][31][32][33] In this study, we present results of a detailed investigation of the synthesis and structural characterization of dimethyl-(4-{4-{3-methyl-3-phenyl-cyclobutyl)-thiazol-2-yl]-hydrazonomethyl}-phenyl)-amine by using single crystal X-ray, 1 H-NMR, 13 C-NMR and UV-vis. and quantum chemical methods.…”

STRUCTURAL REASSIGNMENT OF EPIERYTHRATIDINE, AN ALKALOID FROM Erythrinafusca, BASED ON NMR STUDIES AND COMPUTATIONAL METHODS

“…method minimizes the error in this type of calculation. 21 The NMR results are summarized in Table 2 …”

“…[17][18] Moreover, extensive spectroscopic analyses and quantum mechanical (QM) methods have been used for the reassignment of some structures, 19 and can be very helpful to confirm both rigid and flexible molecular scaffolds. 20,21 On the other hand, this methodology has been used to derive stereostructures by comparing the experimental NMR spectroscopic data with the corresponding results of calculations for all the possible stereoisomers. [22][23][24] In this work we report the isolation of erysotrine (1), erysodine (2), erythraline (3), erytharbine (4), erysotrine N-oxide (5) and epierythratidine (6) from the seeds of Erythrina fusca Lour.…”