Assigning the stereochemistry of pairs of diastereoisomers using GIAO NMR shift calculation

Assigning stereochemistry to single diastereoisomers by GIAO NMR calculation: the DP4 probability

GIAO NMR shift calculation has been applied to the challenging task of reliably assigning stereochemistry with quantifiable confidence when only one set of experimental data are available. We have compared several approaches for assigning a probability to each candidate structure and have tested the ability of these methods to distinguish up to 64 possible diastereoisomers of 117 different molecules, using NMR shifts obtained in rapid and computationally inexpensive single-point calculations on molecular mechanics geometries without time-consuming ab initio geometry optimization. We show that a probability analysis based on the errors in each (13)C or (1)H shift is significantly more successful at making correct assignments with high confidence than are probabilities based on the correlation coefficient and mean absolute error parameters. Our new probability measure, which we have termed DP4, complements the probabilities obtained from our previously developed CP3 parameter, which applies to the case of assigning a pair of diastereoisomers when one has both experimental data sets. We illustrate the application of DP4 to assigning the stereochemistry or structure of 21 natural products that were originally misassigned in the literature or that required extensive synthesis of diastereoisomers to establish their stereochemistry.

GIAO DFT 13C/15N chemical shifts in regioisomeric structure determination of fused pyrazoles

The combined use of two-dimensional NMR correlation experiments and gauge including atomic orbital density functional theory in (13)C NMR chemical shift (CS) calculations allowed reliable and simple structural determination of regioisomeric heterocyclic systems that originate from the reactions of acylquinolinones with substituted hydrazines. Moreover, the results of differential analysis between the calculated (15)N NMR CSs for hypothetical structures and the experimental data of the title azaheterocyclic systems were even more advantageous with respect to (13)C because there was no need for correlational analysis: structures of the regioisomeric compounds could be determined just by direct comparison.

DFT-GIAO(1)H NMR chemical shifts prediction for the spectral assignment and conformational analysis of the anticholinergic drugs (-)-scopolamine and (-)-hyoscyamine

The relatively large chemical shift differences observed in the (1)H NMR spectra of the anticholinergic drugs (-)-scopolamine 1 and (-)-hyoscyamine 2 measured in CDCl(3) are explained using a combination of systematic/molecular mechanics force field (MMFF) conformational searches and gas-phase density functional theory (DFT) single point calculations, geometry optimizations and chemical shift calculations within the gauge including/invariant atomic orbital (GIAO) approximation. These calculations show that both molecules prefer a compact conformation in which the phenyl ring of the tropic ester is positioned under the tropane bicycle, clearly suggesting that the chemical shift differences are produced by the anisotropic effect of the aromatic ring. As the calculations fairly well predict these experimental differences, diastereotopic NMR signal assignments for the two studied molecules are proposed. In addition, a cursory inspection of the published (1)H and (13)C NMR spectra of different forms of 1 and 2 in solution reveals that most of them show these diastereotopic chemical shift differences, strongly suggesting a preference for the compact conformation quite independent of the organic or aqueous nature of the solvent.

A multi-standard approach for GIAO (13)C NMR calculations

The influence of the reference standard employed in the calculation of (13)C NMR chemical shifts was investigated over a large variety of known organic compounds, using different quantum chemistry methods and basis sets. After detailed analysis of the collected data, we found that methanol and benzene are excellent reference standards for computing NMR shifts of sp(3)- and sp-sp(2)-hybridized carbon atoms, respectively. This multi-standard approach (MSTD) performs better than TMS in terms of accuracy and precision and also displays much lower dependence on the level of theory employed. The use of mPW1PW91/6-31G(d)//mPW1PW91/6-31G(d) level is recommended for accurate (13)C NMR chemical shift prediction at low computational cost.