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DFT‐GIAO 1 H and 13 C NMR prediction of chemical shifts for the configurational assignment of 6β‐hydroxyhyoscyamine diastereoisomers
Author(s) -
Muñoz Marcelo A.,
JosephNathan Pedro
Publication year - 2009
Publication title -
magnetic resonance in chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0749-1581
DOI - 10.1002/mrc.2432
Subject(s) - chemistry , chemical shift , diastereomer , density functional theory , computational chemistry , atomic orbital , ring (chemistry) , carbon 13 nmr , crystallography , stereochemistry , organic chemistry , physics , quantum mechanics , electron
1 H and 13 C NMR chemical shift calculations using the density functional theory–gauge including/invariant atomic orbitals (DFT–GIAO) approximation at the B3LYP/6‐311G++(d,p) level of theory have been used to assign both natural diastereoisomers of 6β‐hydroxyhyoscyamine. The theoretical chemical shifts of the 1 H and 13 C atoms in both isomers were calculated using a previously determined conformational distribution, and the theoretical and experimental values were cross‐compared. For protons, the obtained average absolute differences and root mean square (rms) errors for each comparison showed that the experimental chemical shifts of dextrorotatory and levorotatory 6β‐hydroxyhyoscyamines correlated well with the theoretical values calculated for the (3 R ,6 R ,2′ S ) and (3 S ,6 S ,2′ S ) configurations, respectively, whereas for 13 C atoms the calculations were unable to differentiate between isomers. The nature of the relatively large chemical shift differences observed in nuclei that share similar chemical environments between isomers was asserted from the same calculations. It is shown that the anisotropic effect of the phenyl group in the tropic ester moiety, positioned under the tropane ring, has a larger shielding effect over one ring side than over the other one. Copyright © 2009 John Wiley & Sons, Ltd.