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Stereospecificity of 1 H, 13 C and 15 N shielding constants in the isomers of methylglyoxal bisdimethylhydrazone: problem with configurational assignment based on 1 H chemical shifts
Author(s) -
Afonin Andrei V.,
Pavlov Dmitry V.,
Ushakov Igor A.,
Keiko Natalia A.
Publication year - 2012
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.3828
Subject(s) - chemistry , chemical shift , intramolecular force , hydrogen bond , nmr spectra database , coupling constant , crystallography , molecule , stereochemistry , computational chemistry , spectral line , physics , organic chemistry , astronomy , particle physics
In the 13 C NMR spectra of methylglyoxal bisdimethylhydrazone, the 13 C‐5 signal is shifted to higher frequencies, while the 13 C‐6 signal is shifted to lower frequencies on going from the EE to ZE isomer following the trend found previously. Surprisingly, the 1 H‐6 chemical shift and 1 J (C‐6,H‐6) coupling constant are noticeably larger in the ZE isomer than in the EE isomer, although the configuration around the –CH═N– bond does not change. This paradox can be rationalized by the C–H⋯N intramolecular hydrogen bond in the ZE isomer, which is found from the quantum‐chemical calculations including Bader's quantum theory of atoms in molecules analysis. This hydrogen bond results in the increase of δ ( 1 H‐6) and 1 J (C‐6,H‐6) parameters. The effect of the C–H⋯N hydrogen bond on the 1 H shielding and one‐bond 13 C– 1 H coupling complicates the configurational assignment of the considered compound because of these spectral parameters. The 1 H, 13 C and 15 N chemical shifts of the 2‐ and 8‐(CH 3 ) 2 N groups attached to the –C(CH 3 )═N– and –CH═N– moieties, respectively, reveal pronounced difference. The ab initio calculations show that the 8‐(CH 3 ) 2 N group conjugate effectively with the π‐framework, and the 2‐(CH 3 ) 2 N group twisted out from the plane of the backbone and loses conjugation. As a result, the degree of charge transfer from the N‐2– and N‐8– nitrogen lone pairs to the π‐framework varies, which affects the 1 H, 13 C and 15 N shieldings. Copyright © 2012 John Wiley & Sons, Ltd.