title: 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.
abstract: 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.
authors.affiliation: Institute of Chemistry, Siberian Branch of the Russian Academy of Science, Favorski St 1, 664033, Irkutsk, Russia.
authors.forename: Andrei V
authors.initials: AV
authors.lastname: Afonin
authors.forename: Dmitry V
authors.initials: DV
authors.lastname: Pavlov
authors.forename: Igor A
authors.initials: IA
authors.lastname: Ushakov
authors.forename: Natalia A
authors.initials: NA
authors.lastname: Keiko
mesh_terms: D002247:Carbon Isotopes; D006835:Hydrazones / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy / Q000592:standards; D015394:Molecular Structure; D009587:Nitrogen Isotopes; D011522:Protons; D011765:Pyruvaldehyde / Q000031:analogs & derivatives* / Q000737:chemistry; D012015:Reference Standards; D013237:Stereoisomerism
chemical_list: D002247:Carbon Isotopes; D006835:Hydrazones; D009587:Nitrogen Isotopes; D011522:Protons; C576441:methylglyoxal bisdimethylhydrazone; D011765:Pyruvaldehyde