Study of stereospecificity of 1H, 13C, 15N and 77Se shielding constants in the configurational isomers of the selenophene-2-carbaldehyde azine by NMR spectroscopy and MP2-GIAO calculations

• Published in: Magnetic resonance in chemistry 2011-11, Vol.49 (11), p.740-748
• Main Authors: Afonin, Andrei V., Pavlov, Dmitry V., Albanov, Alexander I., Levanova, Ekaterina P., Levkovskaya, Galina G.
• Format: Article
• Language: English
• Subjects: 13C NMR; 15N NMR; 1H NMR; 77Se NMR; Azines; Constants; GIAO calculations; Isomers; Isotopes; Magnetic Resonance Spectroscopy; Mathematical analysis; Molecular Conformation; NBO analysis; NMR; NMR spectroscopy; Nuclear magnetic resonance; Organoselenium Compounds - chemistry; Protons; Quantum Theory; Selenium - chemistry; selenophene-2-carbaldehyde azine; Shielding; shielding constants; Spectra; Stereoisomerism; stereospecificity
• ISSN: 0749-1581; 1097-458X; 1097-458X
• DOI: 10.1002/mrc.2824

In the 1H and 13C NMR spectra of selenophene‐2‐carbaldehyde azine, the 1H‐5, 13C‐3 and 13C‐5 signals of the selenophene ring are shifted to higher frequencies, whereas those of the 1H‐1, 13C‐1, 13C‐2 and 13C‐4 are shifted to lower frequencies on going from the EE to ZZ isomer or from the E moiety to the Z moiety of EZ isomer. The 15N chemical shift is significantly larger in the EE isomer relative to the ZZ isomer and in the E moiety relative to the Z moiety of EZ isomer. A very pronounced difference (60–65 mg/g) between the 77Se resonance positions is revealed in the studied azine isomers, the 77Se peak being shifted to higher frequencies in the ZZ isomer and in the Z moiety of EZ isomer. The trends in the changes of the measured chemical shifts are reasonably reproduced by the GIAO calculations at the MP2 level of the 1H, 13 C, 15 N and 77Se shielding constants in the energy‐favorable conformation with the syn orientation of both selenophene rings relative to the C = N groups. The NBO analysis suggests that such an arrangement of the selenophene rings may take place because of a higher energy of some intramolecular interactions. Copyright © 2011 John Wiley & Sons, Ltd. The 1H, 13 C, 15 N and 77Se chemical shifts of the selenophene ring change in a regular manner on going from the 1‐EE to the 1‐ZZ isomer or from the E moiety of 1‐EZ isomer to the Z moiety that is substantiated by gauge‐included atomic orbital (GIAO) calculations. The discovered trends in change of chemical shifts reproduce that observed previously for a series of oximes. This allows the use of the chemical shifts for conformational and configurational analysis of azines. The NBO analysis shows that the π,π*‐conjugation as well as σ,σ*‐ hyperconjugation favors the syn‐arrangement of selenophene rings relative to the C = N group in all isomers. Very large difference (60–65 mg/g) between 77Se resonance in the 1‐EE and 1‐ZZ isomers as well as the E and Z moieties of 1‐EZ isomer is explained by the joint action of steric and electronic effects.