Study of conformations and hydrogen bonds in the configurational isomers of pyrrole-2-carbaldehyde oxime by 1H, 13C and 15N NMR spectroscopy combined with MP2 and DFT calculations and NBO analysis

✍ Scribed by Andrei V. Afonin; Igor A. Ushakov; Dmitry V. Pavlov; Andrei V. Ivanov; Al'bina I. Mikhaleva

• Publisher: John Wiley and Sons
• Year: 2010
• Tongue: English
• Weight: 173 KB
• Volume: 48
• Category: Article
• ISSN: 0749-1581
• DOI: 10.1002/mrc.2650

✦ Synopsis

Abstract

The ^1^H, ^13^C and ^15^N NMR studies have shown that the E and Z isomers of pyrrole‐2‐carbaldehyde oxime adopt preferable conformation with the syn orientation of the oxime group with respect to the pyrrole ring. The syn conformation of E and Z isomers of pyrrole‐2‐carbaldehyde oxime is stabilized by the NH···N and NH···O intramolecular hydrogen bonds, respectively. The NH···N hydrogen bond in the E isomer causes the high‐frequency shift of the bridge proton signal by about 1 ppm and increase the ^1^J(N, H) coupling by ∼3 Hz. The bridge proton shows further deshielding and higher increase of the ^1^J(N, H) coupling constant due to the strengthening of the NH···O hydrogen bond in the Z isomer. The MP2 calculations indicate that the syn conformation of E and Z isomers is by ∼3.5 kcal/mol energetically less favorable than the anti conformation. The calculations of ^1^H shielding and ^1^J(N, H) coupling in the syn and anti conformations allow the contribution to these constants from the NH···N and NH···O hydrogen bondings to be estimated. The NBO analysis suggests that the NH···N hydrogen bond in the E isomer is a pure electrostatic interaction while the charge transfer from the oxygen lone pair to the antibonding orbital of the NH bond through the NH···O hydrogen bond occurs in the Z isomer. Copyright © 2010 John Wiley & Sons, Ltd.