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

The ¹H, ¹³C and ¹⁵N 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 ¹J(N, H) coupling by ∼3 Hz. The bridge proton shows further deshielding and higher increase of the ¹J(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 ¹H shielding and ¹J(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.