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

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.