CH···N and CH···O intramolecular hydrogen bonding effects in the¹H,¹³C and¹⁵N NMR spectra of the configurational isomers of 1-vinylpyrrole-2-carbaldehyde oxime substantiated by DFT calculations

According to the 1H, 13C and 15N NMR spectroscopic data and DFT calculations, the E-isomer of 1-vinylpyrrole-2-carbaldehyde adopts preferable conformation with the anti-orientation of the vinyl group relative to the carbaldehyde oxime group and with the syn-arrangement of the carbaldehyde oxime group with reference to the pyrrole ring. This conformation is stabilized by the CH···N intramolecular hydrogen bond between the α-hydrogen of the vinyl group and the oxime group nitrogen, which causes a pronounced high-frequency shift of the α-hydrogen signal in 1H NMR (∼0.5 ppm) and an increase in the corresponding one-bond 13C–1H coupling constant (ca 4 Hz). In the Z-isomer, the carbaldehyde oxime group turns to the anti-position with respect to the pyrrole ring. The CH···O intramolecular hydrogen bond between the H-3 hydrogen of the pyrrole ring and the oxime group oxygen is realized in this case. Due to such hydrogen bonding, the H-3 hydrogen resonance is shifted to a higher frequency by about 1 ppm and the one-bond 13C–1H coupling constant for this proton increases by ∼5 Hz. Copyright © 2008 John Wiley & Sons, Ltd.