Unusual spectroscopic behaviour of the carbonyl and carbon–carbon double-bond groups in the infrared spectra of simple nitroenaminoketone derivatives

The assignment of the vibrational spectra of several isomers of the simple nitroenamine Me(H)N–C(H)C(NO2)COMe has been theoretically examined (DFT at the B3LYP level using cc-pvtz basis sets) and a normal mode analysis of vibrations has been undertaken. Contrary to the usually expected, analysis of the computed IR spectra shows the inversion of the carbonyl stretching band and the `enamine bandʹ (a coupled mode that is mainly a combination of the CC and C–N stretchings) for the configurational isomer where the amino and carbonyl groups are forming an intramolecular hydrogen bonding.