A comparative ab initio and semi-empirical study of the vibrational spectra of a series of nitroenamines with intramolecular hydrogen-bonds has been performed. Although the semi-empirical MNDO/H calculations reproduce qualitatively some of the observed spectral properties, the ν(NO₂) frequencies are considerably overestimated. The minimal basis set STO-3G gives the correct order for the frequencies of the characteristic vibrations of the nitroenamine system, and reproduces many of the observed structural effects. Better results still are obtained with the split-valence 3-21G basis set. The calculation of force constants, infrared and Raman intensities, and isotope frequency shifts has confirmed the previous assignment of the experimental spectra. The analysis of the normal modes reveals the complex vibrational couplings occurring in these molecules and explains the unusual vibrational spectra of the nitroenamine system. The comparison between theoretical and experimental spectra also allowed an independent assignment of the nature of the isomers present in the molecules studied.