Vinylogous systems. Part 5. Vibrational spectroscopy of the nitroenamine system

Abstract

Vibrational frequencies have been assigned to those bonds which comprise the conjugated core (IIa) of a nitroenamine. The results are compared and contrasted with recent infrared studies of the aminoenone system (Ia). The analysis for (IIa) rests upon the postulate that vibrational coupling of the multiply bonded units is reinforced by the accompanying electronic flux, leading to an unprecedentedly low value (1 280–1 250 cm^–1) for the symmetric nitro vibration of the NCCNO₂ unit. No dramatic differences between the i.r. spectra of cis- and trans-isomers were apparent. The high degree of electronic polarisation that exists in nitroenamines is reflected in extremely weak Raman bands for all major functional groups.