Infrared absorption and isomerism of 3-aminocrotonic esters. Part I. 3-(Alkylamino)crotonic esters

The i.r. spectra of 3-(monoalkylamino)crotonic esters indicate that these substances exist either in the liquid state or in solution as equilibrium mixtures of the isomers with the (Z)- and (E)-configuration. Each isomeric form has characteristic frequencies which can be used in assigning compounds of this type to their correct configuration. The rate of interconversion between the isomers and the position of the equilibrium depend mainly on the polarity of the solvent, non-polar solvents favouring the chelated (Z)-form. The spectra show complexities which are interpreted in terms of the existence in each isomeric form of rotamers arising from restricted rotations around the C–N and C–CO2R single bonds. The spectra of the N-deuteriated derivatives have revealed the existence of extensive mechanical coupling affecting mainly the ν(CC), ν(C–N) and δ(N–H) vibrations.