Spectroscopic Study of the Conformational Isomerism of 2-Formylfuran Derivatives

Abstract The conformational equilibria of 5-X-2-formylfurans (X = CH3, Br, CN, NO2, 4-O2NC6H4) were studied by infrared (IR), Raman and 1H-NMR spectroscopy. The ratios of the conformers were determined from the solvent dependence of the IR spectra, and the energy differences of the conformers from the temperature dependence of liquid-state Raman spectra and 1H-NMR spectra. Conclusions are drawn concerning the relative magnitudes of the rotational barriers. CNDO/2 calculations are included for comparison. All compounds investigated exist as equilibrium mixtures of the O⋯O trans and O⋯O cis forms, the former being predominant in non-polar media, the latter in polar solvents and the liquid state. The effect of substituents on the conformational properties are discussed.