Fermi resonance and solvent dependence of the CO moiety of 4‐(dimethylamino) benzaldehyde

The Raman spectra of 4‐(dimethylamino)benzaldehyde in the 1550–1750 cm⁻¹ region in seventeen solvents and as a molten liquid were measured. In all cases a multicomponent band was observed in the CO stretching region and these components were analyzed by the solvent variation method for Fermi resonance interactions. Unperturbed vibrational frequencies were calculated, and the frequencies attributed to ν(CO) were plotted against the solvent electron acceptor, number. A dual‐trace plot was observed. These traces were attributed to extrinsic interactions involving both the carbonyl oxygen and the aldehydic hydrogen in solvents of low acceptor number and predominantly the carbonyl oxygen in solvents of high acceptor number. The spectrum of molten 4‐(dimethylamino)benzaldehyde was used to predict an acceptor number of 14 for this molecule. This value was used to differentiate between high and low acceptor numbers for the solvents used in this study.