The infrared spectra of substituted ω-bromoacetophenones were recorded in solvents of varied dielectric constants. All the compounds exhibited two carbonyl bands. The splitting of the bands was attributed to existence of rotomers. The higher and lower frequency bands were assigned to cis and gauche conformations respectively. A linear plot was obtained between the frequencies of the bands and Hammett σ constants. The ratio of the cis and gauche conformers increased with increase in the dielectric constant of the solvent. The results suggest that the relative stabilities of the conformers are determined by the electrostatic repulsion between the C=O and C-Br dipoles in the cis form and electrostatic attraction between negative bromine atom and the carbon atom para to the ring substituent which is rendered positive by the electron withdrawing groups.

The conformational analysis of cinnamanilides has been carried out using IR spectroscopy. All the anilides studied were found to exist as equilibrium mixtures ofs-cis ands-trans forms in benzene.

Thes-cis form was predominant over thes-trans in all the anilides except in thep-nitro anilide in which thes-trans form was predominant. The relative stabilities of the conformers were found to depend upon the electrostatic repulsions between the anilide nitrogen and the β-carbon atom in thes-trans form and those between the π-electrons of the C=O and C=C bonds in thes-cis form.