Theoretical and experimental investigation on the rotational isomerism in alpha-fluoroacetophenones.

The geometries involved in the conformational equilibria of alpha-fluoroacetophenone, p-nitro-alpha-fluoroacetophenone, and p-methoxy-alpha-fluoroacetophenone were investigated. Theoretical calculations showed that cis and gauche forms (F-C-C=O) are their most stable conformers and that in the vapor phase the gauche conformer is predominant. The three compounds were synthesized, and the conformational behavior in solution was estimated from infrared (IR) and nuclear magnetic resonance (NMR) spectra obtained in solvents of different polarity. Their IR spectra showed one carbonyl absorption for the cis and one for the gauche conformer, and that the cis conformer was preferred in the more polar solvents. (1)J(CF), (2)J(C(O)F), and (2)J(HF) coupling constants were obtained from their NMR spectra, and they also showed a preference for the cis conformer when more polar solvents were used. The vapor phase calculations showed a conformational preference for the gauche form. However, when the solvent effects were included in the calculations, the results were in complete agreement with the experimental data (NMR and IR), the cis conformer being the most stable one.