Conformational analysis. Part 27. NMR, solvation and theoretical investigation of conformational isomerism in fluoro- and 1,1-difluoro-acetone

Abstract

The solvent and temperature dependence of the 1H and 13C NMR spectra of fluoroacetone (FA), 1,1-difluoroacetone (DFA) and 1,1,1-trifluoroacetone (TFA) are reported and the 4JHF, 1JCF and 2JCF couplings analysed using ab initio calculations and solvation theory.In FA the energy difference (Ecis–Etr) between the cis(F–C–CO 0°) and trans(F–C–CO 180°) conformers is 2.2 kcal mol–1 in the vapour, decreasing to 1.0 kcal mol–1 in CCl4 solution and to –0.6 kcal mol–1 in the pure liquid.In DFA the conformational equilibrium is between the less polar cis(H–C–CO 0°) and a gauche conformation (H–C–CO 104°). The energy difference (Eg–Ecis) is +0.8 kcal mol–1 in the vapour, decreasing to 0.1 kcal mol–1 in CCl4 solution and to –0.9 kcal mol–1 in the pure liquid.The vapour state energy difference for FA compares well with that calculated (2.8 kcal mol–1 at MP4/6–31G*). DFA calculations at this level gave only one minimum in the potential surface corresponding to the cis form. A minimum for the gauche conformer was only found when solvation was included in the ab initio calculations, or at much larger basis sets (6-311++G**).The conformer couplings obtained show that the 4JHF coupling (F–C–C–CH3) is proportional to cos2θ, where θ is the F–C–C–C dihedral angle. The 1JCF and 2JCF couplings also show a pronounced orientation dependence which could be of particular utility in those cases where other couplings are not present.