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

The solvent and temperature dependence of the H-1 and C-13 NMR spectra of fluoroacetone (FA), 1,1-difluoroacetone (DFA) and 1,1,1-trifluoroacetone (TFA) are reported and the (4)J(HF), (1)J(CF) and (2)J(CF) couplings analysed using ab initio calculations and solvation theory. In FA the energy difference (E(cis) - E(tr)) between the cis (F-C-C=O 0 degrees) and trans (F-C-C=O 180 degrees) 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-C=O 0 degrees) and a gauche conformation (H-C-C=O 104 degrees). The energy difference (E(g) - E(cis)) is +0.8 kcal mol(-1) in the vapour, decreasing to 0.1 kcal mol(-1) in CCl4 solution and to -0.6 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 sol solvation was included in the ab initio calculations, or at much larger basis sets (6-311++G**). The conformer couplings obtained show that the (4)J(HF) coupling (F-C-C-CH3) is proportional to cos(2) theta, where theta is the F-C-C-C dihedral angle. The (1)J(CF) and (2)J(CF) couplings also show a pronounced orientation dependence which could be of particular utility in those cases where other couplings are not present.