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 ⁴J_HF, ¹J_CF and ²J_CF couplings analysed using ab initio calculations and solvation theory. In FA the energy difference (E_cis–E_tr) 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 (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 –0.9 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 ⁴J_HF coupling (F–C–C–CH₃) is proportional to cos²θ, where θ is the F–C–C–C dihedral angle. The ¹J_CF and ²J_CF couplings also show a pronounced orientation dependence which could be of particular utility in those cases where other couplings are not present.