13C NMR, infrared, solvation and theoretical investigation of the conformational isomerism in 1-haloacetones (X = Cl, Br and I)

The solvent dependence of the 13C NMR spectra of chloroacetone (CA), bromoacetone (BA) and iodoacetone (IA) are reported and the 3JCH couplings analysed using ab initio calculations and solvation theory. In CA the energy difference (Ecis − Egauche) between the cis (ClCCO 0°) and gauche (ClCCO 155o) conformers is 1.7 kcal mol−1 in the vapour, decreasing to 0.8 kcal mol−1 in CCl4 solution and to −1.0 kcal mol−1 in the pure liquid. The conformational equilibrium, in BA, is between the more polar cis (BrCCO 0o) and gauche (BrCCO 132°) conformations. The energy difference (Ecis − Egauche) is 1.8 kcal mol−1 in the vapour, decreasing to 0.9 kcal mol−1 in CCl4 solution and to −0.4 kcal mol−1 in the pure liquid. The energy difference (Ecis − Egauche), in IA, between the cis (ICCO 0°) and gauche (ICCO 104°) conformers is 1.1 kcal mol−1 in the vapour phase, decreasing to 0.5 kcal mol−1 in CCl4 solution and to −0.5 kcal mol−1 in the pure liquid. The vapour state energy difference for BA [1.4 kcal mol−1 at B3LYP/6-311++G(d,p)] and for IA [1.6 kcal mol−1 at B3LYP/6-311++G(d,p)/LANL2DZ)] are in very good agreement with the above values. For CA the agreement is also satisfactory [1.4 kcal mol−1 at B3LYP/6-311++G(d,p)].