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

Abstract

The solvent dependence of the 13C NMR spectra of chloroacetone (CA), bromoacetone (BA) and iodoacetone (IA) are reported and the 3 J CH couplings analysed using ab initio calculations and solvation theory. In CA the energy difference ( E cis − E gauche ) between the cis (Cl C C O 0°) and gauche (Cl C C O 155 o) conformers is 1.7 kcal mol −1 in the vapour, decreasing to 0.8 kcal mol −1 in CCl 4 solution and to −1.0 kcal mol −1 in the pure liquid. The conformational equilibrium, in BA, is between the more polar cis (Br C C O 0 o) and gauche (Br C C O 132°) conformations. The energy difference ( E cis − E gauche ) is 1.8 kcal mol −1 in the vapour, decreasing to 0.9 kcal mol −1 in CCl 4 solution and to −0.4 kcal mol −1 in the pure liquid. The energy difference ( E cis − E gauche ), in IA, between the cis (I C C O 0°) and gauche (I C C O 104°) conformers is 1.1 kcal mol −1 in the vapour phase, decreasing to 0.5 kcal mol −1 in CCl 4 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)].