Conformational preferences and orbital interactions for methyl haloacetates

Conformational preferences and orbital interactions of methyl chloroacetate (1), methyl bromoacetate (2) and methyl iodo-acetate (3) were analyzed using experimental infra-red data, theoretical calculations and NBO analyses. The conformational equilibria of compounds 1–3 can be represented by their cis and gauche rotamers. The gauche form of 1 is stable in the vapour phase and in a non-polar solvent, but the cis is predominant in a polar solvent. For 2 the gauche form is more stable than the cis, in both the vapour and liquid phases, but for compound 3 only the gauche   form was observed both in vapour phase as in solution. These conformational preferences were attributed to the orbital interaction between two antibonding orbitals πCO*→σCX*. This unexpected interaction was possibly due to the high (0.2) electron density on πCO*, which results from the interaction between ether oxygen lone pair and πCO*.