Conformational preferences for N,N-dimethyl-2-haloacetamides (halo=F, Cl, Br and I) through theoretical and experimental studies: An unexpected orbital interaction

Conformational preferences and orbital interactions of N,N-dimethyl-2-fluoroacetamide (1), N,N-dimethyl-2-chloroacetamide (2), N,N-dimethyl-2-bromoacetamide (3) and N,N-dimethyl-2-iodoacetamide (4) were analyzed using experimental infra-red data, theoretical calculations and NBO analysis. The conformational equilibria of compounds 1-4 can be represented by their cis and gauche rotamers. The gauche form of 1 is stable in the vapor phase and in a non-polar solvent, but the cis is predominant in a polar solvent. For 2-4 the gauche form is more stable than the cis, in both the vapor and liquid phases. These conformational preferences were attributed to the orbital interaction between two antibonding orbitals, π*CO→σ*C-X. This unexpected interaction was possibly due to the high (0.3) electron density on π*CO, which results from the interaction between one nitrogen lone pair and π*CO.