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

Conformational preferences and orbital interactions of N,N-dimethyl-2-fluoroacetamide (1), N,N-dimethyl-2-chloroacetamide (2), N,Ndimethyl-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, p* C]O /s* C-X . This unexpected interaction was possibly due to the high (0.3) electron density on p* C]O , which results from the interaction between one nitrogen lone pair and p* C]O .