Conformational and electronic interaction studies of 2-fluoro-substituted N, N-dimethylacetamides

Abstract

The ν CO IR analysis of the 2-fluoro-substituted N,N-dimethylacetamides RC(O)NMe 2 [RFCH 2 ( 1), F 2CH ( 2) and F 3C ( 3)] and of the parent N,N-dimethylacetamide [RCH 3 ( 4)] in solvents of increasing polarity suggests the occurrence of Fermi resonance for 2, 3 and 4 and of cis– gauche rotational isomerism for 1. The 1st overtone analysis along with ab initio calculations at MP2/6−31+G(d,p) level are in agreement with the existence of the cis and gauche conformers for 1 and indicate the occurrence of cis– gauche and gauche– gauche conformers for 2 and of a single cis– gauche– gauche rotamer for 3. The stabilisation of the gauche rotamer with respect to the cis rotamer for 1 and of the gauche– gauche rotamer over the cis– gauche rotamer for 2, along with the unique cis– gauche– gauche rotamer for 3 is discussed in terms of a −I inductive effect of the CF n group, a Repulsive Field Effect between the CO and C−F dipoles and the orbital interaction between one or two fluorine 2p lone pairs (in the gauche conformation) and the π CO ∗ orbital. Eigenvector analysis shows that the fluorine 2p orbital coefficient at the LUMO increases progressively, going from the cis– gauche– gauche rotamer of 3 to the cis– gauche rotamer of 2, to the gauche– gauche rotamer of 2 and to the gauche rotamer of 1. Carbonyl frequency gauche shifts (Δ ν) are in line with this trend and support the π CO ∗/ n F orbital increases in the same direction.