The nu(CO) IR analysis of the 2-fluoro-substituted NN-dimethylacetamides RC(O)NMe2 [R=FCH2 (1), F2CH (2) and F3C (3)] and of the parent NN-dimethylacetatnide [R=CH3 (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/b - 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 CF12 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 pi(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 (Deltanu) are in line with this trend and support the pi(CO)*/n(rho) orbital increases in the same direction. (C) 2002 Elsevier Science B.V. All rights reserved.607417008799