Negative hyperconjugation of some fluorine containing groups

Energies of some aromatic or conjugated aliphatic compounds fluorine containing were calculated within the framework of the density functional theory at the level B3LYP/6-311 + G(d.p)// B3LYP/6-311 + G(d.p). Evaluation proceeded in terms of isodesmic reactions, in which the groups CF3, SF5 and SCF3 acted as substituents. All these groups behave as rather strong resonance acceptors. The pertinent canonical structures must be formulated with one C-F bond broken-no-bond resonance or negative hyperconjugation. In agreement with these formulae, the C-F bond lengths are elongated: they increase particularly strongly if a conjugated donor group is present. The principle of negative hyperconjugation is thus able to predict qualitatively several observable facts (energy. geometry), even when the pertinent formulae much exaggerate the real Structural features.