Datensatz

Zusammenfassung

Compared to ethylene and its nonfluorinated derivatives, C₂F₄ is peculiar in many reactions. It very easily adds to radicals and prefers formation of four-membered rings over Diels-Alder reactions. This has been rationalized by the preference of fluorine for carbon sp³ hybridization, which is possible on opening of the double bond. Another property, the thermal dissociation of the C=C bond, has been explained by the stabilization of the product (CF₂) by back-bonding. Here, it is attempted to correlate such properties with vibrational constants, in particular for C=C stretching and twisting and for carbon pyramidalization. The only force constant found to be lowered compared to ethylene is that for trans pyramidalization (ν₈), and CC bond softening on ν₈ distortion is indicated by the conspicuously large magnitude of anharmonic constant, x₁₈. Both observations can be rationalized by a valence-bond model that predicts a trans bent structure on weakening the CC bond. Conclusions are drawn about the dissociation path and peculiarities of the potential. Other anharmonicities, both experimental and calculated and some in ¹²C¹³CF₄ and ¹³C₂F₄, are also discussed. In particular some strong Fermi resonances are identified and their effects accounted for.