Abstract: | Carbon-carbon bond deformation curves for fluorinated ethylene molecules and the corresponding carbocations were calculated
by the ab initio self-consistent field method in the 5-31G basis set. The maximum force required for bond cleavage was taken
as a criterion for bond strength. It has been found that for ethylene, replacement of hydrogen by fluorine insignificantly
strengthens the C=C bonds in symmetric molecules. However, in molecules with an asymmetric arrangement of fluorine atoms,
the bond is slightly weakened due to different charges on the carbon atoms. The configuration of the corresponding carbocations
also depends on the bond polarity: an assymmetric distribution of electron density in the C=C bond region leads to the formation
of σ-complexes, while a symmetric distribution of electron density (pure covalent bonding) gives π-complexes. Since the carbon-carbon
bond in the σ-complexes is essentially weaker, one should expect significant weakening of the bond in high-acidity media if
the bond exhibits any kind of asymmetry (chain branching, defects, etc.). For the considered molecules, an antibatic correlation
has been established between the strength criterion Fmax (unlike the dissociation energy) and the bond length.
Institute of Physical Chemistry, Russian Academy of Sciences, Moscow. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 1, pp. 34–41, January–February, 1995.
Translated by I. Izvekova |