CALCULATION OF CARBON-CARBON BOND DEFORMATION CURVES FOR FLUORINATED ETHYLENES AND THE CORRESPONDING CARBOCATIONS

Carbon-carbon bond deformation curves for fluorinated ethylene molecul es 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 . dr has been found that for ethylene, replacement of hydrogen by fluo rine insignificantly strengthens the C = C bonds in symmetric molecule s. However, in molecules with an asymmetric arrangement of fluorine at oms, the bond is slightly weakened due to different charges an the car bon atoms. The configuration of the corresponding carbocations also de pends on the bond polarity: an asymmetric distribution of electron den sity in the C = C bond region leads to the formation of sigma-complexe s, while a symmetric distribution of electron density (pure covalent b onding) gives pi-complexes. Since the carbon-carbon bond in the sigma- complexes is essentially weaker, one should expect significant weakeni ng of the bond in high-acidity media if the bond exhibits any kind of asymmetry (chain branching, defects, etc.). For the considered molecul es, an antibatic correlation has been established between the strength criterion F-max (unlike the dissociation energy) and the bond length.