Mechanism of addition of fluoromethyl radicals to fluoroethylenes

The self-consistent charge and configuration method for subsystems (SCCCMS), charge sensitivity analysis, and our new scheme for energy partitioning were applied to discuss the mechanism of addition of fluoromethyl radicals to fluoroethylenes. A hybrid density functional method, i.e., the combination of Becke's three-parameter functional and the Perde/Wang 91 gradient-corrected functional, was used in the calculations with the 6-31G* basis set. Charge sensitivity characteristics (chemical potentials and hardnesses) of the reactants have been obtained. Chemical potentials clearly showed that alkenes act as donors of electrons (bases), while radicals act as accepters (acids). The systems "electronic activation energies" were decomposed into deformation, electrostatic, polarization, charge-transfer, and exchange components. Steric effect was identified with the sum of deformation and exchange energies, while polar effect was related to the sum of electrostatic, charge-transfer, and polarization contributions. A correlation between the magnitude of polar effect and inter-reactant distance in transition states was found. It has been demonstrated that the anomaly in regioselectivity in methyl or fluoromethyl radical addition to trifluoroethylene is understandable as an electrostatically controlled process.