A diradical mechanism for the addition of F2 to ethene: a density functional theory study

The long disputed mechanism for the F(2) addition to ethene is elucidated by density functional theory calculations. With hybrid functionals and a large basis set, DFT provides an excellent description for the dissociation of ground state F(2), and a reasonable account for the F(2)...C(2)H(4) van der Waals complex, which makes it possible for the modeling of reactions between elemental fluorine and organic molecules. The attack of F(2) on ethene first produces a diradical intermediate, which then dissociates into two radicals CH(2)F-CH(2) and F. The first step is exothermic with a low barrier around 1.8 kcal/mol, and the exothermic energy is more than enough to overcome the barrier in the second step for the homolysis of the dangling F-F bond in the diradical, although the presence of solvents and matrices environment could stabilize the diradical. Our calculations provide a coherent framework to understand this reaction not only in the gas and solution phases, but also in the matrices environment where mode-specific enhancement is observed for the addition process induced by infrared radiation.