CF2XCF2X and CF2XCF2 center dot radicals (X = Cl, Br, I): Ab initio and DFT studies and comparison with experiments

1,2-dihalotetrafluoroethanes (CF2XCF2X, X = I, Br and Cl) and halotetrafluo roethyl radicals (CF2XCF2., X = I, Br, and Cl) have been studied by ab init io molecular-orbital techniques using restricted Hartree-Fock and Density f unctional theory (DFT-B3PW91). For the optimized HF geometries, we carried out local MP2 calculations to account for electron correlation effects. Eac h CF2XCF2X molecule and CF2XCF2. radical has two conformational minima (ant i and gauche) and two rotational transition structures in the rotational en ergy surface along the C-C bond. The rotational barriers of the radicals ar e smaller than those of the parent molecules due to the absence of the nonb onded interaction between two halogen atoms. In contrast, the conformationa l energy difference between two stable rotamers (anti and gauche) of each r adical is larger than that in the corresponding parent molecules. This stab ilizing effect on the anti conformers of the radicals is rationalized in te rms of hyperconjugation between the radical center and the sigma*(C-X) mole cular orbital. The dissociation energies for breaking the first and second C-X bonds of CF2XCF2X were also calculated and compared with available expe rimental data. The CF2XCF2. radicals show dramatically different behavior c ompared with haloethyl radicals (CH2XCH2.). The CF2XCF2. radical has two mi nima and two saddle points, whereas CH2XCH2. radical has only one minimum a nd one saddle point in the rotational energy surface. In addition, the brid ged structures are not stable for CF2XCF2. radicals in contrast with CH2XCH 2. radicals. The origin of these differences is attributed to differences i n the environment of the radical center. The calculated structures of the C F2ICF2. radical were used in interpreting a recent experimental observation (Cao et al. Proc. Natl. Acad. Sci. 1999, 96, 338) and are compared with qu antitative results from a new experiment (Ihee et al. Science 2001, 291, 45 8) using the ultrafast electron diffraction technique.