Conformations and Barriers of Haloethyl Radicals (CH₂XCH₂, X = F, Cl, Br, I): Ab Initio Studies

Radicals such as CH₂XCH₂^•, where X is a halogen, play an important role in the stereochemical control observed in many chemical reactions. To elucidate the origin of the stereoselectivity, we calculated the structures and potential energy surfaces of the haloethyl radicals (CH₂XCH₂^•, X = F, Cl, Br, I) using ab initio quantum mechanics [HF, local MP2, DFT (both B3PW91 and B3LYP)]. We find that the CH₂BrCH₂^• and CH₂ICH₂^• radicals strongly favor the symmetrically bridged structures while the CH₂ClCH₂^• radical leads to similar energy for symmetric bridging and classical structures. (In contrast, X = H and F leads to dramatically different structures). This confirms the Skell hypothesis of symmetric bridging to explain the stereochemical control of the CH₂BrCH₂^• and CH₂ICH₂^• radicals, indicating that such bridged structures play an important role in the dissociation processes involving CH²XCH₂^• with X = Cl, Br, and I. The trends in the rotational barriers and structural parameters are consistent with hyperconjugation between the singly occupied carbon 2p orbital and the σ*(C-X) MO. We find that the rotational barrier, bridged structure, and dissociation of the radicals are described much more accurately using DFT (with GGA) than with HF or LMP2.