Hyotcherl Ihee,† Ahmed H. Zewail,† and William A. Goddard III‡*
† Arthur Amos Noyes Laboratory of Chemical Physics (127-72)
‡ Materials and Process Simulation Center, Beckman Institute (139-74)
Division of Chemistry and Chemical Engineering
California Institute of Technology, Pasadena, California 91125
Halo radicals such as CH2XCH2•, where X is a halogen, play an important role in the stereochemical control observed in many chemical reactions. The stereoselectivity can be explained either by high population of the anti conformer with high rotational barriers to other rotamers or by bridging of the halogen atom between two carbon atoms. To elucidate the origin of the stereoselectivity, we calculated the structures and potential surfaces of the haloethyl radicals (CH2XCH2•, X = F, Cl, Br, I) using ab initio quantum mechanics (QM). The QM methods include HF, local MP2, DFT (both B3PW91 and B3LYP). We find that the rotational barrier, bridged structure, and dissociation of the radicals are described much more accurately using the DFT than with HF or LMP2.
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 (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 of X = Cl, Br, and I. As will be discussed later, for halofluorinated radicals (CF2XCF2•), the theoretical and experimental results indicate a different picture; the bridged structure is no longer favorable.
The trends in the rotational barriers and structural parameters are consistent with hyperconjugation between the singly occupied carbon 2p orbital and the s *(C-X) MO.