Radicals such as CH 2 XCH 2• , 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 (CH 2 XCH 2 • , X ) F, Cl, Br, I) using ab initio quantum mechanics [HF, local MP2, DFT (both B3PW91 and B3LYP)]. We find that the CH 2 BrCH 2• and CH 2 ICH 2• radicals strongly favor the symmetrically bridged structures while the CH 2 ClCH 2 • 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 CH 2 BrCH 2• and CH 2 ICH 2 • radicals, indicating that such bridged structures play an important role in the dissociation processes involving CH 2 XCH 2• 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. IntroductionThe class of halo radicals such as CH 2 XCH 2 • is important in a number of chemical processes [1][2][3][4] and determines the stereoselectivity of the reaction products from halogenation 1,2,5-9 of alkenes and alkanes. Their role in stereoselective control is determined by whether the radical is classical (Figure 1c) or bridged and if bridged whether the structure is symmetric ( Figure 1a) or asymmetric (Figure 1b). The possible minima and transition structures for rotation around the C-C bond are schematically represented in Figure 2. Anti (I) and gauche (III) rotamers are candidates for local energy minima on the rotational energy surface because Pauli repulsion between bonding pairs would be minimized at these two conformations. The other two structures (II and IV) possessing eclipsed bonds are also possible transition states on the rotational energy surface.If the structure of the radical is bridged, we expect retention of the stereochemistry. However, if the radical prefers a classical asymmetric conformation, additional conditions must be fulfilled to exert stereochemical control. Namely, a strong preference for high population of the anti conformer and nonplanarity of the radical center are required. Experimental 10-20 and theoretical 21-43 studies have led to a good understanding of the structure and energetics for the cases of X ) H, F, and Cl; however, little is known about X ) Br and I. The motivation for our studies was to elucidate the X ) Br and I systems. But we also studied X ) H, F, Cl in cases to compare to previous experiments and theory.In this paper we use four techniques of first principles quantum mechanics (QM) methods to examine the potential surface for CH 2 XCH 2 • with X ) H, F, Cl, Br, and I. These QM met...