A THEORETICAL INVESTIGATION OF THE STRUCTURES AND PROPERTIES OF CLASSICAL AND BRIDGED BETA-SUBSTITUTED ETHYL RADICALS

The structural parameters of beta-substituted ethyl radicals HnM-CH2-CH2. (HnM = H3C, H2N, HO, F, H3Si, H2P, HS, Cl) have been determined by ab initio calculations including electron correlation. At the UMP2/DZP + BF level of theory, the absolute energy minimum is found in all radicals for a classical structure 1a in which the radical center C(alpha) is slightly pyramidal. Substituents containing second-row heteroatoms adopt a gauche conformation (omega = 60-degrees) with respect to the singly occupied 2p carbon orbital (SOMO). Rotation about the C(alpha)-C(beta) bond is nearly free. Substituents containing third-row heteroatoms adopt the eclipsed conformation (omega = 0-degrees), having a rotation barrier of about 2 kcal mol-1. The symmetrically bridged structure 1b is less stable than the classical structure 1a by 40-60 kcal mol-1 and 10-20 kcal mol-1 in radicals bearing second- and third-row substituents, respectively. However, third-row heteroatoms form a very loose bridged complex except for M = Cl, the energy minimum being only ca. 0.3 kcal mol-1 lower in energy relative to dissociation. The rotational motion about the C(alpha)-C(beta) bond is always favored as against a shuttle motion of the substituent between the two carbon atoms. Shuttling is, however, likely only for the chlorine atom. There is no theoretical evidence of either static or dynamic asymmetric bridging from third-row substituents as proposed to explain the unexpectedly low values of the beta-proton hyperfine coupling constant a(beta) observed in radicals bearing third-row substituents as well as the stereochemical control exerted by third-row elements in free-radical reactions. The values of a(beta) computed as a Boltzmann weighted average over the rotational states agree qualitatively with the experimental findings. Low values of a(beta) in the eclipsed conformation are due to a sizable reduction in spin density at H(beta) with increasing electronegativity of the heteroatom. The high population of the eclipsed conformation in radicals bearing third-row substituents in conjunction with the nonplanarity of the radical site and/or steric hindrance of the beta-substituent could account for the observed stereochemical control.