MECHANISM OF HYDROGEN ABSTRACTION REACTIONS BY FREE-RADICALS - SIMPLE METATHESIS OR INVOLVING INTERMEDIATE COMPLEX

Hydrogen abstraction reactions, R + HX --> RH + X (R = CFyH3-y (y = 0-3), C2H5; X = F, Cl, Br, CH3), have been investigated by ab initio methods. It is found that with reactants which are polarized with a net positive charge on the hydrogen of HX and a net negative charge on the carbon at the radical site, reactions proceed via a weakly hydrogen-bonded intermediate complex (R..H.X) prior to the formation of the transition state (R.H.X). All geometries (reactants, intermediate complexes, transition-state structures) were optimized at the (U)MP2/6-31G* level. Electron correlation energies were evaluated at the (U)MP4/6-311G**//(U)-MP2/6-31G* level. For the reaction CH2F + HBr --> CH3F + Br, the Gaussian-1 (G1) theory was employed. The stability of the intermediate complexes (R..H.X) depends mainly on the dipole interaction between the polarized reactants as well as the one-electron, two-orbital interaction between the SOMO of the radical and the sigma*HX orbital of HX. The theoretical results also furnish indirect support for negative activation energies found experimentally for R + HBr --> RH + Br (R = alkyl radical). Absolute rate constants and kinetic isotope effects for the reaction C2H5 + HBr(DBr) --> C2H6(C2H5D) + Br are evaluated by transition-state theory (TST) and RRKM theory as applied to the dissociation of the intermediate complex.