MECHANISM OF HYDROGEN ABSTRACTION REACTIONS BY FREE-RADICALS - SIMPLEMETATHESIS 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 met hods. It is found that with reactants which are polarized with a net p ositive charge on the hydrogen of HX and a net negative charge on the carbon at the radical site, reactions proceed via a weakly hydrogen-bo nded intermediate complex (R..H.X) prior to the formation of the trans ition state (R.H.X). All geometries (reactants, intermediate complexes , transition-state structures) were optimized at the (U)MP2/6-31G lev el. Electron correlation energies were evaluated at the (U)MP4/6-311G //(U)-MP2/6-31G* level. For the reaction CH2F + HBr --> CH3F + Br, th e Gaussian-1 (G1) theory was employed. The stability of the intermedia te complexes (R..H.X) depends mainly on the dipole interaction between the polarized reactants as well as the one-electron, two-orbital inte raction between the SOMO of the radical and the sigmaHX orbital of HX . The theoretical results also furnish indirect support for negative a ctivation energies found experimentally for R + HBr --> RH + Br (R = a lkyl radical). Absolute rate constants and kinetic isotope effects for the reaction C2H5 + HBr(DBr) --> C2H6(C2H5D) + Br are evaluated by tr ansition-state theory (TST) and RRKM theory as applied to the dissocia tion of the intermediate complex.