Yh. Chen et E. Tschuikowroux, MECHANISM OF HYDROGEN ABSTRACTION REACTIONS BY FREE-RADICALS - SIMPLEMETATHESIS OR INVOLVING INTERMEDIATE COMPLEX, Journal of physical chemistry, 97(15), 1993, pp. 3742-3749
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.