The role of bridged structures in the mechanism of the reaction between chlorine atom and ethylene

The potential energy surface for the reaction between a chlorine atom and e thylene was extensively explored by using ab initio methodologies. Two diff erent routes for the 1,2 migration of the chlorine atom were identified. On e of them involves a C-2v (B-2(2)) transition structure (TSsb) that directl y connects two equivalent structures (P and P') of the 2-chloroethyl radica l with the chlorine atom attached either to C1 (P) or to C2 (P') carbon ato ms in ethylene (shuttling motion). In the second pathway, the 2-chloroethyl radical (P) coverts into a C-2v ((2)A(1)) intermediate (I-add) through a C -s ((2)A') transition structure (TSadd). Then I-add leads to the 2-chloroet hyl radical (P') through a transition structure equivalent to TSadd (TSadd' ). The 'indirect shuttling motion' described along this latter route is not ably lower in energy and allows one to rationalize some mechanistic aspects experimentally observed in reactions involving haloethyl radicals. (C) 200 0 Elsevier Science B.V. All rights reserved.