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 ethylene was extensively explored by using ab initio methodologies. Two different routes for the 1,2 migration of the chlorine atom were identified. One of them involves a C2v (2B2) transition structure (TSsb) that directly connects two equivalent structures (P and P′) of the 2-chloroethyl radical with the chlorine atom attached either to C1 (P) or to C2 (P′) carbon atoms in ethylene (shuttling motion). In the second pathway, the 2-chloroethyl radical (P) coverts into a C2v (2A1) intermediate (Iadd) through a Cs (2A′) transition structure (TSadd ). Then Iadd leads to the 2-chloroethyl radical (P′) through a transition structure equivalent to TSadd (TSadd′ ). The `indirect shuttling motionʹ described along this latter route is notably lower in energy and allows one to rationalize some mechanistic aspects experimentally observed in reactions involving haloethyl radicals.