The potential energy surface for the reaction between a chlorine atom and ethylene was extensively explored by using ab initio methodologies.Two different rutes for the 1,2 migration of the chlorine atom were identified.One of them involves a C_(2v) (~2B_2) transition structure (TS_(sb) 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 C_(2v) (~2A_1) intermediate (I_(add) through a C_s (~2A') transition structure (TS_(add).Then I_(add) leads to the 2-chloroethyl radical (P') through a transition structure equivalent to TS-(add) (TS'_(add).The indirect shuttling motion'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.
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