The role of complexes in hydrogen abstraction from haloethanes by the hydroxyl radical. A case of guided reactions

Analysis of the minimum energy path (MEP) of hydrogen abstraction from ethane, fluoroethane and chloroethane by the hydroxyl radical (OH) has shown that hydrogen-bonded intermediate complexes are present on the reactants and products side of the MEP. A significant stabilization (2-3 kcal mol(-1)) is observed at the G2(MP2) level of theory for complexes formed in the haloethane reactions. They are characterized by an interaction between halogen atoms and hydrogen from OH and differ significantly from the transition-state structures. Vibrational analysis demonstrated that reactant complexes are a reaction branching point where three thermodynamically accessible channels are competing; alpha-abstraction, beta-abstraction and dissociation to reactants. (C) 1997 Elsevier Science B.V.