Detalle de: Potential Energy Surface for the Chlorine Atom Reaction with Ethylene: A Theoretical Study

The potential energy surface of the reaction between chlorine atom and ethylene was explored at the MP2/6-31G(d,p), Becke3LYP/6-31G(d,p), QCISD/6-31G(d,p), MP2/6-311+G(d,p), MP2/6-311++G(3df,3pd), and MP2/aug-cc-pVDZ levels of theory. Further QCISD(T)/6-31G(d,p) and QCISD(T)/cc-pVDZ optimizations were performed for some structures of special interest. The geometrical parameters computed for the different structures located on the potential energy surface do not differ too much when employing different methods and basis sets with the only exceptions of those structures involving long distance interactions (van der Waals structures). The pronounced flatness of the potential energy surface in the regions where these structures appear seems to be the responsible for the observed discrepancies. The full optimized QCISD structures tend to become less stable than those computed at the MP2 level, whereas the opposite is true for the Becke3LYP structures. At the MP2 and QCISD levels, the transition structure associated with a direct shuttle motion in the addition channel is too high in energy to be involved in the dissociation mechanism. The existence of two bridged structures Iadd (minimum) and TSadd (transition structure) on the potential energy surface helps to explain the experimentally detected stereochemical control exercised by the chlorine atom in reactions involving haloethyl radicals. Contrarily, the Becke3LYP calculations suggest a mechanism in which the direct shuttle motion could play a relevant role, although the competing mechanism of rotation around the C−C bond is lower in energy. The MP2 and QCISD abstraction channels also differ considerably from the Becke3LYP one. However, in this case all the different potential energy surfaces seem to be consistent with the reported experimental data on the activation energy and endothermicity for the abstraction reaction. The QCISD(T)/aug-cc-pVDZ//QCISD/6-31G(d,p) relative energies and barrier heights are consistent with the experimental data available on exo/endothermicities and activation barriers for the addition and abstraction reactions.