Potential energy surface for the chlorine atom reaction with ethylene: A theoretical study

The potential energy surface of the reaction between chlorine atom and ethy lene was explored at the MP2/ 6-3 1G(d,p), Becke3LYP/6-31G(d,p), QCISD/6-31 G(d,p), MP2/6-31 1SG(d,p), MP2/6-31 1++G(3df,3pd), and MP2/aug-cc-pVDZ leve ls of theory. Further QCISD(T)/6-31G(d,p) and QCISD(T)/cc-pVDZ optimization s were performed for some structures of special interest. The geometrical p arameters computed for the different structures located on the potential en ergy surface do not differ too much when employing different methods and ba sis sets with the only exceptions of those structures involving long distan ce interactions (van derWaals structures). The pronounced flatness of the p otential energy surface in the regions where these structures appear seems to be the responsible for the observed discrepancies. The full optimized QC ISD structures tend to become less stable than those computed at the MP2 le vel, whereas the opposite is hue for the Becke3LYP structures. At the MP2 a nd 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 (minim um) 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, th e Becke3LYP calculations suggest a mechanism in which the direct shuttle mo tion could play a relevant role although the competing mechanism of rotatio n around the C-C bond is lower in energy. The MP2 and QCISD abstraction cha nnels also differ considerably: from the Becke3LYP one. However, in this ca se all the different potential energy surfaces seem to be consistent with t he reported experimental data pn the activation energy and endothermicity f or the abstraction reaction. The QCISD(T)/ aug-cc-pVDZ//QCISD/6-31G(d,p) re lative energies,and barrier heights are consistent with the experimental da ta available on exo/endothermicities and activation barriers for the additi on and abstraction reactions.