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

Abstract

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.

Theoretical approach to the mechanism of reactions between halogen atoms and unsaturated hydrocarbons: The Cl + propene reaction

“…These levels of theory were chosen bearing in mind the results of previous studies on similar systems. 15,16,19 The structures located on the PESs were characterized as minima or transition structures by computing the corresponding Hessian matrices. Graphical analysis of the imaginary frequency associated with the transition structures and intrinsic reaction coordinate (IRC) 25,26 calculations provided the required topological information to interconnect all the structures.…”

“…According to our previous experience, 15,16,19 we improved the energetics by performing QCISD(T)/aug-cc-pVDZ//MP2/aug-ccpVDZ calculations. The MP2/aug-cc-pVDZ geometries were preferred to the MP2/6-31G(d,p) ones used previously 16,19 because in the present case the Dunning augmented basis set leads to a more uniform mechanistic picture for the abstraction process (see below).…”

“…The MP2/aug-cc-pVDZ geometries were preferred to the MP2/6-31G(d,p) ones used previously 16,19 because in the present case the Dunning augmented basis set leads to a more uniform mechanistic picture for the abstraction process (see below). Further, the presence of diffuse functions in the basis set allows for an appropriate representation of the dispersion forces that should play an important role in the stabilization of the weakly bound structures considered in this work.…”

“…Some important consequences arising from the presence of weakly bound complexes on the potential energy surfaces (PESs) for these reactions have been reported in recent studies. 15,16 Intermediate van der Waals complexes have been shown to become specially relevant for systems with significant tunneling effects 17 or in the cases where negative Arrhenius activation energy parameters are observed. 18 It has also been shown that the existence of intermediate complexes on the PES allows one to rationalize the pressure dependence experimentally observed in the abstraction reaction between chlorine atoms and isoprene.…”

How does a reaction path branching take place? A classification of bifurcation events

“…For example, this is explicitly said in Ref. [26], or drawn in papers of Sordo et al [27]; but it is not true.…”