M.T. Rayez

Three-atom indirect exchange reactions. II. Dynamical behaviours explained by a simple model

Abstract A quasi-classical trajectory (QCT) treatment of the role of the topological factors of the intermediate well in the nascent distribution of the energy on the products of indirect reactive exoergic processes is presented. We show that there exists a closed domain of regular behaviour of the trajectories which is connected to a continuous relationship between the average vibrational energy channeled on the new diatomic molecule and the anisotropy and the location of the well. An optical model is presented to explain simply this behaviour. This model carries some predictive character.

A theoretical study of the decomposition of halogenated alkoxy radicals. I. Hydrogen and chlorine extrusions

Abstract The reaction paths for the extrusion of a Z  H or Cl atom from an alkoxy radical CXYZO where X and Y are H, Cl and F in all possible combinations have been studied using the MNDO/CI method. Radicals of this type have been implicated in the upper atmosphere reactions that are thought to be involved in the decomposition of the ozone layer. The results are discussed using Marcus theory and compared to existing experimental observations. Correlation diagrams are used to illustrate the effect of the position along the reaction coordinate of a surface crossing upon the activation energy for the extrusion of Cl reaction, due to the presence of a p-type orbital on the departing atom.

Three-atom indirect exchange reactions. I. 1d QCT study of the topological factors influencing the energetic distribution on the products

Abstract In this paper, we analyze the topological factors which play a role on the energetic distribution on the products of a three-atom indirect reaction: A + BC → AB + C. In order to realize a systematic investigation we built an analytical expression for model surfaces exhibiting a well along the route from reactants to products. With the help of the one-dimensional quasi-classical trajectory approach, it has been found that the anisotropy of the well is a dominant factor which governs the final distribution between the vibration of the newly formed diatomic molecule AB and the relative translation between C and AB.

Theoretical study of the reaction C(3P) + N2O(X1Σ+). II. Dynamical trajectory studies on a three-atom model

Abstract A three-atom analytical model for the ground-state potential-energy surface for the reaction C + N 2 O is proposed. A classical trajectory study is performed on this reaction. It shows a much higher vibrational excitation with population inversion in the vibrational levels of CN(X 2 Σ + ) than expected from experiment and, therefore, suggests a distribution of the energy of the reaction not only into the newly formed CN bond but also into the pre-existing NO bond.

Ab initio study of the potential energy surfaces for the reaction N(4Su +CH(X 2IIr) → CN(X 2Σ+, A 2IIi + H(2Sg)

Abstract The three lowest triplet potential energy surfaces of HCN have been investigated with the complete active space multiconfigurational self consistent field (CAS/MCSCF) method. Two surfaces are issued from the ground state reactants, while the third one is issued from the first excited state of the reactants, and is coupled to the others by a conical intersection. Of the two surfaces issued from the ground state reactants, one possesses a very small potential barrier in the entrance channel, while the other surface has none, thus making the reaction possible at very low temperature. For each surface, two potential wells have been found, corresponding to bent structures HCN and HNC. These potential wells are separated by high isomerisation saddle points. The approach of H toward CN shows a potential barrier for each of the three surfaces. Some parts of the conical intersection have been calculated and found energetically lower than the ground state reactants. Thus, transitions between surfaces should play an important role in the reaction dynamics.

A theoretical study of the decomposition of halogenated alkoxy radicals. II. Fluorine extrusion

Abstract The extrusion of a fluorine atom from alkoxy radicals is analyzed theoretically and compared with experiment. Although the MNDO/CI method gives qualitatively correct information about the nature of the wavefunction, the MNDO method with or without CI is inadequate to describe the dissociation reaction path, presumably due to parameterization problems. On the contrary, ab initio SCF coupled with extended CI seems to correctly mimic the barrier for fluorine extrusion. Using modified Marcus theory in conjunction with the ab initio results for CH 2 FO , we evaluate the barriers for the other radicals investigated.

Four-atom exoergic indirect reactions A+BCD → AB+CD. 1D-QCT study of some topological factors influencing the energetic distribution of the products

Abstract Some results concerning the influence of two topological factors - the anisotropy and the depth of the intermediate potential well ABCD - on the final energy disposal are presented in the case of an indirect exchange exothermic reaction A + BCD → AB + CD, the BCD molecule being linear in its equilibrium geometry and the four atoms keeping a collinear conformation during the course of the process. Analytical models of the potential energy surface lead to the conclusion that these two topological factors essentially govern the partitioning between the vibrational motion of the two diatomic molecules formed, AB and CD. On the contrary, only a weak (and perhaps not significant) influence is shown for the sharing between the relative recoil energy of the products and the total vibration. A detailed analysis of the dependence of the relative percentages of vibrational energy of AB and CD on the vibrational phases of the two normal stretching modes of BCD suggests that there is a loss of predictability of models of elementary chemical reactions as the number of atoms increases.

Ab initio calculation of nuclear spin—spin coupling constant of HF and CN−

Abstract The pertubation—variational method introducing singularities has been used for the calculation of the Fermi contact contribution to the NMR spin—spin coupling constants of HF and CN − . The method employed in this paper cannot definitely solve the convergence problem, although an improvement has been observed.

Preliminary experimental and theoretical results on the dynamics of the reaction N + CH → CN + H

Owing to its exoergicity, the reaction N + CH → CN + H might produce CN in 3 electronic states: X2Σ+, A2Πi and B2Σ+. However, the correlation diagram rules out the production of B2Σ+ state for any symmetry of the reaction complex, either linear or planar. It has been checked experimentally. A vibrational population inversion in A2Πi state has been observed. The excitation limit at v’=16 agrees with the total energy available to the products. Theoretical calculations found no potential energy barrier as expected for an exoergic atom + radical reaction.