J.C. Rayez

Quasiclassical trajectory method for molecular scattering processes: necessity of a weighted binning approach

Abstract It is shown that the standard binning method utilized in the classical study of scattering processes dealing with more than three atoms, which consists in rounding the product vibrational actions (the x i s) to their nearest (the n i s), is not appropriate when the total energy disposal in the products is too low for the classical and quantum densities of vibrational states to be equal. Two alternative approaches are proposed which account as simply as possible for both the whole set of trajectories performed and the quantal nature of the vibrational energy.

Classical dynamics of dissociative adsorption for a nonactivated system: The role of zero point energy

We present dissociative adsorption probabilities of H2 on Pd(111) computed with the classical trajectory method. We perform both classical (C) and quasiclassical (QC) calculations, the latter including, by contrast with the former, the initial zero point energy (ZPE) of H2. We analyze in detail the role played by the ZPE and demonstrate the strong and weak points of both C and QC calculations. We show that ZPE is crucial in accelerating the molecules toward the surface through vibrational softening. However, at low energies, dynamic trapping is quenched in QC calculations by processes of vibration to rotation energy transfer that would be associated with closed channels in a quantum approach. In this study we use a new representation of the H2/Pd(111) potential energy surface (obtained by interpolation of ab initio data) with a significantly better accuracy in the entrance channel region which plays a decisive role in the dissociation dynamics.

A theoretical study of the dynamics of the reaction C(3P)+NO(X2Π)→CN(X2Σ+)+O(3P)

Abstract A quasi-classical trajectory (QCT) treatment of the reaction C ( 3 P) + NO (X 2 Π)→CN(X 2 Σ + ) + O ( 3 P) is presented on two different analytical and global representations of the lowest potential energy surface of 2 A′ symmetry. We compare the dynamic information obtained theoretically with that obtained from the recent supersonic pulsed crossed beam scattering experiment performed in our laboratory. A satisfactory agreement is obtained, notably for the percentage of the vibrational energy and the population of the vibrational levels of CN(X 2 Σ + ).

A crossed-beam study of the reaction C(1D)+H2(X1Σ+, v=0)→CH(X2Π, v′)+H(2S)

Abstract Product angular and time-of-flight distributions have been measured for the first time for the prototypical insertion reaction C( 1 D)+H2 using the crossed-beam technique with mass spectrometric detection at the collision energy of 1.86 kcal mol−1. Center-of-mass angular and kinetic energy distributions have been derived for CH(X 2 Π , v′=0)+H products and compared with those of statistical calculations based on phase-space theory.

Analytical representations of high level ab initio potential energy curves of the C2 molecule

Abstract Realistic analytical representations of the twelve lowest singlet and triplet electronic adiabatic potential energy curves of C2 molecule are given in this article. The corresponding electronic states are correlated with C atoms both in their 3P state. A new set of high level MRCI calculations coupled with a double many-body expansion analytical fitting based on the extended Hartree–Fock approximate correlation energy model have been used in this work. Using RKR data available in the literature, comparison is made between our results and RKR turning points concerning the four lowest singlet states X1Σ+g, A1Πu, B1Δg and B′1Σ+g of C2. The agreement is very satisfying.

Theoretical approach to the reaction C(3P)+NO(X 2Π)

Correlation diagram and extended ab initio calculations are used to obtain the main features of the dynamical aspects of the reaction C(3P)+NO(Xu20092Π). The lowest surface of 2Π symmetry leading to the ground state via an expected intermediate CNO is found to be 4.2 eV below the reactant energies. There is no potential energy barrier for the collinear approach along the 2Π surface. The first excited state of CNO has 2Σ+ symmetry. The high energy barrier appearing for the collinear C+NO approach yielding this state disappears for a bent configuration. A spin orbit avoided crossing between the 2Σ+ and 4Π surfaces is computationally investigated for the collinear approach. It is concluded that any such surface crossing will not play a significant role in the generation of excited CN(Au20092Π). Due to the deep wells for the ground and first excited state of CNO, the vibrational populations in the products CN(Xu20092Σ+) and CN(Au20092Π) are expected to be thermalized. However, the rotational populations are expected to be so...

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.

Theoretical study on the comparative fate of the 1-butoxy and β-hydroxy-1-butoxy radicals

Theoretical n high level ab initio BAC-MP4 and DFT calculations followed by a kinetic RRKM analysis have been performed n in this work for the study of unimolecular reactions of the 1-butoxy and β-hydroxy-1-butoxy radicals. We have n shown that the substitution of H by OH on the carbon in the β position of the 1-butoxy radical (leading to the n β-hydroxy-1-butoxy radical) results in an important lowering of the decomposition barrier and a slight increase of the n isomerisation barrier. Coupled to the rate constant calculations, this study suggests that, contrary to the fate n of the 1-butoxy radical, the thermal decomposition is the major pathway for the β-hydroxy-1-butoxy radical. We n have also shown that, under atmospheric conditions (760 Torr and 298 K), both isomerisation and decomposition processes n are still in the fall-off range for the hydroxy radical. These behaviors have been interpreted in terms of n electronic structures and intramolecular hydrogen bonding. This is the first theoretical study of the β-hydroxy-1-butoxy n radical unimolecular reactions. As there are no experimental measurements on the β-hydroxy-1-butoxy radical n rate constants, this theoretical study is the first to predict kinetic parameters for the decomposition and n isomerisation n reactions of this compound.

Dynamics simulation of N2 scattering onto W(100,110) surfaces: A stringent test for the recently developed flexible periodic London-Eyring-Polanyi-Sato potential energy surface

An efficient method to construct the six dimensional global potential energy surface (PES) for two atoms interacting with a periodic rigid surface, the flexible periodic London-Eyring-Polanyi-Sato model, has been proposed recently. The main advantages of this model, compared to state-of-the-art interpolated ab initio PESs developed in the past, reside in its global nature along with the small number of electronic structure calculations required for its construction. In this work, we investigate to which extent this global representation is able to reproduce the fine details of the scattering dynamics of N(2) onto W(100,110) surfaces reported in previous dynamics simulations based on locally interpolated PESs. The N(2)/W(100) and N(2)/W(110) systems are chosen as benchmarks as they exhibit very unusual and distinct dissociative adsorption dynamics although chemically similar. The reaction pathways as well as the role of dynamic trapping are scrutinized. Besides, elastic/inelastic scattering dynamics including internal state and angular distributions of reflected molecules are also investigated. The results are shown to be in fair agreement with previous theoretical predictions.

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 ue5fb 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.