Classical trajectory calculations on gas-phase reactive collisions

Accurate Reaction Probabilities for Translational Energies on Both Sides of the Barrier of Dissociative Chemisorption on Metal Surfaces

Molecular dynamics simulations are essential for a better understanding of dissociative chemisorption on metal surfaces, which is often the rate-controlling step in heterogeneous and plasma catalysis. The workhorse quasi-classical trajectory approach ubiquitous in molecular dynamics is able to accurately predict reactivity only for high translational and low vibrational energies. In contrast, catalytically relevant conditions generally involve low translational and elevated vibrational energies. Existing quantum dynamics approaches are intractable or approximate as a result of the large number of degrees of freedom present in molecule-metal surface reactions. Here, we extend a ring polymer molecular dynamics approach to fully include, for the first time, the degrees of freedom of a moving metal surface. With this approach, experimental sticking probabilities for the dissociative chemisorption of methane on Pt(111) are reproduced for a large range of translational and vibrational energies by including nuclear quantum effects and employing full-dimensional simulations.

Trajectory dynamics study of collision-induced dissociation of the Ar + CH4 reaction at hyperthermal conditions: vibrational excitation and isotope substitution

We investigate the role of vibrational energy excitation of methane and two deuterated species (CD(4) and CH(2)D(2)) in the collision-induced dissociation (CID) process with argon at hyperthermal energies. The quasi-classical trajectory method has been applied, and the reactive Ar + CH(4) system has been modeled by using a modified version of the CH(4) potential energy surface of Duchovic et al. (J. Phys. Chem. 1984, 88, 1339) and the Ar-CH(4) intermolecular potential function obtained by Troya (J. Phys. Chem. A 2005, 109, 5814). This study clearly shows that CID is markedly enhanced with vibrational excitation and, to a lesser degree, with collision energy. In general, CID increases by exciting stretch vibrational modes of the reactant molecule. For the direct dissociation of CH(4), however, the CID cross sections appear to be essentially independent of which vibrational mode is initially excited. In all situations studied, the CID cross sections are always greater for the Ar + CD(4) reaction than for the Ar + CH(4) one, the Ar + CH(2)D(2) being an intermediate situation. A detailed analysis of the energy transfer processes, including their relation with CID, is also presented.

Exact quantum dynamics study of the O(+)+H2(v=0,j=0)-->OH(+)+H ion-molecule reaction and comparison with quasiclassical trajectory calculations

The close-coupling hyperspherical (CCH) exact quantum method was used to study the title barrierless reaction up to a collision energy (E(T)) of 0.75 eV, and the results compared with quasiclassical trajectory (QCT) calculations to determine the importance of quantum effects. The CCH integral cross section decreased with E(T) and, although the QCT results were in general quite similar to the CCH ones, they presented a significant deviation from the CCH data within the 0.2-0.6 eV collision energy range, where the QCT method did not correctly describe the reaction probability. A very good accord between both methods was obtained for the OH(+) vibrational distribution, where no inversion of population was found. For the OH(+) rotational distributions, the agreement between the CCH and QCT results was not as good as in the vibrational case, but it was satisfactory in many conditions. The kk(') angular distribution showed a preferential forward character, and the CCH method produced higher forward peaks than the QCT one. All the results were interpreted considering the potential energy surface and plots of a representative sampling of reactive trajectories.

Ab initio and direct quasiclassical-trajectory study of the Cl+CH4→HCl+CH3 reaction

We present an electronic structure and dynamics study of the Cl + CH(4)--> HCl + CH(3) reaction. We have characterized the stationary points of the ground-state potential-energy surface using various electronic structure methods and basis sets. Our best calculations, CCSD(T) extrapolated to the complete basis-set limit based on geometries and harmonic frequencies obtained at the CCSD(T)/aug-cc-pvtz level, are in agreement with the experimental reaction energy and indirect measurements of the barrier height. Using ab initio information, we have reparametrized a semiempirical Hamiltonian so that the predictions of the improved Hamiltonian agree with the higher-level calculations in various regions of the potential-energy surface. This improved semiempirical Hamiltonian is then used to propagate quasiclassical trajectories and characterize the reaction dynamics. The good agreement of the calculated HCl rotational and angular distributions with the experiment indicates that reparametrizing semiempirical Hamiltonians is a promising approach to derive accurate potential-energy surfaces for polyatomic reactions. However, excessive energy leakage from the initial vibrational energy of the CH(4) molecule to the reaction coordinate in the trajectory calculations calls into question the suitability of the standard quasiclassical-trajectory method to describe energy partitioning in polyatomic reactions.

Cross sections of the O++H2→OH++H ion-molecule reaction and isotopic variants (D2, HD): Quasiclassical trajectory study and comparison with experiments

A dynamics study [cross section and microscopic mechanism versus collision energy (E(T))] of the reaction O+ + H2 --> OH+ + H, which plays an important role in Earth's ionosphere and interstellar chemistry, was conducted using the quasiclassical trajectory method, employing an analytical potential energy surface (PES) recently derived by our group [R. Martinez et al., J. Chem. Phys. 120, 4705 (2004)]. Experimental excitation functions for the title reaction, as well as its isotopic variants with D2 and HD, were near-quantitatively reproduced in the calculations in the very broad collision energy range explored (E(T) = 0.01-6.0 eV). Intramolecular and intermolecular isotopic effects were also examined, yielding data in good agreement with experimental results. The reaction occurs via two microscopic mechanisms (direct and nondirect abstraction). The results were satisfactorily interpreted based on the reaction probability and the maximum impact parameter dependences with E(T), and considering the influence of the collinear [OHH]+ absolute minimum of the PES on the evolution from reactants to products. The agreement between theory and experiment suggests that the reaction mainly occurs through the lowest energy PES and nonadiabatic processes are not very important in the wide collision energy range analyzed. Hence, the PES used to describe this reaction is suitable for both kinetics and dynamics studies.

Ab initiopotential energy surface, variational transition state theory, and quasiclassical trajectory studies of the F+CH4→HF+CH3 reaction

In this work we present a study of the F+CH(4)-->HF+CH(3) reaction (DeltaHdegrees(298 K)=-32.0 kcal mol(-1)) using different methods of the chemical reaction theory. The ground potential energy surface (PES) is characterized using several ab initio methods. Full-dimensional rate constants have been calculated employing the variational transition state theory and using directly ab initio data. A triatomic analytical representation of the ground PES was derived from ab initio points calculated at the second- and fourth-order Møller-Plesset levels with the 6-311+G(2df,2pd) basis set, assuming the CH(3) fragment to be a 15 a.m.u. pseudoatom in the fitting process. This is suggested from experiments that indicate that the methyl group is uncoupled to the reaction coordinate. A dynamics study by means of the quasiclassical trajectory (QCT) method and employing this analytical surface was also carried out. The experimental data available on the HF internal states distributions are reproduced by the QCT results. Very recent experimental information about the reaction stereodynamics is also borne out by our QCT calculations. Comparisons with the benchmark F+H(2) and analogous Cl+CH(4) reactions are established throughout.

Ab initio analytical potential energy surface and quasiclassical trajectory study of the O+(4S)+H2(X 1Σg+)→OH+(X 3Σ−)+H(2S) reaction and isotopic variants

An analytical potential energy surface (PES) representation of the O(+)((4)S)+H(2)(X (1)Sigma(g) (+)) system was developed by fitting around 600 CCSD(T)/cc-pVQZ ab initio points. Rate constant calculations for this reaction and its isotopic variants (D(2) and HD) were performed using the quasiclassical trajectory (QCT) method, obtaining a good agreement with experimental data. Calculations conducted to determine the cross section of the title reaction, considering collision energies (E(T)) below 0.3 eV, also led to good accord with experiments. This PES appears to be suitable for kinetics and dynamics studies. Moreover, the QCT results show that, although the hypotheses of a widely used capture model are not satisfied, the resulting expression for the cross section can be applied within a suitable E(T) interval, due to errors cancellation. This could be a general situation regarding the application of this simple model to ion-molecule processes.

THEORETICAL STUDIES OF CHEMICAL DYNAMICS: Overview of Some Fundamental Mechanisms

Recent remarkable progress in theoretical studies of (a) quantum dynamics of chemical reactions, (b) characteristics and dynamics of superexcited states of molecules, (c) nonadiabatic transitions at potential curve crossings, and (d) multidimensional tunneling is reviewed briefly. Underlying common basic concepts and fundamental mechanisms such as adiabaticity and nonadiabatic transition are extracted and discussed in order to facilitate a comprehensive understanding of chemical dynamics. Not only the basic theoretical methodologies but also the intriguing dynamical aspects of each subject are explained as simply as possible.