Trajectory Dynamics Study of the Ar + CH₄ Dissociation Reaction at High Temperatures:  the Importance of Zero-Point-Energy Effects

Abstract

Large-scale classical trajectory calculations have been performed to study the reaction Ar + CH 4 f CH 3 + H + Ar in the temperature range 2500 e T/K e 4500. The potential energy surface used for ArCH 4 is the sum of the nonbonding pairwise potentials of Hase and collaborators (J. Chem. Phys. 2001, 114, 535) that models the intermolecular interaction and the CH 4 intramolecular potential of Duchovic et al. (J. Phys. Chem. 1984, 88, 1339, which has been modified to account for the H-H repulsion at small bending angles. The thermal rate coefficient has been calculated, and the zero-point energy (ZPE) of the CH 3 product molecule has been taken into account in the analysis of the results; also, two approaches have been applied for discarding predissociative trajectories. In both cases, good agreement is observed between the experimental and trajectory results after imposing the ZPE of CH 3 . The energy-transfer parameters have also been obtained from trajectory calculations and compared with available values estimated from experiment using the master equation formalism; in general, the agreement is good.

Quasiclassical trajectory study of the SiH4+H→SiH3+H2 reaction on a global ab initio potential energy surface

“…However, in classical trajectories, the energy can flow freely among all the modes without ZPE constraint, possibly yielding behavior which is not allowed in quantized real world ͑e.g., a molecule with a vibrational energy below its ZPE͒. To fix this problem in QCT calculations, some strategies [60][61][62][63][64][65] have been proposed but no completely satisfactory scheme has emerged. Here, in order to correct the ZPE leakage, we employed a so-called nonactive method, 65,66 which follows the genuine QCT approach but discards from statistics any nonphysical trajectory that is found to violate the specified physical criteria.…”

Trajectory Dynamics Study of Collision-Induced Dissociation of the Ar + CH4 Reaction at Hyperthermal Conditions: Vibrational Excitation and Isotope Substitution

“…The main goal of most of these studies has been the determination of dissociation rate coefficients in the high-temperature regime, which is considered to be important for the CH 4 /O 2 combustion chemistry. 33 Particularly, we have performed 25 the first QCT study of the title reaction where the rate coefficient has been calculated in the temperature range 2500 e T/K e 4500; general agreement between experimental 30 and our theoretical 25 rate coefficients has been achieved after adequate correction of the CH 3 zeropoint energy (ZPE), as calculated by using the corresponding normal-mode harmonic frequencies. In addition, we have also used the QCT method to obtain 25 energy-transfer parameters from which the 〈∆E〉 (the average energy transferred in all collisions) and 〈∆E 2 〉 1/2 (the root-mean-squared energy transferred in all collisions) values show good agreement with values deduced 24 from experiment, 30 while overestimating them in the case of 〈∆E d 〉 (the average energy transferred in deactivating collisions).…”

“…In addition, the intermolecular Ar-CH 4 interactions have been modeled by a pairwise generalized exponential function whose parameters have been obtained by a fit to CCSD(T)/cc-pVTZ ab initio points. 26 This intermolecular potential accurately describes the ab initio data 26 up to energies of ∼10 eV, while the range of applicability of the one 44 used in our previous work 25 extends only up to ∼5 eV. Since both intramolecular 43,25 and intermolecular 26 potential functions have been extensively described in the original papers, no further details are given here.…”

“…In the present dynamics study, we have applied an improved version 25 of the CH 4 potential energy surface of Duchovic et al, 43 which proved 25 to be more reliable than the original one, 43 to reproduce the vibrational excitation of methane. In addition, the intermolecular Ar-CH 4 interactions have been modeled by a pairwise generalized exponential function whose parameters have been obtained by a fit to CCSD(T)/cc-pVTZ ab initio points.…”

“…16 Schatz and collaborators applied the direct dynamics version of the QCT method in which the energy and its gradients are calculated "on-the-fly" as the trajectory is integrated, while the experiments at Minton's laboratory have been performed with the use of a crossed molecular beams apparatus that incorporates a fast-atom beam source. [17][18][19] The Ar + CH 4 collisional system has been the subject of various theoretical [20][21][22][23][24][25][26] and experimental [27][28][29][30][31][32] studies concerning both CID and ET processes. The main goal of most of these studies has been the determination of dissociation rate coefficients in the high-temperature regime, which is considered to be important for the CH 4 /O 2 combustion chemistry.…”