A classical trajectory study of the intramolecular dynamics, isomerization, and unimolecular dissociation of HO2

Abstract

The classical dynamics and rates of isomerization and dissociation of HO2 have been studied using two potential energy surfaces (PESs) based on interpolative fittings of ab initio data: An interpolative moving least-squares (IMLS) surface [A. Li, D. Xie, R. Dawes, A. W. Jasper, J. Ma, and H. Guo, J. Chem. Phys. 133, 144306 (2010)] and the cubic-spline-fitted PES reported by Xu, Xie, Zhang, Lin, and Guo (XXZLG) [J. Chem. Phys. 127, 024304 (2007)]. Both PESs are based on similar, though not identical, internally contracted multi-reference configuration interaction with Davidson correction (icMRCI+Q) electronic structure calculations; the IMLS PES includes complete basis set (CBS) extrapolation. The coordinate range of the IMLS PES is limited to non-reactive processes. Surfaces-of-section show similar generally regular phase space structures for the IMLS and XXZLG PESs with increasing energy. The intramolecular vibrational energy redistribution (IVR) at energies above and below the threshold of isomerization is slow, especially for O-O stretch excitations, consistent with the regularity in the surfaces-of-section. The slow IVR rates lead to mode-specific effects that are prominent for isomerization (on both the IMLS and XXZLG) and modest for unimolecular dissociation to H + O2 (accessible only on the XXZLG PES). Even with statistical distributions of initial energy, slow IVR rates result in double exponential decay for isomerization, with the slower rate correlated with slow IVR rates for O-O vibrational excitation. The IVR and isomerization rates computed for the IMLS and XXZLG PESs are quantitatively, but not qualitatively, different from one another with the largest differences ascribed to the ~2 kcal/mol difference in the isomerization barrier heights. The IMLS and XXZLG results are compared with those obtained using the global, semi-empirical double-many-body expansion DMBE-IV PES [M. R. Pastrana, L. A. M. Quintales, J. Brandão, and A. J. C. Varandas, J. Chem. Phys. 94, 8073 (1990)], for which the surfaces-of-section display more irregular phase space structure, much faster IVR rates, and significantly less mode-specific effects in isomerization and unimolecular dissociation. The calculated IVR results for all three PESs are reasonably well represented by an analytic, coupled three-mode energy transfer model.

Dynamics of Complex-Forming Bimolecular Reactions: A Comparative Theoretical Study of the Reactions of H Atoms with O2(3Σg–) and O2(1Δg)

“…The derivatives of both potential energy surfaces are discontinuous at the C 2v arrangement. Since this may distort the outcome of trajectories and destroy energy conservation along trajectories, we tested a smoothing technique similar to the symmetrization proposed by Dawes et al 46 The details of our method are summarized in the Appendix. We found that, while individual trajectories changed their course in some cases if they happened to visit the C 2v region, the ensemble average cross sections, angular and product state distributions and even the fraction of isomerized trajectories (see below) remained the same as with the original PES.…”

Inefficient intramolecular vibrational energy redistribution for the H + HO2 reaction and negative internal energy dependence for its rate constant

“…The effect of inefficient IVR on reactivity was previously characterized for reactions with HO 2 , O+ OH, and H + O 2 as reactants [45][46][47][48]; these systems share features with the present system and indeed similar effects were quantified. The present nonstatistical effect is one of a variety of nonstatistical effects that have drawn notable attention as potentially limiting the accuracy of statistical approaches like TST, including inefficient thermalization of reactants [49-53], "non-RRKM" unimolecular isomerization and dissociation [54-59], and post bottleneck bifurcations [60][61][62][63].…”

Accurate Full-Dimensional Global Diabatic Potential Energy Matrix for the Two Lowest-Lying Electronic States of the H + O2 ↔ HO + O Reaction

“…There have been several versions of the global reactive PES based on ab initio calculations for the ground electronic state HO 2 ( X̃ 2 A ″), − as well as an excited state HO 2 ( Ã 2 A ′) . The ground state PES developed more than 15 years ago by Xu, Xie, Zhang, Lin, and Guo (XXZLG) has been extensively used to understand the HO 2 spectrum and the H + O 2 ↔ O + OH reaction dynamics and kinetics. − …”