A Quasiclassical Trajectory Study of the Reaction of H Atoms with O₂(¹Δ_g)

Abstract

The kinetic and dynamic characteristics of the reaction of H atoms with electronically excited O 2 were studied using the quasiclassical trajectory (QCT) method and the potential energy surface of Li et al. (J. Chem. Phys. 2010, 133, 144306-144314.). The reaction takes place via a deep potential well that can be entered by climbing a barrier in the reactant valley and can be left without a barrier on the product side. In this reaction the basic assumptions of statistical rate theories are not fulfilled: i) 80% of the trajectories cross the barrier region twice and are nonreactive; b) The energy is not equilibrated in the HO 2 potential well. The QCT cross sections agree well with those from the existing exact quantum mechanical data and extend them to vib-rotationally excited reactants. The thermal rate coefficients agree well with measurements of pure reactive quenching of O 2 ( 1 Δ g ) and are 2 lower than those involving both reactive and electronic quenching. The temperature dependence is described as k 2 = 5.81×10 -16 T 1.45 exp(-2270/T) cm 3 molecule -1 s -1 . Based on comparison of the QCT data with the two kinds of experiments we estimate that electronic quenching is faster than reaction by a factor of about 10 at low and 2 at high flame temperatures.

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

“…The height of the barrier is well below the dissociation energy, which means that all HO 2 molecules formed by capture have much more energy than needed for isomerization. The occurrence of H-atom transfer from one O to the other has been observed on both the ground-and the excited-state potential surfaces 25,39 . Detailed Table 1 shows the following ratios for both reactions R1 and shows that reactive events are intimately connected to isomerization: uniformly in about 90% of reactive collisions the H atom first makes a bond with one of the oxygens and switches to the other.…”

“…and recently the kinetics 31,32,33,34,35,36 and dynamics 37,38,39 of the reaction of electronically excited molecular oxygen, O 2 ( 1 Δ g ) with H atoms,…”

“…On the other hand, in the exited state, the reaction is exoergic by 41.5 kJ/mol and the energetic threshold is determined by the relatively low barrier in the entrance channel (the classical height of which is 31.7 kJ/mol). Electronic excitation has been found to significantly increase the reactivity of O 2 towards H atoms 39 . In accordance with the expectations based on the endoergicity of reaction R1 and the barrier height of reaction R2, the excitation functions are characterized by a threshold energy near 58 kJ/mol and 30 kJ/mol, respectively for R1 and R2, when the reactant diatom is in vib-rotational ground state.…”

“…The complexes formed, even when having relatively long lifetimes, were found to disobey several expectations for the applicability of statistical rate theories. First, in the majority of collisions, the system, after entering the potential well, cross the potential barrier twice, once on the way in, once on the way back to reactants 39 . Second, the lifetime distribution of the complex is not exponential in contrast to the requirements of the applicability of statistical theories.…”

A surface-hopping method for semiclassical calculations of cross sections for radiative association with electronic transitions

“…The semiclassical methods are based on classical trajectories and therefore do not contain the resonance structure and the tunneling in the cross sections. In spite of the lack of resonance and tunneling effects, trajectory based methods can provide reasonable results in many cases [25][26][27] . The extension of the theoretical studies for polyatomic systems is highly desirable.…”