Mechanism Change in the Dynamics of the O' + O2 → O'O + O Atom Exchange Reaction at High Collision Energies.

György Lendvay

doi:10.1021/acs.jpca.9b07393

Abstract

The extreme velocity and the large available energy of atoms with hyperthermal kinetic energies can give rise to novel mechanisms and behavior of chemical reactions unseen at thermal conditions. Crossed-molecular-beams experiments combined with isotope labeling on the reaction of hyperthermal O atoms with O2 molecules have provided an example of the arising complexity of such systems. Quasiclassical trajectory (QCT) calculations proved to be instructive in the exploration of the microscopic mechanism of the reactive and inelastic scattering observed, and a new mechanism has been identified: there are reactive collisions in which the potential energy remains repulsive during the entire encounter ("direct" reactions in which, in a sense, no complex is formed). In this work, the effect of the magnitude of the collision energy on this mechanism is explored. At hyperthermal collision energies, the reaction is characterized by a unique impact parameter window favorable for reaction through complex formation, while the direct collisions take place exclusively at small impact parameters. In direct reactive collisions, contributing as much as 12% to the reaction cross section, first the existing bond is broken, and the new bond is formed afterward. This kind of collision is unique to extremely high collision energies. Analysis of various correlations was used to find out the details of the reaction dynamics. The observed phenomena indicate that when the collision energy is extremely high, one can expect deviation from what an extrapolation from the more familiar energy ranges would predict.

Highlights

The reaction of O atoms with O2 molecules has been widely studied both experimentally and theoretically
Another reason for the interest in the reaction is the so-called mass-independent isotope effect, which leads to anomalous isotope abundance of oxygen in stratospheric ozone.[4−9] Collisional energy transfer plays an essential role in ozone formation,[10−12] and the fate of the collision complex depends on the lifetime and the internal dynamics of the complex, which may be isotope-dependent.[13]
Studies of mechanistic questions are less popular because they are timeconsuming, we have been engaged in using the method to understand the connection between the features of the potential energy surface (PES) and the dynamics of bimolecular reactions,[22−26] with particular interest to complex-forming reactions.[18−20,27−31] In connection with the picture that emerged from the Quasiclassical trajectory (QCT) calculations on the O + O2 system[20] at very high collision energies, we found intriguing the possibility of mechanism change of bimolecular reactions when Ecoll covers a wide range, such as the appearance of the dual mechanism that may arise at some energy threshold

Summary

Introduction

The reaction of O atoms with O2 molecules has been widely studied both experimentally and theoretically. The importance of the reaction lies in the fact that this is the way of stratospheric ozone formation:[1−3] ozone can be stabilized when the initially formed O3 collision complex lives long enough to be hit by a collision partner that reduces its internal energy so that it cannot dissociate Another reason for the interest in the reaction is the so-called mass-independent isotope effect, which leads to anomalous isotope abundance of oxygen in stratospheric ozone.[4−9] Collisional energy transfer plays an essential role in ozone formation,[10−12] and the fate of the collision complex depends on the lifetime and the internal dynamics of the complex, which may be isotope-dependent.[13] The question of the origin of the mass independence of enrichment of isotopes 17O and 18O has not been answered yet and is the subject of recent experimental and theoretical activity. Theoretical work[17] has demonstrated that the O + O2 atom exchange reaction displays nonstatistical behavior, independently of its isotope constitution

Methods

Results

Discussion

Conclusion