State-to-state quantum dynamics of O + O ₂ isotope exchange reactions reveals nonstatistical behavior at atmospheric conditions

Abstract

The O þ O 2 exchange reaction is a prerequisite for the formation of ozone in Earth's atmosphere. We report here state-to-state differential and integral cross sections for several O þ O 2 isotopeexchange reactions obtained by dynamically exact quantum scattering calculations at collision energies relevant to atmospheric conditions. These reactions are shown to be highly nonstatistical, evidenced by dominant forward scattering and deviation of the integral cross section from the statistical limit. Mechanistic analyses revealed that the nonstatistical channel is facilitated by shortlived osculating resonances. The theoretical results provided an in-depth interpretation of a recent molecular beam experiment of the exchange reaction and shed light on the initial step of ozone recombination.complex-forming reactions | isotope effects | ozone | reactive scattering M ost chemical reactions need be activated to overcome the energy barrier, but many others require little or no activation because the intrinsic reaction path is barrierless (1). The latter type of reactions typically involves free radicals. Even at very low energies, their encounters lead to a relatively stable intermediate complex, which eventually breaks up to form products. Such complex-forming reactions are prevalent in interstellar clouds, in atmospheres, and in other gas-phase conditions such as combustion. The complex-forming mechanism manifests in various measurable attributes, such as a non-Arrhenius rate constant and a product angular distribution peaked in both forward and backward directions. exchange reaction is a prototypical complex-forming reaction via the metastable ozone (O Ã3 ) intermediate, supported by a ∼1.13 eV potential well (2). The importance of this reaction derives from the fact that it is a prerequisite for the formation of ozone in Earth's atmosphere (3, 4):in which the metastable ozone formed during the exchange reaction is stabilized by collisions with a third body (M). An in-depth understanding of the bimolecular isotope exchange reactions will not only advance our knowledge of complex-forming reactions in general, but also shed light on the surprising and significant enrichment of heavy ozone isotopomers discovered in the stratosphere (5) and in laboratory settings (6) more than 20 years ago. A key issue for a complex-forming reaction is whether it is statistical or not. If the reaction is dominated by a sufficiently longlived intermediate, the reactivity is completely determined by the availability of open channels at a given energy (7,8), and dynamics becomes unimportant. This simple statistical picture has been very successful in understanding many complex-forming reactions (9). On the other hand, ample examples can be found for nonstatistical complex-forming reactions (10-12), often due to the short lifetime of the so-called "osculating" intermediate complex (13).Recently, van Wyngarden et al. (11) reported an important crossed molecular beam study of the 16 O þ 36 O 2 → 34 O 2 þ 18 O exchange reaction at 0.32 eV of co...

Communication: Highly accurate ozone formation potential and implications for kinetics

“…We note here that the QSM model is an approximation, since the exchange reaction is known to have a non-negligible nonstatistical component. 23 While exact quantum dynamics calculations require a full-dimensional PES, the QSM results based on the 2D PES reported here strongly suggest that a new full-dimensional PES at the same level of theory will close the gap between theory and experiment in this respect.…”

“…Here, we will focus on the rate constant of the O + O 2 exchange reaction, which is related to that of the ozone formation reaction in the high pressure limit. The cross section for the nominally barrierless reaction is dominated by large impact parameter collisions, 23 where the centrifugal potential transforms the submerged "reef" into a real barrier. Two important consequences are the following.…”

Communication: Rigorous quantum dynamics of O + O2 exchange reactions on an ab initio potential energy surface substantiate the negative temperature dependence of rate coefficients

“…29 In our recent work, quantum statistical model (QSM) calculations on the DLLJG PES have already produced a negative temperature dependence for the O + O 2 rate coefficients. 22 However, this QSM-based conclusion is tentative as the O + O 2 exchange reactions are known to have a strong non-statistical character, [30][31][32] accurate description of the kinetics with any form of statistical theory. To provide a conclusive assessment of the DLLJG PES, we report here an extensive quantum dynamics investigation, which computes the rate coefficients and their temperature dependence from state-to-state and initial-state selected integral cross sections.…”

“…All necessary helicity channels, up to 2J/3, labeled by the projection of the total angular momentum J on the body-fixed axis (K), were included. 31 The initial state-specified thermal rate coefficients were obtained by Boltzmann averaging their ICSs:…”

Dynamics of the O-Atom Exchange Reaction 16O(3P) + 18O18O(3Σg–) → 16O18O(3Σg–) + 18O(3P) at Hyperthermal Energies

“…[4][5][6][7] Very interesting isotope effects have been reported in ozone chemistry. 8,9 Several groups have reported unusual enrichment of heavy ozone, 50 O 3 , in stratospheric ozone samples. [10][11][12] _ENREF_13_ENREF_16 Numerous explanations for the unexpected enrichment have been offered in the literature since then.…”