Kinetic isotope effect of the 16O + 36O2 and 18O + 32O2 isotope exchange reactions: Dominant role of reactive resonances revealed by an accurate time-dependent quantum wavepacket study

The Journal of Chemical Physics

2016

Calculations of energy transfer in the recombination reaction that forms ozone are carried out within the framework of the mixed quantum/classical theory and using the dimensionally reduced 2D-model of ozone molecule, with bending motion neglected. Recombination rate coefficients are obtained at room temperature for symmetric and asymmetric isotopomers of singly and doubly substituted isotopologues. The processes of resonance formation, spontaneous decay, collisional dissociation, and stabilization by bath gas (Ar) are all characterized and taken into account within the steady-state approximation for kinetics. The focus is on stabilization step, where the mysterious isotopic η-effect was thought to originate from. Our results indicate no difference in cross sections for stabilization of scattering resonances in symmetric and asymmetric isotopomers. As practical results, the general and simple analytic models for stabilization and dissociation cross sections are presented, which can be applied to resonances in any ozone molecule, symmetric or asymmetric, singly or doubly substituted. Present calculations show some isotope effect that looks similar to the experimentally observed η-effect, and the origin of this phenomenon is in the rates of formation/decay of scattering resonances, determined by their widths, that are somewhat larger in asymmetric isotopomers than in their symmetric analogues. However, the approximate two-dimensional model used here is insufficient for consistent and reliable description of all features of the isotopic effect in ozone. Calculations using an accurate 3D model are still needed. Published by AIP Publishing. [http://dx

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

On stabilization of scattering resonances in recombination reaction that forms ozone

Ivanov

The Journal of Chemical Physics

2016

“…In what follows the symbol S is used for the most abundant sulfur isotope, 32 S, and Q is used for one of the rare isotopes ( 33 S, 34 S, or 36 S). It is assumed that those are present in trace amounts, and each can be treated independently from other rare isotopes.…”

Section: Recombination Kineticsmentioning

confidence: 99%

Recombination reactions as a possible mechanism of mass-independent fractionation of sulfur isotopes in the Archean atmosphere of Earth

Proc. Natl. Acad. Sci. U.S.A.

2017

A hierarchy of isotopically substituted recombination reactions is formulated for production of sulfur allotropes in the anoxic atmosphere of Archean Earth. The corresponding system of kinetics equations is solved analytically to obtain concise expressions for isotopic enrichments, with focus on massindependent isotope effects due to symmetry, ignoring smaller mass-dependent effects. Proper inclusion of atom-exchange processes is shown to be important. This model predicts significant and equal depletions driven by reaction stoichiometry for all rare isotopes: 33 S, 34 S, and 36 S. Interestingly, the ratio of capital Δ values obtained within this model for 33 S and 36 S is −1.16, very close to the mass-independent fractionation line of the Archean rock record. This model may finally offer a mechanistic explanation for the striking mass-independent fractionation of sulfur isotopes that took place in the Archean atmosphere of Earth.S-MIF | mass-independent fractionation | Archean atmosphere | sulfur recombination | isotope effect M ass-independent fractionation of sulfur isotopes (S-MIF) in the Archean rock record (1-6) serves as strong evidence of an anoxic atmosphere (7-9), indicating a low-oxygen gasphase chemistry before 2.3 billion y ago. However, the actual chemical process, or processes, responsible for generation of significant enrichments of sulfur-bearing species in heavy isotopes of sulfur, and leading to mass-independent fractionation, still remains unidentified. Some mass-independent fractionation was observed experimentally in SO 2 photolysis (10-12) and in nonadiabatic dynamics of SO 2 photodissociation (13,14). Another important chemical pathway, specific to a low-oxygen atmosphere (7,8,(15)(16)(17), is a chain of recombination reactions that start from recombination of photolytically produced sulfur atoms (S + S → S 2 ), go through formation of larger sulfur allotropes (S + S 2 → S 3 , S 2 + S 2 → S 4 , and S + S 3 → S 4 ), and end up at the elemental sulfur (S 4 + S 4 → S 8 ) that can be deposited, preserved, and could contribute to S-MIF in the Archean atmosphere. Unfortunately, experimental studies of gas-phase sulfur chemistry and photochemistry are complicated, and their results are often not entirely certain. Moreover, sulfur has four stable isotopes ( 32 S, 33 S, 34 S, and 36 S), which leads to a multitude of possible isotopic combinations of reagents and products in the recombination reactions listed above. It is unlikely that all these processes will be characterized experimentally at the required level of detail any time soon. Thus, it is desirable to analyze these reactions theoretically to determine whether a significant enrichment of rare sulfur isotopes is at all possible through these processes, and what steps are the most important.We build upon our experience (18-21) with the recombination reaction that forms ozone, O + O 2 → O 3 , and is known to produce significant mass-independent fractionation of oxygen isotopes 17 O). In this paper we attempt to export these two major ele...

“…Examples include surfaces for isomerization reactions of small polyatomic molecules, 1-3 for molecule-molecule interactions, 4,5 and very accurate surfaces for several triatomic and tetra-atomic molecules. 2,6 One example is a new and rather accurate potential energy surface (PES) of O 3 that has already been used recently to improve our understanding of the O atom exchange processes O + O 2 → O 2 + O, [7][8][9][10] and of the ozone forming recombination reaction 11 O + O 2 ⇔ O 3 * +bath gas − −−−−−− → O 3 .…”

Section: Introductionmentioning

confidence: 99%

Efficient method for calculations of ro-vibrational states in triatomic molecules near dissociation threshold: Application to ozone

Teplukhin

The Journal of Chemical Physics

2016

A method for calculations of rotational-vibrational states of triatomic molecules up to dissociation threshold (and scattering resonances above it) is devised, that combines hyper-spherical coordinates, sequential diagonalization-truncation procedure, optimized grid DVR, and complex absorbing potential. Efficiency and accuracy of the method and new code are tested by computing the spectrum of ozone up to dissociation threshold, using two different potential energy surfaces. In both cases good agreement with results of previous studies is obtained for the lower energy states localized in the deep (∼10 000 cm −1 ) covalent well. Upper part of the bound state spectrum, within 600 cm −1 below dissociation threshold, is also computed and is analyzed in detail. It is found that long progressions of symmetric-stretching and bending states (up to 8 and 11 quanta, respectively) survive up to dissociation threshold and even above it, whereas excitations of the asymmetric-stretching overtones couple to the local vibration modes, making assignments difficult. Within 140 cm −1 below dissociation threshold, large-amplitude vibrational states of a floppy complex O· · · O 2 are formed over the shallow van der Waals plateau. These are assigned using two local modes: the rocking-motion and the dissociative-motion progressions, up to 6 quanta in each, both with frequency ∼20 cm −1 . Many of these plateau states are mixed with states of the covalent well. Interestingly, excitation of the rocking-motion helps keeping these states localized within the plateau region, by raising the effective barrier. Published by AIP Publishing. [http://dx