O(3P) + CO2 Collisions at Hyperthermal Energies: Dynamics of Nonreactive Scattering, Oxygen Isotope Exchange, and Oxygen-Atom Abstraction

Dynamics of Inelastic and Reactive Collisions of 16O(3P) with 15N18O

The dynamics of O(³P) + NO collisions were investigated at a collision energy of ⟨E_coll⟩ = 84.0 kcal mol^-1 with the use of a crossed molecular beams apparatus employing a rotatable mass spectrometer detector. This experiment was performed with beams of ¹⁶O atoms and isotopically labeled ¹⁵N¹⁸O molecules to enable the products of reactive and inelastic scattering to be distinguished. Three scattering pathways were observed: inelastic scattering (¹⁶O + ¹⁵N¹⁸O), O-atom exchange (¹⁸O + ¹⁵N¹⁶O), and O-atom abstraction (¹⁸O¹⁶O + ¹⁵N). All product channels exhibited a preponderance of forward scattering, but scattering over a broad angular range was also observed for all products. For inelastic scattering, an average of 90% of the collision energy is retained in the translation of ¹⁶O and ¹⁵N¹⁸O. On the other hand, for O-atom exchange (which also leads to O + NO products), the collision energy is partitioned roughly evenly between the translation of ¹⁸O + ¹⁵N¹⁶O and the internal excitation of ¹⁵N¹⁶O. The available energy for O-atom abstraction is significantly lower than the collision energy because of the endoergicity of this reaction, but the available energy is again roughly evenly partitioned between the translation of ¹⁸O¹⁶O + ¹⁵N and the internal excitation of the molecular (O₂) product. The relative yields of the three scattering pathways were determined to be 0.751 for inelastic scattering, 0.220 for O-atom exchange, and 0.029 for O-atom abstraction.

Quasiclassical Trajectory Study of the O(3P) + CO2(1Σg+) Reaction at Hyperthermal Energies

This paper presents the reaction mechanism, cross sections, and product energy partitioning for the O + CO₂ reaction, calculated using Born-Oppenheimer molecular dynamics simulations with the quasiclassical trajectory (BOMD-QCT) method. At collision energies up to 9.5 eV, three reactions, oxygen exchange (above ∼1.5 eV), abstraction (above ∼5.5 eV), and dissociation (above ∼7.5 eV) occur, with abstraction and dissociation involving either an insertion-elimination mechanism or a stripping mechanism. The insertion-elimination mechanism involves the formation of a planar CO₃ intermediate which lies 0.52 eV above the ground-state CO₂; the energetic barrier for oxygen abstraction via this mechanism is 3.52 eV. Interestingly, the insertion-elimination mechanism predominately contributes to the cross sections at collision energies just above the effective energetic threshold for the abstraction and dissociation reactions; at higher collision energies, the contribution from the stripping mechanism increases and eventually dominates. At a collision energy of 9.5 eV, the cross sections for oxygen exchange, abstraction, and dissociation are 4.17 a₀², 1.58 a₀², and 0.68 a₀², respectively. The lower reaction cross sections, higher effective reaction barrier, and product energy distribution of the stripping mechanism were attributed to the short lifetime (28 fs) of the OCOO species compared with that of the CO₃ species (45 fs) that arises in the insertion-elimination mechanism. For the exchange reaction, it is found that roughly 40% of the reactant translational energy ends up in CO₂ vibration, which provides a single-collision mechanism to produce highly excited CO₂. We also studied intersystem crossing effects using trajectory surface hopping calculations and find no changes compared to single surface (triplet) calculations at energies below 7.5 eV; however, at 7.5 eV and higher the abstraction cross sections are changed by as much as 20%, and the (very small) dissociation cross sections are changed by factors of four or more.

S2O2q+ (q = 0, 1, and 2) Molecular Systems: Characterization and Atmospheric Planetary Implications

We use accurate ab initio methodologies at the coupled cluster level ((R)CCSD(T)) and its explicitly correlated version ((R)CCSD(T)-F12) to investigate the electronic structure, relative stability, and spectroscopy of the stable isomers of the [S₂O₂] system and of some of its cations and dications, with a special focus on the most relevant isomers that could be involved in terrestrial and planetary atmospheres. This work identifies several stable isomers (10 neutral, 8 cationic, and 5 dicationic), including trigonal-OSSO, cis-OSSO, and cyc-OSSO. For all these isomers, we calculated geometric parameters, fragmentation energies, and simple and double ionization energies of the neutral species. Several structures are identified for the first time, especially for the ionic species. Computations show that in addition to cis-OSSO and trans-OSSO proposed for the absorption in the near-UV spectrum of the Venusian atmosphere other S₂O₂, S₂O₂⁺, and S₂O₂²⁺ species may contribute. Moreover, the characterization of the stability of singly and doubly charged S₂O₂ entities can also be used for their identification by mass spectrometry and UV spectroscopy in the laboratory or in planetary atmospheres. In sum, the quest for the main UV absorber in Venus' atmosphere is not over, since the physical chemistry of sulfur oxides in Venus' atmosphere is far from being understood.

Barriometry – an enhanced database of accurate barrier heights for gas-phase reactions

The kinetics of many reactions are critically dependent upon the barrier heights for which accurate determination can be difficult. More than 100 accurate barriers are obtained with the high-level W3X-L composite procedure.

Hydrogen Isotope Exchange of Chlorinated Ethylenes in Aqueous Solution: Possibly a Termolecular Liquid Phase Reaction

This work reports an experimental study of the hydrogen/deuterium exchange in the basic aqueous solutions of trichloroethylene, trans-1,2-dichloroethylene, and cis-1,2-dichloroethylene using (1)H NMR as a monitoring method. 1,1-Dichlorethylene was also investigated but found not to exchange hydrogen isotopes with water. The kinetics of isotope exchange features two different pathways, the first is first order with respect to hydroxide ion, whereas the second is second order. The first pathway is interpreted as a straightforward bimolecular reaction between chloroethylene and hydroxide ion, which leads to the deprotonation of chloroethylene. The second pathway involves a transition state with the association of one molecule of the chloroethylene and two hydroxide ions. It is shown that the second pathway could involve the formation of a precursor complex composed of one chloroethylene molecule and one hydroxide ion, but a direct termolecular elementary reaction is also feasible, which is shown by deriving a theoretical highest limit for the rate constants of termolecular reactions in solution.