The asymmetric dimer N2–O2: Characterization of the potential energy surface and quantum mechanical calculation of rotovibrational levels

Thermodynamics of the van der Waals Dimers of O2, N2 and the Heterodimer (N2)(O2) and Their Presence in Earth's Atmosphere

The dimerization thermodynamics of N2 and O2, the principal components of Earth's atmosphere, have been determined from the respective second virial coefficients of the bound and metastable dimers calculated using the method of Stogryn and Hirschfelder that utilizes the Lennard-Jones (LJ) potential to account for intermolecular interactions. In addition, the thermodynamic properties of the heterodimer (N2)(O2) have been obtained using the same approach, employing combining rules to construct the LJ potential. Thus, Keq, ΔH, and ΔS for the three dimers are reported between 80-120 K. Over this temperature range, the ranking of Keq is (N2)(O2) > (O2)(O2) > (N2)(N2). The same trend is found for the exoethalpicity of dimer formation. For example, at 100 K, the Keq values are, respectively, 0.0406(14), 0.0215(5), and 0.0181(10), and the corresponding ΔH values are -2401(5), -2344(7), and -2279(1) J/mol. The mole fraction composition of the dimers in the atmosphere was calculated for altitudes up to 20 km. These calculations show that in the troposphere and the lower stratosphere (up to 20 km), the three dimers rank fifth to seventh in abundance, between CO2 and Ne. In this region, the average mole fractions of (N2)(N2), (O2)(O2), and (N2)(O2) are calculated to be 3.4(2) × 10-4, 2.80(9) × 10-5, and 1.95(7) × 10-4, respectively.

Global and Full-Dimensional Potential Energy Surfaces of the N2 + O2 Reaction for Hyperthermal Collisions

The energy transfer, dissociations, and chemical reactions between O₂ and N₂ play an important role in the re-entry process of aircraft and many atmospheric, combustion, and plasma processes. Recently, Varga et al. (J. Chem. Phys., 2016, 144, 024310) developed a full-dimensional high-precision potential energy surface (PES) of the ground triplet electronic state for the O₂ and N₂ system based on ca. 55,000 data points, whose energies were calculated by multi-state complete-active-space second-order perturbation theory/minimally augmented correlation-consistent polarized valence triple-zeta electronic structure calculations plus dynamically scaled external correlation. The fitting function adopted the many-body expansion form with the four-body interactions fitted by the permutationally invariant polynomial in terms of bond-order functions of the six interatomic distances (MB-PIP). In this work, we refit the PES of the N₂O₂ system by two methods based on the same data set that was used by Varga et al. The first uses the permutation invariant polynomial-neural network (PIP-NN) method to fit the entire energy of the 55,000 data points. In the second approach, the PIP-NN method is used to fit only the four-body interaction component, a similar treatment in the MB-PIP method, and the resulting PES is named MB-PIP-NN. Then, the performances of these new PESs and the MB-PIP PES are comprehensively and systematically compared, such as comparisons of various scans, properties of stationary points, and dynamics simulations. Possible improvements for the PES of N₂O₂ are suggested. A more reliable PES of the system can be constructed in terms of data sampling range, electronic structure calculation level, and fitting method for high-temperature calculation and simulation in the future.

New Aspects of the Airglow Problem and Reactivity of the Dioxygen Quintet O2(5Πg) State in the MLT Region as Predicted by DFT Calculations

Dioxygen in the quintet O₂(⁵Π_g) state is a weakly bound species near the entrance of the O(³P) + O(³P) recombination channel. It was predicted by ab initio calculations in 1977 and detected experimentally in 1999. Meantime, the O₂(⁵Π_g) species was tentatively assumed as intermediate in transport properties calculations for the rarefied gases of the Earth's upper atmosphere, though its potential energy curve is still debated. Besides six other strongly bound low-lying states of dioxygen, the O₂(⁵Π_g) state is an important potential candidate for modeling energy transfer and airglow of the upper atmosphere. A number of photochemical kinetic schemes designed to simulate energy flow upon atomic and molecular oxygen collisions in the rarefied mesosphere take into account a participation of the O₂(⁵Π_g) state in energy relaxation processes responsible for terrestrial nightglow. All mechanisms of energy redistribution are based on the hard-sphere collision models. The possibility of chemical interactions between the quintet excited state of dioxygen and other atmospheric components has not been considered so far in photochemistry of the upper atmosphere. In the present paper, the chemical reactivity of the quintet O₂(⁵Π_g) species is calculated for the first time in the framework of the density functional theory. Definitely, O₂(⁵Π_g) is the most reactive species among all other metastable dioxygen states below 5.1 eV. Quintet products of the O₂(⁵Π_g) state association with heavy inert gases, H₂O, N₂, and CO₂ are predicted to be chemically significant, while the complexes with abundant H₂ and He species are rather weak and not important even in the mesopause low-temperature region. The complex with N₂ molecule is unexpectedly stable with dissociation energy 4 kJ/mol, which can strongly influence the abundant termolecular association O + O + N₂ → O₂ + N₂ process. Reaction with meteoritic ablated Mg atom produces metastable ⁵A₁ excited state of MgO₂ being more strongly bound than the ground ³A₂ state of magnesium peroxide.

Vibrational energy transfer and dissociation in O2-N2 collisions at hyperthermal temperatures

Simulation of vibrational energy transfer and dissociation in O₂-N₂ collisions is conducted using the quasi-classical trajectory method on an ab initio potential energy surface. Vibrationally resolved rate coefficients are obtained in a high-temperature region between 8000 and 20 000 K by means of the cost-efficient classical trajectory propagation method. A system of master equations is constructed using the new dataset in order to simulate thermal and chemical nonequilibrium observed in shock flows. The O₂ relaxation time derived from a solution of the master equations is in good agreement with the Millikan and White correlation at lower temperatures with an increasing discrepancy toward the translational temperature of 20 000 K. At the same time, the N₂ master equation relaxation time is similar to that derived under the assumption of a two-state system. The effect of vibrational-vibrational energy transfer appears to be crucial for N₂ relaxation and dissociation. Thermal equilibrium and quasi-steady state dissociation rate coefficients in O₂-N₂ heat bath are reported.

Efficiency of Collisional O2 + N2 Vibrational Energy Exchange

By following the scheme of the Grid Empowered Molecular Simulator (GEMS), a new O2 + N2 intermolecular potential, built on ab initio calculations and experimental (scattering and second virial coefficient) data, has been coupled with an appropriate intramolecular one. On the resulting potential energy surface detailed rate coefficients for collision induced vibrational energy exchanges have been computed using a semiclassical method. A cross comparison of the computed rate coefficients with the outcomes of previous semiclassical calculations and kinetic experiments has provided a foundation for characterizing the main features of the vibrational energy transfer processes of the title system as well as a critical reading of the trajectory outcomes and kinetic data. On the implemented procedures massive trajectory runs for the proper interval of initial conditions have singled out structures of the vibrational distributions useful to formulate scaling relationships for complex molecular simulations.