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(5Πg) 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(5Πg) 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(5Πg) 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(5Πg) 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(5Πg) species is calculated for the first time in the framework of the density functional theory. Definitely, O2(5Πg) is the most reactive species among all other metastable dioxygen states below 5.1 eV. Quintet products of the O2(5Πg) 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.