Preliminary ab initio study of the quartet states of the complex formed between NO(X2Π) and O2(X3Σg−)

Molecular properties and excited state van der Waals potentials in the NO A2Σ+ + O2 XΣg- collision complex

We characterize NO A²Σ⁺ + O₂ X³Σ_g^- van der Waals (vdW) Potential Energy Surface (PES) with RHF/RCCSD(T) and CASSCF/CASPT2 calculations. To do this, we first assess our computational setup to properly represent the individual molecular properties of O₂ X³Σ_g^-, NO X²Π, and NO A²Σ⁺. Specifically, we show that highly augmented basis sets are necessary to properly represent the NO A²Σ⁺ polarizability. Then, we optimize different vdW geometries, and provide BSSE corrected plots of the quartet vdW PES. The surfaces show a confined channel at a distance of approximately 6 Å with a depth of at least 20 cm^-1 that we believe is caused by NO A²Σ⁺ hyper-polarizability. At shorter distances, the channel is connected to a vdW basin centered around the O-N O-O linear geometry with an inter-molecular separation of 4.3 Å, and a depth of 95 cm^-1 at the RCCSD(T) level. A CASPT2 scan along the linear geometry show that this vdW basin exists on both the doublet and quartet excited surfaces. These results infer the existence of a collision complex between NO A²Σ⁺ and O₂ X³Σ_g^-, as predicted by earlier experiments.

Angular distributions for the inelastic scattering of NO(X2Π) with O2(X3Σg-)

The inelastic scattering of NO(X²Π) by O₂(X³Σ_g^-) was studied at a mean collision energy of 550 cm^-1 using velocity-map ion imaging. The initial quantum state of the NO(X²Π, v = 0, j = 0.5, Ω=0.5, 𝜖 = -1, f) molecule was selected using a hexapole electric field, and specific Λ-doublet levels of scattered NO were probed using (1+1^') resonantly enhanced multiphoton ionization. A modified "onion-peeling" algorithm was employed to extract angular scattering information from the series of "pancaked," nested Newton spheres arising as a consequence of the rotational excitation of the molecular oxygen collision partner. The extracted differential cross sections for NO(X) f→f and f→e Λ-doublet resolved, spin-orbit conserving transitions, partially resolved in the oxygen co-product rotational quantum state, are reported, along with O₂ fragment pair-correlated rotational state population. The inelastic scattering of NO with O₂ is shown to share many similarities with the scattering of NO(X) with the rare gases. However, subtle differences in the angular distributions between the two collision partners are observed.

Structure, stability, and properties of the trans peroxo nitrate radical: the importance of nondynamic correlation

We report a comparative single-reference and multireference coupled-cluster investigation on the structure, potential energy surface, and IR spectroscopic properties of the trans peroxo nitrate radical, one of the key intermediates in stratospheric NOX chemistry. The previous single-reference ab initio studies predicted an unbound structure for the trans peroxo nitrate radical. However, our Fock space multireference coupled-cluster calculation confirms a bound structure for the trans peroxo nitrate radical, in accordance with the experimental results reported earlier. Further, the analysis of the potential energy surface in FSMRCC method indicates a well-behaved minima, contrary to the shallow minima predicted by the single-reference coupled-cluster method. The harmonic force field analysis, of various possible isomers of peroxo nitrate also reveals that only the trans structure leads to the experimentally observed IR peak at 1840 cm(-1). The present study highlights the critical importance of nondynamic correlation in predicting the structure and properties of high-energy stratospheric NOx radicals.