Stereodynamic Control of Collision-Induced Nonadiabatic Dynamics of NO (A²Σ⁺) with H₂, N₂, and CO: Intermolecular Interactions Drive Collision Outcomes

Abstract

Intermolecular interactions, stereodynamics, and coupled potential energy surfaces (PESs) all play a significant role in determining the outcomes of molecular collisions. A detailed knowledge of such processes is often essential for a proper interpretation of spectroscopic observations. For example, nitric oxide (NO), an important radical in combustion and atmospheric chemistry, is commonly quantified using laser-induced fluorescence on the A 2 Σ + ← X 2 Π transition band. However, the electronic quenching of NO (A 2 Σ + ) with other molecular species provides alternative nonradiative pathways that compete with fluorescence. While the cross sections and rate constants of NO (A 2 Σ + ) electronic quenching have been experimentally measured for a number of important molecular collision partners, the underlying photochemical mechanisms responsible for the electronic quenching are not well understood. In this paper, we describe the development of high-quality PESs that provide new physical insights into the intermolecular interactions and conical intersections that facilitate the branching between the electronic quenching and scattering of NO (A 2 Σ + ) with H 2 , N 2 , and CO. The PESs are calculated at the EOM-EA-CCSD/d-aug-cc-pVTZ//EOM-EA-CCSD/aug-cc-pVDZ level of theory, an approach that ensures a balanced treatment of the valence and Rydberg electronic states and an accurate description of the open-shell character of NO. Our PESs show that H 2 is incapable of electronically quenching NO (A 2 Σ + ) at low collision energies; instead, the two molecules will likely undergo scattering. The PESs of NO (A 2 Σ + ) with N 2 and CO are highly anisotropic and demonstrate evidence of electron transfer from NO (A 2 Σ + ) into the lowest unoccupied molecular orbital of the collision partner, that is, the harpoon mechanism. In the case of ON + CO, the PES becomes strongly attractive at longer intermolecular distances and funnels population to a conical intersection between NO (A 2 Σ + ) + CO and NO (X 2 Π) + CO. In contrast, for ON + N 2 , the conical intersection is preceded by an ∼0.40 eV barrier. Overall, our work shines new light into the impact of coupled PESs on the nonadiabatic dynamics of open-shell systems.

Anisotropy Measurements from the Near-Threshold Photodissociation of the N2–NO Complex

“…The barrier separates the conical intersection of the D 1 and D 2 states from the region of the excited state readily accessed during a vertical electronic transition from the minimum N 2 −NO ground state geometry. 3 We also note that while the conical intersection may funnel population from the D 2 excited state to the D 1 state, it is not accessible at the energies in our current experiments due to the barrier. Finally, Guardado et al found that the minimum energy geometry for the (A 2 Σ + A′) excited state is linear with an R NN of 3.19 Å and a binding energy of 335.02 cm −1 .…”

“…3−5 In particular, a very recently published potential energy surface found the N 2 −NO (A) excited state to have a conical intersection that may funnel population to the ground state of the complex, leading to quenching. 3 The ground state for the N 2 −NO complex has been extensively studied using both electronic structure calculations and experiments. 4,6−10 The overall results for the ground state are consistent with relatively low barriers to internal rotation for the diatomic moieties, resulting in a floppy complex having a minimum geometry with the NO bond approximately perpendicular to the Jacobi vector between the NO center of mass (CM) and N 2 CM.…”

“…11 The observed feature in NO (A) + Ar resulted from a well on the excited potential near the N atom side, and so they postulated that NO (A) + N 2 also has a well near the N atom side of NO, which was consistent with the calculations of Lozeille et al 4,5 Very recently, Guardado et al published potential energy curves for the ground and lowest excited states of N 2 −NO. 3 They used a variant of the equationof-motion coupled-cluster singles and doubles method, EOM-EA-CCSD/aug-cc-pVDZ, to perform constrained optimizations and then determine the energy from a single point calculation with a larger basis set, EOM-EA-CCSD/d-aug-cc-pVTZ. 3 The N 2 moiety splits the degenerate NO (X 2 Π) ground state potential energy surface into a pair of states labeled as D 0 (X 2 Π A″) and D 1 (X 2 Π A′).…”

“…3 They used a variant of the equationof-motion coupled-cluster singles and doubles method, EOM-EA-CCSD/aug-cc-pVDZ, to perform constrained optimizations and then determine the energy from a single point calculation with a larger basis set, EOM-EA-CCSD/d-aug-cc-pVTZ. 3 The N 2 moiety splits the degenerate NO (X 2 Π) ground state potential energy surface into a pair of states labeled as D 0 (X 2 Π A″) and D 1 (X 2 Π A′). 3 In their calculation they located a conical intersection between the D 1 state and the excited D 2 (A 2 Π A′) state, which is accessed during the transition.…”

“…3 The N 2 moiety splits the degenerate NO (X 2 Π) ground state potential energy surface into a pair of states labeled as D 0 (X 2 Π A″) and D 1 (X 2 Π A′). 3 In their calculation they located a conical intersection between the D 1 state and the excited D 2 (A 2 Π A′) state, which is accessed during the transition. They also found an angle-dependent barrier in the range of 0.2 eV, which is due to an avoided crossing between the D 2 state and the higher energy D 4 (C 2 Π A′) state.…”