Mechanism of Chemiluminescence in the Air Afterglow Reaction

Chemiluminescent emission resulting from the thermal reaction between nitric oxide and atomic oxygen to yield NO₂ is known as an air afterglow. The product forms in both the ground (²A₁) and excited (²B₂) states, with the latter being responsible for the chemiluminescence. Despite the reaction being recognized for several decades, the underlying mechanism, particularly the formation of the ²B₂ state of NO₂, remains unclear. The ground- and excited-state PESs describing the NO-O reaction have been explored using multireference electronic structure methods (CASSCF combined with XMS-CASPT2 energy corrections). Our results suggest that the formation of NO₂ in two distinct electronic states involves a ridge-mediated bifurcation of the ground-state pathway. Additionally, a thermally accessible excited-state channel has been identified on the excited-state PES. Molecular orbital analysis along the bifurcated pathways has been performed to elucidate how a single reactant transforms into a product with two distinct electronic natures. Surface-hopping-based nonadiabatic dynamics simulations reveal that the excited-state pathway plays a significant role in generating the NO₂ in the ²B₂ state. Energetic analysis of computed MEPs indicates that the NO₂ in the excited state forms in the vibronic states associated with the ²A₁ and ²B₂ states, which undergo dipole-allowed radiative emission. The resulting frequencies are in good agreement with the observed broad chemiluminescent spectrum. The mechanism of the air afterglow reaction is strikingly different from that of cycloperoxides, the sole class of chemiluminescent systems studied in depth to date. This study provides new fundamental insights into the chemiluminescence phenomenon.