Mechanism of the Chemiluminescent Reaction between Nitric Oxide and Ozone

The gas phase reaction of nitric oxide with ozone to give chemiluminescence is used extensively for detection of nitrogen oxides. The molecular mechanism of chemiluminescence in this reaction is not known. So far, the only chemiluminescent systems studied in depth are certain cycloperoxides, which emit light following decomposition. Given our understanding of the mechanism of chemiluminescence in those molecules, one would expect by extension that in the NO + O₃ reaction the chemiluminescent species (NO₂ in this case) is formed in the excited state through a reaction pathway that diverges from the ground state pathway near the transition state. A systematic search for such a pathway leads us to conclude that such a mechanism is unlikely. Instead, our study suggests that chemiluminescence in the NO + O₃ reaction is due to emission from the NO₂ vibronic states associated with the ground (X̃ ²A₁) and first excited (à ²B₂) electronic states, which are populated in the nascent NO₂ produced in the reaction. The vibronic coupling between the X̃ ²A₁ and à ²B₂ states of NO₂ is due to a conical intersection (CI), which is geometrically and energetically close to the à ²B₂ minimum energy geometry and only 1.3 eV higher than ground state NO₂. Further, the CI is 1.2 eV lower than the energy of the NO + O₃ reactants and therefore thermodynamically accessible following the reaction. An analysis of the product energy distribution indicates that the major fraction of the reaction energy is channeled into the vibrational modes of NO₂, sufficient to populate the vibronic states of NO₂ around the X̃/à CI. These vibronic states show dipole-allowed emission in a frequency range that is consistent with the observed broad chemiluminescence spectrum.