Mechanism of the Chemiluminescent Reaction between Nitric Oxide and Ozone.

Abstract

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.