Mechanism of chemiluminescent combination reactions involving oxygen atoms

Abstract

Abstract The intensities Ic and Ia respectively of chemiluminescent emission by CO2 in the O+CO reaction and by NO2 in the O+NO reaction have been measured from 200 to 300 °K in a fast flow system. Both Ia and Ic were found to obey an expression of the type I = I0[O] [XO], where I0 was independent of total pressure over a similar range of low pressures. I0c was found to depend on the nature of the inert gas M used as carrier. I0a was found to have a small negative temperature coefficient similar to that of the overall reaction O + NO + M-> NO2 + M. (1a) I0c had a positive activation energy of 3.7 + 0.5 kcal/mole. The pre-exponential factors of I0a and I0a were similar and the rate constant at 293° K for the overall combination O + CO + M -> CO2 + M (1c) was less than 0.002 of that of reaction (1a). The chemiluminescent reactions proceed via three body processes, since I0 and k1 showed dependences on the nature of M for both the O + NO and O + CO reaction. A considerable fraction of the product molecules formed in both reactions is produced through the excited state from which emission occurs and the negative temperature coefficients of k1a and I0a are apparently due to redissociation of excited NO2 molecules from vibrational levels close to the dissociation limit. O + NO are stabilized into an excited state of NO2 by a third body. This state, which is the one responsible for the predissociation in the NO2 absorption spectrum , crosses the state from which emission occurs and into which excited NO2 molecules undergo rapid radiationless transitions. A similar radiationless transition occurs between triplet CO2 molecules formed in the initial combination step and the singlet state from which emission to the ground state (1E+g) takes place. Spin reversal in the O + CO reaction is therefore not a rate-controlling step for light emission nor for combination, and the activation energy observed for I0c is due to the presence of an energy barrier over which the CO2 molecule must pass to reach the stable triplet state.