Mechanism of chemiluminescent combination reactions involving oxygen atoms

Abstract

The intensities I$_c$ and I$_a$ respectively of chemiluminescent emission by CO$_2$ in the O+CO reaction and by NO$_2$ in the O+NO reaction have been measured from 200 to 300 $^\circ$K in a fast flow system. Both I$_a$ and I$_c$ were found to obey an expression of the type $I=I_0[\mathrm{O}] [\mathrm{XO}],$ where I$_0$ was independent of total pressure over a similar range of low pressures. I$_{0c}$ was found to depend on the nature of the inert gas M used as carrier. I$_{0a}$ was found to have a small negative temperature coefficient similar to that of the overall reaction \begin{equation*}\tag{1a} \mathrm{O+NO+M}\rightarrow \mathrm{NO}_2+M.\end{equation*} I$_{0c}$ had a positive activation energy of 3.7 $\pm$ 0.5 kcal/mole. The pre-exponential factors of I$_{0a}$ and I$_{0c}$ were similar and the rate constant at 293$^\circ$ K for the overall combination \begin{equation*}\tag{1c}\mathrm{O+CO}+M\rightarrow \mathrm{CO}_2+M\end{equation*} was less than 0.002 of that of reaction (1a). The chemiluminescent reactions proceed via three body processes, since I$_0$ and k$_1$ 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 k$_{1a}$ and I$_{0a}$ are apparently due to redissociation of excited NO$_2$ molecules from vibrational levels close to the dissociation limit. O+NO are stabilized into an excited state of NO$_2$ by a third body. This state, which is the one responsible for the predissociation in the NO$_2$ absorption spectrum, crosses the state from which emission occurs and into which excited NO$_2$ molecules undergo rapid radiationless transitions. A similar radiationless transition occurs between triplet CO$_2$ molecules formed in the initial combination step and the singlet state from which emission to the ground state ($^1\Sigma^+_g$) 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 I$_{0c}$ is due to the presence of an energy barrier over which the CO$_2$ molecule must pass to reach the stable triplet state.