Kinetics of CN reactions with N 2 O and CO 2

The rate constants for the reaction of CN with N 2 O and CO 2 have been measured by the laser dissociation/laser‐induced fluorescence (two‐laser pump‐probe) technique at temperatures between 300 and 740 K. The rate of CN + N 2 O was measurable above 500 K, with a least‐squares averaged rate constant, k = 10 −11.8±0.4 exp(−3560 ± 181/T) cm 3 /s. The rate of CN + CO 2 , however, was not measurable even at the highest temperature reached in the present work, 743 K, with [CO 2 ] ⩽ 1.9 × 10 18 molecules/cm 3 . In order to rationalize the observed kinetics, quantum mechanical calculations based on the BAC‐MP4 method were performed. The results of these calculations reveal that the CN + N 2 O reaction takes place via a stable adduct NCNNO with a small barrier of 1.1 kcal/mol. The adduct, which is more stable than the reactants by 13 kcal/mol, decomposes into the NCN + NO products with an activation energy of 20.0 kcal/mol. This latter process is thus the rate‐controlling step in the CN + N 2 O reaction. The CN + CO 2 reaction, on the other hand, occurs with a large barrier of 27.4 kcal/mol, producing an unstable adduct NCOCO which fragments into NCO + CO with a small barrier of 4.5 kcal/mol. The large overall activation energy for this process explains the negligibly low reactivity of the CN radical toward CO 2 below 1000 K. Least‐squares analyses of the computed rate constants for these two CN reactions, which fit well with experimental data, give rise to\documentclass{article}\pagestyle{empty}\begin{document}$$ k_{{\rm N}_{\rm 2} {\rm O}} \, = \,6.4 \times 10^{- 21} {\rm T}^{{\rm 2}{\rm .6}} \exp (- 1860/{\rm T)cm}^{\rm 3} /{\rm s} $$\end{document}\documentclass{article}\pagestyle{empty}\begin{document}$$ k_{{\rm C} {\rm O}_{\rm 2}} \, = \,6.1 \times 10^{- 18} {\rm T}^{{\rm 2}{\rm .2}} \exp (- 13530/{\rm T)cm}^{\rm 3} /{\rm s} $$\end{document}for the temperature range 300–3000 K.