Kinetics of CN reactions with N₂O and CO₂

The rate constants for the reaction of CN with N₂O and CO₂ 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 + N₂O was measurable above 500 K, with a least‐squares averaged rate constant, k = 10^−11.8±0.4 exp(−3560 ± 181/T) cm³/s. The rate of CN + CO₂, however, was not measurable even at the highest temperature reached in the present work, 743 K, with [CO₂] ⩽ 1.9 × 10¹⁸ molecules/cm³. 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 + N₂O 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 + N₂O reaction. The CN + CO₂ 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 CO₂ 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 (Formula Presented.) (Formula Presented.) for the temperature range 300–3000 K.