On the mechanism and branching ratio of the CN + O₂ → CO + NO reaction channel using transient IR emission spectroscopy

The contribution of the title reaction to the total reaction of CN + O₂ has been determined, using time-resolved IR emission spectroscopy of the product CO(v″). This product channel is found to contribute up to 29% with 2% experimental variance to the total reaction. A contribution of this magnitude to the total reaction of CN + O₂ by this product channel proceeding via a four-center transition state is energetically improbable. Ab initio calculations have been performed on two- and four-center transition states which are presumably formed in the reaction between CN radicals and molecular oxygen. The results of these calculations confirm the existence of a very high barrier to the formation of a four-center transition state. Consequently, a new mechanism for the formation of CO in reaction 1 is proposed. This new mechanism consists of two sequential steps rather than two parallel steps to explain the formation of the products NCO, O, and CO in the reaction CN + O₂.