A theoretical study on the potential energy surface of the NCO + NO ₂ reaction

The singlet potential energy surface of the NCO+NO₂ reaction system, including four isomers and four transition states, has been studied theoretically at the Gaussian-3 level of theory in order to explore the possible mechanism of the reaction. By means of the ab initio MO calculations, we found that the pathway (1), NCO+NO₂⇌O(O)NNCO→ON(NC(O)O) →NNO+CO₂, is the major pathway, while the pathway (2), NCO+NO₂ ⇌cis-trans-ONONCO→cis-cis-ONONCO→CO+2NO, is the minor pathway on the singlet potential energy surface. The pathways on the triplet potential energy surface are less competitive than those on the singlet potential energy surface. Thus, they are negligible. Both pathways (1) and (2), involving the barrierless entrances to the first adduct isomers and the tight transition states to the products, can give a reasonable explanation for the experimentally observed negative temperature dependence of rate constants.