Reaction mechanism and kinetics of the NCN+NO reaction: comparison of theory and experiment.

ABSTRACT

The rate constants for the NCN + NO reaction have been measured by laser photolysis/laser-induced fluorescence technique in the temperature range of 254-353 K in the presence of He (40-600 Torr) and N2 (30-528 Torr) buffer gases. The NCN radical was produced from the photodissociation of NCN3 at 193 nm and monitored with a dye laser at 329.01 nm. The reaction was found to be strongly positive-pressure dependent with negative-temperature dependence, as was reported previously. The experimental data could be reasonably accounted for by dual-channel Rice-Ramsperger-Kassel-Marcus calculations based on the predicted potential-energy surface using the modified Gaussian-2 method. The reaction is predicted to occur via weak intermediates, cis- and trans-NCNNO, in the 2A" state which crosses with the 2A' state containing more stable cis- and trans-NCNNO isomers. The high barriers for the fragmentation of these isomers and their trapping in the 2A' state by collisional stabilization give rise to the observed positive-pressure dependence and negative-temperature effect. The predicted energy barrier for the fragmentation of the cis-NCNNO (2A') to CN + N2O also allows us to quantitatively account for the rate constant previously measured for the reverse process CN + N2O --> NCN + NO.