Kinetics of the Reaction of the Cyclopentadienyl Radical with Nitrogen Dioxide.

Abstract

The kinetics of the reaction of the cyclopentadienyl radical (c-C5H5) with nitrogen dioxide (NO2) was studied by laser photolysis/photoionization mass spectroscopy. Overall rate constants were obtained in direct real-time experiments in the temperature region 305-800 K and at bath gas densities of (3.0-12.0) × 1016 molecules cm-3. The overall rate constant is independent of temperature between 300 and 400 K but decreases by a factor of approximately 7 above 400 K, without any discernible pressure dependence. A potential energy surface study of the reaction was performed, and an RRKM/master equation model was created. The reaction proceeds via initial addition to one of the two types of atoms of the NO2 molecule (nitrogen or oxygen). The N-bonded adduct can isomerize and decompose back to the reactants; this channel is significantly affected by falloff above 400 K and, although dominant at room temperature, becomes negligible at 600 K and above. The O-bonded adduct undergoes chemically activated isomerizations and decomposition, with a minor contribution from stabilization at low temperatures; this channel dominates at high temperatures and is effectively pressure-independent. The model provides a quantitative explanation for the observed temperature dependence of the rate constant.