Determination of the rate constant for the radical-radical reaction NCO(X2Pi) + CH3(X2A2 ') at 293 K and an estimate of possible product channels

The rate constant for the reaction of the cyanato radical, NCO(X2Pi), with the methyl radical, CH3(X2A2' '), has been measured to be (2.1 +/- 1.3(-0.80)) x 10(-10) cm3 molecule(-1) s(-1), where the uncertainty includes both random and systematic errors at the 68% confidence level. The measurements were conducted over a pressure range of 2.8-4.3 Torr of CH4 and at a temperature of 293 +/- 2 K. The radicals were generated by the 248-nm photolysis of ClNCO in a large excess of CH4. The subsequent rapid reaction, Cl + CH4, generated the CH3 radical. The rate constant for the Cl + CH4 reaction was measured to be (9.2 +/- 0.2) x 10(-14) cm3 molecule(-1) s(-1), where the uncertainty is the scatter of one standard deviation in the data. The progress of the reaction was followed by time-resolved infrared absorption spectroscopy on single rovibrational transitions from the ground vibrational level. Multiple species were detected in these experiments, including NCO, CH3, HCl, C2H6, HCN, HNC, NH, and HNCO. Temporal concentration profiles of the observed species were simulated using a kinetic model, and rate constants were determined by minimizing the sum of the squares of the residuals between experimental observations and model calculations. Both HCN and HNC seem to be minor products (<0.3% each) of the NCO + CH3 reaction. The peak concentrations of NH and HNCO were small, accounting for <1% of the initial NCO concentration; however, their temporal profiles could not be fit by the model kinetics. The observed C2H6 temporal profile always peaked at significantly higher concentrations than the model predictions, and several reaction models were constructed to help explain these observations. The most likely product channel seems to be the recombination channels, producing CH3NCO and CH3OCN.