KINETIC-STUDY OF THE REACTIONS OF CN WITH O2 AND CO2 FROM 292-K TO 1500-K USING HIGH-TEMPERATURE PHOTOCHEMISTRY

Absolute rate constants for the reaction of CN with O2 have been measured as a function of temperature (292-1565 K) and pressure (5-80 Torr), by combining the laser-induced fluorescence detection technique with high-temperature reactor technology. CN radicals were generated by using 193-nm excimer laser photolysis of dilute C2N2/Ar mixtures. Decays of the radical concentration were monitored by using laser-induced fluorescence (LIF) near 387 nm under pseudo-first-order conditions. The reaction of CN with O2 has a negative temperature dependence and is independent of pressure. These data contain minimal curvature and are adequately fit by the expression k (+/- 2-sigma) = (1.02 +/- 0.12) x 10(-11) exp[(+220 +/- 25)/T] cm3/(molecule s). This result represents the widest continuous temperature range over which this rate constant has been measured to date. For the reaction with CO2, the CN decays are complex and are strongly dependent on the gas temperature. No detectable reaction is observed for temperatures below 900 K. For temperatures above about 1150 K, CN decays are biexponential. Because of the complexities in the behavior of the data, no reliable Arrhenius rate parameters were determined.