KINETIC-STUDY OF THE PRESSURE-DEPENDENCE OF THE REACTION CF3O- RATE-CONSTANT MEASUREMENTS (P=0.5-9 TORR) AND COMPETITION BETWEEN ASSOCIATION (CF3ONO2) AND DISPROPORTIONATION (CF2O+FNO2) CHANNELS(NO2 AT 298 K )

The kinetics of the reaction of the CF3O radical with NO2 has been stu died at 298 K at low pressure (0.5-9 Ton of helium) by fast-low-laser- induced fluorescence (LIF), pyrolysis of a dilute mixture of CF3OOCF3 and He being used as the source of the CF3O radical. In good agreement with the results of Zellner and co-workers (obtained in the pressure range 5-100 Ton), the rate constant has been found to be pressure depe ndent. All the experimental data were analysed by a multichannel RRKM procedure using the results of ab initio calculations as input data. T his revealed that two reaction channels, the association CF3O +/- NO2 --> CF3ONO2 (1a) and the disproportionation CF3O + NO2 --> CF2O + FNO2 (1b), must be invoked and they proceed via a common energized adduct CF3ONO2. The pressure dependence of the branching ratio was predicted by this calculation: the disproportionation channel would be negligib le near atmospheric pressure and becomes the major channel at pressure below ca. 0.3 Torr, with a calculated second-order limiting low-press ure value of the rate equal to 3.2 x 10(-12) cm(3) molecule(-1) s(-1). The high-pressure limit rate constant obtained is k(infinity) = (1.65 +/- 0.2) x 10(-11) cm(3) molecule(-1) s(-1). An analytical representa tion of the pressure dependence of the rate constant at 298 K is propo sed using the conventional Tree expression with an added constant.