Kinetics of the Reactions of CH3CH2, CH3CHCl, and CH3CCl2 Radicals with NO2 in the Temperature Range 221−363 K

Abstract

The gas-phase kinetics of three ethyl radical reactions with NO(2) have been studied in direct measurements using a laser photolysis/photoionization mass spectrometer (LP-PIMS) coupled to a temperature controlled tubular flow reactor. Reactions were studied under pseudo-first-order conditions with NO(2) always in large excess over initial radical concentrations. All the measured rate coefficients exhibit a negative temperature dependence, which becomes stronger as the chlorine substitution in the alpha-carbon of the ethyl radical increases. No pressure dependence of the rate coefficients was observed within the experimental range covered (0.5-6 Torr). The obtained results can be expressed conveniently as follows: k(CH(3)CH(2) + NO(2)) = (4.33 +/- 0.13) x 10(-11) (T/300 K)(-0.34 +/- 0.22) cm(3) s(-1) (221-365 K), k(CH(3)CHCl + NO(2)) = (2.38 +/- 0.10) x 10(-11) (T/300 K)(-1.27 +/- 0.26) cm(3) s(-1) (221-363 K), and k(CH(3)CCl(2) + NO(2)) = (1.01 +/- 0.02) x 10(-11) (T/300 K)(-1.65 +/- 0.19) cm(3) s(-1) (248-363 K), where the given error limits are the 1sigma statistical uncertainties of the plots of log k against log(T/300 K). Overall uncertainties in the measured rate coefficients were estimated to be +/-20%. The observed reactivity toward NO(2) decreases with increasing chlorine substitution at the radical site as was expected with respect to our previous measurements of chlorine containing methyl radical reactions with NO(2). A potential reason for the observed reactivity differences is briefly discussed, and a possible reaction mechanism is presented.