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

The gas-phase kinetics of three ethyl radical reactions with NO₂ 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 NO₂ 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 α-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(CH₃CH₂ + NO₂) = (4.33 ± 0.13) × 10⁻¹¹ (T/300 K)^{−0.34 ± 0.22} cm³ s⁻¹ (221−365 K), k(CH₃CHCl + NO₂) = (2.38 ± 0.10) × 10⁻¹¹ (T/300 K)^{−1.27 ± 0.26} cm³ s⁻¹ (221−363 K), and k(CH₃CCl₂ + NO₂) = (1.01 ± 0.02) × 10⁻¹¹ (T/300 K)^{−1.65 ± 0.19} cm³ s⁻¹ (248−363 K), where the given error limits are the 1σ 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 NO₂ 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 NO₂. A potential reason for the observed reactivity differences is briefly discussed, and a possible reaction mechanism is presented.