Rate coefficients for the reactions CH₃ + Br₂ (224–358 K), CH₃CO + Br₂ (228 and 298 K), and Cl + Br₂ (228 and 298 K)

Abstract

Rate coefficients for the reactions of CH 3 + Br 2 (k 2 ), CH 3 CO + Br 2 (k 3 ), and Cl + Br 2 (k 5 ) were measured using the laser-pulsed photolysis method combined with detection of the product Br atoms using resonance fluorescence. For the reactions involving organic radicals, the rate coefficients were observed to increase with decreasing temperature and within the temperature range explored, were adequately described by Arrhenius-like expressions: k 2 (224-358 K) = 1.83 × 10 −11 exp(252/T) and k 3 (228-298 K) = 2.92 × 10 −11 exp(361/T) cm 3 molecule −1 s 1 . The total, temperature-independent uncertainty for each reaction (including possible systematic errors in Br 2 concentration measurement) was estimated as ∼7% for k 2 and 10% for k 3 . Accurate data on k 5 was obtained at 298 K, with a value of 1.88 × 10 −10 cm 3 molecule −1 s −1 obtained (with an associated error of 6%). A limited data set at 228 K suggests that k 5 is, within experimental uncertainty, independent of temperature. C 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: [575][576][577][578][579][580][581][582][583][584][585] 2010

Temperature‐dependent rate constants for the reactions of chlorine atom with methanol and Br2

“…Nicovich and Wine 21 employed a laser flash photolysis‐resonance fluorescence technique to measure the rate constant of reaction (3) as a function of temperature at T = 298‐401 K. k 3 = (1.58 ± 0.22) × 10 −10 cm 3 molecule −1 s −1 was found to be virtually independent of temperature, although a slight positive temperature dependence was observed ( E / R = 144 K) 21 . The k 3 data of Nicovich and Wine 21 shown in Figure 5 have been adjusted upward by 11%, as proposed by Khamaganov and Crowley, 23 considering currently recommended 31 absorption cross section for Br 2 at 415.8 nm of 6.5 × 10 −19 cm 2 molecule −1 instead of 5.87 × 10 −19 cm 2 molecule −1 used by Nicovich and Wine 21 in optical measurements of the Br 2 concentrations. In previous study from our group, 22 essentially similar (to those of the present work) experimental equipment and experimental approach were used except that Cl atoms were titrated at the end of the reactor with C 2 H 3 Br to be monitored at m / z = 62/64 (C 2 H 3 Cl + ): …”

“…A limited data set obtained by the authors at 228 K provided k 3 = (2.1 ± 0.3) × 10 −10 cm 3 molecule −1 s −1 at this temperature. Khamaganov and Crowley 23 combined their data with those of Nicovich and Wine 21 (corrected for absorption cross section) and recommended a value of k 3 = (1.81 ± 0.10) × 10 −10 cm 3 molecule −1 s −1 , independent of temperature from 200 to 400 K. This value is in excellent agreement with the mean of all the measurements of k 3 carried out in the present study, k 3 (±1 σ ) = (1.8 ± 0.1) × 10 −10 cm 3 molecule −1 s −1 at T = 225‐960 K (dotted line in Figure 5), although we observe a slight decrease of the rate constant with increasing temperature. Least‐squares fit to k 3 data from the present work (solid line in Figure 5) provides the following Arrhenius expression: k 3 = 1.64 × 10 −10 exp(34/ T ) cm 3 molecule −1 s −1 at T = 225‐960 K. …”

“…The reason for this disagreement is most likely due to a systematic error in previous measurements, which is difficult to identify at present. Khamaganov and Crowley 23 measured the rate coefficient of the Cl + Br 2 reaction, k 3 = (1.94 ± 0.12) × 10 −10 cm 3 molecule −1 s −1 at T = 298 K, using laser‐pulsed photolysis of Cl 2 in the presence of Br 2 with detection of the reaction product, Br atoms, using resonance fluorescence. A limited data set obtained by the authors at 228 K provided k 3 = (2.1 ± 0.3) × 10 −10 cm 3 molecule −1 s −1 at this temperature.…”

“…Previously, the reaction rate constant was measured only at relatively low temperatures ( T < 400K): the available data for k 3 are spread within a factor of 1.4 19‐23 . In the present work, we report the measurements of the rate constant in an extended temperature range, T = 225‐960 K.…”

Pressure dependent photolysis quantum yields for CH3C(O)CH3 at 300 and 308 nm and at 298 and 228 K

“…We make use of the reactions of the CH 3 and CH 3 CO photoproducts with Br 2 to generate Br atoms. These reactions have rate coefficients of k 9 E 1.83 Â 10 À11 exp(252/T) cm 3 molecule À1 s À1 , and k 10 E 2.92 Â 10 À11 exp(361/T) cm 3 molecule À1 s À1 , which were recently determined in this laboratory 20 and are known to generate Br atoms with 100% yield, 21,22 independent of temperature and pressure.…”