J. Phys. Chem. 1993,97, 5050-5053
Laser Photolysis/Laser-Induced Fluorescence Studies of the Reaction of OH with 1,1,1,2-and l,l,Z,Z-Tetrachloroethane over an Extended Temperature Range Zben Jiang, Philip H. Taylor,: and Barry Dellinger Environmental Sciences and Engineering, University of Dayton Research Institute, 300 Coliege Park, Dayton, Ohio 45469-01 32 Received: December 22, 1992; In Final Form: February 17, 1993
Absolute rate coefficients are reported for the gas-phase reaction of OH radicals with 1,1,1,2- (kl)and 1,1,2,2tetrachloroethane (k2) over an extended temperature range. Employing a laser photolysis/laser-induced fluorescence technique, experiments were conducted with a flow system at a total pressure of 740 f 10 Torr using H e as diluent and carrier gas. The temperature dependence of the rate coefficients was best described by the modified Arrheniusexpressions kl(293-882 K) = (3.36 f 0.52) X 10-12(T/300)1.21 exp[(-1553 f 92)/Tl cm3 molecule-’ s-1 and k2(295-701 K) = (2.72 f 0.42) X 10-12(T/300)0.22exp[(-915 f 62)/Tl cm3 molec-I s-I. The room temperature reactivity of kl is nearly an order of magnitude less than k2. The difference in reactivity is discussed in terms of C-H bond dissociation energies and polarity changes in the transition state induced by the presence of j3 C1 substitution. Revised substituent factors for Atkinson’s structureactivity relationship model derived from our experimental measurements are shown to improve the predictions of this model.
Introduction The high-temperature reactivity of hydroxyl (OH) radicals with chlorinated hydrocarbons (CHCs) is significant for two reasons. First, accurate modified Arrhenius parameters describing the rate behavior over extended temperature ranges represent an important component of the successful modeling of the combustion of these compounds. Second, these measurements can further test semiempirical models describing the temperature dependence of these reactions. In a continuation of previous studies,’-7 we report absolute rate coefficients for the reactions
CH2C1CC13 O H
+H20 CC12CHC12+ H20 CHClCCl,
using a refined laser photolysis/laser-inducedfluorescence (LP/ LIF) technique. Data were obtained at a total pressure of 740 f 10 Torr over a temperature range of 293-882 K for kl and 295-701 K for k2. The motivation for this study was 2-fold. First, there are no previously reported measurementsof kl .Second, the only reported measurements of k2 are -50% lower than predictionsfrom Atkinson’s structureactivity relationship (SAR) and were obtained over a limited temperature range (292-365 K). This study was thus performed to extend our data base of the reaction of OH with chlorinated ethanes and to hopefully clarify the reactivity of k2. The room temperature reactivity of these compoundsobtained from these experimentsis discussed in terms of C-H bond dissociation energies and changesin the polarization of the transition states induced by B C1substitution. Comparisons of the temperature dependence of kl and k2 with the predictions of Atkinson’s SAR model are also presented. Experimental Technique and Data Reduction All experiments were performed using a refined laser photolysis/laser-induced fluorescence technique. A detailed descriptionof this techniqueand its applicationto OH kinetic studies
* Author to whom correspondence should be addressed. 0022-3654/93/2097-5050$04.00/0
has been reported previously;1*2 hence, we only briefly summarize the experiment. All experiments were carried out under ‘slow flow”conditions; Le., the buildup of reaction products was minimized. Individually controlled gas flows of CHC/N20/H20/He were thoroughly mixed before entering the reactor. Thecomposite flow conditioned the reactor for 45-90 s prior to the onset of data collection, thereby minimizing any effects due to reactant adsorption on the reactor walls. All experiments were conducted at a total pressure of 740 f 10Torr. The gas temperature in the reaction zonewas measured with two chromel/alumel thermocouples and was observed to be constant within f 2 K over both the dimensions of the probed volume and the duration of the experiment. Hydroxyl radicals were produced by 193.3-nm photodissociation of CHC/N20/H20/He gas mixtures with a ArF excimer gas laser (Questek Model 2320). Initial OH concentrations, estimated using publishedvaluesof the N20 absorptioncoefficient and photodissociation quantum yieldlo and based on the rapid reaction of O(lD) with H20 (95% conversion in 1000[OH] in all reactive experiments, exponential “reaction only” OH dependences, of pseudo-first-order decay constant k’ = kl [substrate] kd, were observed. (kd is the first-order rate coefficient for OH disappearance because of diffusion from the reaction volume and reaction with background impurities.) Bimolecular rate coefficients, kl, were obtained from the slope of the least-squares straight line through the (CHC, k’) data points (cf. Figures 1 and 2). Values of k’ranged from about 75 to 2300 SI, depending on the reactant concentration and the temperature. The first-order decay constants in the
0 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97, No. 19, 1993 5051
LP/LIF Studies of OH and Tetrachloroethane
TABLE Ik Absolute Bimolecular Rate Coefficients for 4
1013k2,cm3 molecule-' s-I
295 316 326 340
1.09 f 0.27" 1.38 f 0.20 1.54 0.19 1.98 f 0.26
T,K molecule-' s-I
1O13k2,cm3 molecule-' s-I
382 402 422 442
520 562 640 701
5.22 f 0.57 6.36 f 0.63 8.06 f 0.78 8.96 f 0.81
2.71 f 0.10 3.06 f 0.21 3.20 f 0.21 3.74 0.34
Uncertainties represent f 2 a estimates of the random experimental error from the least-squares analysis. 0
TABLE IU: Absolute Bimolecular Rate Coefficients for 4 and Corrected Rate Coefficients for kla T.K 10I2k3,cm3 molecule-l s-I 1OI3kl,cm3 molecule-' s-I 0
CH,CICCI~ 1 c'3(molrc cm-5)
Figure 1. Pseudo-first-order rate constant, k', as a function of [CH2CICC13] at various reaction temperatures. Error bars for the 663 K data are shown to indicate the typical uncertainty in the k'measurement.
295 3 26 335 362 403 452 460 a
1 @ I 3 (molec cm.5)
Figure 2. Pseudo-first-order rate constant, k', as a function of [CHCl2CHCI2] at various reaction temperatures.
TABLE I: Absolute Bimolecular Rate Coefficients for kl lo%, cm3
T,K molecule-] s-I
1013kl,cm3 molecule-' s-I
lo%, cm3 molecule-] s-I
0.69 f 0.14"
480 501 544 575 579 623 663
2.42 f 0.32 2.62f0.72 3.99f0.65 5.78* 1.1 6.04f0.48 6.61 fO.39 8.50f0.66
696 701 737 777 783 832 882
11.8 f 3.4 8.46f 1.8 11.9f 3.3 15.3 f 2.0 14.8f 1.2 17.1 f 1.5 21.5 f 3.0
293 326 335 362 403 452 460
0.69f0.10 0.74f0.15 0.85 f 0 . 2 0 1.11 fO.17 1.68 f 0.40 1.81 f 0.19
Above 460 K, correction to k , is 99.9% with no detectable olefinic contaminants. The remaining chemicals used in this study had the following stated minimum purities: He, 99.999+%; N20,99.9%; H20, HPLC organic-free reagent grade. Results and Discussion Absolute rate coefficient values for kl and k2 are listed in Tables I and 11. Random error limits (Za), derived from a
between temperatures of 293 and 460 K were conductedto correct the CH2ClCC13 rate measurements. Although previous measurementsof k3 have been reported,I2this reaction is hypothesized to occur by a radical addition mechanism at these temperatures and, as a result, may be pressuredependent. Direct measurements under our experimental conditions of high pressure were thus conducted to ensure that the proper correction was applied. Bimolecular rate coefficients for k3 and corrected rate coefficients for kl are shown in Table 111. Room temperature measurements of k3 were significantly larger (65%) than the recommendedratecoefficient (2.36 X 1W2cm3molecule-' s-I),l2 presumably due to enhanced stabilization of the C2HC13-OH adduct at the higher pressures of our experiments. As the reaction temperature was increased, k3 exhibited a negative temperature dependenceand converged with the recommended literature values at 400 K. The negative temperature dependence of k3 between 293 and 400 K is consistent with the observed lack of a temperature dependence of kl over a similar temperature range based on the level of impurity within the CH2ClCC13 sample. The corrected rate coefficients for kl thus appear reasonable and result in an Arrhenius plot exhibiting modest curvature. Several experimental studies were conducted to verify that additional systematic errors were not affecting the rate measurements for k l (and k2). The possibility of high-temperature thermal artifacts was investigated experimentallyby varying the total gas flow rate. Both kl and k2 were found to be independent of residence time in the high-temperature region, implying the lack of secondaryreaction chemistry associated with the thermal decomposition of the substrate. These variable flow studies produced upper temperature limits of 882 and 701 K for kl and kZ,respectively, above which Substrate thermal decomposition resulted in much larger rate coefficients. The possibility that OH consumption could be due to reaction with photolytically generated, but unreacted, 0 atoms was investigated by varying the H 2 0 concentration. Bimolecular rate determinations were unaffected by a factor of 5 change in H2O concentration,indicating that unreacted 0 atoms had no effect on the observed measurements.
Jiang et al.
5052 The Journal of Physical Chemistry, Vol. 97, No. 19, 1993
TABLE Iv: Arrhenius Rate Expressions for 4-ks rate expression, om3 molecule-' s-I coefficient k4 1.7 X 10-" ks 1.7 X 10-" k6 2.5 X l o L 3exp(-19900/RQb ki 2.5 X loi3exp(-17900/RQC ks 2.3 X 10-l2exp(938/RVd kn, 2.5 X lo-" exp(-6020/RQd LI A factor estimated by analogy with other chlorocarbon radical recombination reactions, refs 13 and 14. Reference 24. A factor calculated from transition-state theory, ref 14. Activation energy calculated from following expression: E, = AHr + 2 kcal mol-'. Rate expressionsestimated from previous measurements of other chloroolefin OH reactions, refs 15-17.
r . . . . l . . . . l . . . . l . . . . I . . . ~ l 1 .o
Figure 4. Arrhenius plot of kinetic data for k2. Also shown are the experimental measurements of Qui et a1.I2and the SAR prediction of Atkinson.8
Figure 3. Arrhenius plot of kinetic data for kl. Also shown is the SAR prediction of Atkinson.s
Possible effects due to substrate photolysis were also investigated by attenuating the excimer laser output. No photolytic effects were evident, as measured rate constants were independent of photolysis laser intensity at the low intensities employed (1-4 mJ cm-2). The importance of thermally induced secondary reactions was further evaluated by numerical analysis of additional OH decay routes. These decay routes were postulated to result from collisional stabilization or decomposition of the primary tetrachlorotthyl radicals produced from the H abstraction process: CCl,CHCl,
CCl,CHCl, CHClCCl, C,HCl, C,HCl,
+ C1 + C1
(k4) (k6) (k,)
Arrhenius parameters used in this analysis are given in Table IV. The numerical results indicated no pronounced increase in OH decay rates (