J. Phys. Chem. 1992,96, 1293-1296
and propagating the microexplosions.
Conclusions Photochlorination of ethene and propene in solid solutions with chlorine in the range 35-50 K occurs mainly via trans addition to the carbon-carbon double bond, yielding the anti conformer of the respective dichloroalkane. The quantum yield for the photochlorination of ethene at 48 K is 116 f 15. For propene at 50 K,the yield is 740 f 120. While these values are large compared with those of other solid-state chain reactions, they are still small compared with typical values found in liquid solutions and gases. The site and orientational disorder characteristic of binary solid solutions inhibits chain growth but can be overcome by transient melting of the samples at sufficiently high laser fluence. The activation barrier to addition of C1 to the alkene appears to be an important factor in determining the chain length of the reaction (kinetic control). Heat released in the reaction does not
1293
appear to be a significant factor for promoting chain growth at low laser fluence. Photolysis of these solid solutions at temperatures near 10 K results in a microexplosion. Initiation requires as little as 0.9 mJ/cm2 cumulative fluence. As much as 67% of the reactants are oonverted to products during one of these events. The products are formed as mixtures of anti and gauche conformers, indicating that transient melting occurs. A new technique has been developed for determining the relative intensities of IR absorption bands of conformational isomers based on changes in conformer populations during microexplosions. Acknowledgment. This work is supported by the National Science Foundation under Grant CHE-8918733 and by the US. Air Force Phillips Laboratory (Edwards AFB) under Contract FO4611-90-K-0036. Registry No. Ethene, 74-85-1; propene, 115-07-1; 1,2-dichlorocthane, 107-06-2; 1,2-dichloropropane, 78-87-5.
Laser PhotolysWLaser- I nduced Fluorescence Studies of the Reaction of Hydroxyl with 1,I,2-Trichloroethane over an Extended Temperature Range Philip H.Taylor,* Zhen Jiang, and Barry Dellinger University of Dayton Research Institute, Environmental Sciences Laboratories, 300 College Park, Dayton, Ohio 45469-0132 (Received: August 15, 1991; In Final Form: October 4, 1991)
Absolute rate coefficientsare determined for the gas-phase reaction of OH radicals with 1,1,2-trichloroethaneover an extended temperature range using a laser photolysis/laser-inducedfluorescence technique. Experiments were performed in a flow system at a total pressure of 740 f 10 Torr using He as diluent and camer gas. The rate coefficients, obtained over a temperature range of 295-850 K, exhibited pronounced non-Arrhenius behavior and were best described by the modiied Arrhenius equation k(T) = (1.63 f 0.22) X (T/300)2.Mexp ((70 f 5 5 ) / 7 ) cm3molecule-’ s-l. Comparison of the data with one previous low-temperature measurement is presented. The temperature dependence of the data is in good agreement with the s t r u m c t i v i t y relationship proposed by Atkinson. Similar comparisons yielded much poorer agreement with the semiempirical calculations of Cohen and Benson. Based on these results and our previous studies of CH3CH2Cl+ OH and CH2ClCH2Cl + OH, revised substituent factors, F ( X ) , for CH2Cl and CHC12are presented.
Introductioo Previous measurements of OH reactions with chlorinated hydrocarbons have been obtained predominantly at subambient and near-ambient temperatures emulating tropospheric and stratospheric chemistry.’ The need for a more fundamental understanding of the chemical processes involved in the incineration of hazardous wastes has resulted in a renewed interest in measurements of OH-chlorinated hydrocarbon reaction rates, in this case at elevated temperatures.” The impetus of these studies is to reduce the uncertainty generated upon extrapolation of experimentally measured rate coefficients to combustion temperatures. Specifically, an increase in the data base of high-temperature rate coefficient measurements is needed for validation and improvement of semiempirical models.&* The predictive ability of these techniques significantly affects the viability of chlorinated hydrocarbon combustion modeling efforts. Since all possible reactants cannot be studied, the accuracy of estimation methods is important and must be verified by comparison with high-temperature experimental data. A second, more fundamental objective of these studies is to gain insight into the molecular parameters that appear to most strongly influence the reactivity of these compounds. Previous studies have focused particular attention on the relative strengths of the C-H *To whom corrapondence should be addressed.
0022-3654/92/2096-1293$03.00/0
bonds being broken during the abstraction We have also considered how differences in the partial polarization of the activated complexes may be related to differences in reactivity? In this paper, we report atmospheric pressure (740 f 10 Torr) absolute rate coefficient measurements for the reaction of OH with 1,l ,24richloroethane: CH2ClCHClz + OH
+
CHzClCCl2 (+CHClCHCl,)
+ H2O (k,)
over the temperature range 295-850 K. Comparisons of the temperature dependence of the rate data with semiempirid model predictionsbs are discussed. Revised substituent factors, F(X), (1) Atkinson, R. A. Chem. Reu. 1986, 86, 69 and references therein. (2) Taylor, P.H.; DAngelo, J. A.; Martin, M. C.; Kasner, J. H.; Dellinger, B. In?. J . Chem. Kine?. 1989, 21, 829. (3) Kasner, J. H.; Taylor, P. H.; Dellinger, B. J. Phys. Chem. 1990, 91,
3250.
(4) Taylor, P. H.; McCarron, S.;Dellinger, B. Chem. Phys. Lett. 1991, 177, 27. (5) Liu, A.; Mulac, W. A,; Jonah, C. D. J. Phys. Chem. 1989, 93,4092. (6) Atkinson, R. A. Int. J. Chem. Kine?. 1986, 18, 555. (7) Cohen, N.; Benson, S. W. J . Phys. Chem. 1987, 91, 162. (8) Cohen, N.; Benson, S. W. J . Phys. Chem. 1987, 91, 171. (9) Benson, S.W. Thermochemicul Kinetics, 2nd ed.; Wiley and Sons: New York, 1976; pp 90-100.
Q 1992 American Chemical Society
1294 The Journal of Physical Chemistry, Vol. 96, No. 3, 1992
Taylor et al. TABLE I: Absolute Rate CoefficieatS (hc d mdeeuk-' s-') for k, T, K 10I3kl 1013k, T, K 295 1.84 f 0.07' 480 6.30 A 0.50 299 1.96 f 0.06 540 8.60 0.70 325 2.70 & 0.15 600 10.80 f 0.44 339 3.20 0.09 651 12.90 f 0.47 360 3.12 f 0.21 14.20 .+ 0.55 670 400 4.28 f 0.23 722 19.70 0.42 404 4.30 f 0.25 775 22.10 .+ 1.62 447 4.67 0.35 850 25.80 f 3.12 ~
*
*
*
'Uncertainties represent f2u estimates of the random experimental error from the least-squares analysis. 0
1
2
3
5
4
6
7
[Ct4ClCHC&]x lW*(molec cr")
Figure 1. Plot of pseudo-first-order rate constant, k', as a function of [CH2C1CHCl2]at various reaction temperatures.
for CH2Cl and CHC12 are presented based on these results and our previous studies of CH3CH2Cl+ OH3 and CH2ClCH2Cl+
OH.^
Experimeatpl Tech~~iqw and Data Reduction All experiments were performed using a refined laser photolysis/laser-induced fluomtxnce technique. A detailed description of this technique and its application to OH kinetic studies has been previously r e p ~ r t e d ;hence, ~ . ~ we only briefly summarize the experiment. All experimentp were carried out under "slow flow" conditions; i.e., the buildup of reaction products was minimized Individually controlled gas flows of CH2CICHC12/N20/H20/Hewere thoroughly mixed before entering the reactor. The composite 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 atmospheric pressure, 740 f 10 Torr. The gas temperature in the reaction zone was measured with a retractable chromel/ alumel thermocouple 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-nm PhotodisPociation of CHzClCHC12/NzO/H20/Hegas mixtures. Following reaction initiation, time-resolved OH profiles were measured as functions of [CH2C1CHC12]using laser-induced fluorescence. OH decays were obtained over 2-3 decay lifetimes by signal-averaging 200&3000 laser shots over a time interval of 0.8-30 ms after the photolysis pulse. Over the entire temperature range, reactive and diffusive [OH] decay profiles exhibited exponential behavior and were fit by the following nonlinear expression [OH] = [OHIOexp(-k't) + y (1)
.
where y is the background (constant) signal level. Because [CH2C1CHCl2]> 5OO[OH] in all reactive experiments, exponential "reaction only" OH dependences of pseudo-first-order decay constant k' = kl[CH2C1CHCl2] 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, k,,were obtained from the slope of the least-squares straight line through the (CHzClCHCl2,k? data points (cf. Figure 1). Values of k' ranged from about 80 to 1200 s-l, depending on the reactant concentration. The first-order OH decay rate constants in the absence of reactant, kd,ranged from 70 to 140 s-' and increased with increasing reactor temperature. The chemicals used in this study had the following stated minimum purities: He, 99.999+%; N20, 99.9%; H20, HPLC
+
(10) Zhang, Z.; Liu, R.; Huie, R.; Kurylo, M. J. J. Phys. Chem. 1991, 95, 194. (11) Danis, F.; Caralp, F.; Veyret, B.; Loirat, H.; Leaclaux, R. Int. J . Chem. Kinet. 1989, 21, 715.
TABLE Ik Arrbenilrs Rate Expressions (In cm3 molecule-' 6')for kek, rxn rate expression 1.7 X 3.2 X l o f 3e~p(-23000/RT)~ 2.3 X exp(938/RT)e e~p(-602O/Rl")~ 2.5 X 1.7 X 3.2 X 10" e~p(-21500/RT)~ 9.4 X lo-]' exp(562iRT)' 2.0 X lo-" e~p(-5000/RT)~ 'A factor estimated by analogy with other radical recombination reactions, ref 9. b A factor calculated from transition-state theory, ref 9. Activation energy calculated from following expression: E, = AH, + 2 kcal mol-I. cReference 10. dRate expression estimated by analogy with other chlorohydrocarbon H atom transfer reactions, ref 5.
organic-free reagent grade. 1,1,2-Trichloroethane was subjected to acid-washing followed by fractional distillation prior to use. GC/MS analysis indicated a purity of >99.9%.
Reeulcs and Discussion Absolute rate coefficients for kl are listed in Table I. Random error limits ( 2 4 , derived from a propagation of error analysis, were generally less than &lo%. In the absence of reactant impurities, sources of systematic error are expected to be limited to thermally and photolytically induced secondary reactions. Photolytically derived systematic error was tested by variation of the intensity of the pump laser from 2 to 12 mJ/cm2. OH decays were invariant, within experimental error, for intensities 1 8 mJ/cm2 for all temperatures. At 12 mJ/cm2, a negligible (
