Communications to the Editor
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2303
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Log(p(E) joule/cm2) Figure 2. Dependence of the conversion per flash (in the Irradiated volume) upon the lncdenf laser energy density: (0)pumping CH3CH2F with the P(18) line of the 00’1-02’0 transition with 12% CH3CF3;(W) pumping CH3CF3with the R(16) line of the OOo-lOrrO transition with 50% CH3CH2F; (A)pumping CH3CF, with 25% cyclopropane. The slope of the lines drawn through the data points are 9 f 1 for CPHSF and 6 f 1 for CH3CF3. The total pressure for these experiments was 0.6 Torr.
ponent was negligible; however, at the maximum energy a minor “thermal” contribution was identified by the monitoring molecules. Thus, the slopes are upper limits to the true values for the laser induced reaction. Even at the highest energy, HF eliminationlo was still the only reaction observed for C2HsFor CH3CF3. Based upon the sensitivity of our detection system, the practical threshold for observation of reaction was 2 J/cm2. Variation of the wavelength of irradiation by selection of different C02lines only showed effects consistent with the change in energy density illustrated in Figure 2. The question of intramolecular relaxation of the absorbed laser energy was studied by irradiating bromofluoroethane with the P(36) line of the 00’1-02’0 band. If intramolecular energy randomization were minimal, excitation by absorption of laser energy would be expected to excite the molecule to favor HF elimination. In contrast the RRKM statistical4 yields would greatly favor HBr because of the lower threshold energy. Experiments at 0.6 Torr using 3.1 J/cm2 showed a pronounced favoring for HBr elimination over HF elimination; the measured ratio was bout 101. This ratio is consistent with an RRKM calculationll if the mean excitation energy in the CH2FCH2Br molecule was 75 kcal mol-’. If randomization has occurred, the RRKM calculated k~ values also can be used to estimate the mean energy in CH3CH2Fand CH3CF3. At 1 Torr the collision rate is lo7 s-l which can be taken as the lower limit to the rate For this limit the molecules must have -70 and -85 kcal mol-’ for C2H5F and CH3CF3,respectively?* Providing that these reactions occur via a RRKM mechanism, these molecules evidently acquire energy from the laser field in significant excess of the threshold energy. Acknowledgment. We thank Dr. Wayne C. Danen (Kansas State University) for useful suggestions and discussions. This work was supported by the National Science Foundation (MPS 75-02793).
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References and Notes (1) R. V. Ambartzumian and V. S. Letokov, Acc. Chem. Res., 10, 61 (1977). (2) D. F. Dever and E. Grunwald, J. Am. Chem. Soc., 98, 5055 (1976). (3) (a) I. Glatt and A. Yogev, J. Am. Chem. Soc., 98, 7087 (1976); (b) A. Yogev and R. M. J. Benmair, Chem. Phys. Left., 46, 290 (1977). (4) (a) H. W. Chang, N. L. Craig, and D. W. Setser, J . Phys. Chem., 76, 954 1972); (b) K. C. Kim, D. W. Setser, and B. E. Holmes, ibld.,
77, 725 (1973); (c) P. J. Marcoux, E. E. Seifert, and D. W. Setser, Int. J. Chem. Kinet., 7, 473 (1975); J. Phys. Chem., in press (1977). (a) E. Tschulkow-Roux and W. J. Qulring, J. Phys. Chem., 75, 295 (1971); (b) P. Cadman, M. Day, A. W. Kirk, and A. F. TrotmanDickenson, Chem. Commun., 203 (1970); (c) S.W. Benson and H. E. O’Neal, “Kinetic Data on Gas Phase Unimolecular Reactions”, National Bureau of Standards, Washington, D.C., 1970, p 89. (a) D. S.Frankel. J. Chem. Phvs.. 65. 1696 (1976) (b) S.Makamel and J. Jortner, lbM., 65, 5204i1976);‘ (c) M.‘F. ododman, J. Stone, and D. A. Dows, ibid., 65, 5052, 5062 (1976). M. J. Cosalola, P. A. Schulz, Y. T. Lee, and Y. R. Shen, Phys. Rev. Lett., 38,-17 (1977). (a) W. C. Danen, W. D. Munslow, and D. W. Setser, J . Am. Chem. Soc., 99, 6961 (1977). (b) Our finding of no laser induced decomposition for cyclopropane differs from reports in the literature, E. Grunwald, Chem. Eng. News, 54 (48), 18 (1976); M. L. LeSieckl and W. A. Guillory, J . Chem. Phys., 66, 4317 (1977), presumably because of our less severe focusing conditions. M. C. Gower and K. W. Billman, Appi. Phys. Lett., 30, 514 (1977); Opt. Commun., 20, 123 (1977). W. Braum and W. Tsang, Chem. Phys. Lett., 44,354 (1976). These authors observed mainly elimination reactions from alkyl chlorides and bromaes, but for the ioddes radicals were formed from C-I bond rupture. T. H. Richardson, unpublished results; the threshold energles used in the calculatlon were 60 and 54 kcai/mol for HF and HBr eiiminatlon, respectively. Depattment of Chemistw Kansas State University Manhattan, Kansas 66506
T.
H. Rlchardson D. W. Setser’
Received July 11, 1977
Kinetics of the CI 4- C2H2Reaction. Stratospheric Implication Publicatlon costs assisted by Centre Natlonal de la Recherche Sclentifique, France
Sir: Termination of C1 atom chains in the stratosphere would essentially occur via C1 t CH,
-+
CH, t HCl
(1)
which would result, at least temporarily, in the interruption of ozone depletion in the earth’s strat0sphere.l Another hydrocarbon, acetylene, has been recently suggested by Lee and Rowland2as a species which would also remove C1 atoms. To estimate the effect of C2H2, these authors have studied the kinetics of the C1+ C2H2 reaction by a competitive method. We have also performed in 1972 a kinetic study of this reaction which led to an absolute rate constant determ i n a t i ~ n . This ~ study was made in relation to the inhibition of hydrocarbon flames by chlorinated compounds4 in order to explain the cl emical mechanism of the inhibition. The Cl + C2Hzreaction was studied by the flow discharge-mass spectrometer method. Typical experimental conditions for the rate constant measurements were as follows: pressure, 0.50-2.00 Torr (diluent, helium); temperature, 295-500 K; (C,H,), 0.1-5 X 1014mol cmM3; . rate constant for the (Cl), 1-3 X 1015atoms ~ m - ~The elementary reaction C1 + C,H,
+
products
(2)
was found to be temperature independent in the 295-500 K range: kz = (2.0 f 0.5) X cm3 molecule-1 s-l. The products of the overall reaction were also identified. The main products HC1, C2H2C1,, and C2HCl were simultaneously detected in the flow reactor; when the excess of C1 atoms was increased, other chlorinated compounds were also observed as CzHzC14and C2Cl4. Stoichiometry The Journal of Physical Chemistry, Vol. 81, No. 24, 1977
Communications to the Editor
2304
was measured (C1 c o n ~ u m e d / C ~consumed) H~ and reached high values (6 or 8) when chlorine atoms concentrations were increased. Fast secondary reactions must exist in agreement with the experimental observation of the products and the value of stoichiometry. The initial step proposed was unambiguously the addition reaction Cl t C,H,
--f
C,H,Cl*
(3)
because the zero activation energy observed makes negligible the abstraction path C1+ C,H,
4
HC1 t C,H
(4)
which is 9 kcal/mol endothermic. Following the initial addition step (3), fast secondary reactions occurring in our system were considered and a mechanism of the overall reaction was suggested, where the excited or stabilized C2HzClradical could undergo reactions with chlorine atoms leading to the formation of C2HCl (by an abstraction reaction) or C2H2C12(by an addition reaction). A comparison of our kinetic results with the measurements of Lee and Rowland allows to indicate the actual stratospheric significance of the reaction of chlorine atoms with acetylene. Our value for the rate constant of reaction 2 is lower than the determination of Lee and Rowland. The value of kz estimated by these authors is k 2 = cm3 molecule-l s-l, that corresponds to effective addition in 2-5 collisions. Our value, k2 = 2 X cm3molecule-' s-l, gives an efficiency of the addition reaction of Following reaction 3 Lee and Rowland considered that CzH2C1*could undergo either stabilization ( 5 ) or decomposition ( 6 ) : C,H,Cl* t M -+ C,H,Cl t M C,H,Cl*
-+
C,H, t C1
In our system, since a pressure dependence study of k 2 could not be carried out, it is not possible to exclude the occurrence of the decomposition reaction of C2HzC1*(6) which may not be negligible, compared to stabilization reaction 5 at pressures near 1Torr. Furthermore reaction 6 could also not be negligible compared to the reactions of C2H2C1*with excess of C1 atoms. A comparison with the similar addition reaction H + CzH2(7) does not give any clear indication about the pressure dependence of the rate constant: from the results of Volpi and Zocchi5 it seems that the stabilization reaction of CzH3*became predominant at 1-2 Torr (with H2and Ar as third bodies), while the works of Michael and Weston! and Michael and Niki7 have shown a pressure dependence on k7 in the pressure ranges 2-17.3 and 0.52-1.67 Torr, respectively. Thus, if the decomposition reaction of C2HzC1*is not negligible near 1 Torr with He as a diluent the rate constant of step 3 would be higher than 2 X cm3 molecule-' s-l and the difference with the data of Lee and Rowland2would be consequently reduced. In addition the agreement between the two determinations would be good if we consider the efficiency of stabilization of C2HzC1*by He to be 1/500 at 1-2 Torr. Considering the stratospheric impact of the C1+ C2Hz reaction, we can compare it with the C1+ CH4 reaction for which the rate constant has been also determined in our experimental system.8 At 30 km altitude using the stratospheric mixing ratios of and lo4 for CzHzand CH4, respectively, and considering our values of the rate constants calculated at T = 226 K, the rate ratio kz(C2H2)/k1(CH4)is 5 X loT4. However, if we admit there is a pressure dependence of k2 at low pressures, then kz would equal about 2 X Tbe Journal of Pbyslcal Cbemktry, Vol. 8 1, No. 24, 1977
cm3 molecule-l s-l at 9 Torr which is the pressure corresponding to an altitute of 30 km. That gives for lzz(C2Hz)/kl(CH4)a value of 5 X which also agrees with the upper limit of given by Lee and Rowland.2 This upper limit was calculated considering that about 1%of the CzH2C1*radicals would be stabilized at 30 km. Finally, whatever the fraction of stabilized C2H2Clcould be, the rate constant found for k2 makes insignificant the C1+ CzHzreaction in stratospheric processes compared to the C1 CH4 reaction.
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References and Notes (1) F. S. Rowland and M. J. Molina, Rev. Geophys. Space Phys., 13, 1 (1975). (2) F. S. C. Lee and F. S. Rowland, J. Phys. Chem., 81, 7, 684 (1977). (3) G. Poulet, J. Barassln, G. Le Bras, and J. Combourieu, Bull. SOC. Chlm. Fr., 1, 1 (1973). (4) G. Le Bras, I. Hajai, J. Combourieu, and P. Laffitte, J . Chlm. Phys., 64, 1153 (1967). ( 5 ) G. G. Volpi and F. Zocchi, J . Chem. Phys., 44, 4010 (1966). (6) J. V. Michael and R. E. Weston, Jr., J. Chem. phys., 45, 3632 (1966). (7) J. V. Michael and H. Niki, J . Chem. Phys., 46, 4969 (1967). (8) G. Poulet, G. Le Bras, and J. Combourleu, J . Chlm. Phys., 71, 101 (1974). CNRS Centre de Recherches sur la Chlmle de la Combustion et des Hautes Temphratures 45045 Orleans Cedex, France
6.Poulet 6. Le Bras* J. Combourleu
Received July 22, 1977
The Role of the Termination Reactlon H 4- CH:, CH4 in the Pyrolysis of Propane in the Temperature Range 1100-1300 K -+
Sir: Propane pyrolysis is a famous industrial process. Besides ethylene, the most important fine chemical feedstock in the chemical industry, hydrogen, methane, ethane, and propylene have also been found to be major products. This process has been investigated with many techniques and at various temperature~.l-~Recently Lifshitz et al. studied this pyrolysis with a shock tube in the temperature range 1100-1300 K using mixtures of propane and argon as reactantx~~ This mixture was heated to a desired temperature by a reflected shock wave, maintained and allowed to react at that temperature for a period of 700 ps, and rapidly cooled by expansion waves. A series of such experiments was carried by them for different reactant compositions and reaction temperatures. Lifshitz and Frenklach then proposed the following reaction scheme as a mechanism of the generation of these products:2 C,H,
-+
CH,
+ C,H,
CH, t C,H, CH, t i-C,H, CH, + C,H, -+ CH, + n-C,H, H + C,H, -+ H, + i-C,H, H + C,H, -+ H, + n-C,H, i-C,H, C,H, + CH, n-C,H, C,H, + CH, i-C,H, C,H, + H n-C,H, C,H, + H C,H, C,H, + H CH, + CH, C,H, +
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