T H E
J O U R N A L
O F
PHYSICAL CHEMISTRY Registered in U.S. Patent Office
0 Copyright, 1978, by the American Chemical Society
VOLUME 82, NUMBER 16
AUGUST 10,1978
The Rate Constant for the Reaction of O~ygen(~P) Atoms with Dichlorine Monoxide Andrzej W. Mizlolek’ and Mario J. Mollna** Depaflrnent of Chemistry, University of California, Irvine, California 92717 (Received January 3, 1978; Revised Manuscript Received May 30, 1978) Publication costs assisted by the National Aeronautics and Space Administrat/on
The reaction between ground state oxygen atoms and dichlorine monoxide has been studied in a fast flow system over the temperature range from 236 to 295 K under pseudo-first-order conditions. The oxygen atom concentration was monitored by NOz* chemiluminescent emission. The Arrhenius expression for the rate constant was found to be 2.7 f 0.3 X exp[(-560 f 80)/T]cm3 molecule-l s-l, and at 295 K the rate constant value cm3 molecule-l s-’. is 4.1 f 0.5 X
Introduction The reaction of oxygen atoms with C120 has been studied previously by two groups. Freeman and Phillips3 employed a fast flow discharge system coupled to a mass spectrometer and obtained a rate constant value of 1.4 f 0.2 X cm3 molecule-l s-l at room temperature. Basco and Dogra4 employed an indirect flash photolytic technique to obtain a value of 8.6 X cm3 molecule-’ s-’, also at room temperature. In the present work we have used a fast flow discharge apparatus to measure the rate constant for the reaction o(3~ +)c i 2 o 2c10 (1)
-
We employed the air afterglow technique (NO2chemiluminescence) for the detection of oxygen atoms.6 We have also extended the temperature range of the measurements (236-295 K) in order to obtain values for the parameters of the Arrhenius expression. Hydrolysis of C120 produces HOC1, a species which is of some interest in connection with the chemistry of chlorine in the ~tratosphere.~ Reaction with oxygen atoms is a likely stratospheric destruction mechanism for HOC1. In order to estimate the rate of this process the rate constant for the 0 + ClzO reaction has to be known, because HOC1 samples are almost invariably prepared in the laboratory as an equilibrium mixture with ClzO and HzO. 0022-3654/78/2082-1769$01 .OO/O
Experimental Section The basic discharge-flow apparatus has been described earliere7 The oxygen atoms were produced by the reaction of nitrogen atoms (generated by a microwave discharge through N2)with nitric oxide; the total pressure was 1 Torr. The dichlorine monoxide was introduced to the system through a moveable injector. An RCA-1P28 photomultiplier tube monitored the intensity of the afterglow from excited NO2 produced by the reaction of excess NO with unreacted O(3P). The logarithm of the intensity was plotted vs. the distance of the injector from an arbitrary zero position, and the rate constants were calculated from the slope of the resulting straight lines. Initial [ 01 was estimated by chemiluminescent titration of N with NO.fi The 1P28 tube responds only to X
The Journal of Physical Chemistry, Vol. 82,No. 16, 1978
Reaction of O(3P) Atoms with Ci20
IO DISTANCE,
15
cm
*L
Flgure 3. First-order decays of 0 atom signals at 273 K: 0, [CI 0 = 7.7 x 1013molecule om3; A, [cI,o], = 12.7 X 1013molecuie c m ; 0 ,[C120]o = 17.8 X loi3molecule ~ m - ~ .
cm3molecule-l s-l) then a significant depletion of ClzO might occur. The most likely products are ClNO and C10: ClzO
+ NO
ClNO
+ C10
value a t room temperature is about three times smaller than the one reported by Freeman and Phillips3 and about two times smaller than the one reported by Basco and D ~ g r a .On ~ the other hand, the complications arising from side reactions would tend, if at all, to increase the apparent rate constant measured in our experiments. The value of kl reported by Basco and Dogra was obtained rather indirectly from an analysis of a complex reaction mechanism, and later worklZhas since shown that the kinetic scheme proposed by Basco and Dogra requires reinterpretation. In the experiments of Freeman and Phillips a tenfold excess of oxygen atoms over ClzO was employed and the reason for the discrepancy with ow results is not clear: the rate of the C1 C120 reaction is not sufficiently fast to deplete ClzO significantly in their system, so that the catalytic effect of reactions 2 and 3 does not appear to be responsible for the discrepancy. Some other complications which might have affected the work of Freeman and Phillips are (1)truly pseudo-first-order conditions were not attained, so that absolute concentrations of both oxygen atoms and ClzO had to be determined, and (2) the use of a microwave discharge through oxygen-argon mixtures, and the overall low pressures (-0.25 Torr) in their flow tube might have resulted in the presence of rather large concentrations of 02(lAg).
+
4t11111111 a11111 5
1771
(7)
The effect of this reaction on our experiments would be to increase the apparent primary reaction rate due to the fast, subsequent reactions of the C10 radicals discussed earlier; on the other hand, no trend in kl with increasing [NO] was observed (see Table 11). Furthermore, a nonnegligible reduction in the C120 concentration resulting from reaction 7 would give rise to curvature in the plots of the logarithm of the afterglow intensity vs. injector distance; no such behavior was noted under any of a wide range of experimental conditions (see Figure 3). The temperature dependence of kl shown in Figure 2 can be expressed as 2.7 f 0.3 X exp[(-560 f 80)/T] cm3 molecule-l s-l, The error limits are based on a consideration of probable systematic errors and typical scatter in the data. The magnitude of the preexponential factor is comparable to that of other reactions of oxygen atoms with triatomics; e.g., for 0 + 03,COS, COz, and CS2 the A values are respectively 1.9, 2.6,2.8, and 3.7 X cm3 molecule-l s-l.ll These values are in agreement with expectations from transition state theory. There is some discrepancy between our results and those which appeared earlier in the literature: our rate constant
Acknowledgment. We thank Mr. Brian Jacobs for technical assistance. Also, discussions with Dr. J. E. Spencer are gratefully acknowledged. This research has been supported by the National Aeronautics and Space Administration under Grant No. NSG-7208.
References and Notes (1) Geological Research Division, Scripps Institution of Oceanography, UCSD, La Jolia, Calif. 92093. (2) Alfred P. Sloan Foundation Research Fellow; Dreyfus Teacher-Schohr. (3) C. G. Freeman and L. F. Phillips, J. Phys. Chem., 72, 3025 (1968). (4) N. Basco and S. K. Dogra, Proc. R . Soc. London, Ser. A , 323, 29 (1971). (5) R. D. Hudson, Ed., "Chlorofluoromethanes and the Stratosphere", National Aeronautics and Space Administration Reference Publication No. 1010,Washington, D.C., 1977. (6) F. Kaufman, Prog. React. Kinet., 1, 1 (1961). (7) L. T. Molina, J. E. Spencer, and M. J. Molina, Chern. Phys. Lett., 45, 158 (1977). (8) C. J. Schack and C. B. Lindahi, Inorg. Nucl. Chem. Lett., 3, 367
(1967). (9) M. M. Rochkind and G. C. Pimentel, J. Chem. phys., 42, 1361 (1965). (10)C. L. Lin, J . Chem. Eng. Data, 21, 411 (1976). (11) R. F. Hampson and D. Garvin, Natl. Bur. Stand. (U.S.),Spec. Pub/., No. 513 (1978). (12) P. P. Bemand, M. A. A. Clyne, and R. T. Watson, Chem. Soc., Faraday Trans. 7 , 69, 1356 (1973);see also ref 13. (13) R. T. Watson, J . Phys. Chem. Ref. Data, 6,871 (1977).
