T H E J O U R N A L OF
PHYSICAL CHEMISTRY Registered in U.S.Patent Office 0 Copyright, 1978, by the American Chemical Society
VOLUME 82, NUMBER 19
Rate Constant for the Reaction CIO
+ NO
-
CI
SEPTEMBER 21,1978
+ NOp
M. T. Leu* and W. B. DeMore Jet Propulsion Laboratory, Callfornla Institute of Technology, Pasadena, California 9 1103 (Received March 13, 1978) Publication costs assisted by the Jet Propulsion Laboratory
+
-
The rate constant for the reaction C10 NO C1+ NO2 has been determined over the temperature range 226.7-415.4 K in a discharge flow system using a mass spectrometer as a detector. The results, expressed in the Arrhenius form kl = (5.72 f 0.18) X exp[(296 f 20)/27 cm3 s-l, are compared with previous measurements. I. Introduction Possible ozone depletion in the stratosphere due to the catalytic cycle of C10, has generated considerable interest in chlorine chemistry.l The reaction
admitting a trace of ozone (510l2molecules/cm3) into the flow tube, leading to the reaction
kl
The rate constant k2 has been measured in our laborato$ and elsewhere."-1° The reactant NO, premixed with helium, was then admitted into the flow tube through a moveable Pyrex injector (0.6 cm 0.d.) with multiple holes around the tip. Both the discharge tube and the flow tube were coated with phosphoric acid aged under vacuum conditions. Flow rates of gases were measured by linear mass flowmeters (Teledyne-Hastings-Fhydist).Flowmeters were calibrated by a Hastings bubble-type calibrator (Model HBM-1). The flow rate of the premixed NO/He mixture was calibrated by the method of pressure drop at constant volume and temperature. The total pressure of the reaction zone was measured with a calibrated MKS Baratron pressure meter (Model 310 AHS-10) which was connected in the middle of the reaction zone. The temperature of the flow tube was controlled by a high-capacity Haake circulator with fluids flowing through the jacket and measured by a chromel-constantan thermocouple in the middle of the cooling jacket. Most of gases used for this research were supplied by Matheson Gas Products, including helium (UHP), oxygen (UHP), chlorine (Research Grade), and nitric oxide (CP Grade). Helium was further purified by passage through a molecular sieve trap at liquid nitrogen temperature, which was believed to be effective in removing H 2 0 and
C10 + NO
C1+ NO2
(1) is important in determining the efficiency of the catalytic cycle. This reaction has been studied by two groups in recent years. First, Clyne and Watson2reported kl = (1.70 f 0.21) X cm3 s-l at 298 K using a discharge flow/mass spectrometer apparatus. Second, Zahniser and Kaufman3 investigated this reaction by a discharge flow/resonance fluorescence apparatus. They reported kl = (1.13 f 0.14) X exp[(2OO f 3 0 ) / q cm3 s-l over the temperature range 230-298 K. We have undertaken the direct study of reaction 1using an apparatus which is similar to that used by Clyne and Watson. +
11. Experimental Section The apparatus and the experimental principle used for this research have been described in detail in a previous p~blication.~ Briefly, all rate constant measurements were made by observing the decay of C10 ( m / e 51) in a large excess of NO in a Pyrex flow tube 120 cm in length and 2.5 cm i.d. In a side arm of the flow tube chlorine atoms ( w 1014 atoms/cm3) were generated by passing a small amount of molecular chlorine in a helium carrier through a quartz discharge tube, with approximately 30 W of microwave power. The C10 radicals were produced by 0022-3654/78/2082-2049$0 1.OO/O
c 1 + 03
k2
@ 1978 American Chemical Society
c10
+ 02
2050
The Journal of Physical Chemlstty, Vol. 82, No. 19, 1978 Io5
I
I
I
I
I
M. T. Leu and W. B. DeMore
I
CPO + N O + C P + NO,
'0°
T = 299 K V = 2570 cm I - '
x
3
A
1 0 3 1 1 0 5
I
I 15
IO
I
IN01 (x I O l 3 ~ r n - ~ ) 0 0.51 -k I .53
I 20
0 1.91
1.01 1.40
0
2.31
I 25
I 30
REACTION LENGTH, cm
Figure 1. CIO decay resulting from the reaction CIO
NO,.
I
35
+ NO
-
0
CI
+
Figure 2. Plot of k,' vs. [NO] at T = 299 K.
C10 + NO2 + He C1+ NO
--
+ He-.
I
I
+ NO2
CIO + N O - C I
another molecular sieve trap at dry ice temperature to remove higher oxides of nitrogen. The mass spectrometer was used to prove the absence of NO2 ( m / e = 46). Ozone was prepared in a small laboratory ozonizer with oxygen carrier.
C10 + C10
4
3
[NO], (x IOl3 rnoleculer/crn3)
02.Nitric oxide was purified by passing the gas through
111. Results and Discussion In the present experiment the gas concentrations in the reaction zone were adjusted such that 3 X 10l6 I [He] I 8X [Cl] = 1014,5 X 1O1O 1 [ClO] I 10l2,and 5 X 10l2 I [NO] I 3.8 X 1013( ~ m - ~The ) . initial C10 concentration was at least one order smaller than the NO concentration to satisfy the pseudo-first-order conditions. Also, interference from the reactions
2
1
THISWORK X
0
2
CLYNE A N D WATSON ZAHNISER A N D KAUFMAN 4
3
ID (x IO3
K-I)
products
(2)
C10N02 + He
(3)
Flgure 3. Arrhenius plot of k , vs. 1/ Tof our data, and the data of Clyne and Watson, and Zahniser and Kaufman.
ClNO + He
(4)
TABLE I: Summary of Measurements of k ,
is believed to be negligible based on the reported rate constants for these reactionsa4J0 The radicals C10 were detected at m/e 51 by a mass spectrometer operated at 60 V of electron energy. Figure 1shows examples of the experimental data, taken under the following conditions: T = 299 K, P = 1.08 Torr, fl = 2570 cm s-l, and [CIO]oN 5 X loll molecules ~ m - ~ The . concentrations of nitric oxide were varied from 0.51 X 1013 . was observed to decay to 2.31 X 1013molecule ~ m - ~[ClO] exponentially and was defined as [ c ~ o ]=~ [~10]~e-k1't
(5)
where k: is the first-order rate constant for reaction 1and t is the reaction time. k? was then calculated from the slopes of the straight lines in Figure 1. The rate constant kl was determined as kl = kll/[NO]
(6)
All data at T = 299 K are summarized in Figure 2, which shows the variation of k: vs. [NO]. The data can be represented approximately by a straight line with zero intercept. The average of these data is (1.53 f 0.11) X cm3 s-l. The uncertainty represents the first standard deviation of kl. All data were corrected for the effect of concentration gradient due to axial diffusion, with the maximum correction being about 10%. The correction for the effect due to radial diffusion was small and was not made (15%). The results of a total of 70 experiments taken in the temperature range 226.7-415.4 K are shown in Figure 3,
T,K 226.7 231.0 234.7 265.6 299.0 360.7 392.0 415.4
k , , lo-" cm3 s-' 2.04 1.95 2.09 1.83 1.53 1.31 1.24 1.14
i: i: i: ?
no. of experiments
0.06
8
0.06 0.10 0.18
3 9 6 27 6 4
i 0.11 i 0.07 i: 0.07 i 0.04
7
and are listed in Table I. A weighted least-squares fitting computer program was used to obtain the Arrhenius expression kl = (5.72 f 0.18) X exp[(296 f 20)/T] cm3 s-l (7) Again, the uncertainties represent the first standard deviation of the A factor and the activation temperature. The systematic error of all data was estimated to be 15%, which includes the uncertainties of absolute pressure, flow rates, the corrections of axial and radial diffusions, and the geometry of the flow tube. Comparison of the present results and those of previous investigators is shown in Figure 3 and also in Table 11. Our data at T = 299 K are in excellent agreement with those obtained by Clyne and Watson. The agreement with the results of Zahniser and Kaufman, which are 30-40% higher than our data, is also good. The negative temperature dependence observed in this work, equivalent to !Po, agrees reasonably well with the dependence found by Zahniser and Kaufman.
Rate Constant for the Reaction CIO
+ NO
-
CI
+ NO2
The Journal of Physical Chemistry, Vol. 82, No. 19, 1978 2051
TABLE 11: ComDarison of Measurements of k , Arrhenius expressions
k , , 298 K
technique ref
(1.70 i 0.21) x 10'" DFMS 2 (2.18 f 0.23) x l o - ] , (1.13 f 0.14) x 10'''- DFRP 3 exp[(2OO i 30)/T] (1.53 i 0.11) X lo-" (5.72 i 0.18) X lo-'*. DFMS this exp[(296 i 20)/T] work
The existence of a negative temperature dependence in a simple bimolecular reaction requires some explanation. Recent trajectory calculations by Jaffell indicate that small inverse temperature dependences are possible for the cross sections of reactions such as C10 + NO, but not of a magnitude sufficient to overcome the positive temperature dependence of the collision frequency. Thus, according to this treatment, the overall temperature dependence should be positive. Observations of a negative temperature dependence in the reactions of atomic oxygen with certain olefins have previously been discussed by Singleton and Cvetanovicl2 and by Davis et al.13 As concluded by those workers, the temperature dependence of the entropy of activation is inadequate to explain the effect, because this would require the postulation of improbable properties for the transition state. For the olefin reactions, Singleton and Cvetanovic proposed instead the formation of an intermediate complex which is collisionally stabilized, and which may either redissociate to the reactants or otherwise dissociate in a different mode to give products. A negative temperature dependence of the overall process is then expected if the activation energy of the reverse process giving back the reactants is greater than the forward process giving products. As discussed in a later paragraph, this explanation requires the assumption that the reverse process is comparable in rate to the forward process, despite the fact that the activation energy of the reverse reaction is postulated to be substantially greater than that of the forward reaction. This imposes restrictions on the relative A factors for the two processes. A mechanism involving formation of an intermediate complex was also proposed by Smith and Zellner14and Smith15 to explain the temperature14 and pressure16 dependence of the OH + CO reaction: OH + CO e [HOCO]* H + COZ
+
tM
kf
k,
kri
C1+ NO2
-
N
-
+
OClmNO (Ilb) If the intermediate having the isomeric form OC1-NO is formed to a significant extent in the gas phase C10 NO reaction, then this species may have a low A factor for dissociation into C1 + NOz, due to the rearrangement required [OCl*NO]*-+ [ClO*NO]* C1 + NO2 (12) -+
HOC0 (8) A somewhat analogous mechanism can be suggested for the present reaction, which does not necessarily involve a collisionally stabilized intermediate. The mechanism is C10 + NO S [ClO.NO]
critical, or threshold, energy required for the given process to occur, and n is a parameter identified with the number of effective oscillators in the molecule. For the reverse reaction, the excess energy E* - E, is thermal in nature and therefore strongly temperature dependent. By contrast, the excess energy for the product-forming path consists of both thermal energy and overall reaction exothermicity, the latter quantity being 9 kcal/mol. (This is based on the assumption that there is no energy barrier for the reaction C1+ NOz [ClO-NO]. Support for this assumption is found in the matrix experiments of Tevault and Smardzewski,16in which the C1+ NOz reaction evidently occurred at temperatures as low as 10 K.) Because of the fact that k, increases more rapidly with temperature than does k,, the overall reaction rate constant k f [ k , / ( k , + k,)] decreases with increasing temperature, Le., shows a negative temperature dependence. All of the foregoing mechanisms involving intermediate complex formation have one important feature in common with regard to the required behavior of the intermediate. That is, they all require that the rate constant for dissociation of the intermediate into original reactants must be at least comparable in magnitude to the rate constant for dissociation into products. For the present case, k,. Otherwise, no mechanism 9, this means that K, temperature (or pressure effects) on the overall rate constant via this mechanism can exist. However, eq 10 would predict that k, >> k, if the corresponding preexponential factors, A , and A,! are similar in magnitude. In k,, it is necessary for the preexpoorder to obtain k, nential factor A, to be one or two orders of magnitude greater than A,. It is difficult to predict with certainty whether or not this condition is probable, although if the intermediate were simple chlorine nitrite it would appear unlikely. The splitting of a species such as ClONO into two fragments C10 and NO may have a somewhat higher A factor than that for splitting off a C1 atom, but it is not obvious that the A factor should be higher by one or two orders of magnitude. A partial answer to this paradox may be found in the previously referenced work of Tevault and Smardzewski,16 who studied the C10 + NO reaction under low temperature matrix conditions, and observed two products C10 + NO C10.NO (W
(9)
The intermediate, which may not be identical with chlorine nitrite (see later discussion of other possibilities), can be viewed as a chemically activated species which undergoes decomposition to give either C10 + NO or C1 + NOz. The effective second-order rate constant for the overall C10 + NO reaction is k f [ k , / ( k , + k,)]. According to RRK theory, both k, and k , are given by expressions of the form
+
+
Thus under these circumstances the effective A factor for redissociation into C10 + NO may be much higher than that for product formation. It should be noted that the proposed mechanism, eq 9, is not incompatible with the fact that the rate constant for the reaction is large, Le., 1.5 X cm3/s at 298 K. If k, = k,,as required to explain the negative temperature dependence, then it is only necessary that kf = 3 x lo-" cm3/s, which is not an unreasonable rate constant. An implication of the present mechanism of the C10 NO reaction is that the reaction rate may be somewhat pressure dependent. This can be seen from a simple comparison of mechanisms 8 and 9. The pressure range of the present experiments was not sufficient to test this possibility.
+
where A is the preexponential factor for the process in question (not necessarily equal for k, and k,), E* is the total excitation energy of the intermediate, E, is the
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The Journal of Physlcal Chemlstry, Vol. 82, No. 79, 1978
K. J. Olszyna and E. Grunwald
Finally, the effect of the present result on the model calculations of ozone depletion in the stratosphere needs to be discussed briefly. The calculations in the reports of the Atmospheric Chemistry Panel of the National Academy of Sciences17and the NASA Chlorofluoromethanes and the Stratosphere Assessment18 have used the rate constant for C10 + NO as obtained by Zahniser and Kaufman. For kl at 230 K, the present result, 2.1 X cm3 s-l, is about 20% smaller than the rate constant, 2.7 X cm3s-’, used in both the NAS and NASA studies. However, based on the sensitivity to this reaction in the model calculations, as given in the respective reports, it appears that the predicted stratospheric ozone reduction due to chlorofluoromethane would be increased only by about 3.5% in the NAS study and about 3.8% in the NASA study. The small difference between our result and that of Zahniser and Kaufman thus has no practical consequence for stratospheric modeling of CFM effects. Indeed, the possible pressure dependence of this reaction rate, as previously discussed, may constitute a more significant source of uncertainty.
References and Notes (1) F. S. Rowland and M. J. Molina, Rev. Geophys. Space Phys., 13,
l(1975). (2) M. A. A. Clyne and R. T. Watson, J. Chem. SOC.,Faraday Trans. 1, 70, 2250 (1474). (3) M. S. Zahniser and F. Kaufman, J. Chem. Phys., 66, 3673 (1977). (4) M. T. Leu, C. L. Lin, and W. B. DeMore, J . Phys. Chem., 81, 190 (1977). (5) M. T. Leu and W. B. DeMore, Chem. Phvs. Left.. 41. 121 (1976). (6) M. A. A. Clyne and W. S. Nip, J. Chem.-Soc., Faraday Trios. 2 , 72.838 (1976). (7) M.’S. Zahniser,’F. Kaufman, and J. 0. Anderson, Chem. Phys. Lett., 37, 226 (1976). ( 8 ) M. Kurylo and W. Braun, Chem. Phvs. Left., 37. 232 (1976). (9) R. T. Watson, E. Machado, S. Fischer, and D. D. Davis,‘J. Chem. Phys., 65, 2126 (1976). (IO) R. T. Watson, J. Phys. Chem. Ref. Data Ser., 6, l(1977). (1 1) R. L. Jaffs, “Can a Bimolecular Gas Phase Reaction Have a Negative Activation Energy?” presented at the 175th National Meeting of the American Chemical Society, Anahelm, Calif., 1978. (12) D. L. Singleton and R. J. Cvetanovic, J. Am. Chem. Soc., 98, 6812
(1976). (13) D. D. Davis, R. E. Hule, and J. T. Herron, J. Chem. Phys., 59,628 (1973). (14) I. W. M. Smith and R. Zellner, J. Chem. Soc., Faraday Trans. 2 , 69, 1617 (1973). (15) I. W. M. Smith, Chem. Phys. Left., 49, 112 (1977). (16) D. E. Tevauit and R. R. Smardzewski, J . Chem. Phys., 67, 3777 (1977). (17) “Halocarbons: Effects on Stratospherlc Ozone”, Panel on At-
Acknowledgment. This paper presents the results of one phase of research carried out at the Jet Propulsion Laboratory, California Institute of Technology, under Contract No. NAS7-100, sponsored by the National Aeronautics and Space Administration.
mospheric Chemistry, Natlonai Academy of Sciences, Washlngton, D.C., 1976. (18) “Chlorofluoromethanesand the Stratosphere”, NASA Ref. Pub/., No. 1010 (1977).
Megawatt Infrared Laser Chemistry of CClF2CClFpon Excitation of Two Distinct Vibrational Modes‘ Kenneth J. Olszyna and Ernest Grunwald” chemistry Department. Brandeis University, Wakham, Massachusetts 02 154 (Received April 3, 1978) Publication costs assisted by the National Science Foundation
CCIFzCCIFzat pressures of 12 and 150 Torr was flash irradiated with a pulsed C 0 2 laser at 921 and 1052 cm-’. These wavenumbers correspond to two different C-F stretching bands of the gauche isomer. Absorbed energy (E&) ranged up to 41 kcal/mol. Decomposition per flash (100 f ) ranged up to 21%. A plot of In f vs. based on the original data showed dispersion into two curves, one for each wavenumber. However, correction for beam inhomogeneity, based on a model which neglects mixing of the inhomogeneously excited volume elementa during the reaction time, caused this dispersion largely to disappear. Principal reaction products are CC12F2, CClF, CzF4, CF3CF=CF2, and CFzCFC1. The nature of these products and the reaction stoichiometry indicate that C-C bond breaking is the principal laser-inducedprimary reaction step. At 150 Torr and E* > 30 kcal/mol, C-C bond breaking is dominant. At 12 Torr or Eab< 30 kcal/mol, the stoichiometry becomes more complicated and primary C-C1 bond breaking may be significant. After correction for beam inhomogeneity, data under all conditions are reproduced approximately by In f,,, = 4.85 - 288/Eaba.The negative slope, 288 kcal/mol, is markedly greater than the activation energy for C-C or C-C1 bond breaking.
A current issue in infrared laser chemistry is whether excitation of different vibrational modes of a given chemical species will lead to different chemical results. For CC12F2at 12 Torr, excitation of two different normal modes with pulsed megawatt laser radiation gave practically the same results: yields per flash and product compositions depended only on the amounts of energy absorbed per of CC12F2.2 mole (Eaba) On the other hand, for CClF2CClF2at 200 Torr, irradiation with a 4-W continuous laser of two different ab0022-3654/78/2082-2052$01 .OO/O
sorption bands centered at 921 and 1052 cm-’ gave reaction rates that varied widely. At equal energy absorption, yields of C2F4per unit time varied with laser frequency not only between, but also within each of the two absorption bandsq3s4 We now report a photochemical and photophysical study o f pulsed megawatt laser excitation of CClFzCClF2at 1 2 and at 150 Torr. The COzlaser was tuned to 921 and 1052 cm-’l, respectively, and thus excited two different C-F stretching modes of the gauche i ~ o m e r .Reaction ~ products 0 1978 American Chemical Society
