1743
J . Phys. Chem. 1992, 96, 1743-1748
+
Time-Resolved Observation of the Formation of CF,O and CFClO in the CF2CI O2 and CFC12 O2 Reactions. The Unimolecular Elimination of CI Atoms from CF2C10 and CFCI2O Radicals
+
Fuxiang Wu and Robert W. Cam* Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455 (Received: July 1 , 1991; In Final Form: October 31, 1991)
The unimolecular elimination of chlorine atoms from CF2C10 and CFC1,O radicals has been observed at 238 and 298 K, and at 4-20 Torr total pressure by measurement of the rate of formation of the carbonyl halide product in real time. The fragment ion (CFO)' was found to be specific for the detection of CF,O and CFClO and was monitored by time-resolved mass spectrometry in flash photolysis of mixtures of CF2C1Br or CFCI, with 0, and NO. These two C1 atom elimination reactions were verified to be very facile, even at low temperatures. The rate constants were estimated by regression analysis of experimentally measured formation curves of CF20and CFC10, and decay curves of CF2CI02and CFCI2O2were determined in the same experiments. The estimated 298 K rate constants are (6.4 f 1.4) X lo4 s-I for CF2CI0 and (1.2 0.4) X los s-I for CFC120.
*
Introduction The currently accepted mechanisms for stratospheric photooxidation of chlorofluorocarbons 11 (CFC13) and 12 (CF2C12) involve the formation of the oxy radicals CFC120 and CF2C10, and their unimolecular decomposition by C-Cl bond scission.1-3 In the case of CFZCl2,reactions 1-9 occur.
-
+ hv CF,Cl+ C1 CF2Cl + 0 2 + M CF2C102 + M CFzClO2 + NO2 + M CF2ClOzNO2 + M CF2C102N02 + M CF2C102 + NO2 + M CF2C102N02 + hv CF2C102 + NO2 CF2ClO + NO2 CFzClO2 + N O 2CFzC102 2CF2C10 + 0 2 CF2ClO2 + C1- CFzClO + C10 CF2Cl2
+
+
+
+
-
+
CF2C10
CFzO
+ C1
(l) (2) (3) (4) (5)
(6) (7) (8) (9)
A similar set of reactions for CFC13 can be written (reactions 10-18). CFC13 + hv CFCl2 + C1 (10) CFCl2 + 0 2 + M CFC1202 + M (11) +
-
+
CFC1202 + NO2
+M
CFC1202N02 + M
+
CFC1202N02 + hv
(12)
+ NO2 + M
(1 3)
CFC1202
CFCl202 + NO2
+ NO2 2CFC1202 2CFC120 + 0 2 CFCl202 + C1- CFCl2O + C10
CFClzOz
+ NO
CFC1202N02 + M
+
CFC120
+
CFCl2O
CFClO
(14) (15) (17)
+ C1
(1) Simonaitis, R.; Glavds, S.; Heicklen, J. Geophys. Res. Lerr. 1979.6 (5). 385.
(2) Lesclaux, R.; Dognon, A. M.; Caralp, F. J. Phorochem. Phorobiol. 1987, 41, 1.
(3) Moore, S . 8.; Carr, R. W. J . Phys. Chem. 1990, 94, 1393.
- -
Experimental Section The experiments were conducted over the temperature range of 298-238 K, and 4-20 Torr total pressure with a broad-band flash photolysis reactor and time-resolved mass spectrometry system. The flash duration is about 5 ps. The experimental apparatus and procedures are the same as described Reactant gas mixtures used in the experiments were CF2ClBr-02, CF2C1Br-02-NO, CFC13-02, and CFC13-02-N0. The
(16)
(18) Reactions 7, 8, 16, and 17 are unimportant in the stratosphere but may occur in laboratory studies. The association reaction of NOz and CFzC102 or CFClz02 is an important stratospheric removal process for the peroxy radicals. The kinetics of reactions +
3,4, 12, and 13 have been experimentally determined.3-7 A 2D Photochemical was introduced7 in Order to an limit for the amount of chlorine stored in the form of peroxynitrate Using absorption CrOSS Section and temperature-dependent rate w&kknts of reaction 12. The reaction of CF2C102and CFC1202 radicals with N O competes with reactions 3 and 12 and siphons chlorine out of the CF2C102N02and CFC1202N02reservoirs. The kinetics of reactions 6 and 15 were determined bv Donnon et a1.8 and Tuckerman et aL9 Although the formation of CF20 and CFClO from the photolysis of chlorofluoromethanes in the presence of oxygen has been confirmed earlier,I0J1only a little kinetic data are available for reactions 9 and 18. A lower limit estimate on the decomposition rate of CF2C10 was obtained by Carr et al. in a flash spectroscopy study.12 A theoretical study of the extrusion of C1 atom shows that the C-C1 bond in halomethoxy radicals is quite weak.I3 Ab inito molecular orbital calculations were carried out by Li et al.14 for C1 and F elimination reactions from CCl3-,F,0. They found that F atom extrusion from C1-containing species of CCl3,F,O radicals is unlikely. An investigation of the photooxidation of halomethanes permitted indirect measurement of the decomposition rate constants of CC130and CFC120 radicals? showing that they are also highly unstable. In the present work, the unimolecular elimination of C1 atoms from CF2C10 and CFC120 has been directly observed for the first time.
(4) Caralp, F.; Lesclaux, R.; Rayez, M. T.; Rayez, J. C.; Forst, W. J. Chem. SOC.,Faraday Trans. 1988,84, 569. ( 5 ) Wu, F.; Carr, R. W. Inr. J . Chem. Kinet. 1991, 23, 701. ( 6 ) Koppenkastrop, D.; Zabel, F. Inr. J . Chem. Kiner. 1991, 23, 1. (7) Lesclaux. R.; Caralp, F.; Dognon, A. M.; Cariolle, D. Geophys. Res. Lett. 1986, 13, 933. ( 8 ) Lesclaux, R.; Caralp, F. Inr. J . Chem. Kinet. 1984, 16, 1117. (9) Tuckerman, R. T.; Whittle, E. J . Phorochem. 1985, 31, 7. (10) Jayanty, R. K. M.; Simonaitis, R.; Heicklen, J. J . Photochem. 1975, 4, 381. (11) Lupo, D. W.; Quack, M. Chem. Phys. Lett. 1986, 130 ( 5 ) , 371. (12) Carr, R. W.; Peterson, D. G.; Smith, F. K. J . Phys. Chem. 1986, 90, 607. (13) Rayez, J. C.; Rayez, M. T.; Halvick, P.; Duguay, 9.; Lesclaux, R.; Dannenberg, J. J. Chem. Phys. 1987, 116, 203. (14) Li, 2.;Francisco, J. S. J . Am. Chem. SOC.1989, 111, 5660.
0022-365419212096- 1743%03.00/0 0 1992 American Chemical Society
1744 The Journal of Physical Chemistry, Vol. 96,No. 4, 1992
following radicals were monitored through their fragment ions to measure their decay or formation rate constants: CF2C102by CF202+at m / z = 82; CFC1202by CFC102+ at m / z = 98; C F 2 0 by CFO+ at m / z = 47; and CFClO by CFO+ at m / z = 47. A lower electron energy, 21 eV, instead of 30 eV employed for peroxy radical measurements, was used in the measurement of CFO at m / z = 47 to increase the signal to noise ratio. The large background signal at m / z = 47 is due to C3TIwhich is produced by fragmentation of CF2C1Br in the ion source region. Formation of NO2 and C10 was also observed in this work. All of the reactant gas mixtures except the experiments with N O were prepared and stored in a glass vacuum system. Gas mixtures continuously flowed through the reactor at 20 cm/s. In the experiments with NO, N O was degassed and stored in a separate bulb and was added and mixed with the gas mixtures before they entered the reactor (total mixing time is about 0.5 s). According to the rate constants of reaction 1915 2 N 0 0 2 2N02 (19)
+
-+
the conversion of N O to NO2 is negligible during this mixing time. N O was used diluted (0.64%) in N2 and the partial pressures of N O in the reactor were only roughly estimated since there is no need to know the exact partial pressures of N O in these particular experiments. N2 and O2were used directly from the cylinders without further purification. The NO was supplied by Matheson Gas Products with a minimum purity of 99.0%. The N 2 was from Linde Specialty Gases with a minimum purity of 99.99%. CFC13 was provided by Matheson Gas Products at a purity of 99.0%. CF20and CFClO used for measurement of their mass spectra were used directly from the cylinders provided by SCM Specialty Chemicals. All the other gases used and the preparation and purification procedures of the reactant gases are the same as the previous work.5 CF2C1O2and CFC1202radicals were generated by flash photolysis of CF2ClBr3and CFC13* in the presence of oxygen, respectively. CF2C10 and CFC120 radicals were then formed mainly through reactions of CF2C102and CFC1202with N O and C1. With sufficient N O present, reactions 6 and 15 dominate, and we may consider these as the consecutive reactions:
-
CF2C102 CFC1202
kd
CF2C10 -?. C F 2 0
(20)
CFC120 -% CFClO
(21)
where, according to the mechanism, kgl and kl< are pseudofirst-order rate coefficients for reactions 6 and 15. For the general consecutive first-order reactions kl
A-B-C
kll
species B is a product of reaction I from species A and species C is the product of reaction I1 from species B, the concentrations of species A, B, and C can be expressed as eqs 23, 24, and 25 if at the beginning of experiment, only A is present and is equal to [A01: [AI = [A01 exp(-k,t) (23)
Wu and Carr
TABLE I: Decomposition Rate Constants (T= 298 K) total press., gas composition, 96 8 10 10 15 8 10 15 10 4 4 6 6 6 8 10 10 10
m / z kgl,' ms-l 47/82 1.09 47 j82 1.39 1.08 47/82 47/82 1.54 47/82 1.58 1.76 47/82 47/82 2.54 47/82 1.51 47/82 1.60 47/82 0.62 47/82 1.57 47/82 0.69 2.15 47/82 47/82 6.64 47/82 1.24 47/82 1.24 47/82 2.65
10 15 20 10 15 20 10 15 20 10 15 20 4 6 6 10 15 20
47/82 47/82 47/82 47/82 47/82 47/82 47/82 47/82 47/82 47/82 47/82 47/82 47/82 47/82 47/82 47/82 47/82 47/82
Torr
0.56 0.89 1.01 0.79 0.89 1.06 0.68 1.09 1.11 0.78 0.88 0.89 0.62 0.46 0.75 0.78 0.89 1.08
k9," ms-l CF2CIBr O2 84 95 4.3 4.3 102 95 5.0 36 62 36 5.0 36 49 6.0 94 71 6.0 94 62 6.0 94 6.0 94 53 20 65.0 80 52.5 20 20 20 65.6 80 20 20 56.5 73.6 80 20 20 21.3 80 70.1 80 20 20 100 80 20 60.2 80 av 64 f 14 > 1000 97.4 2.6 > 1000 2.6 97.4 > 1000 2.6 97.4 > 1000 5.0 95 > 1000 5.0 95 434 5.0 95 > 1000 6.0 94 102 94 6.0 6.0 34 94 > 1000 10.4 89.6 >loo0 10.4 89.6 >loo0 10.4 89.6 56.5 80 20 52.2 20 20 57.0 20 80 > 1000 20 80 > 1000 80 20 80 80 20
NOb 0.1 0.1 0.1 0.1 0.11 0.11 0.11 0.11 0.073 0.081 0.073 0.081 0.442 1.11 0.156 0.156 0.67 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0
"CF2C102% CF2C10 4 C F 2 0 . bApproximatevalues.
18 would be nearly equal to the rate of reactions 6 and 15. Reaction of oxy radicals with N O can be dismissed, since taking the rate coefficient as 1 X lo-" cm3 molecule-' s-l from ref 2 the decay rate of CF2C10 and CFC120 would be far too slow to compete with the observed rate of decay by C1 atom elimination reported below. In the absence of NO, the formation rate of CF20 and CFClO would be nearly equal to the rate of reactions 8 and 17, provided the concentration of peroxy radicals is low enough to neglect peroxy radical self reactions 7 and 16. This kinetic scheme suggests that if kI is associated with kgl or kl
