UV absorption spectra, kinetics and mechanisms of the self-reaction of CHF2O2 radicals in the gas phase at 298 K

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CHEMICAL PHYSICS LETTERS

Volume 192, number 1

UV absorption spectra, kinetics and mechanisms

of the self-reaction of CHF202 radicals in the gas phase at 298 K Ole J. Nielsen ‘, Thomas Ellermann,

Elzbieta Bartkiewicz

*

Section for Chemical Reactivity, Environmental Science and Technology Department, Ris0 National Laboratory, DK-4000 Roskilde, Denmark

Timothy

J. Wallington

’ and Michael D. Hurley

Research Staff; E308j7, Ford Motor Company, P.O. Box 2053, Dearborn, MI 48121-2053,

USA

Received 6 November 199 1; in final form 18 February 1992

The ultraviolet-absorption spectrum and the self-reaction of CHF202 radicals have been studied in the gas phase at 298 K using the pulse radiolysis technique and long-pathlength Fourier transform infrared spectroscopy. Absorption cross sections were quantified over the wavelength range 220-280 nm. The measured cross section near the absorption maximum was ucHFIOl(240 nm) = (2.66 kO.46) x IO-‘* cm* molecule-‘. The absorption cross section data were used to derive the observed self-reaction rate constant for the reaction CHF202+CHF,02-+products, defined as d[ R] /dt=2k,obs[CHFZOZ]2, klob,= (5.OkO.7) x lo-‘* cm3 molecule-’ s-r ( k 20). The only carbon-containing product observed by FTIR spectroscopy was FC(O)F. These results are discussed with respect to previous studies of peroxy radicals,

1. Introduction Stratospheric ozone loss over the Antarctic in October [ 1,2] has caused the atmospheric chemistry of chlorofluorocarbon (CFC) compounds to assume increased importance. There is an international effort to replace CFCs with environmentally acceptable alternatives. Hydrofluorocarbons (HFCs) are one class of CFC substitutes. There is relatively little information on the fate of HFCs in the atmosphere. As part of a collaboration between our laboratories to study the chemistry of peroxy radicals and the atmospheric fate of halogen-containing compounds [ 3-61 we have performed an experimental study of the CHF202 radical, UV absorption spectrum and self reaction: CHF2 O2 + CHF2 0, --,products .

(1)

HFCs will react with OH radicals in the lower at’ To whom correspondence should be addressed. ’ Permanent address: Agricultural and Teachers University, Siedlce, Poland.

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mosphere to produce fluorinated alkyl radicals, which will react with O2 to give a peroxy radical. For example in the case of CH2F2 CHFz +02 +M-+CHF202 +M .

(2)

The atmospheric fate of fluorinated alkyl peroxy radicals is uncertain.

2. Experimental Two different experimental systems were used in the present work. The pulse radiolysis transient UVabsorption spectroscopy setup at Rise National Laboratory was used to study the UV-spectra and the selfreaction kinetics, while the FTIR spectrometer at Ford Motor Company was used to investigate the products of the self-reaction of CHF,OI radicals. Both experimental systems have been described in detail [ 7-91, and are only briefly discussed here.

0009-2614/92/$ 05.00 0 1992 Elsevier Science Publishers B.V. All rights reserved.

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2.1. Pulse radiolysis system

CHFzOz radicals were generated by the irradiation of SF6/02/CH2F2 gas mixtures in a 1 P stainless steel reactor using a 30 ns pulse of 2 MeV electrons from a Febetron 705B field emission accelerator. SFs was always in great excess and was used to generate fluorine atoms SF6 +e--SE

,

(3)

Sq -+F + products ,

(4)

F+CH2F2-CHF2+HF,

(5)

CHF2 +O, +M+CHF202

+M .

(2)

To monitor the transient UV absorption, the output of a pulsed 150 Watt xenon arc lamp was multipassed through the reaction cell (120 cm pathlength). Reagent concentrations used were: SF6 50950 mbar, O2 20-40 mbar, and CH2F2 0.8-20 mbar. All experiments were performed at 298 K. Ultra high purity O2 was supplied by L’Air Liquide, SF6 (99.9%) was supplied by Gerling and Holz and CHzFz (99.3%) was obtained from Fluorochem. All reagents were used as received. 2.2. FTIR system The FTIR system was interfaced to a 150 QPyrex reactor. Radicals were generated by the UV irradiation of mixtures of CHzFz and Cl* in air at 920 mbar total pressure at 298 K using the output of 22 blacklamps (Ge-BLB-40). The loss of reactants and the formation. of products were monitored by FAIR spectroscopy, using an analyzing pathlength of 26.6 m and a spectral resolution of 0.25 cm-‘. Ultra pure synthetic air was supplied by Matheson Gas Products, CHzFz and FC(O)F were obtained from PCR Inc. at a stated purity of >99%.

3. Results 3.1. UV absorption spectrum of CHF202

Measurement of absolute absorption cross sections for the CHFzOz radicals requires absolute calibration of the initial F atom yield. This calibration

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was achieved by monitoring the transient absorption at 2 16.4 nm of methyl radicals produced by radiolysis of SF,/CH, mixtures, as described in detail elsewhere [ 10,111. In the present series of experiments, using a value of 4.12 X lo-” cm* molecule-’ for a(CH3) at 216.4 nm [ 121, the yield of F atoms at 550 mbar of SF6 and full irradiation dose was 1.9 1 x 1015cmm3. We estimate the F atom yield calibration to be accurate to better than + 15%, including a 10% uncertainty in a( CH3). Before investigating the absorption spectrum of the CHF202 radical, initial reagent concentrations have to be chosen to minimize the production of FO, radicals in our system via F+O, +FO, .

(6)

F02 radicals absorb strongly in the region 220-240 nm [ 131 where CHF202 radicals also absorb. The amount of F02 formed in our experiments depends upon the rate constant ratio k,lk, and the concentration ratio [ CH,F,] / [ 0, ] . To measure k5 we performed a series of experiments in which the maximum in the transient absorption at 220 nm was observed following the pulse radiclysis of SFJ CH2Fz/02 mixtures. In these experiments, the radiolysis dose and the SF6 and O2 concentrations were held fixed (half dose, [ SF61 = 550 mbar, [O,] =40 mbar) while the CHZF2 concentration was varied over the range O-20 mbar. Fig. 1 shows the observed variation of the maximum absorption as a function of the concentration ratio [ CH2F2] / [ 0, ] . k5/k6 was then determined as described in one of our recent papers [6] to be 70223. Errors are 20. Using k6=1.4~10-‘3 cm3 molecule-’ s-’ leads to kg= (9.8+ 3.2) x lOI* cm3 molecule-’ s-‘, consistent with the value reported by Clyne and Hodgson [ 14 1. To minimize complications caused by the presence of F02 radicals, experiments were performed using initial concentrations of [ CH2F2 ] = 10 mbar, [ O2 ] = 40 mbar and [ SF6 ] = 550 mbar. Using these conditions, the initial absorption maximum was measured as a function of the radiolysis dose as shown in fig. 2. The initial absorption was linear with the radiolysis dose. This linearity shows that the formation of CHF202 was proportional to the initial yield of F atoms. Hence, secondary radical-radical reactions such as the reaction of F atoms with CHF2 83

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0.11

0.0

’

’

0.1

0.2

I

’

0.3

0.5

0.4

Fig. 1. Maximum transient absorption at 220 nm following the pulsed radiolysis of mixtures of O-20 mbar CH2F2, 40 mbar of O2 and SF6 to a total pressure of 600 mbar as a function of the concentration ratio [CH2Fz] / [O,]. The solid line is a lit to the data.

0.2

-

,i! 0.1 -

I

0.0 0.0

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0.2

0 3 Fraction

I

I

I

I

I

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0.4

0.5

0.6

0.7

0.6

0.9

of

maximum

1 .o

full radiolysis dose. Combining this value with three additional pieces of information: (i) the calibrated yield of F atoms of 1.91 x lOi cmv3 (550 mbar of SF6, full radiolysis dose), (ii) the calculated conversion of 95Ohof F into CHF,02 and 5O/6into FOz, and (iii) the absorption cross section of FOz at 240 nm, 0=3.0X lo-i8 cm* molecule-’ [ 151, we calculate an absorption cross section (base e) of crcHFzoz(240 nm)=(2.66+0.06)x10-‘8 cm2 molecule-‘. Errors quoted thus far represent the statistical uncertainty associated with our measurements. We estimate that, in addition, there is 15O/6uncertainty in the absolute calibration of the F atom yield. Combining the statistical and possible systematic errors, we arrive at ~Curzoz(240 nm) = (2.66-t0.46) X lo-* cm2 molecule-‘. To map out the absorption spectrum of the CHF202 radical, experiments were performed to measure the initial absorption between 220 and 280 nm following the pulsed irradiation of SF6/CH2F2/ O2 mixtures. Initial absorptions were then scaled to that at 240 nm and hence converted into absolute absorption cross sections. Corrections were then applied to account for the formation of FO, radicals, using F02 absorption cross sections from ref. [ 151. Values so obtained are given in table 1 and shown in fig. 3. The spectra of CH302 [ 16 ] and CH2F02 [ 17 ] have been included in fig. 3 for comparison. 3.2. Kinetic data for self-reaction

dose

Fig. 2. Maximum transient absorption at 240 nm following the pulsed radiolysis of mixtures of 10 mbar CH2F2, 40 mbar of O2 and 550 mbar SF, as a function of the radiolysis dose. The solid line is a linear regression.

or CHFzOz radicals are of negligible importance. From the value of k5/k6 derived above, we calculate that using [ CH2F2] = 10 mbar and [O,] =40 mbar there is 95% conversion of F atoms into CHFzOz radicals, and a 5% conversion into FO1. The initial absorption of CHFzOz at 240 nm has a linear dependence on dose all the way up to full dose, as seen in fig. 2. The line drawn through the data in fig. 2 is a linear least-squares analysis ofthe data which has a slope of 0.266 k 0.006 (errors are 20). From this slope a predicted absorbance (log,,) of 0.266 + 0.006 is calculated for experiments using 84

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CHEMICAL PHYSICS LETTERS

Fig. 4 shows typical transient absorption data obtianed for the self-reaction of CHF202 radicals, together with a nonlinear least-squares second-order fit. The kinetic analysis of the second-order decays Table 1 Measured UV-absorption cross sections

(nm)

ux 1020 ( cm2 molecule- ’ ) CHF20Z

220 230 240 250 260 270 280

449 392 266 149 76 39 23

Wavelength

-

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CHEMICAL PHYSICS LETTERS

I

I

I

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I

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I

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220

230

240

250

260

270

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290

500

400

300

::

200

”

100

b

0 200

210

Wavelength.

300

nm

Fig. 3. Absorption cross section data for CHFrOr (0 ) measured in this work. For comparison the spectra of CH302 (0) [ 161 and CH2F02 ( V ) [ 171 have been included.

was performed as discussed in previous publications [ 18,191. The decay of the transient absorption by CHF202 radicals was monitored at a variety of wavelengths between 220 and 300 nm. At the four wavelengths where we had sufficient signal-to-noise ratio (220, 230, 240 and 250 nm) the decay was well represented by the second-order least-squares fit. These results can be converted to rate constants by multiplying the k&o values by our measured absorption cross-sections at the appropriate wavelengths. Systematic variation of each of the following parameters by at least a factor of two from a base case ( [ SF6], 545 mbar; [ CH,F, ],4 mbar; [O,], 40 mbar;

24 April 1992

dose, maximum) had no observable effect on the values of k/a derived. In experiments conducted at shorter wavelengths an appreciable residual absorption was observed; see fig. 4. This residual is caused by the formation of F02 radicals which, over the time scales used in this work, are essentially unreactive. Variation of the ratio [ CH,F,] / [O,] by a factor of six had no effect on our measured k/a values, showing reaction of FO1 with CHF202 radicals is unimportant. No residual absorption other than that ascribed to FOz formation was observed. No effect of pressure on k, was observed over the range 150-950 mbar total pressure of SF6 diluent. Values of k, derived at the four wavelengths were indistinguishable within the experimental uncertainties and can be averaged to yield klobs= (5.0? 0.7) X lo- ‘* cm3 molecule--’ s- ’ at 298 K. Quoted errors represent 2a. 3.3. Product study of the self-reaction of CHF202 radicals

The value of the observed second-order rate constant reported above may be an overestimate of the true bimolecular rate constant for this reaction due to the formation of HO2 radicals in the reaction cell and the subsequent reaction of these radicals with CHF202 radicals: CHF202+CHFz02+CHF20+CHFzO+02, (la)

O’l7T---T

CHFz0z+CHF202+FC(0)F+CHF20H+02, (lb)

CHF,O+O+FC(O)F+HO,, HO1 + CHFz 02 + CHF, OOH + O2 .

0.0

tw I1

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/

400

600

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Time

,

1 1000

(us)

Fig. 4. Absorption at 220 nm following the pulsed radiolysis of a mixture of 4 mbar CH2F2, 40 mbar of O2 and 556 mbar SF,. Single pulse, half dose, no signal averaging. The solid line represents a non-linear least-squares second-order tit.

(7) (gal

To quantify this effect, and to provide mechanistic data for the self-reactions of CHF202 radicals, a product study of the self-reaction of CHF202 was performed. In this product study, photolysis of molecular chlorine in the presence of CH,F,/air mixtures was used as a source of CHF202 radicals. To assess the magnitude of possible loss of any product through secondary reactions with Cl atoms it is necessary to know the reactivity of Cl atoms towards CH2F2 and the products of interest. To pro85

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24 April 1992

vide the first piece of information, a relative rate technique was used to measure the kinetics of C1+CH2F,+HC1+CHFz,

(9)

relative to that of Cl+CH,,+HCl+CH,.

(10)

The experimental procedure has been described [ 201. The decay of CHzFz was observed as a function of that of CH, following the photolysis of CH,F,/CH,/ Cl* mixtures in either air or nitrogen diluent at 920 mbar total pressure and 295 K. There was no observable difference between experiments performed in air or N2. Linear leastsquares analysis of the data yields a rate constant ratio kg/k 1,,=0.32f0.02. Using the literature value of k10=(l.0+0.1)~10-‘3 cm3 molecule-’ s-’ [21] gives kg=(3.2f0.2)~10-‘4 cm3 molecule-’ s-‘. Quoted errors represent 2~. This value is a factor of two lower than the previous relative rate determinations of k9=(6.8+0.2)x lo-l3 cm3 molecule-’ s-l reported by Tschuikow-Roux et al. [ 221, who also used CH, as the reference compound. The origin of the discrepancy between our result and that of Tschuikow-Roux et al. [ 22 ] is unknown. In view of the low reactivity of Cl atoms towards CH2FZ, secondary reactions of Cl atoms with products of the self-reaction of CHF,02 radicals may become important. To test for this complication, two sets of experiments were performed, one using a low [ CH,F,] ,, ( 30 x 10P3 mbar) and another using a high [ CH2F2 ] o ( 13 mbar ). Both sets of experiments were performed at 920 mbar total pressure and 295 K. On irradiation of CHzFJCIJair mixtures with low initial CH,F2 concentrations, HCl and FC(O)F were the only observable products. The yield of FC(O)F is plotted as a function of the loss of CH,F2 in fig. 5; linear least-squares analysis gives a yield of 104 f 2% for FC (0)F. Quoted errors are 20 and do not include systematic uncertainties. We estimate that potential systematic errors could add an additional 1O”/ouncertainty. The fact that only one carbon-containing product is observed in experiments with low initial CH2F2 concentration suggests that, either the majority of reaction ( 1) proceeds via channel ( la) and the resulting HO* radicals do not complete effectively with the self-reaction for loss of CHF202, or that other products, such as CHF*OOH and 86

0

2

4 D [CH~FJ

6

.Y

10

(mTorr)

Fig. 5. The yield of FC( 0)F versus loss of CH2F2. The solid line is a linear regression to the data. (0 ) 14.9 mTorr CH2F2, ( ‘I ) 16.9 mTorr CH2F2. ( A ) 6 I .7 mTorr CH2F2.

CHF,OH, are formed but are rapidly converted into FC(O)F by secondary reactions with Cl atoms, for example: CHF100H+C1-,CF200H+HCl,

(11)

CF200H+FC(0)F+OH,

(12)

CHFzOH+C1+CF20H+HCl,

(13)

CHFOH+O,+HO,

(14)

+FC(O)F.

These possibilities were tested by the irradiation of CHzF,/CIJair mixtures with a large [CH2FZ10 ( 13 mbar) and small conversions of CH*F, (0.2-0.8%). Under these conditions secondary reaction of Cl atoms with products should be of minor importance. When 13 mbar of CHzFz is admitted to the long-path cell a significant fraction of the infrared spectrum is blocked out. Fortunately windows in the IR exist at around 1800,2700, and 3600 cm-’ enabling the formation of FC (0) F, HCl, and CHF200H to be monitored. We do not have a reference spectrum of CHF*OOH. Thus, CHF200H was searched for by comparing the integrated absorption over the region 3600-3650 cm-’ with that of a calibrated spectrum of C,H,OOH acquired earlier. No evidence for the formation of CHFzOOH was observed. The yield of CHFzOOH was less than 10% of that of FC(O)F. The yield of HCl was 102 -t 8% of that of FC (0 ) F. On standing in the dark for 20 min there was no observable loss ( < 2%) of either HCl or FC(O)F products. Irradiation of mixtures of FC(O)F in air with, or without, added Clz for periods of 90 s (typical of the present work) produced no observable loss

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(~2%) of FC(O)F. Thus, FC(O)F neither photolyses nor reacts with Cl atoms to any significant extent under our conditions. These results combine to show that FC(O)F is the major, if not sole, carboncontaining product of the self reaction of CHFzOz radicals.

4. Discussion The spectroscopic, kinetic, and product data in the present work are the first such results available for CHF202. It is of interest to compare with literature data for analogous radicals. The absorption spectrum of CHF102 obtained in this work is compared in fig. 3 to those of the CHS02 [ 15 ] and CH2F02 [ 161 radicals. The substitution of H atoms with F atoms seems to have little or no effect on the shape and magnitude of the absorption spectra. However, F atom substitution moves the absorption maximum to progressively shorter wavelengths by approximately 15 nm per F atom. The value of k’ measured in this work, (5.0*0.7)x lo-l2 cm3 molecule-’ s-‘, is comparable to those published for the self-reaction of k=(3.07+0.65)~10-‘* [16] and CHZFOZ, (4.01+0.52)x lo-l2 cm3 molecule-’ s-’ [6]. The self-reaction of peroxy radicals usually proceeds via two channels: CHF202+CHF202-+CHF20+CHF20+02, (la) CHF202+CHFZ02-+FC(0)F+CHF20H+02. (lb) The alkoxy radicals CHF20 will then react with O2 to give HO2 radicals and FC(O)F. The generation of HO2 radicals in turn leads to the possible formation of the alkyl hydroperoxide:

LETTERS

action of CHF202 radicals proceeds to give FC( 0)F and HO2 radicals, it is interesting to note the absence of CHF200H (the expected product of reaction of CHF202 with HO*). This can be rationalized in three ways. The simplest explanation is to conclude that the rate of reaction (8a) is too slow to compete with the self-reaction for CHF202 radicals. Simulations of the chemistry in the chamber using the Acuchem program [ 23 ] and a chemical mechanism consisting of reactions ( 1 ), (7) and (8) and the HO2 self-reaction with rate constants k,,=5~ lo-‘* and k,=2.0x lo-l5 and kH02+HOz=3.0x lo-‘* cm3 molecule-’ s- ’ showed that a value of kg, 6 1.5X lo- I2 cm3 molecule-’ s- ’ is necessary to reproduce the < 10% yield of CHF,OOH that we observe. This rate constant is significantly lower than any measured rate constant for the reaction of peroxy radicals with HO, which are typically in the range (36)x10-I2 cm3 molecule-’ s-’ [24-261. Alternatively, the lack of CHF200H can be explained by invoking an additional channel for the reaction of CHF202 radicals with HO2 CHF202+H02-,FC(0)F+H20+02.

HO2 + CHFz02 +CHF200H

(8b)

In this latter case, there is no restriction on the choice of the rate of channel (8b). However, for a given choice of overall rate kg there is a restriction for the branching ratio k,Jk,. With a rate of k8= 6.0 X 1O-l2 cm3 molecule-’ s-‘, k,,/k,, must be 22.5 to reproduce the experimental observation of < 10% CHF200H yield. Reaction of peroxy radicals with HO2 is known to give alkyl hydroperoxides as the dominant, if not exclusive, products [ 27,281. Thus a ratio of k,.Jk,> 2.5 would be surprising. Finally, a third explanation for no observable CHF200H is that the hydroperoxide is formed, but that it decomposes to form FC(O)F CHF200H-+FC(0)F+H20.

CHF20+02-+FC(0)F+H02,

24 April 1992

(15)

(7) + 02.

(8a)

Expected carbon-containing products then include FC(O)F, CHF,OH, and CHF200H. Experimentally, we observe only FC(O)F, suggesting that reaction ( 1) proceeds predominately, if not exclusively, via channel ( la). In view of the fact that the majority of the self-re-

Previous observations that CH,OOH, C2HSOOH and CH,FOOH do not decay in the dark in our chamber [5,20,29] suggest that rapid decomposition of CHF200H in the chamber is not likely. At the present time, in the absence of kinetic or mechanistic data for reaction (8), an assessment of the contribution of this reaction to the observed decay of CHF202 is not possible. Hence, we are unable 87

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to correct our observed second-order rate constant k lobs.

It is of interest to compare the product yields for the self-reaction of CHFIOz radicals reported here, with the analogous study of CH2C102 and CH2F02 radicals reported by Niki et al. [ 301 and Wallington et al. [ 5 1. In all three studies the aldehyde accounts for the majority of the products and the hydroperoxide is present at a yield of approximately 10% or less. The low hydroperoxide yield in these systems is interesting as it suggests that the reaction of HO2 radicals with these halogenated methyl peroxy radicals is either much slower, or that it proceeds via a different mechanism, than previously thought for this type of reaction. Clearly, further experimental work is required to understand the role of HO2 radicals in the atomospheric oxidation of haloalkanes.

Acknowledgement

Thanks are due to Jette Munk (Riser) for Technical assistance and Steve Japar (Ford) for helpful comments. OJN would like to thank the Commission of the European Communities for financial support.

References [ 11 J.D. Farman, B.C. Gardiner and J.D. Shanklin, Nature 315 (1985) 207. [2] S. Solomon, Nature 347 (1990) 6291, and references therein. [3] T.J. Wallington and O.J. Nielsen, Intern. J. Chem. Kinetics 23 (1991) 785. [ 41 O.J. Nielsen, J. Munk, G. Locke and T.J. Wallington, J. Phys. Chem. 95 (1991) 8714. [ 5] T.J. Wallington, J.C. Ball, O.J. Nielsen and E. Bartkiewicz, J. Phys. Chem., in press.

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[ 61 T.J. Wallington and O.J. Nielsen, Chem. Phys. Letters 187 (1991) 33. [ 71 K.B. Hansen, R. Wilbrandt and P. Pagsberg, Rev. Sci. Instr. 50 (1979) 1532. [ 81 O.J. Nielsen, Rise-R-480 ( 1984). [9] T.J. Wallington and S.M. Japar, J. Atmos. Chem. 9 ( 1989) 399. [lo] T. Ellermann, Rise-M-2932 (1991). [ 111 R.A. Cox, J. Munk, O.J. Nielsen, P. Pagsberg and E. Ratajczak, Chem. Phys. Letters 173 ( 1990) 206. [ 121 T. Macpherson, M.J. Pilling and M.J.C. Smith, J. Phys. Chem. 89 (1985) 2268. [ 131 P. Pagsberg, E. Ratajczak and A. Sillesen, Chem. Phys. Letters 141 (1987) 88. [ 14] M.A.A. Clyne and A. Hodgson, J. Chem. Sot. Faraday Trans. 1181 (1985)443. [ 151 P. Pagsberg, private communication ( 1990). [ 161 T.J. Wallington, M.M. Maricq, T. Ellermann and O.J. Nielsen, J. Phys. Chem., in press. [ 171 P. Dagaut, T.J. Wallington and M.J. Kurylo, Intern. J. Chem. Kinetics 20 (1988) 815. [ 181 M.J. Kmylo, P.A. Ouellette and A.H. Laufer, J. Phys. Chem. 90 (1986) 437. [ 191 S.P. Sander and R.T. Watson, J. Phys. Chem. 86 ( 198 1) 2960. [20] T.J. Wallington, J.M. Andino, J.C. Ball and S.M. Japar, J. Atmos. Chem. 10 ( 1990) 30 1. [21] W.B. DeMore, M.J. Molina, S.P. Sander, D.M.Golden, R.F. Hampson, M.J. Kurylo, C.J. Howard and A.R. Ravishankara, JPL Publication 87-41 ( 1987). [ 22 ] E. Tschuikow-Roux, T. Yano and J. Niedzielski, J. Chem. Phys. 82 (1985) 65. [23] W. Braun, J.T. Herron and D.K. Kahaner, Intern. J. Chem. Kinet. 20 (1988) 51. [24] P. Dagaut, T.J. Wallington and M.J. Kurylo, J. Phys. Chem. 92 (1988) 3833. [25] P. Dagaut, T.J. Wallington and M.J. Kurylo, J. Phys. Chem. 92 (1988) 3836. [ 261 P. Lightfoot, B. Veyret and R. Lesclaux, J. Phys. Chem. 94 ( 1990) 708. (271 T.J. Wallington and S.M. Japar, Chem. Phys. Letters 166 (1990) 495. [28] T.J. Wallington and S.M. Japar, Chem. Phys. Letters 167 (1990) 518. [29] T.J. Wallington, C.A. Gierczak, J.C. Ball and S.M. Japar, Intern. J. Chem. Kinetics 21 (1989) 1077. [ 301 H. Niki, P.D. Maker, C.M. Savage and L.P. Breitenbach, Intern. J. Chem. Kinetics 12 (1980) 1001.