J. Phys. Chem. 1995, 99, 10847-10852
Elementary Reactions of CF2(X) with 0 3 and 0
10847
2
W. Hack* and M. Wagner Max-Planck-Institut f i r Stromungsforschung, Bunsenstrasse IO, 0-37073 Gottingen, Germany
K. Hoyermann Institut fur Physikalische Chemie, Tamannstrasse 6, 0-37077 Gottingen, Germany Received: April 5, 1999
The reaction of CF~(%'AI)with 03 has been investigated in an isothermal flow reactor at temperatures between 300 and 600 K and pressures in the range 4 I P/mbar I5. CF2(%) was produced by a microwave discharge of CF,Brz/He mixtures and detected by laser-induced fluorescence (LIF). [CFz(%)](t) profiles were measured under pseudo-first-order conditions ([CF,(%)]o *: [03]0). The rate constant of the reaction CF2(%) 0 3 ( % ) products (1) was determined, kl(T) = 1.9 x 10l2exp(-(25 f 8) kJ mol-'/RT) cm3 mol-' s-' (300 IT/K d 600). An estimation of the rate constant for the reaction CF2(%) 02(X) products (2) can be given as k2(8OO K) I5 x lo6 cm3 mol-' s-I.
-
The tropospheric release of chlorofluoromethanes (CFCs) results in a transportation of these molecules into the upper atmosphere and stratosphere, due to their negligible removal in the lower atmosphere.' In the stratospherethe CFCs are broken down via photolysis by solar UV-radiation. The chlorine formed in the UV-photodissociation, leading to the catalytic chain reactions initiating the destruction of 0 3 by C1 atoms, was in the past the main Many fewer investigations deal with the fluorine-containing CFC radicals in the stratospheric photochemistry. Difluorocarbene in its electronic ground state (CF2(%)) is formed by photolysis of chlorofluorohydrocarbons in particular CFzClz in the strato~phere,63~ which is known as F-12, and released into the atmospheric in large quantities. The direct reaction of CF2(%) with 03, CF2(%)
+ 0, -products
(1)
could be a significant sink of stratospheric CF@) since the depletion by oxygen molecules, CF,(%)
+ 0, -products
(2)
and other stable molecules at stratospheric temperatures is slow. A direct catalytic chain reaction,
+ 0, -.CF,Ot + 0, CF20t + 0 - CF, + 0,
CF2(%)
(a) (b)
is, however, energetically possible, only if 185 k.I/mol of excess energy remains in the CF20f from its production (ARH(1a) = -539 kJ/mol). But even if CFz(X) does not act as a catalyst, the ozone destruction in reaction 1 can be of interest since CF@) is unlikely to react with other stratospheric trace molecules. The fast reaction of CF&) with 0 atoms present in the stratosphere leads to the destruction of "odd oxygen". This reaction, which influences the ozone concentration indirectly, @
-
+
Introduction
Abstract published in Advance ACS Absrracrs, June 1, 1995.
+
can be compared with the direct ozone depletion via reaction 1 only if kl(T) is known (see the Discussion Section). In reaction 1 the CF20t decompositionand thus the formation of very reactive fluorine atoms are energetically feasible. The rate constants of the direct reactions of CF2(2) with 0 3 and of 02 are essential for atmospheric models to make meaningful predictions, but also the products of these reactions have to be known.8 Besides the importance for the atmospheric photochemistry and the role of CF@) in the etching processes?Sl0 there is a basic interest in the reactivity of CF~(%'AI) as being one of the small carbenes. In comparison with the singlet carbenes CH2(1) and CHF(%), CF2(2) is a quite unreactive species. Whereas most reactions of CH2(9 and CHF(X) are fast, for the CF& only the rate constants of the reactions with 0,N, and H atoms are large, being on the order of 10l2 ~ m ~ / ( m o l * s ) . ' ~In -'~ comparison, the rate constants of reactions with stable molecules such as Cl2 ( k = 2.1 x lo9 cm3/(mol.s)), Br2 ( k = 1.6 x lo9 ~m~/(moP*s)),'~ and the oxidizing reagent NO2 (k = 4.53 x lo9 ~m~/(mol.s))'~ are low. For the reactions with the species 0 2 (k < 1 x lo7 cm3/(mol.s)), H2, or ethylene only upper limits are given in the literature.I4-l6 The reactivity of CF2 can be placed between the very reactive CHz radical, which quite often reacts in nearly every collision, and the isovalent CO molecule. At room temperature CO does not react with a measurable rate with stable species such as 0 2 ( k = 2.52 x 10l2exp(-199 ~ZJIRT)'~ or 0 3 (k(298 K) < 0.241 cm3/(moh)).'*. Therefore, it is of practical and theoretical interest to investigate the temperature dependence of the rate constant of the reactions of CF2(2) with ozone and oxygen. For the reaction CF@) 0 3 there are to our knowledge no direct rate data available in the literature. The reaction of CF@) with 02(X)has been measured at high temperature^'^-^' in shock wave experiments and at room temperature, leading to estimations for an upper limit.'0,'6322923 The aim of this work was to measure the temperature dependence of the rate of the reaction between CF2 and ozone,
+
CF,(%'A,)
-
+ 03(X1A1)
and the rate of the reaction
0022-3654/95/2099-10847$09.00/0 Q 1995 American Chemical Society
products
(1)
Hack et al.
10848 J. Phys. Chem., Vol. 99, No. 27, I995
probe Imovable)
flowreoctor
fluorescence cell
Figure 1. Isothermal flow reactor with ozone production and CFz(X) laser-induced fluorescence detection device.
+
CF2(%) 02(X)
-
products
(2)
at a medium temperature. The high absorption of CFz(A-3) in the ultraviolet region and the high fluorescence quantum yield of this A’BI state makes the laser-induced fluorescence (LIF) an ideal detection meth~d.~~-~’
Experimental Section The experimental arrangement is shown in Figure 1. An isothermal flow reactor consisting of a glass tube with an inner diameter of 5.5 cm and a length of 70 cm was used for the measurements. A movable probe, in which the CF2 radicals were produced, had an inner diameter of 2.2 cm. The total reactor was heated by a high-temperature thermostat (Julabo HT 3 (silicon oil)). The CF2 radicals were produced by dissociation of CF2Br2 diluted with He in a microwave discharge (Schafer Mikrotron 200; power 15 W). At concentrations smaller than 1% CF2Br2 in He, the dissociation of CF2Br2 is nearly c0mp1ete.I~ The CF2 radicals were detected by LIF, using a tunable dye laser (Lambda Physik LPD 3002) with Coumarin 307 (Ama = 508 nm) as the laser dye. The dye laser was pumped by the third harmonic (2 = 355 nm) of a Nd:YAG laser (Spectra Physics GCR 3). The dye laser frequency was doubled by a BBO crystal (Lambda Physik BBOI), separated from the fundamental by a Pellin Broca prism and coupled into the detection cell by a quartz plate at the Brewster angle with respect to the laser beam. In this work the A(0,2,0) %(O,O,O) transition was used with an excitation wavelength of 1 = 262.7 nm. The absorption cross section of CF@ A) is u = 1.37 x cm2.21 The spectral integral fluorescence was detected by a photomultiplier (Hamamtsu R 955) with an integrated preamplifier.
-
-
The signals were processed by a gated integrator and boxcar averager (Stanford Research System, SRS) and digitized and stored in a computer (IBM PC/AT). In order to increase the signal to noise ratio, signal averaging over 90 samples was performed. The ozone was produced from molecular oxygen with a commercially available ozonier (Argentox GL 10, 220 V, 0.7 A). O3 was absorbed on silica gel at the temperature of solid CO2 in two traps and thus separated from the oxygen. The ozone was then desorbed from the silica gel and transported by He into the reactor. In this way maximum 0 3 concentrations of [ 0 3 ] I3 x lo-’ mol/cm3 were achieved. The other gases with highest commercially available purity (He =- 99.9999% Messer Griesheim, 0 2 2 99.996% UCAR/ Messer Griesheim, CF2Br2 2 98% Merck-Schuchardt) were used without further purification. The absolute concentration of ozone was determined by UVabsorption measurements (250 I1/nm I3OOz8)by a photometer (Carl Zeiss PMQ3). The OdHe mixture passed a gas cuvette with an absorption length of Z = 30 cm (see Figure 1). The absolute concentration of ozone in the absorption cell was ]~ calculated using Lambert-Beer’s law: log Z/Zo = - E Z [ O ~with the parameters e(254 nm) = 3.22 x IO6 cm2/mol and ~(289.36 nm) = 4.232 x lo5 cm2/mol; I , transmission; lo, transmission of helium; and [O,],, concentration of ozone in the absorption cell (mol/cm3). The flow of ozone into the reactor is then @03 = [O~],@HJ(P,/RT, - [o,],), whereby P, is the total pressure and T, the temperature in the cell. Results The rate of the elementary reaction
-
CF2(R) -I- 0, products (1) was followed by the depletion of CF&. The CF2(3) source
Elementary Reactions of CF2(2) with 03 and
J. Phys. Chem., Vol. 99, No. 27, 1995 10849
0 2
TABLE 1: Experimental Details for the Determination of ki(T)"
t
[CFzJ 10-12 T P 0 (mol/ (K) (mbar) ( c d s ) runs cm3) 301 373 403 433 473 523 573 598
4.5 4.5 4.5 4.5 4.5 4.5 4.5 5.0
502 664 66 1 71 1 842 93 1 1019 887
22 9 9 8 10 12 10
9
3.2 2.0 4.4 2.1 2.2 2.0 2.3 2.1
[031
10-10 (mol/ cm3)
w10-9
kerf
k,
(s-l)
(SKI)
2.8-29.3 0.6-2.0 2.7-27.7 1.5-3.8 2.6-28.5 1.4-4.8 2.2-17.6 2.0-4.8 1.5-30.1 1.7-10.7 1.9-16.3 4.6-15.0 2.3-13.9 9.5-25.2 1.7-17.1 12.0-43.5
(cm3/ mobs)
0.3 0.4 f 0.3 1.1 0.9 f 0.3 1.2 1.3 f 0.5 1.4 1.9 f 0.7 1.8 3 . 0 f 0 . 8 3.3 7.2 f 1.8 5.0 10 f 4 7.7
The experimental errors are determined from the extreme slopes to describe the experimental points in Figure 3.
+
-d ln[CF2]/dt = k,[O,] k,
\ 1
I
I
1
0
20
LO
60
-
ms
reaction time
Figure 2. [CFz] depletion as a function of the reaction time (Ar) at T = 573 K in the absence and the presence of 03 for various concentrations: (0)LO31 = 0; (e)[ 0 3 l = 2.3; (A) [O,] = 4.2; (W) [ 0 3 ] = 8.3; (0)[O3] = 13 (in units of mol/cm3).
(microwave discharge in diluted CFzBr2 mixtures (1% CFzBr2 in He)) has been studied in discharge flow reactors with continuous molecular beam sampling and mass spectrometric dete~ti0n.I~It was found that the dissociation of CF2Br2 can be described by the reaction CF2Br2 CF2(%) Br2 and that the formation of F atoms is negligibleI5 and the dissociation of CF2Br2 was c0mp1ete.I~ Thus, the initial CF2Br2 concentration can be inserted as [CF2]0. In the absence of 03 the main CF& sink is the heterogeneous depletion
-
CF2(R)
+
+ wall -products
(3)
with a rate constant in the range 0.3 5 k,/s-' 5 5 for the temperature range applied to determine kl(T). The accommodation coefficient for the CF2 radicals, y = (2RkW/vcq)(with V C F ~ = mean molecule velocity, R = reactor radius), is found to be between y(300 K) = 2 x and y(600 K) = 2.3 x The combination reaction CF2(R)
+ CF,(R) - C2F4
(4)
(k4(298 K) = 2.56 x 1Olo ~m~/(mol.s)~') and the reaction CF,(%)
+ Br, -.products
(5)
(k5(500K) = 1.6 x lo9 cm3/(mol s)I4) are too slow to be of importance under the experimental conditions ([CF2(%)]0 r 2 x IO-', mol/cm3 and [CF,] r [Brz]). As mentioned above, the F atom concentration in the system is small; thus, the reaction F CF2 products (6) is unimportant for the CF2 profiles under the experimental conditions applied. In the absence of 0 3 a straight line in the ln[CF2] vs At plot is obtained (Figure 2), indicating that a first-order CF&) decay is dominant over the second-order contributions of reaction 4 and 5. The ozone was added to the reactor in large excess over CF2(X) (59 I[O&f[CF& 5 1.3 x IO4). Under these pseudofirst-order conditions the CF2(2) concentration profile is described by
+
-
keff
with k, describing the first-order depletion of CF2(%) in the absence of 0 3 . The fiirst-order rate constant (k,) was determined before and after each run. The difference between these values was negligible, indicating that the reactor wall was not changed by 0 3 with respect to the catalytic activity of the wall toward CF2(8). The average values of kw for each temperature are given in Table 1. The temperature dependence of the heterogeneous reaction, Le. y(T), can be described by an Arrhenius activation energy of approximately EA = 12 kJ/mol. The CF2(%) concentration profiles in the presence of 0 3 are shown in Figure 2, for four chosen 0 3 concentrations. The experimental details, flow velocity 0,initial CF2 concentration, and temperature between 300 and 600 K are summarized in Table 1. The pressure in all measurements was 4-5 mbar. The ln([CF21/[CF2]0} vs At plots are shown in Figure 2; [CF~IO is the initial CF2 concentration obtained by extrapolation. From the slope of the measured plots like those shown in Figure 2, the first-order rate constants were obtained. The plots of these first-order constants (keff) vs the ozone concentration are shown in Figure 3. For ozone the thermal decomposition has to be taken into consideration. The homogeneous decomposition
+
+ +M
0, M- 0, 0
(7)
proceeds with k7(Q = 4.31 x lOI4 exp(-92.9 kJ mol-'/RT) cm3/(mol s) for M = This leads to a first-order rate constant under the experimental conditions of k7(573 K) = 1.17 x lo-' s-I. The depletion Of 03 at the maximum reaction time (At = 50 ms) is given by [03] = 0.9942[03]0, which is equivalent to a maximum 0 atom concentration of [O] = 1.4 x mol/cm3. The 0 atoms can react either with CF@) or with OS,besides heterogeneous depletion and combination. The reaction CF2(2)
+ 0 -products
(8)
is a fast reaction with kg(298 K) = 1.1 x loi3~m~/(mol.s).'~~" The 0 3 , which is present in the system in excess over CF2(2), can scavange the 0 atoms via 0
+ 0, -.20,
(9)
with a rate constant kg(T) = 4.8 x 10l2 exp(- 17.1 kJ mol-'/
RT) ~ m ~ / ( m o l . s )The . ~ ~relative importance of reactions 8 and 9 is given by k[CF2]/k9[03]. At low 0 3 concentrations the CF2 depletion is accelerated by reaction 8, whereas at high O3 concentrations the 0 atoms react preferably with 0 3 and thus the CF2 depletion is less affected. The change in the Os
Hack et al.
10850 J. Phys. Chem., Vol. 99, No. 27, 1995
I
4
Icm3/molSI lnk
A523K
T=573K &
/
/”
I
2.0
I
10-3
-’
3.0
T I K” 1 Figure 5. Arrhenius plot of log kl(T) vs W i n the temperature range
Figure 3. Plots of kerf vs [03]at various temperatures: T
= 301 K (0);T = 373 K (0);T = 473 K (U);T = 523 K (W); T = 573 K (e); underlining the experimental points at low 0 3 concentrations (.* -); line used to determine the second-order rate constant (- - -) (see text).
/
r/
/
373 5 T K 5 573. The error bars are due to Table 1.
the 0 atoms by 0 3 . At T = 573 K the fist-order rates as a function of [03] can be interpreted in the same way as indicated in Figure 3. The experiments at high 0 3 concentrations are used to determine kl(573 K). The effects at T = 598 K are so large that a determination of kl(598 K) was not attempted. Straight lines in the bffvs [03]plots are observed for temperatures below T = 573 K. At very low temperatures, however, reaction 1 becomes too slow to be observed in the isothermal flow reactor since the 0 3 concentrations were limited in the experimental arrangement used. The results determined at T = 301 K are not included to determine k,(Q. The temperature dependence of kl follows an Arrhenius behavior, as shown in Figure 5. The following Arrhenius expression for the reaction between difluorocarbene and ozone was obtained: k,(T) =
1.9 x IOi2 exp((-25 f 8) kJ mol-’/RT) cm3/(mol-s) in the temperature range 373 The rate of the reaction 01
I
I
10
0
,
1
20
I031 [lO”Omolc m”I
Figure 4. First-order rate constants vs O3 concentrations at T = 598 K ; (0) experimental points; (- -) line calculated for T = 598 K, [CF& = 2.1 x mol/cm3, [MI = 8 x lo-* mol/cm3, with kl(598 K) = 1.25 x 1Olo cm3/(mols) from the Arrhenius expression, with an intercept of k, = 7.7 s-I; (- - -) simulated values taking into account 0 2 0 M, k~(598K) = 3.3 x lo6 cm3/ in addition 0 3 M product, k = 1.05 x 10” cm3/(mol*s),and 0 -t (moles), 0 CF2 0 3 2 0 2 , k = 1.53 x 10” cm3/(mol-s).
-
+
+
- -.
CF,(%)
*
+ O,(X) -products
(2)
was determined at T = 803 K and a total pressure of P = 12 mbar. A large 0 2 concentrationin the reactor ([02] = 1.1 x lod7mol/cm3) changed the CFz(X) concentration only marginally within the reaction time 21 5 tRlms 5 136. From these results an upper limit of
+ +
concentration is, under the conditions of this work, too small to be of any importance. This behavior is illustrated in Figure 4 for T = 598 K. At low 0 3 concentrations the first-order rate constants measured in the flow reactor are significantly larger than expected from the rate constants obtained from the Arrhenius expression for that temperature. At high O3 concentrations the deviation is marginal due to the consumption of
ITK I573.
k2(803 K) 5 5 x lo6 cm3/(mol*s) can be determined.
Discussion For the rate of the reaction
CF,(%)
+ O3-.products
(1)
to our knowledge, no values have been published. The reaction
Elementary Reactions of CF2(%) with 03 and 0
J. Phys. Chem., Vol. 99, No. 27, I995 10851
2
proceeds with a significant activation energy of EA = 25 kl/ mol and a small preexponential factor of ko = 1.2 x 10l2cm3/ (moles). The oxidation reaction of CF2(%) with N02, CF2(%)
+ NO, - C F 2 0 + NO
26
(10)
t
(klo = 4.5 x lo9 cm3/(moh)l5) is 1 order of magnitude faster than reaction 1. In comparison with other oxidation reactions of CF2 the reactivity of 0 3 lies between the reactive 0 atoms,
CF,
+ 0 -.CFO + F
(8)
and the unreactive 0 2 molecule (reaction 2). The reaction of 03 with another singlet carbene CHF(%IA') is significantly faster, k(298 K) = 5.9 x 10l2 ~ m ~ / ( m o l * s ) , ~ ' with virtually no activation energy.31 For the reaction products of CF@) with 03(%)one would expect energetically the pathway CF,(%:'A,)
+ O,(%'A,)
-
CF,O(%'A,)
0, (X,a,b)
ARH = -539 kJ/mol
The reaction enthalpy ARH is calculated for the electronic ground state of 02(X). The spin conservation rule, however, predicts that 0 2 is not formed in its (3Cg-)electronic ground state but in a singlet excited state. The reaction energy is sufficient to produce both 0 2 (a'A,) or 02(b'Cg+). There is also sufficient energy for decomposition of CF2O. CF20
-
CFO
+F
ARH = +494 Wmol
Mass spectrometric studies for the products of reaction 1 are underway.8 For reaction 2, CF2(2)
+ 0, - products
(2)
some qualitative statements are found in the literature concerning the rate constant at room t e m p e r a t ~ r e ' ~and . ~ ~some , ~ ~ measurements at high temperatures (T > 1200 K).'9,20,32 No direct measurements, however, were found in the medium temperature range. CF2(2) does not react readily with 0 2 at room temperature.22 The rate coefficient k2 was determined at room temperature by following the transient absorption of CF2(%) after flash photolysis of C2F4 and C&, respectively.I6 The rate of the CF2 disappearance (via CF2 CF2 C2F4 (4)) was found to be independent of 0 2 pressure up to P(02) = 146 mbar. From this observation an upper limit k2(300 K) < 1 x lo7 cm3/(mol.s) was determined. The rate constant for reaction 2 has been measured at high temperature from shock wave experiments following the CF2(%) concentration by a b s ~ r p t i o n . ' ~The , ~ ~ values . ~ ~ k2(7') at high temperature obtained in those experiments are sketched in Figure 6, together with the upper limit of this work obtained at a medium temperature. The rate constant, namely, the upper limit, of this work matches the extrapolation from high temperature. The upper limit obtained at room temperature is much higher than a value estimated from the shock tube data as well as from the upper limit of this work. The reaction products of reaction 2 could be CF~O(%IA_I) and O(3P),which correlates electronically adiabatic with CF2(X) and 02(X). Small quantities of carbonyl fluoride were observed by gas chromatography as a reaction product.16 The reactants do not correlate with the stable intermediate difluorodioxirane, CF202(2)33in its 'AI electronic ground state.34 Due to ab initio calculations for difluor~dioxirane,~~ the first electronic excited
+
-
"I
+
t t
l6
0.4
I
1
I
0.6
0.8
1.0
103
1.2
I
T-'IK'' 1 Figure 6. Reaction rate constants kz(7). ML = (ref 20), KM = (ref 19), and BHR = (ref 8); (0)upper limit of this work.
triplet state of CF202(ii3B1) is energetically below the reactants CF2(%) and 02(X). Thus, the observed activation energy points to a barrier for the triplet surface. The relative removal rates of CF2(%) by odd oxygen O(3P) and 0 3 can now be estimated for stratospheric conditions, i.e. at temperatures in the range 240-250 K at an altitude of about 30 km. The concentration ratio of ozone over 0 atoms during the daytime is about [03]/[0] = 105.35 After sunset the O(3P) concentration falls very rapidly, but also CF2(%) which is formed photolytically will disappear. An extrapolation of kl(T) of this work to stratospheric temperatures leads to kl(250 K) = 1.2 x IO7 cm3/(mol*s). The rate of the atom-radical reaction O CF2 products (8) can be assumed to be temperature independent. Thus, the destruction of odd oxygen in reaction 8 is still a factor of about 10 faster than in reaction 1; however, the total effect of reaction 1 on the ozone layer has to be determined in a detailed kinetic model.
-
+
Acknowledgment. We are greatly indebted to Prof. H. Gg. Wagner for his generous support and stimulating interest. Financial support by the DFG, SFB 357 ''Molekulaxe Mechanismen Unimolekularer Prozesse" is acknowledged. References and Notes (1) Rowland, F. S.; Molina, M. J. Rev. Geophys. Space Phys. 1975, 13, 1. ( 2 ) Chou, C. C.; Milstein, R. J.; Smith, W. S.; Ruiz, H. V.; Molina, M. J.; Rowland, F. S. J . Phys. Chem. 1978, 82, 1. (3) Cox,R. A. J . Photochem. 1984, 25, 43. (4) Cox,R. A.J . Photochem. Photobiol. A: Chem. 1990, 51, 29. (5) Molina, M. J.; Rowland, F. S. Nature 1974, 249, 810. (6) Kirmse, W. Curbene Chemistry, 2nd ed.; Academic Press: New York, 1971. (7) Rebbert, R. E.; Ausloos, P. J. J . Photochem. 1975, 4 , 419. (8) Hack, W.; Beiderhase, Th.; Hoyermann, K. H. To be published. (9) Gilbert, J. R.; Slagle, I. R.; Graham; R. E.; Gutman, D. J . Chem. Phys. 1976, 80, 14. (10) Ryan, K. R.; Plumb, I. C.Plasma Chem. Plasma Processes 1984, 4 , 271.
10852 J. Phys. Chem., Vol. 99, No. 27, 1995 (11) Hancock, G.; Harrison, P. D.; MacRobert, A. J. J . Chem. Soc., Faraday Trans. 2 1986, 82, 647. (12) Hancock, G.; Heard, D. E. J. Chem. Soc., Faraday Trans. 1991,8, 1039, 1045. (13) Tsai, C.; McFadden, D. L. J . Phys. Chem. 1989, 93, 2471; 1990, 94, 3298. Tsai, C.; McFadden, D. L. Chem. Phys. Lett. 1990, 173, 241. (14) Sugawara, K.; Nakanaga, T.; Takeo, H.; Matsumura, C. Chem. Phys. Lett. 1987, 134, 347. (15) Edelbuttel-Einhaus, J.; Hack, W.; Hoyermann, K. H.; Rhode, G.; Wagner, H. Gg. Ber. Bunsen-Ges. Phys. Chem. 1989, 93, 1413. (16) Dalby, F. W. J . Chem. Phys. 1964, 41, 2297. (17) Tsang, W.; Hampson, R. F. J. Phys. Chem. Ref. Data 1986, 15, 1087. (18) Arin, L. M.; Wameck, P. J . Phys. Chem. 1972, 76, 1514. (19) Keating, L. R.; Matula, R. A. J . Phys. Chem. 1977, 66, 1237. (20) Modica, A. P.; La Graff, J. E. J . Chem. Phys. 1965, 43, 3383. (21) Sharpe, S.; Hartnett, B.; Sethi, H. S.; Sethi, D. S. J. Photochem. 1987, 38, 1. (22) Heicklen, J. In Advances in Photochemistry Pitts, J. N., Hammond, G. S., Noyes, W. A., Eds.; Interscience: New York, 1969; pp 57-148. (23) Tyerman, J. R. Trans. Faraday SOC. 1969, 65, 1188.
Hack et al. (24) Verkateswarlu, P. Phys. Rev. 1950, 77, 676. (25) Mathews, C. W. Can. J . Phys. 1967, 45, 2355. (26) King, D. S.; Schenk, P. K.; Stephenson, J. C. J . Mol. Spectrosc. 1979, 78, 1. (27) Domhofer, G.; Hack, W.; Langel, W. J. Phys. Chem. 1983, 87, 3456. (28) Griggs, M. J . Phys. Chem. 1968, 49, 857. (29) Haimerl, J. M.; Coffe, T. P. Combust. Flame 1979, 35, 117. (30) Atkinson, R.; Baulch, D. L.; Cox, R. A.; Hampson, R. F.; Kerr, J. A.; Troe, J. J . Phys. Chem. Ref. Data 1992, 21, 1125. (31) Hack, W.; Wagner, M. To be published. (32) Bauer, S. H.; Hou, K. C.; Resler, E. L., Jr. 6th International Shock Tube Symposium; 1967; p 504. (33) Russo, A.; DesMarteau, D. D. Angew. Chem. 1993, 105, 956. (34) Rahman, M.; McKee, M. L.; Shevlin, P. B.; Sztyrbicka, R. J . Am. Chem. SOC. 1988, 110, 4002. (35) Wayne, R. P. Chemistry of Atmospheres; Clarendon Press: Oxford, 1985.
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