J. Phys. Chem. 1995,99, 454-4557
4554
The 193 and 248 nm Photodissociation of CF&(O)Cl M. Matti Maricq” and Joseph J. Szente Research Laboratory, Ford Motor Company, P.O. Box 2053, Drop 3083, Dearbom, Michigan 48121 Received: December 22, 1994; In Final Form: January 17, 1995@
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The photodissociation of trifluoroacetyl chloride is examined by time-resolved IR and UV spectroscopy. These show that CF3C(0)C1 CF3 CO C1 is the major dissociation pathway at both 193 and 248 nm. This pathway appears to proceed via a primary C-C1 bond fission leaving an internally excited CF3CO fragment that rapidly dissociates. At 193 nm the CO is formed with extensive internal excitation (Tvib = 3800 f 900 K), whereas at 248 nm it is formed predominantly in v = 0. The photolysis yield is a193= 1.01 f 0.1 1 at 193 nm. At 248 nm the overall yield is @248 = 0.92 f 0.08; however, about 14% of the C F F O fragment remains intact.
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I. Introduction Awareness of the adverse effects stemming from the atmospheric release of chlorofluorocarbon compounds has brought intense scrutiny upon their potential replacements.’a2 The current candidates include partially halogenated alkanes, the so-called hydrochlorofluorocarbons (HCFCs). Because they are “designed” to be susceptible to tropospheric degradation, which is initiated by reaction with hydroxyl radicals, it is important to determine the HCFC degradation products and to examine their environmental impact. Among these products are a variety of carbonyl compounds of the form CX3C(O)X, where the X’s represent a combination of H, F, and C1 atoms. For example, trifluoroacetyl chloride is formed from the degradation of HCFC-123 (CF3CClzH) via the sequence of reactions CF3CC1,H
OH
CF,CCl,
01
-NO
CF3CC1202 CF3CC120
CF,COCl
+ C1
If the carbonyl compound contains hydrogen, it is susceptible to further oxidation initiated by hydroxyl radical attack. The fully halogenated species, however, do not readily undergo additional gas-phase chemistry. Instead, they are removed either by uptake into water droplets or by photodissociation. Understanding the relative importance of these removal mechanisms requires knowledge of the UV spectra and photodissociation products of the carbonyl species. Rattigan et aL3have measured temperature-dependent UV absorption cross sections for CSC(0)Cl and concluded that photodissocaitionis competitive with hydrolysis in the troposphere assuming unit quantum efficiency for decomposition. FIlR examination of the photolysis products at 254 nm by Meller et aL4 indicates a quantum yield of 1.0 and suggests that dissociation into CF3 CO C1 is the principal decomposition pathway. The present paper presents real-time measurements from the flash photolysis of trifluoroacetyl chloride that confirm these conclusions at 248 nm, although a minor channel for CF3CO formation is also noted. At 193 nm each photon absorbed leads to fission of both the C-C and C-Cl bonds as has been observed previously for acetyl ~hloride.~
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11. Experimental Section
The apparatus is the same as used in our previous study of CH3C(O)C1 photodi~sociation.~A sample of trifluoroacetyl @
Abstract published in Advance ACS Abstracts, March 1, 1995.
chloride in a slowly flowing gas mixture containing -2 Torr of C2H6, 10-70 Torr of 0 2 , and N2 at a total pressure of 120 Torr is photolyzed by a UV light pulse from an excimer laser. Absorption of a W photon can lead to a number of possible decomposition channels:
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CF3C(0)C1
+ CO + C1 CF3C0 + C1 CF, + COCl CF3C1+ CO CF,
(1) (2)
(3)
(4)
and a variety of techniques are employed to interrogate the products. Any CF3 radicals that are formed are converted to CF3O2 via CF,
+ 0, + M -.CF30, + M;
k = 8.5 x
cm3 s-l (ref 6) (5)
and are detected 20 ,us after the excimer laser pulse using time resolved W spectroscopy.7 Similarly, CF3C0, if present, adds oxygen to form CF3C(0)02 and is detected by its W absorption in the 200-240 nm range. Chlorine atoms react rapidly with ethane
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C1+ C2H6
C,H,
+ HC1; k = 5.7 x lo-” cm3 s-’ (ref 6) (6)
to form HC1, which is monitored by transient IR absorption of the P(4) line (UIR = 1.05 x cm2 at 120 Torr of N2) using an infrared diode laser.8 In addition, the ethyl radical is converted into C2H5O2 upon the addition of molecular oxygen and is detected by its W absorption at 238 nm. Finally, CO(v=O-1) is detected via the P(9) (UIR = 1.18 X lo-’’ cm2 at 120 Torr of N2) and P(15) (UIR = 2.1 x cm2 at 12 Torr) lines by IR diode laser absorption and CO(v=1-2) is monitored via the P(9) line (UIR = 8.0 x cm2 at 12 Torr). At the 0 2 and ethane concentrations employed in these experiments, the lifetimes of the above conversion reactions are on the order of 100 ns. Thus, while we do not observe nascent products, we can discern “prompt” photodissociation events from collision-induced dissociation and from secondary reactions between photolysis fragments. Quantum efficiencies are derived by comparing the HCl and RO2 yields from CSC(0)Cl to those from the photodissociation
0022-365419512099-4554$09.0010 0 1995 American Chemical Society
Photodissociation of CF3C(O)Cl
J. Phys. Chem., Vol. 99, No. 13, 1995 4555
TABLE 1: CF3C(O)CI Photolysis Yields
A ‘
g v)
wavelength(nm) 193 193 193 248 248 248 248 248
1.0
0.5
HCI“
4.2 1.2 2.1 2.3 1.8 2.2
CO
4.2 1.8 2.0 1.9
CF302
C2H502
ab
2.3‘ 1.8d
2.2
0.95 1.08 0.89 0.99 0.82 0.97
Concentration units are lOI4 ~ r n - ~ Relative . to CH3C1 for 193 nm; to Cl2 for 248 nm dissociation. With added ethane. Without added ethane. a
0.0 200
250
300
20 ps after 193 nm photolysis of
in CH ,,
200
250
300
wavelength (nm) Figure 1. (A) UV absorption spectrum of trifluoroacetyl chloride. (B) UV spectrum 20 ps after the photolysis of CF3C(O)Cl both with and without the presence of added ethane. The dotted curves show the contributions of CF302 and C2H502 to the spectrum marked “in C2H;’.
of CH3C1 at 193 nm and Clz at 248 nm, assuming the latter to have unit efficiencies. For CF3C(O)Cl dissociation in the presence of ethane, RO2 represents the sum of CF302 and C2HsO2; for CH3Cl it represents the sum of the ethylperoxy and methylperoxy radicals formed. The composite RO2 spectrum is deconvolved into its components by fitting it to a linear combination of reference spectra9for CF302 and C2H502. The quantum yield is determined as [HC1l,,,,,,,Abs(Ref)
a)= [HC1],,,Abs(CF3C(0)C1) where Ref represents either Cl2 or CH3Cl and Abs is the absorbance of the relevant gas mixture at 193 or 248 nm, as appropriate. At 193 nm the quantum yield was also determined from an analogous expression by measuring the relative formation of RO2 radicals from CH3Cl versus trifluoroacetyl chloride. For reference, the W cross sections determined from the absorbances used to calculate @ are as follows: ul93(CF3C(0)Cl) = 1.6 x cm2, ul93(CH3Cl) = 7.2 x cm2, u248(CF3C(O)Cl) = 5.8 x lo-’’ cm2, and U248(c12) = 7.6 x cm2.
111. Results The UV spectrum of CSC(0)Cl is shown in Figure 1A. It displays two broad absorption features, a weak one with a maximum at 255 nm and a much stronger band having an onset of 210 nm and rising steeply to shorter wavelengths. The absorption maximum of u = 6.6 x cm2 for the 255 nm band compares very well with the value of 6.8 x cm2 reported by Rattigan et aL3
Information about products from the 193 nm photolysis of trifluoroacetyl chloride was obtained both by U V and IR spectroscopy (the results are collected in Table 1). Figure 1B shows the W spectrum obtained following the photolysis of CF3C(O)Cl/Oz/N2 and C F ~ C ( O ) C ~ / C Z H ~ Ogas ~ / Nmixtures. Z The contribution to the measured absorbance by ozone produced from the 193 nm dissociation of 0 2 (predominantly in the beam path outside the cell) has been corrected for by subtracting a “blank” spectrum obtained by omitting the CF3C(O)Cl (or the CH3C1 for yield calibration) from the gas mixture. The broad absorption feature constituting the spectrum marked “in C2H6(’in Figure 1B is attributed to peroxy radicals formed from the addition of 0 2 to various CF3C(O)Cl photodissociation fragments. Peroxy radicals typically exhibit strong unstructured absorption bands (UUV= 4 x lo-’* cm2) in the 200-250 nm range. Present possibilities include CF3O2, CF3C(0)02,ClC(O)O2, and C2H5O2, with the last radical originating from 0 2 addition to the ethyl radical produced from reaction 6. Fitting the spectrum marked “in C2H.5” to reference spectra9 of these radicals indicates the formation of 2.3 x 1OI4~ m CF3O2, - ~ 2.2 x 1014cmV3C2H502 (their contributions are shown by the dotted curves), and 0.1 x lOI4 ~ m CF3C(O)O2. - ~ The calibration experiment run under identical conditions, but with 1.7 Torr of methyl chloride replacing 0.1 1 Torr of CF3C(O)C1, showed a total peroxy radical (CH302 f C2H5O2) concentration of 3.2 x l O I 4 ~ m - implying ~, a yield of @I93 = 0.95 f 0.16. The error is a statistical combination of 10% errors in the determinations of [R02] and 5% errors for the absorbances of the CF3C(O)C1 and CHF1 gas mixtures at 193 nm. The curve in Figure 1B marked “w/o C2H6)’ shows the spectrum obtained from the photolysis of CSC(0)Cl in the absence of ethane. As expected the spectrum closely resembles that of CF302. The CF302 yield of 1.8 x lOI4 cm-3 is lower than the yield obtained in the presence of ethane because of a fast reaction between chlorine atoms (scavenged by ethane when present) and CF302.I0 The origin of the small absorbance in the 250-280 nm range is due to the C10 formed by this reaction. Transient IR absorption was utilized to probe for the CO and C1 fragments of CSC(0)Cl photolysis, the latter via the HCl formed by reaction 6. The results for 193 nm are reported in Table 1 and displayed in Figure 2A. The figure shows prompt formation ( t 5 ps) of HCl, but a much slower formation of an equal quantity of CO. Comparison of the HC1 yield from 0.095 Torr CF3C(O)Cl to that from 1.6 Torr CH3Cl indicates that @I93 = 1.08 & 0.16, in agreement with the W experiments. The prompt and stoichiometric formation of HCl eliminates C1C(0)02from the list of possible peroxy radicals given above. Two reasons could explain the relatively slow appearance of CO. The first is that 193 nm photolysis produces CF3CO C1 and that CO is subsequently formed by collisional dissociation of the CFsCO fragment. The second is that the CO is formed immediately, but that the majority is bom in vibrationally excited
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4556 J. Phys. Chem., Vol. 99, No. 13, 1995
CF,C(O)CI
Maricq and Szente
+ hv + CF, + CO + C1
6
I
193 nm
5 4
3 F
El0
f
0
2
1
r(
W
g 3o E
. I
-
c,
4f
6
2
1 248 nm
0 0 0
50
100
150
200
250
time (ps) Figure 2. Time dependence of HCl and CO formation from the photodissociation of trifluoroacetyl chloride. The total pressure is 120 Torr. (A)193 nm. (B)248 nm.
states that are not probed by the diode laser. Two arguments favor the latter explanation: First, the CO appearance rate of k = 2.9 x lo4 s-l, while somewhat faster than expected from the vibrational relaxation",'* of CO(v=l) CO(v=O) (krel = 2.3 x lo4 s-I at 120 Torr of N2) is considerably faster than the CF3CO dissociation ratel3 of kdis = 1.2 x lo4 s-' at PtOt120 Torr. Second, the formation of CF3CO is not consistent with the rapid appearance of CF3O2 (
