J. Phys. Chem. 1989, 93, 3647-3649 reaction mechanisms can be derived, thereby allowing the observed product distributions to be used for elucidation of a reaction mechanism. Furthermore, the distribution function derived here provides a starting point for the quantitative analysis of the complex behavior recently reported for high-frequency laser pulse initiated free-radical polymerizations. Due to the complexity of
3647
the high-frequency experiment, it will be treated as a separate case in a subsequent paper, as will the influence on the distribution of subtle changes in the reaction mechanism.
Acknowledgment. This work was supported by the National Science Foundation Grant DMR 85- 14424 (Polymer Program).
Infrared Multiphoton Decomposition of Hexafluorobenzene Investigated by Diode Laser Kinetic Spectroscopy: Detection of CF and CF, Ko-ichi Sugawara,* Akio Watanabe, Yoshinori Koga, Harutoshi Takeo, Kenzo Fukuda, Jiro Hiraishi, and Chi Matsumura National Chemical Laboratory for Industry, Tsukuba, Ibaraki, 305 Japan (Received: October 12, 1988)
Infrared multiphoton decomposition of hexafluorobenzene (C6F6) was investigated by diode laser kinetic spectroscopy. Two transient species, CF and CF2, were detected, and the time evolution of their signals was observed. The signal of CF rose within 10 p s after a C 0 2 laser pulse and decreased with lifetime of about 200 p s , while that of CF2 rose slowly as CF decayed out. Final products observed by an FTIR spectrometer were mostly C2F4and C6FsCF3.These experimental results suggested that CF was produced at the early stage of the reaction and that CF2 was produced by the reaction of CF with a certain fluorine-containing species and then decayed out through the dimerization and/or the reaction with C6F6.
Introduction Infrared-laser-induced reactions with a pulsed CO, laser have been extensively studied for many molecules that absorb the infrared light in the 9-11-pm region. The reactions of the molecules that do not appreciably absorb the light have been made possible by adding an infrared sensitizer which strongly absorbs the C 0 2 laser light without participating in the chemical reactions. As sensitizers, sulfur hexafluoride (SF,) and silicon tetrafluoride (SiF4) have been widely used. Infrared multiphoton absorption of hexafluorobenzene (c6F6), which was thought to be another candidate for the sensitizer, has been investigated by several workers.’” Their results showed, however, that C6F6 was decomposed by the C 0 2 laser quite easily and that this molecule could be used as the sensitizer only when the fluence of the CO, laser was low. Recently, Koga et aL6 found that a considerable amount of CsF6 was decomposed by the CO2 laser with fluence as low as 0.7 J and consequently, they recommended that when this molecule was the fluence should be below 0.3 J used as the sensitizer. Duignan et al.4 investigated the decomposition reaction of C6F6 with high laser fluence, in the range and reported the observation of C2 and C3 as 100-900 J transient species by emission spectroscopy. The final gaseous products identified in their study were CzF4, C6FSCF3,C3F6,C4Fs, and C2F6. Their results strongly suggested that, besides C? or C3, CF, or other fluorocarbon species should exist as the transient species. It is of interest to know how is decomposed by the C 0 2 laser. To clarify this problem in further detail, we have studied the laser-induced reactions of C6F6and similar molecules (1) Speiser, S.; Grunwald, E . Chem. Phys. Left. 1980, 73, 438. (2) Starov, V.; Selamoglu, N.; Steel, C. J. Am. Chem. SOC.1981, 103,
7276. (3) Duignan, M. T.; Garcia, D.; Grunwald, E. J . Am. Chem. SOC.1981, 103, 7281. (4) Duignan, M. T.; Grunwald, E.; Speiser, S. J . Phys. Chem. 1983,87, 4387. (5) Mele, A.; Salvetti, F.; Molinari, E.; Terranova, M. L . J . Phofochem. 1986, 32, 265. ( 6 ) Koga, Y . ;Serino, R. M.; Chen, R.; Keehn, P . M. J. Phys. Chem. 1987, 91, 298.
0022-3654/89/2093-3647$0 1.50 IO
TABLE I: Observed and Calculated Wavenumbers of CF, N’ 16
K.‘
6 6 1 4 6 13 6 12 6 1 1 6 1 0 6 9 6 8 6 I 6 15
transition K,’ N” K.” 10 9
16 15
8 1 4 7 13 6 12 5 1 1 4 1 0 3 9 2 8 1 1
5 5 5 5 5 5 5 5 5 5
K,” 11 10 9 8 7 6 5 4 3 2
wavenumber/cm-’ obsd calcd” 1253.221 1253.258 1253.292 1253.323 1253.350 1253.375 1253.396 1253.415 1253.431 1253.447
1253.219 1253.256 1253.290 1253.321 1253.349 1253.375 1253.397 1253.417 1253.435 1253.450
“Calculated by use of the molecular constants reported in ref 13. C6FsX (X = H, CI, Br, I) at various laser fluences. From the detailed analysis of these reactions, the lowest decomposition path of C6F6was found to be C-F bond breaking, i.e., C6F5-t F. The results of this experiment will be published elsewhere.’ We have also applied diode laser kinetic spectroscopy to investigate the decomposition processes of C6F6. The usefulness of this technique was shown in our previous works on the detection of the transient species CF, in the infrared-laser-induced reaction of CHCIF; and the determination of the rate constants for reactions of CF2.9 We have succeeded in observing the transient species and time evolution of their signals in the infrared multiphoton decomposition (IRMPD) process of C6F6 by this technique. The detailed results and discussion on the reaction mechanism will be given in this paper.
Experimental Section The description of equipment and techniques has been presented previous1y.*-l0 In this experiment gaseous C6F6 was flowed (7) Watanabe, A.; Koga, Y.; Sugawara, K.; Takeo, H.; Fukuda, K.; Matsumara, C.; Keehn, P. M., to be published. (8) Sugawara, K.; Nakanaga, T.; Takeo, H.; Matsumara, C. Chem. Phys. Lett. 1986, 130, 560. (9) Sugawara, K . ; Nakanaga, T.; Takeo, H.; Matsumara, C. Chem. Phys. Lett. 1987, 134, 347.
0 1989 American Chemical Societv
3648 The Journal of Physical Chemistry, Vol. 93, No. 9, 1989
Sugawara et al.
0.03t
P
0.02-
0.0I * 4
0.0I
-
/--0.5
0
1.0
TIME I rns
0
0.2
Figure 2. Time evolution of absorbance ( A ) of CF ( 0 )and CF2 (0) produced by the photolysis of 10 Torr of C6F6.
0.4
TIME I rns
Figure 1. Time evolution of absorbance ( A ) of CF,, where 10 Torr of C6F6(0)and CHCIF, ( 0 )was irradiated with the co2laser lines at 1031.5 and 1085.8 cm-', respectively.
through a cell, 15 cm in length and 3 cm in diameter, and irradiated with a pulsed C 0 2 laser (Lumonics, TEA-820). A diode laser spectrometer (Spectra-Physics, LS-3) was used to observe the transient species. The sample was irradiated with the P(36) line of the 9.6-pm band of the CO, laser at 1031.5 cm-I, with output power of 0.3 J pulse-'. The laser beam was focused into the cell by a Ge lens with a focal length of 20 cm. The diode laser beam was introduced into the cell from the opposite direction to the CO, laser beam, and both beams were made collinear in the cell. A NaCl flat was placed between the cell and the Ge lens to reflect the diode laser beam to a HgCdTe detector. Since we had already observed several rotation-vibration lines of CF2 in the laser-induced reaction of CHC1F2,8*9 the frequency of the diode laser was tuned to this range at first and then to those of other possible transient species, C F and CF,, for which accurate molecular constants had been already reported."*12 Product analysis was also carried out with an FTIR spectrometer (Nicolet, 7199). The sample of C6F6 (Tokyo Kasei, >99%) was used after degassing.
Results and Discussion The initial scanning range of the diode laser was set to 1253.20-1253.46 cm-' to detect CF2. When 3-15 Torr of C6F6 was irradiated with the C 0 2 laser, 10 absorption lines appeared in this spectral range. The spectral pattern obtained in this experiment was almost the same as that reported previo~sly.~ These line positions were in good agreement with those for lines ( N = 7-16) of the u1 band of CF, in its electronic ground state, as summarized in Table I, where calculated values were derived by the use of molecular constants reported by Davies et al.', This result gives definite evidence for the existence of CF2 in the IRMPD process of C6F6. Figure 1 illustrates the time evolution of the CF2 signal in the 0-300-c~~ region observed when 10 Torr of C6F6 was irradiated with the C 0 2 laser. The time variation of absorbance of CF, produced by the CO, laser photolysis of 10 Torr of CHC1F2 was also shown in the same figure. Since CHClF, directly decomposed to CF2 and HCl, the signal of CF2 observed in this system rose within 10 ps after the laser pulse. On the cdntrary, the CF2 signal observed in the system of C6F6 rose slowly over the 0-500-ps region. This indicates that CF, is a secondary reaction product of the IRMPD of C6F6. The concentration of CF, produced in the IRMPD of 10 Torr of C6F6 was roughly estimated to be 10l6 molecules cm-, from the absorbance of the CF2 signal. The frequency range of diode laser was tuned to other possible transient species, CF, and CF. N o signals that we could assign
v5(N)
(10) Nakanaga, T.; Takeo, H.; Kondon, S . ; Matsumara, C. Chem. Phys. Lerf. 1985, 114, 88. ( 1 1 ) Kawaguchi, K.; Yamada, C.; Hamada, Y.; Hirota, E. J . Mol. Specrrosc. 1981, 86, 136. (12) Yamada, C.; Hirota, E. J . Chem. Phys. 1983, 78, 1703. (13) Davies, P. 8.;Lewis-Bevan, W.; Russell, D. K. J. Chem. Phys. 1981, 75, 5602.
0.02-
2
I
0
T I M E / rns
Figure 3. Time evolution of absorbance ( A ) of CF2 produced by the photolysis of 3 ( O ) , 5 (A),and 10 (0)Torr of C6F,.
to CF3 were observed in the range 1250-1270 cm-I. In other experiments where CFJ was irradiated with the C 0 2 laser line at 1074.7 cm-I, the CF3 signal in concentrations as low as lOI4 molecules cm-, was observed. This indicates that, in the IRMPD of (36, the CF3 radical could not be produced, or if produced its concentration must be far lower than that of CF2. On the other hand, two transitions were observed at 1306.032 and 1276.620 cm-I, the frequencies of which were in good agreement with 2nlj2R(6.5)e(1306.032cm-I) and 2n1,2P(3.5)f(1276.621cm-') of C F reported by Kawaguchi et al." These two lines showed almost the same time evolution, indicating that the origin of both lines was the same, and therefore, we assigned them to the transitions of CF. Figure 2 shows the time evolution of the signals of C F and CF2 produced by the laser photolysis of 10 Torr of C6F6. The signal of C F rose very fast within 10 ps and decreased exponentially, while that of CF2 rose slowly as C F decayed out. The solid lines in this figure indicate a result calculated under the conditions that the decay rate of C F is equal to the rise rate of CF, and that CF2 decays out by reactions described below. Fair agreement seen in this figure suggests that CF2 is produced by the reaction of C F with a certain fluorine-containing species. The long-term time profiles of the CF, signal produced by the photolysis of 3, 5 , and 10 Torr of C6F6 are given in Figure 3. The solid lines in this figure were obtained as follows. Since the final gaseous products found by an FTIR analysis were mostly C2F4 and C6F5CF3,the following reaction scheme was assumed: C6F6 + nhv CF + R
CF, CF2
+
+
C F residue CF2 + R'
+ CF2
+ C6F6
-+
C2F4
C~FSCF~
(1) (2)
(3)
(4)
where R and R' indicate fluorine-containing species, and means that several reaction steps are required to form products. The rate constant of reaction 3 was fixed to 3.7 X cm3 molecule-' s-I obtained by Tyerman,I4 while that of reaction 4 was determined in this calculation by adjusting its value so as to fit the time profiles of the CF, signal, because no available value for this reaction was found in the literature. The rate constant (14) Tyerman, W. J . R. Trans. Faraday SOC.1968, 65, 1188
J. Phys. Chem. 1989, 93, 3649-3651 of reaction 4 obtained by this procedure was 1 X cm3 molecule-' s-I. The reaction mechanism for the production of CF2 has been clarified to a certain extent by this experiment; however, another problem, the production of C F radical at the early stage of iRMPD of @6, has been raised by this experiment.- Although no detailed information on C F production has been obtained in this study, the following reaction processes are proposed considering all the observed reaction products. The lowest decomposition path of C6F6 is C-F bond breaking, as described in the Introduction. If the resulting highly excited C6F5radicals absorb the same laser light as C6F6 does in a pulse duration, they may decompose to F and C6F4(tetrafluorobenzyne) which subsequently decomposes further to Cz, CF, C2F2,and so on. The production of C2 observed by Duignan et aL4 is explicable by this mechanism. Since difluoroacetylene (C2F2)is known to be chemically active and tends to polymerize or react with other species, the observation of this species in the final products would be quite difficult. The
3649
detailed studies, however, are required to clarify this reaction mechanism, and especially the spectroscopic studies with time resolution less than 1 ps are very important.
Conclusion In this study the usefulness of diode laser kinetic spectroscopy to the analysis of reaction dynamics has been shown in the IRMPD of c6F6. The production of C F and CF2 as intermediates and their time evolutions were successfully observed. The production of C F preceded that of CFz, and the decay rate of C F was close to the rise rate of CF2. These results may indicate that CF2 was produced by the reaction of CF with a certain fluorine-containing species. Final products observed in this study suggest that CF2 decayed out through the dimerization of CF2 and/or the reaction with CsF6. Registry No. C6F6,392-56-3;CF, 3889-75-6;CF2,21 54-59-8;C2F4, 116-14-3;C6FSCF3, 434-64-0.
A Relative Rate Study of the Reaction of CI Atoms with a Series of Chloroalkanes at 295 K Timothy J. Wallington,* Loretta M. Skewes, and Walter 0. Siegl Research S t a f j Ford Motor Company, P.O. Box 2053, Dearborn, Michigan 48121 (Received: October 13, 1988; In Final Form: November 22, 1988)
The relative rate technique has been used to determine rate constants for the reaction of chlorine atoms with a series of chloroalkanes. The decay rates of the organic species were measured relative to that of ethane and/or n-butane. Using rate constants of 5.7 X lo-'' and 2.25 X lo-'' cm3molecule-' s-I for the reaction of Cl with ethane and n-butane, respectively, we derived the following rate constants, in units of lo-" cm3 molecule-l s-I: chloroethane, 1.19 f 0.17; 1-chloropropane, 5.35 f 0.19; 1-chlorobutane, 11.1 & 0.5; 2-chlorobutane, 6.88 i 0.14; 1-chloropentane, 15.9 k 1.1; 2,3-dimethylbutane, 24.8 i 0.9; cyclopentane, 32.6 1.0. Quoted errors represent 2u and do not include possible systematic errors due to errors in the reference rate constants. Experiments were performed at 295 & 2 K and atmospheric pressure of synthetic air. The results are discussed with respect to the mechanisms of these reactions.
*
Introduction Recognition of the important role of alkyl, R, and alkylperoxy, R02, radicals as intermediates in both atmospheric and combustion processes has lead to a considerable research effort aimed at understanding the kinetics and mechanisms of the reactions of these radicals with other reactive species. The photolysis of chlorine in the presence of organic species is a convenient and hence widely used method of producing alkyl and, in the presence of oxygen, alkylperoxy radicals in the laboratory. Cl2 hv 2C1 (1) C1+ R H R + HC1 (2) R 02+M RO2 + M (3) In systems employing the photolysis of molecular chlorine as a source of chlorine atoms, it is anticipated that secondary loss processes of the alkyl radicals will include reaction with molecular chlorine. R + Cl2 RC1 + C1 (4) Reaction 4 leads to the regeneration of C1 atoms and hence alkyl radicals and the formation of chloroalkanes. This raises the possibility of reaction of atomic chlorine with chloroalkanes to yield chloroalkyl radicals and the corresponding chloroalkylperoxy radicals. In order to estimate the impact of such processes on the alkyl and alkylperoxy radical generation systems, accurate kinetic data are needed for the reaction of C1 atoms with chlo-
+
+
+
+
-+
-
0022-3654/89/2093-3649$01 SO10
roalkanes. Additionally, such kinetic data are useful in establishing reactivity trends and substituent effects in the reaction of Cl atoms with organic compounds. However, in contrast to the significant database now available for the reactions of C1 atoms with many classes of organic species (ref 1-6 and references therein), there have been few studies of the kinetics of the gas-phase reaction of C1 atoms with chloroalkanes. As part of a larger experimental effort in our laboratory to elucidate the kinetics and mechanisms of the reactions of C1 atoms with a variety of organic species (alkanes, alkenes, alcohols, ethers, aldehydes, aromatics7g8),we have investigated the kinetics of the reaction of C1 atoms with a series of chloroalkanes using the relative rate technique. Experiments were conducted at atmospheric pressure of synthetic air, -740 Torr, and 295 f 2 K. We report here measurements of the rate constants of the reaction of C1 atoms with chloroethane, 1-chloropropane, 1-chlorobutane, 2-chlorobutane, (1) Fettis, G . C.; Knox, J. H. Prog. React. Kinet. 1964, 2, 1. (2) Manning, R. G . ; Kurylo, M. J. J. Phys. Chem. 1977, 81, 291. (3) Davis, D. D.; Braun, W.; Bass, A. M. Int. J. Chem. Kinet. 1970, 2, 101. (4) Lewis, R. S.; Sander, S. P.; Wagner, S.; Watson, R. T. J. Phys. Chem. 1980, 84, 2009. ( 5 ) Atkinson, R.; Aschmann, S. M. Int. J . Chem. Kinet. 1985, 17, 3 3 . (6) Atkinson, R.; Aschmann, S. M. Znt. J . Chem. Kinet. 1987, 19, 1097. Wu, C. H.; Japar, S. (7) Wallington, T. J.; Skewes, L. M.; Siegl, W. 0.; M. Int. J . Chem. Kinet. 1988, 20, 861. (8) Wallington, T. J.; Skewes, L. M.; Siegl, W. 0. J. Photochem. Photobiol. A 1988, 45, 167.
0 1989 American Chemical Society
