4109
J . Phys. Chem. 1989, 93. 4109-41 16 formation must be greater than any muting effects due to solvation. Potential solvation effects include steric exclusion and energetic effects. The latter can arise from the necessity of stripping the MeCN (possibly from both reactants) and the alteration of the dielectric media as the charged reactants approach each other. Surprisingly, the reactions are little affected by the greatly different reaction media and the species present. Reasons for this are discussed shortly. There are certainly changes in the excited-state properties caused by solvation effects. In water, shifts of 60 and 90 nm are obtained for the bimolecular and termolecular exciplexes, respectively. The emission spectra of the exciplexes are much less red-shifted with increasing [MeCN] relative to their spectra in pure water.2 For Ru(bpy)$+ in 3.8 M MeCN, the shifts are 10 and 50 nm, respectively. There are barely detectible shifts present in the spectrum of R ~ ( b p y ) ~in ~8.9 + M MeCN as a function of concentration of Ag+. Therefore, the energies of the exciplex states in the MeCN media are higher than those observed in pure water and closer to that of the parent complex. Similar to the water system, *DIAg?+ has a lower state energy than *DIAg+. Finally, the yields and T'S of the exciplexes increase with increasing [MeCN]. Indeed, for the exciplexes of the Ru(4,7-Me2phen)32+,the emission yields are higher than for the parent complex in the same media for the 8.9 M MeCN. The primary effect in the higher quantum yields and lifetimes of the exciplexes in the mixed media appears to involve the energy gap law? the lower the state energy the more efficient is radiationless deactivation. Further, our data show that there is nothing inherently detrimental to the emission process on exciplex formation. Indeed, the mono-Ag+ exciplex at higher [MeCN] emits more efficiently than the parent. The Ag+ in this case, coupled perhaps with an associated MeCN solvation sphere, may protect the excited state. Whitten et al.37have observed a much more efficient emission from * R ~ ( b p y ) ~ ( C N ) ~ l Athan g + the (36) Caspar, J. V.; Kober, E. M.; Sullivan, B. P.; Meyer, T . J. J . A m . Chem. SOC.1982, 104, 630. (37) Kinnaird, M. G.; Whitten, D. G. Chem. Phys. Lett. 1982, 88, 275.
Reaction Mechanism of C2H
parent; however, in this case the Ag+ complex has a much higher energy emitting state, which should enhance the yield via an energy gap law effect. As termolecular exciplexes are quite rare, we comment on the driving force for their formation. Rather than attachment of two Ag+'s a t different points on the periphery of the complex, we suggest that the driving force is formation of Ag2+ from partially reduced Ag+ in *DIAg+. Ag2+ formation is w e l l - k n o ~ n and ,~~ we used this explanation for termolecular exciplex formation in the pure water case. We turn now to the driving forces involved in exciplex formation in mixed media. The smaller red shifts in the mixed media suggest that there is a smaller enthalpic driving force for exciplex formation. If this were the only factor, the equilibrium constants should be reduced, which does not seem to be the case. Thus, the constants for exciplex formation must also involve entropic contributions. Certainly, given the evidence for the existence of species with multiple MeCNs, there is ample opportunity for large entropic effects due to binding or release of MeCN molecules during the reactions. Perhaps the most remarkable result is that in spite of the large changes in media and the multitude of new species present in mixed MeCN/water mixtures there is a remarkably small effect on the exciplex formation on going from pure water to the mixed solvents. There are, however, significant changes in the photophysics including, in some cases, exciplexes with longer lifetimes and higher quantum yields than their parent. Thus, the exciplexes themselves are environmentally sensitive complexes even though their formation constants are largely media insensitive.
Acknowledgment. We gratefully acknowledge the support of the National Science Foundation (Grant C H E 86-00012). We thank Dr. L. A. Sacksteder for making some of the yield measurements. Registry No. R~(4,7-Me~phen)~~*, 24414-00-4;Ru(bpy)32+,1515862-0; Ag', 14701-21-4;M e C N , 75-05-8. (38) Henglein, A. Angew. Chem. 1979, 91, 449.
+ O2
D. R. Lander, Kenneth G. Unfried, James W. Stephens, Graham P. Glass,* and R. F. Curl* Chemistry Department and Rice Quantum Institute, Rice University, Hbuston, Texas 77251 (Received: October 3, 1988)
The reaction system consisting of C2H and O2 has been explored by laser infrared kinetic spectroscopy. In this system, C2H is produced by the flash photolysis of acetylene or trifluoropropyne at 193 nm, and the transient infrared absorption of C2H and possible reaction products are studied with color center and diode laser probes. Only two reaction products were observed, CO and C02, with the amount of CO produced being approximately 5 times larger than the amount of C 0 2 produced. Both products are produced in vibrationally excited states. For CO, infrared absorptions for vibrationally excited state transitions up to u = 7 6 were observed. There appear to be two processes leading to CO formation: a fast, direct process for which the rate of CO appearance approximately matches the rate of C2H decay and a much slower indirect process. The fast process is the only one observed for u = 5 4 and higher, while the indirect process is observed to be dominant for the lower vibrational transitions such that the direct process becomes indiscernible for the v = 1 0 transitions. The rate of the indirect process exhibits saturation with increasing O2pressure. In order to approximate these kinetics, it appears that at least two intermediates between C2H and CO must be involved for the indirect process. +-
+ -
Introduction The ethynyl radical, C2H, has been studied extensively, primarily because of its as an intermediate in a variety of reactions. Ethynyl radicals have been observed in interstellar in planetary atmospheres,2 and in combustion reactions such as ( I ) Tucker, K. D.; Kutner, M. L.; Thaddeus, P. Astrophys. J . 1974, 193,
L115.
0022-3654/89/2093-4109$01.50/0
-
oxygen/acetylene flames, where C2H is thought to be a soot Precursor.3 It is not surprising, therefore, that the C2H -I-0 2 reaction in particular has received a great deal of attention. Despite several previous studies on this reaction,4d it is still un(2) Strobel, D. F. Planet. Space Sci. 1982, 30, 839. (3) (a) Homann, K. H.; Wagner, H. G. Proc. R . SOC.London, A 1967, 307, 141. (b) Warnartz, J. A,; Bockham, H.; Moser, A,; Wenz, H. W. Symp. (Int.) Combust., [Proc.] 1982, 19, 197.
0 1989 American Chemical Society
4110
The Journal of Physical Chemistry, Vol. 93, No. 10, 1989
certain which product channels are important. There are five different sets of products that are energetically accessible in a single step.'
C2H
+ O2
-
+ 0,
C,H
-
+ HCO 2CO + H CH + C02
all products
(1)
AH = -1 57 kcal mol-'
(1 a )
AH = -1 29 kcal mol-'
(Ib)
AH = -80 kcal mol-'
(IC)
C20+ OH
AH = -50 kcal mol-'
(Id)
+0
AH = -10 kcal mol-'
(le)
CO
C2H0
For the more exoergic channels, the products may be vibrationally
or even electronically excited. In previous work on the mechanism of this reaction, Lange and Wagner4 observed C 2 H and several products with mass spectrometry in a fast-flow system, forming the C 2 H radical by a microwave discharge in dilute bromoacetylene/helium mixtures. They observed a signal corresponding the mass of C 2 H 0 and no peaks corresponding to H C O or CO,, indicating that channel l e is present but channels l a and I C are of small importance. Due to a background signal a t m / e = 28 arising from N,, Lange and Wagner could not be certain whether or not C O was produced as would be expected from channels l a and 1b. Renlund et al.5observed the chemiluminescence of CH(A2A), CO(a'3P), and C02(R'Z,+; uj 2 1) from the reaction of O2with C 2 H produced by UV photolysis or IR multiple-photon dissociation, although branching ratios to these products were not determined. They proposed that the reaction proceeded through a peroxy radical intermediate, with the formation of the observed products requiring an interaction of the terminal oxygen atom with the carbon a-electrons. Later work by the same group7 indicated that these measurements may have involved the reaction of an excited state of C2H. Laufer and Lechleider6 have followed the production of CO in reaction 1, preparing C 2 H by flash photolysis. Their yield measurements indicated that two CO's are produced for every C,H consumed. By analyzing the temporal profile of the CO signal, they estimated the branching ratio between reactions ( 1a 1b)/le as 80/20, assuming that for reaction l e CO is ultimately produced by the reaction
+
C,HO
+ 0,
-
2CO
+ OH
(2)
and that for reaction l a an additional C O was produced either by H C O 0, C O HOz (3)
+
+
+
or by spontaneous decomposition of HCO. In previous studies preparing C2H by excimer laser photolysis, we measured the rate constant of the C 2 H + O2 reaction by directly monitoring the C,H infrared absorption signal with color center laser probing.8 The resulting rate constant was much faster than those obtained in previous ~ t u d i e s . ~W - ~e rationalized our faster result by suggesting that mixing was incomplete in the fast-flow study in which C2H was monitored by mass spectrometry4 and that an intermediate might be present in the study6 in which the rate constant was determined by monitoring the rate of appearance of products. Unfortunately, a t the time, we could not follow the I R absorption signals of the products and thus investigate more carefully these rate constant discrepancies. With additional sensitivity provided by the modification of our system (4) Lange, W.; Wagner, H. G . Ber. Bunsen-Ges. Phys. Chem. 1975, 79,
Lander et al. to a multiple-pass arrangement and with the use of a diode laser as an additional infrared probe, we are now able to make a more thorough study of this system. In this work, we attempt to understand the mechanism of the reaction and to determine the branching ratios for the various reaction channels by monitoring HCO, CO, O H , and H 0 2 by color center laser and diode laser kinetic spectroscopy.
Experimental Section The apparatuses have been described in detail elsewhere.9-" Briefly, the reagents were flowed through a glass tube and were photolyzed by the 193-nm (ArF) output of an excimer laser usually operated at 20 Hz. The concentrations of reactants and various products were probed by either a scanning color center laser (Burleigh) operating between 2900 and 3900 cm-l or a diode laser (Laser Analytics) operating between 1850 and 2800 cm-I. In both cases, the photolysis cell consisted of an IR multiple-pass (White) cell of I-m length for the diode laser apparatus and 2-m length for the color center laser apparatus, giving total IR path lengths of 52 and 40 m, respectively. The excimer beam was directed into the multiple-pass cell through a CaF2 window positioned directly below the D-mirrors of the White cell. The UV beam was intercepted on the other end of the cell by an angled beam block placed directly over the White cell notched mirror, resulting in an overlap region of the IR and UV beams consisting of approximately the middle one-third of the cell. With this arrangement, wall reactions cannot affect the observations, since radial diffusion to the walls takes place on a multimillisecond time scale as compared with the C2H reaction time of about 100 ps. The concentration of C2H radicals in the reaction cell can be calculated from the cross section of the trifluoropropyne precursor at 193 nm (-
.
m
+
m
C8
U 0
k
051
m
x
3 4
I_
,
2
4
6
8
'
O 2 pressure (Torr)
C
-
Figure 6. Dependence of the exponential rise rate of CO(o = 1 0) P(12) absorption on 0, pressure for both the acetylene and trifluoropropyne systems. In the acetylene system, the pressures are the same as those in Figure 1. In the trifluoropropyne system, the CF3C2Hpressure is 100 mTorr and the He pressure is fixed at 20 Torr.
Reaction Mechanism of C2H
+ 0,
The Journal of Physical Chemistry, Vol. 93, No. 10, 1989 41 13
c .-0
c I a
4 2 E
Time (Ksec) Figure 7. Comparison of C2H decay signal in the trifluoropropyne system with the CO(u = 1 0) and CO(u = 4 3) formation signal: (a) CO(u = 1 0) P( 11) absorption signal; (b) CO(u = 4 3) R(6) absorption signal; (c) C2H Q2(6) absorption signal. Conditions: trifluoropropyne pressure, 63 mTorr; O2pressure, 271 mTorr; buffer He pressure, 21 Torr; excimer pulse energy, 100 mJ. These scans are the average of 1500 excimer laser shots.
-
-
Thus, there appear to be two types of C O formation. The first is the prompt formation of highly vibrationally excited C O at rates comparable to the C2H decay rate. (For acetylene precursor, the time scale is so short that it is difficult to ascertain whether the CO(u = 5 4) rate matches the C2H decay, but for trifluoropropyne precursor, the times are longer and the match is good. See Figure 7.) The second is a slower formation of the ground state at a rate similar to the rate Laufer and Lechleider6 observed for ground-state CO. Presumably, the slow formation of CO(u = 1 0) involves some intermediate(s). Previously, Stephens et aL8 measured the dependence of the C2H decay rate on oxygen pressure and found the C2H decay rate to be proportional to oxygen pressure. The appearance rate of CO(u = 1 0) in Figure 5 is totally dominated by the indirect process. Therefore, it might be expected to exhibit a different dependence upon oxygen pressure. When the dependence of the appearance rate of CO(u = 1 0) upon oxygen pressure was measured, the results shown in Figure 6 were obtained. From Figure 6, it can be seen that the indirect process as reflected by 0) appearance rate reaches a saturation level the CO(u = 1 with increasing oxygen pressure, implying complex kinetics. With CF3C2Has the precursor, the rate of appearance of CO(u = 1 0) by chemical reaction tends to be mixed with CO(v = 1 0) formed by vibrational relaxation. However, it is worth studying the reaction with this precursor because trifluoropropyne was the precursor used by Laufer and Lechleider.6 Moreover, an indirect mechanism producing C O may very well be different with a different precursor. Figure 7 shows the decay of C2H and the appearance of CO in several vibrational states on a short time scale. Under these conditions, the rate of appearance for CO(u =2 1) (not shown) is 0.17 X 10, s-l, the rate of appearance 3) is 0.37 X 10, s-I, and the rate of decay for for CO(v = 4 C 2 H is 0.34 X lo6 s-I. The rate of decay for C2H that we measured agrees well with that previously measured by Stephens et a1.* (0.37 X lo6 s-l). Although from this plot a quantitative 0) cannot be determined because the asrate for CO(v = 1 ymptote is not reached, the CO(u = 1 0) rate is clearly slower. Since v = 4 3 is not decaying at the maximum time, the slow formation of CO(u = 1 0) cannot be attributed to vibrational relaxation and can only be explained by the presence of an intermediate. The dependence of the u = 1 0 signal appearance rate upon O2pressure can also be measured and is plotted in Figure 6. The CF3C2Hrates in Figure 6 are being affected by contamination with vibrational relaxation that tends to make them slower, but the vibrational relaxation rates appearing in Figure 3 for CF3C2H seem to be much slower than the v = 1 0 formation rate. Thus, the apparent dependence of the C O rate on the precursor in Figure 6 is probably real, indicating that the precursor must play a role in the indirect process. When we measure the CO(u = 1 0) rate under conditions similar to those used by Laufer and Le+
-
-
+
-
-
+
- -
-
+
+
-
-
-
3
A0
80
120
150
240
200
- -
Time (Lsec) Figure 8. Comparison of the CO(u = 1 0) formation signal with the C02(u3= 1 0) formation signal: (a) CO(u = 1 0) P(12) absorption signal with same conditions as Figure 1; (b) C02(u = 1 0) R(0) with 186 mTorr of C2H2,870 mTorr of 02,and a buffer gas of 15 Torr of SF,; (c) CO, ( u = 1 0) R(0) absorption signal with 186 mTorr of CzH2, 4.2 Torr of 02,and a buffer gas of 20 Torr of He.
-
-
chleider,6 we obtain 0.8 X lo4 s-', in reasonable agreement with their result of (1.3 f 0.2) X lo4 s-l. II. Observation of CO, Product. In addition to CO, CO, product is observed for both the acetylene and the trifluoropropyne reaction. In Figure 8, a comparison of CO2(u3 = 1 0) and CO(u = 1 0) formation is shown. The trace with helium as the buffer gas shows an induction period characteristic of an indirect process. Even with the addition of substantial pressures of the suitable CO, vibrational relaxant, SF,, C02(u3= 1 0) formation is still slower than that of CO(u = 1 0). That C 0 2 is produced vibrationally excited (although this was not observed directly) is evidenced by the disappearance of the induction period and the increase in the rate of appearance of C 0 2 ground-state absorption upon the addition of SF,. The ratio of the C O to C 0 2 infrared absorption cross sections determined from an equimolar mixture of C O and C 0 2 was 0.64 f 0.07 for the P( 15) C O and R(0) C 0 2 rotational components. Using this ratio, an overall final product ratio of CO/CO2 = 5/1 is obtained by measuring the relative intensities of these absorptions after photolyzing a static fill containing 300 mTorr of C2H2, 500 mTorr of 02,and 10 Torr of H e with 250 pulses. Therefore, C 0 2 production is a minor channel. III. Search for Other Products. An extensive search for other products was made. Transient signals at the frequencies of known absorptions of the H C O radical were monitored with the diode laser apparatus. The H C O lines in the u3 ( N = 3) bandI5 were used (u3 lines have an intensity similar to those of the previously observed vi band"). The frequencies 1879.1010, 1879.2060, 1879.2139, and 1879.2432 cm-' corresponding to 42 32 F,, 42 32 F2, 4 14 313 F I , and 4,, 313 F,, respectively, were monitored, and no H C O absorptions were observed. Although it is expected that H C O will be converted rather rapidly to H 0 2 by reaction 3, under typical conditions (Po2 = 300 mTorr) the l / e life for H C O as the result of depletion by reaction 3 is 20 Wsi6 while the time required for its formation by reaction 1 is 2.5 p s 8 Therefore, reaction 3 is not expected to interfere with the observation of H C O produced by reaction 1. Nevertheless, it seemed desirable to look for signals from H 0 2 with the color center 606 F, and 121,11 F I lines of H 0 , at laser. The 717 3469.3269 and 3457.637 cm-I, respectively, were monitored, and as expected, no H 0 2 absorption signals were detected. A search for the C H radical was made by using C2D2for photolysis and looking for C D with the diode laser via the C2D 0, reaction. The CD absorptions were predicted from the spectroscopic con-
-
+
-
-
-
+
+-
-
+
-
+
(15) McKellar, A. R. W.; Burkholder, J. B.; Orlando, J. J.; Howard, C. J., to be published.
(16) Timonen, R. S . ; Ratajczak, E.; Gutman, D. J . Phys. Chem. 1987, 91(3), 692.
4114
The Journal of'Physica1 Chemistry, Vol. 93, No. 10, 1989
stantst7obtained from its electronic spectrum. and the P branch of the .Y = 4 level of the C-D stretch was monitored: No absorption signals were observed at the predicted frequencies 2001.875, 2001.948, 2001.978, and 2001.905 cm-I, corresponding to P l , ( 4 ) e, P , , ( 4 ) f, P2,(4) e. and P,,(4) f, respectively. The color center laser was used to look for O H production. No signal was observed in the system CF3CCH + O2at the O H line a t 3484.5957 cm-' L' = 1 0P(2.5)1-.18 However, when 100 mTorr of NO2 was added to the system, a strong O H signal was observed. verifying that O H could be observed under these conditions. The O H observed in this latter case could be due to the reaction NO, H - N O + OH (4) +
+
where the H atoms are produced by reaction 1 b. I t is worth noting that no O H signal was observed in the photolysis of mixtures of trifluo-ropropyne and NO2. The O H signal was quantified by measuring the O H produced in the system acetylene NO,, assumingI9 that the major reactions were C2H2 + hv C2H + H (5)
+
-
-+
C 2 H + C2H2
C4H2+ H
(6)
with the H atoms produced in reactions 5 and 6 then reacting with NO, via reaction 4 to produce O H . Then, a large excess of O2 was added to intercept C 2 H before it is removed by reaction 6 in order to explore the possibility that H could be produced by some set of reactions involving 0,. The measurement with O2 gave an OH signal slightly stronger than the measurement without 0,. I f a total of 2 mol of H are produced for each 1 mol of C2H formed even when C2H is removed by reaction with 02,a slightly stronger OH signal is expected with 0, because 0, competes with NO2as well as C2H2for C2H. This observation is consistent with the direct reaction being channel 1 b (production of 2CO + H ) and suggests even that the indirect reaction producing ground-state C O may also produce H atoms (or that NO2 reacts with intermediates of the indirect reaction to give O H ) . Discussion
Consistent with previous observations by Laufer and Lechleider, carbon monoxide was the major product observed from the reaction of C2H with 0,. However, it is clear from Figure 5 that with acetylene as the precursor C O is formed by two entirely different pathways: one taking place during the time interval over which C2H is removed and populating vibrationally excited CO, and the other populating the lower vibrational states of CO(c = 0-2) over a much longer time scale. Since, as can be seen from Figure 7, vibrationally excited C O is formed a t least as rapidly as C,H is removed, we propose a direct mechanism for its formation. The suggestion that atomic hydrogen is formed in the early stages of the reaction given by the appearance of the O H signal when 0, is added to the C F 3 C 2 H / N 0 2system, and the absence of detectable HCO, favors reaction 1 b as the source of the prompt CO. The production of the lower vibrational levels of C O via the second pathway must take place through the formation of some intermediate(s), since the majority of CO(u = 0) is not formed until long after all the C 2 H is removed. With trifluoropropyne as the precursor, there appears to be a similar situation with highly vibrationally excited CO formed directly by reaction with C 2 H and ground-state C O produced on a much longer time scale, as can be seen in Figure 7. We have already established that the "slow" C O cannot arise from vibrational relaxation because vibrational relaxation is too slow, occurring on a much longer time scale than those considered in Figures 5 and 7. The possibility that CO(u = 0) is produced in highly rotationally excited states and the slow rise of the signal
Lander et al. is due to rotational relaxation must be considered. The slow rise of C O low-J signals in the photolysis of formaldehyde has been accounted for in this way.20 We note that the experiments are carried out at total pressures of over 24 Torr so that each CO molecule experiences over 240 collisions/fis; this should be sufficient for rotational relaxation.20 However, it is conceivably possible that the rotational excitation is so high that the rotational spacings are much greater than the translational energy of collision in which case rotational relaxation might be much slower. Even so, rotational relaxation in these experiments should be dominated by collisions with the buffer He and the 0, present in large excess rather than by collisions with the precursor. We observe that the appearance rate of the slow C O signal depends very strongly on the precursor used (C2H2or CF3C2H). Therefore, the appearance of CO(c = I 0) on a long time scale cannot be the result of relaxation of C O formed in highly excited rotational states by the direct reaction of C 2 H with 02.Moreover, we have not been able to observe CO(r = 1 0) absorptions arising from high rotational states. We note that the photochemistry of acetylene a t 193 nm has been studied previously,2' and this photochemical study might provide some useful information in the present context. In the photochemistry study, an intermediate such as vinylidene or electronically excited acetylene was proposed2I as necessary in addition to C2H to account for the observations. The possible influence of such species on our experiments is discussed in later text. Several features of the overall mechanism of slow C O production can be deduced from a study of Figures 5-7. Since C2H is removed much more rapidly than C O (low u ) is formed, there must be an unobserved intermediate I , that is either a product of the reaction of C,H with 0, or possibly an excited electronic state of the precursor produced by the photolysis. However, if the intermediate is formed from C2H, it cannot react directly with 0, to form CO, since the rate of formation of slow CO differs by 1 order of magnitude when the C2H precursor is changed from CF3C2H to C,H2. This change is readily seen by comparing Figure 5 to Figure 7 and by examining Figure 6. Figure 6 also shows that the rate of slow CO formation exhibits saturation behavior in its dependence upon the partial pressure of 02,leveling off at high O2pressures. If the intermediate is an excited electronic state of C,H, or CF,C2H formed directly by the photolysis, the difference in reactivity found for the two precursors is accounted for, but if such a reaction produces CO(u = 1 0) directly, the leveling off in rate at high 0, pressure is inexplicable. This complex behavior thus suggests that the formation mechanism 0) involves more than one intermediate. for CO(c = 1 A general mechmism that is consistent with all the observations is I , + P ;2 1, (7) + -
-
+
-
I2
+ 0,
-
I,
CO(low u )
+P
-
+ products
(8)
products
(9) where I, is produced by the reaction of C,H (or C2H2') with 02, P represents the precursor (CF3C2Hor C2H2),and I, and I2 are intermediates. The mechanism can be analyzed in a relatively straightforward manner if the steady-state assumption is applied to I2 and if I, is assumed to be formed instantaneously. Applying the steadystate approximation to 12, we have
Substituting expression 10 into 1 1 , we have (17) Herzberg. G. H.; Huber. K. Spectroscopic Constants of Diatomic Molecules; Van Nostrand Reinhold: Dallas. TX, 1979; p 142. (18) Maillard. J . P.; Chauville, J.; Mantz, A . W . J . Mol. Spectrosc. 1976, 63, 120. (19) This assumption is brought into question by the observation of D. Gutman of C4H3 in this system: Gutman. D.. private communication.
(20) Ho. P.: Smith. A . V. Chem. Phvs. Left. 1982. 90. 407 i 2 l j Seki, K.; Nakashima, N.; Nishi: N.; Kinoshita, M . J . Chem. Phys. 1986. 85. 274.
Reaction Mechanism of C2H + O2
The Journal of Physical Chemistry, Vol. 93, No. 10, 1989 4115
where
which can be integrated to give [Ill = [Ill0 exP(-kefft)
(14)
From reaction 8, the rate of formation of C O is given by
W O I /dt = k8[O2I[I2]
(15)
Finally substituting expression 10 for 12, introducing eq 14 for
I,, and integrating, we have [CO] = [CO],[l
- exp(-k,~ft)l
The resulting C O signal has a time dependence consistent with that observed. Within the context of this mechanism, Figure 6 represents a plot of kerfversus O2 partial pressure at fixed values of [PI (16 mTorr of C2H2 and 100 mTorr of CF3C2H,respectively). As predicted by expression 13, keffapproaches an asymptotic limit (of k7[P]) a t sufficiently high partial pressures of 02.From this limit, k7 was determined to be 2 X cm3/s for acetylene. For trifluoropropyne, k7 is less than 2 X lo-” cm3/s. Perhaps it should be reiterated that even the maximum value of kcR ( IO5 s-l) is much smaller than the decay constant for C2H loss (6 X 106/s a t 4.2 , the validity of the assumption of inTorr of 0 2 )confirming stantaneous conversion of C2H to I, that was made in the model. Within the model shown above, k-7 cannot be equal to zero, since this would lead to expression 13 being simplified to the form kerf = k7[P]. However, k9 could be zero. That the effects of reaction 9 are negligible under these conditions is shown by the replotting of the data in Figure 9. Here, the inverse of keff is plotted versus the inverse of the O2 partial pressure. Inverting expression 13 yields
-1- - kerf
k-7 l + b[P] k7[Plbb[Ozl + k9[P])
‘
(16)
(17)
This expression shows that a linear plot in Figure 9 can only be obtained if k9[P]
