J. Phys. Chem. 1982, 86, 4165-4170
4165
Reaction of C,H with 0,: Chemiluminescent Products A. M. Renlund,+ F. Shokoohi, H. Reisler, and C. Wlttig' Departments of Electrical Engineering, Physics, and Chemistry, University of Southern California, Los Angeies, California 90089 (Recelved: December 7, 1981; I n Final Form: June 18, 1982)
We report the direct observation of several chemiluminescentproduct channels associated with the bimolecular reaction of C2H with_O2. Spontaneous emission is detected from the nascent reaction products CH(A2A), CO(a'3Z+),and C02(X'Z,+,u311),and the rate coefficient for the removal of the C2H species which leads to the observed products ( k R = (2.1 f 0.3) X lo-'' cm3molecule-' s-') is independent of the species being monitored, the C2Hprecursor, and the photolysis method (excimer laser at 193 nm, or IR multiple photon dissociation). Reaction is believed to proceed through a peroxy radical intermediate, which provides vibrational energy which can accelerate the nuclei toward the different product channels, and C2Hvibrational excitation does not affect k R but is carried over into product degrees of freedom. The products which we observe require an interaction of the "terminal" oxygen atom with the carbon *-electron system, and there are no apparent inversions or anomalies in the co_ncentrationsor internal state distributions of those species which we detect. It is quite possible that C2H(A21T)is the species responsible for the observed emissions.
+ O2 C2H + O2 C2H + O2 C2H + 0 2
Introduction The gaseous ethynyl radical, C2H, is an important chemical intermediate in several different environments. I t has been identified by its microwave emission and is known to be abundant in interstellar clouds and in the atmospheres of certain carbon-rich stars.l It also plays a central role in combustion and contributes to soot formation, particularly in oxygen/acetylene flames.2 Despite its importance, it is difficult to monitor C2H,as it has no well-documented absorptions or emissions in the UV, visible, or near-IR portions of the spectrum.3-6 It has been positively identified by its ESR spectrum following the photolysis of C2H2in a 4 K Ar matrix? rather convincing assignments of its IR spectra have been made, again using an Ar matrix,' and tentative assignments of its IR spectrum have been reported in the gas phase.8 I t is not practical for us to use ESR or IR spectroscopy to monitor C2H,but these experiments are valuable in that they establish suitable precursors for generating C2H in the environments of concern in the work reported here. Previous experiments by other research groups concerning the kinetics of gaseous C2Hused a discharge/flow apparatus with mass-spectrometric d e t e ~ t i o nclassical ,~ product analyses,1(t12and the time-resolved detection of C4H2 by vacuum-UV absorption following the reaction of C2H with C2H2.13 In our own previous experiments6J4we reported the observation of CH(A2A-X211) chemiluminescence due to the reaction of C2H with O2 and used this chemiluminescent channel in order to obtain several rate coefficients! In general, it is best to monitor both the removal of reactants and the appearance of products in real time when studying kinetic processes. In the case of C2H, this is not yet possible spectroscopically, and only product species can be monitored. It is therefore imperative that the chemiluminescent reactions of C2Hwith O2 be assigned unambiguously if these reactions are to be used as means for obtaining rate coefficients, and one of the aims of the present paper is to do this. The reaction of C2Hwith O2can lead to several chemically distinct products which are listed in reactions l a - le, where the enC2H + O2 all products (1) C2H + O2 CO + HCO AH = -157 kcal mol-' (la)
-
C2H
+H CH + C 0 2 C20 + OH HCCO + 0
AH
2CO
AH
= -129 kcal mol-'
(lb)
AH = -80 kcal mol-'
(IC)
AH = -50 kcal mol-'
(Id)
-
-10 kcal mol-' (ref 15) (le) thalpy changes are computed for 300 K ground-state species by using the JANAF tables,16except where indicated otherwise. Clearly, there is sufficient exothermicity for a number of electronically excited states to be formed in addition to the ground-state products, and the C2H reactant may be in it? ground state, or a low-lying metastable state, such as A211, since it is produced photolytically. Reaction 1 is very interesting in that there are many exothermic product channels in which the nuclei must undergo significant rearrangements during the course of reaction. However, if a peroxy intermediate is formed initially, and/or C2H is in a low-lying excited state, there may be enough vibrational energy to allow access to many of the product channels listed in reaction 1. Nascent product internal excitations can be very revealing insofar as the molecular dynamics of the reaction are concerned, and it is our intention to monitor as many of the reaction products as possible and measure product internal exci(1) K. D. Tucker, M. L. Kutner, and P. Thaddeus, Astrophys. J., 193, 415 (1974). (2) K. H. Homann and H. G. Wagner, h o c . R. SOC.London, Ser. A, 307. 141 (19671. (3) (a)'W.R: M. Graham, K. I. Dismuke, and W. Weltner, Jr., J. Chem. Phys., 60, 3817 (1974); (b) ibid., 63, 2264 (1975). (4) A. H. Laufer, J. Phys. Chem., 83, 2683 (1979); 85, 3828 (1981). (5) G. Herzberg, private communication, 1981. (6) A. M. Renlund, F. Shokoohi, H. Reisler, and C. Wittig, Chem. Phys. Lett., 84, 293 (1981). (7) D. E. Milligan, M. E. Jacox, and L. Abouf-Marguin, J. Chem. Phys., 46, 4562 (1967). (8) M. E. Jacox, Chem. Phys. 7, 424 (1975). (9) W. Lange and H. G. Wagner, Ber. Bumenges. Phys. Chem., 79,165 (1975). (10) A. M. Tam, 0. P. Strausz, and H. E. Gunning, Trans. Faraday Soc., 61, 1946 (1965). (11) C. F. Cullis, D. J. Hucknall, and J. V. Shepherd, Proc. R. SOC. London, Ser. A., 335, 525 (1973). (12) H. Okabe, J. Chem. Phys., 75, 2772 (1981). (13) A. H. Laufer and A. M. Bass, J. Phys. Chem., 83, 310 (1979). (14) H. Reisler, M. Mangir, and C. Wittig, Chem. Phys., 47,49 (1980). (15) This value was estimated from data given in S. Benson, "Thermochemical Kinetics", 2nd ed., Wiley, New York, 1976. (16) D. R. Stull and H. Prophet, 'JANAF Thermochemical Tables", Natl. Stand. Ref. Data Ser. (U.S., Natl. Bur. Stand.), 37 (1971).
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'Present address: Sandia National Laboratories, Division 2516, Albuquerque, NM 87185. 0022-3654/82/2086-4165$01.25/0
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0
1982 American Chemical Society
4188
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The Journal of Physical Chemistry, Vol. 86, No. 21, 7982
A
L
SCREEN
ROOM
Flgure 1. Schematic drawing of the experimental arrangement.
tations when it is reasonably expedient to do so, in order to learn about the shape of the potential surface (e.g., locations of barriers). Below, we report detailed experiments in which we have detected electronically excited CH(A211)and CO(a'3Z+),and vibrationally excited C02(%lZc,u32l),all of which are products of reaction 1. The results are discussed in terms of the details of the channels leading to the observed products.
Experimental Section The experimental technique was described briefly in our previous report6 Here we will give a complete description of the apparatus and techniques, describing separately the generation of C2H and the detection of the chemiluminescent products. The experimental arrangement is shown schematically in Figure 1. Photolytic Preparation of CzH. C2H was prepared by laser photolysis of selected precursor molecules. Mixtures of the precursor molecule, 02,and He or Ar diluent were flowed through an AI vacuum chamber which could be evacuated to 25 mJ cm-2. At 193 nm, O2 absorbs weakly on the edge of the Schumann-Runge bands,17and of the O2molecules were excited at the highest laser fluences used in the experiments. In the case of IR photolysis, the output from a COz TEA laser (Tachisto 215G) was focused with a Ge lens to a maximum fluence -20 J cm-2 at the center of the chamber. C2HCH0was dissociated with the C02laser operating on the (001)-(1OO)P10 transition at 953 cm-I, which overlaps a maximum in the R branch of the u6 (C-C stretch) vibration of C2HCH0.l8 CzHCF3was dissociated with the laser operating on the (001)-(020)R12 transition at 1072 cm-l, which overlaps the 2v6 (CF3 deformation) overtone of C2HCF3.19 Typical ratios of [precursor]:[O2]:[Aror He diluent] in the gas mix were 1-20:20-200:500-2000 with [O,] >> (17) J. R. McNesby and H. Okabe,"Advancesin Photochemistry",Vol. 3, W. A. Noyes, Jr., G. S. Hammond, and J. N. Pitts, Eds., Wiley, New York, 1964, p 157. (18)J. C. D. Brand and J. K. G. Watson, Tram. Faraday Soc., 56,1582 (19fin). \____,.
(19) C. V. Berney, L. R. Cousins, and F. A. Miller, Spectrochim. Acta, 19, 2019 (1963).
[precursor], in order to ensure sensibly first-order kinetics. Total pressures were in the range 0.2-2.0 torr and were measured with a capacitance manometer. Precursor pressures were typically 5-10 mtorr. Detection of Chemiluminescent Products. Visible chemiluminescencefrom the reaction region was detected at right angles to both lasers with a photomultiplier tube (PMT;RCA 31034,200-900 nm) using suitable collecting and focusing optics. In time-resolved experiments, interference filters were used to isolate different spectral regions. Signals from the PMT were processed by using a transient digitizer/signal averager combination with a minimum gate width of 10 ns. Typically, data from 8 to 128 laser firings were summed to achieve a suitable signal-to-noise ratio (S/N). Chemiluminescence spectra were obtained by using a 0.25-m Jarrell Ash monochromator which was mounted between the chamber and the PMT. Spectra were taken point by point (1-nm steps, 2-nm resolution) by scanning the monochromator manually. Typically, 64 time-integrated signals were summed and displayed at each wavelength. IR chemiluminescencewas monitored at right angles to the laser beams with an InSb detector (Spectronics, photovoltaic, 77 K, 1.2 cm2). An interference filter centered at 2300 cm-l (120-cm-' fwhm) was used to isolate a portion of the C02(Au3=1)emission. In some experiments, a gas cell containing C02 was positioned between the interference filter and detector in order to absorb that part of the IR emission due mainly to (001) (OOO), thereby allowing us to monitor vibrationally excited C02 containing more vibrational excitation than a single quantum of u3. Signals from the detector were amplified and processed by the transient digitizer/signal averager combination. Typically, results from 64 to 256 laser firings were averaged in order to obtain sufficient S/N. In separate experiments, we wished to test whether C2(a311,)was also generated as a radical species in the UV photolysis of the precursor. To probe for this species, we used laser-induced fluorescence (LIF) of the d311, a311, system, as has been described previously.20 Reagents. C2HBr and C,HCHO were both synthesized as described previously.6 Both of these, as well as CzHz (Airco) and C2HCF3(PCR Chemicals), were purified by repeated trap-to-trap distillations and were always subjected to freeze-pump-thaw cycles immediately prior to use. Sample purities were confirmed by comparing their IR spectra with published IR spectra21and were checked at frequent intervals to verify that no contamination or degradation had occurred during storage in glass bulbs. O2 (99.995%), Ar (99.998%), and He (99.999%), all from Airco, were used without further purification.
-
-
Results Chemiluminescence Spectra. Figure 2 shows a spectrum of the visible chemiluminescence which was observed when C2H2was dissociated at 193 nm in the presence of OF The gas mixture consisted of 25 mtorr of C2H2,0.5 torr of 02,and 1.5 torr of Ar. The fluence of the ArF laser output was approximately 15 mJ cm-z and fluctuated by less than f20% while the spectrum was being taken. The prominent features in the spectrum can be assigned to the CH(A2A-+X211)systemz2and to the Asundi band system (20) J. D. Campbell, M. H. Yu, and C. Wittig, Appl. Phys. Lett., 32, 413 (1978). (21) For C2HBr, see K. Evans, R. Scheps, S. Rice, and D. Heller, J . Chem. SOC.,Faraday Trans. 2,69,858 (1973). For C2H2,see T. Shimanouchi, 'Tables of Molecular Vibrational Frequencies", Consolidated Vol. 1, NatE. Stand. Ref. Data Ser. (U.S. Natl. Bur. Stand.), 39 (1972). For C,HCHO, see ref 18. For C2HCF,, see ref 19.
Reaction of CzH with 0,
The Journal of Physical Chemistry, Vol. 86, No. 21, 1982 4167
I
co
4 00
500
700
600
A
800
WAVELENGTH (nm)
TIME
+
Figure 2. Spectrum of the chemiluminescence whlch results from the reaction of C,H wlth 0,when 25 mtorr of C,H, is photolyzed at 193 nm in the presence of 500 mtorr of 0, and 1.5 torr of Ar. The monochromator (grating blazed at 600 nm) was scanned in l n m steps with 2 n m resolutlon, and signals from 64 laser firlngs were summed at each wavelength. The spectrum is not corrected for the wavelength response of the detection system, which begins falling at 800 nm.
-
(a’3Z+ a3n) of C0.23 Inspection of the CO emission shows excitation to u’ = 6, with little or no emission from more highly excited CO(a*Z+) or from higher triplet states. The spedral response of the detection system begins falling quite rapidly a t 900 nm, preventing u‘ C 3 from being detected. Clearly identifiable CH(A2A-.X211) spectra were obtained with all precursors dissociated by either UV photolysis or IR MPD, and higher-resolution spectra show that most of the CH emission originates from CH(A2A,~ ’ = 0 , 1 ) .In ~ the cases of the IR MPD of C2HCF3and C2HCH0, the S/N was not sufficient to resolve spectra of the CO(a’3Z+-a311) system, and CO(a’3Z+)emission was detected by using long-wavelength and narrow-band-pass transmission filters. In the case of C2HCF3,“collision-free emission” complicated matters some at h > 700 nm.24-26 For the case of C2HCH0,the ratio [CO(a’38+)]/[CH(A2A)] was largest when C2Hwas produced by 193-nm photolysis. This photolysis is expected to give the maximum amount of vibrationally excited C2H. With IR MPD, which provides the least amount of C2H vibrational excitation, the above ratio was smallest. Time-Resolved Emission. Figure 3 shows time-resolved emission signals for CH(A2A-.X211) and CO(a’3Z+-.a311). A narrow-band-pass interference filter centered at 432.6 nm (7-nm fwhm) was used to isolate the CH(A2A,Au=O) emission, and a similar filter centered at 790 nm (10-nm fwhm) was used to isolate a part of the CO(a’38+,u’=5+a311,u”=0) emission. The time dependence of the chemiluminescence signals is given by6 ki[O~l[C~HIO
I(t) = ((7rad-l
+ 74,D-l)
- (kR[o21
X
+ k,[~recursorl))
{exp(-(kR[02]+ k,[precursor])t) - exp(-(7,ad-1+ 7Q,D-’)t)l
(2)
where [C2Hl0is the concentration of C2Hproduced by laser photolysis, ki refers to specific chemiluminescent product (22)A. M.Bass and H. P. Broida, NBS Monogr. (U.S.), 24 (1961). (23)P. H.Krupenie, Natl. Stand. Ref. Data Ser. (U.S. Natl. Bur. Stand.), 5 (1966). (24)H.Reisler and C. Wittig in ‘Photoselective Chemistry”,Vol. I, J. Jortner, R. D. Levine, and S. A. Rice, Eds., Wiley, New York, 1981,p 679. (25)A. Renlund and C. Wittig, unpublished. (26)M.L. Leeiecki, G. R. Smith, J. A. Stewart, and W. A. Guillory, Chem. Phys., 46, 321 (1980).
TIME
+
Figure 3. Time-resolved chemiluminescencesignals: (A) CH(A2A-. X2n)emission following the 193-nm photolysis of 7.5 mtorr of C,H, in the presence of 130 mtorr of O2and 860 mtorr of Ar; fluorescence was observed through a interferencefilter centered at 432.6 nm (7-nm emission following the 193-nm photolysis fwhm). (B) CO(ar32+-+a3n) of 10 mtorr of C,H, in the presence of 150 mtm of O2 and 840 mtorr of Ar. Fluorescence (prlmarlly v’ = 5 v” = 0) was viewed through an interference fllter centered at 790 nm (10-nm fwhm). Signals from 64 laser flrings were summed for both A and B.
-
channels of reaction 1, kR is the rate coefficient for the removal of the C2H species responsible for the chemiluminescent products, k, is the rate coefficient for the reaction of C2H with the precursor molecule, 7,,dW1 is the spontaneous emission rate of the chemiluminescentspecies being monitored, and 7QD;1 is the combined quenching and diffusion rate of the excited product. Note that kR may include physical quenching as well as reaction. The signals shown in Figure 3 can be analyzed straightforwardly for the cases of electronically excited products, as the decay rates are adequately described by the first exponential term in eq 2. The second exponential term in eq 2 is manifest in the rise of the chemiluminescence signal, and the data are in very good agreement with the known radiative lifetime of CH(A2A),532 n ~ . ~The ’ rise of the CO(a’3Z+-.a311) emission is sensitive to [O,], since CO triplets are very efficiently quenched by 02,% and this is apparent in Figure 3B. Signal decays were monitored over several lifetimes and could be fitted to a single exponential, except when IR MPD induced collision-free emission persisted at long times at the CO emission frequencies.% The measured rates using both CH(A2A-+X2n) and CO(a’32++a311) emissions were independent of the (27)J. Brzozowski, P.Bunker, N. Elander, and P. Erman, Astrophys. J.,207,414 (1976). (28)G. W. Taylor and D. W. Setser, J. Chem. Phys., 58,4840(1973).
The Journal of Physical Chemistry, Vol. 86, No. 21, 1982
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Renlund et ai.
w
2
R A T E = ( 2.0 f.0.3) x t 2
+Pw 9
8
lo-”
- 1201
5.’
1-
I
I 0.2
0.1
0,
1 0.3 PRESSURE
I 0.4
1 0.5
(Torr)
Flgure 4. CO(a’3zf-a3n) chemiluminescence signal decay rate vs. oxygen pressure. The slope gives a rate coefficient, k R ,of (2.0 f 0.3) x 1O-l‘ cm3 molecule-’ s-l.
TIME
‘
W
cm3 molei’
---+
Flgure 5. Time-resolved C02(%’B,+,Av,= 1) IR emission signal following the 193-nm photolysis of 5 mtorr of CpH, in the presence of 80 mtorr of O2 and 210 mtorr of Ar. The emission was observed through an interference filter centered at 2300 cm-’ (120-cm-’ fwhm). The signal shown is the sum of data from 128 laser firings.
CO(X’Z’)tHCO(~‘d)
-1601
Flgure 8. Partial enerp-level diagram showing various product states. The energies of C2H(X2Z+) OdX3E;), and the peroxy intermediate, 02C2H, are indicated by the dashed lines. Not shown is the ‘Emetastable state of C2H, whose energy is uncertain.
+
-
and the sample in order to absorb (mainly) the (001) (000) emission. Approximately 50% of the COz(Au3=l) signal persisted, indicating that a significant amount of v3 excitation is in states other than (001). Since the interference filter transmits only a portion of the Au3 = 1 emission, and there may be signals due to cascading from higher levels, we cannot accurately determine the fractional yield of the (001) state. Detailed analyses of the IR chemiluminescence signals are described elsewhere,30and we obtain an estimate of kR from the C02 Au3 = 1emission of (2 f 1) X lo-” cm3molecule-’ s-l, in agreement with the more accurate values obtained from the CH and CO emissions. The large uncertainty is due to the uniqueness of fitting 7,ise and Tfd, and the different CO, states containing v3 excitation. Since it was our intention to generate C2H in an environment free of other carbonaceous radicals which would complicate the measurements, we performed separate experiments in which we tried to find such species. In particular, using 193-nm photolysis, we tried to detect Cz(a311,) via LIF from the photolysis of C2H2. At the low fluences used throughout our experiments ( [CH(A2A)]. On the basis of our past experiences with InSb detectors,30we feel that it is almost impossible to accurately normalize the InSb detector against the PMT. Thus, only large differences in concentrations can be distinguished. On the basis of the relative signal amplitudes and detector sensitivities, we estimate that [C02(u321)]is at least 2 orders of magnitude larger than [CH(A2A)]. It is inconceivable that these concentrations are comparable. This is reasonable given the enthalpy changes listed in reactions 3-5. (31) H. Okabe, ‘Photochemistry of Small Molecules”, Wiley, New York, 1978. (32) M. Halmann and I. Laulicht, Astrophys. J.,Suppl. Ser., 12, 307 (1966).
The Journal of Physical Chemistry, Vol. 86, No. 21, 1982 4169
In the experiments described above, we have used several C2Hprecursor molecules, two quite distinct photolysis methods, and the detection of three chemically distinct reaction products. The photolytic generation of C2H is accomplished on a short time scale (within 500 ns) relative to the detection time. Furthermore, we have attempted to minimize any effects due to other radical species by generating C2H cleanly and in well-defined environments and by monitoring products which are not likely to be formed from reactions of other species. The consistency of the rate coefficient measurements indicates that the chemiluminescent products derive from reaction 1. The reacting C2H molecules may have unrelaxed vibrational excitation,especially when generated by 193-nm photolysis. However, our measurements of the CO emissions show that, although reagent vibrational excitation appears as product vibrations, kR does not depend on CzH vibrational excitation. A more serious question concerning the C2H species responsible for the obsejved emission involves low-lying electronic states. The A211state has very recently been measured by absorption spectroscopy and is only 3700 cm-’ above the X2Z+ground state.33 Also, there is a low-lying 42-metastable state, whose location has not yet been determined e~perimentally.~~ Since such states are accessible via IR MPD and UV photolysis, we cannot discount the possibility that they are responsible for the observed emissions. Even though the rate coefficients remain constant upon the addition of high pressures of Xe (-100 torr), which is known to induce efficient intersystem crossing in other small carbonaceous radicals with low-lying metastable states (e.g., C2135CHz36),it is still possible that the observed emissions derive from a low-lying electronic state of C2H. If this is the case, then reaction will compete with quenching, and the measured rate coefficient kR will be the sum of the reactive and energy transfer rate coefficients. Thus, if the C2H species which reacts to produce CH(A2A),CO(a’3Z+),and CO2+is a low-lying electronic state, our measured rate coefficient is an upper bound on the rate coefficient for reaction. It seems reasonable that the reaction of C2H with O2 proceeds via a peroxy whose ground state is approximately 50 kcal mol-l lower than separated C2H 0, for ground-state reagents, and 60 kcal mol-’ for C2H(A211) and ground-state 02.15338 If k R = kl,then there is very little, if any, repulsion in the entrance channel, since the reaction rate coefficient is quite large, only 1order of magnitude smaller than the rate coefficient for hard-sphere collisions. The lack of a barrier in the entrance channel is n d surprising for such a radical-radical enc~unter’~ and is in accord with the stability of the 02C2Hspecies. The 02C2Hintermediate can contain 50-60 kcal mol-’ of vibrational energy, which can facilitate the nuclear motions which lead to the observed products. At this time, we have made no estimates concerning the barriers, transition states, and RRKM reaction rates for the different product channels. These are difficult and will be the subject of
+
(33) P. G. Carrick, J. Pfeiffer, R. F. Curl, E. Koester, F. K. Tittel, and
J. V. V. Kasper, J. Chem. Phys., 76, 3336 (1982). (34) S. Shih, S. D. Peyerimhoff, and R. J. Buenker, J.Mol. Spectrosc., 64, 167 (1977). (35) H. Reisler, M. S. Mangir, and C. Wittig, J. Chem. Phys., 73, 2280 (1980). (36) M. N. R. Ashfold, M. A. Fullstone, G. Hancock, and G. W. Ketley, Chem. Phys., 55, 245 (1981). (37) S. W. Benson and P. S. Nangia, Acc. Chem. Res., 12, 223 (1979). (38) S. W. Benson, J. Am. Chem. Soc., 87, 972 (1965). (39) S. W. Benson, Prog. Energy Combust. Sci., 7, 125 (1981). (40) H. Schacke, H. Gg. Wagner, and J. Wolfrum, Ber. Bunsenges. Phys. Chem., 81, 670 (1977).
4170
J. Phys. Chem. 1982, 86, 4170-4175
(a’3Z+,u’=3-6),and the more similar amounts of CH(A2A) and CO(a’3Z+). So far, all of our results derive from products which require an interaction of the terminal oxygen atom with the carbon ?r-electron system. Although we have not yet sampled what we believe may be the major product channels, we have found nothing unusual in the internal states of those species which we have observed.
future research. The interaction of the radical center on the terminal oxygen atom with the carbon ?r-electron system can be significant, since the high vibrational energy content of the 02C2Hintermediate can move the terminal oxygen atom toward the carbon-carbon *-bond system with considerable velocity. For species which derive from this interaction, we see little reason for specificity in the product channels leading to CO + HCO, 2CO + H, and CH + C02,and it is possible that phase space predictions of the chemically distinct product channels and product excitations will provide realistic estimates. The OH + C20 product channel requires that the terminal oxygen seek out the hydrogen atom in preference to its other opportunities, and we will look for this product channel in future experiments. The channel leading to HCCO + 0 is interestingg in that reaction is via a loose transition state, and RRKM estimates of k(E)for this channel may be more accurate than for the other product channels. In our experiments, we found that C2Hvibrational excitation did not influence its rate of removal, but this vibrational excitation was carried over into product degrees of freedom. Vibrational excitation of C2H enhanced both the amount of CO(a’3Z+,u’=5)relative to CH(A2A)and the production of CO(a’3Z+,u’=6),thus showing a tendency to excite more energetic species. The constancy of kR is in agreement with the radical-radical nature of the initial encounter, and such constancy has been noted for systems as simple as CN(v) + H2, in which the reagent vibration does not involve the reaction c ~ o r d i n a t e .The ~ ~ tendency for reagent vibrational excitation to carry over into product excitations, both electronic and vibrational, is in qualitative agreement with phase space predictions, as is the large amount of C02+ relative to both CH(A2A) and CO-
Conclusion We have studied in some detail the chemiluminescent product channels of the reaction of C2Hwith OF The C2H species which is responsible for the products that we obterve is in either the ground X2Z+electronic state or the A211low-lying electronic state which can be produced by both UV photolysis as well as IR MPD of the C2H precursors. Reaction probably proceeds via a peroxy intermediate, whose lifetime may be sufficiently long to allow significant nuclear motion to occur. The reactions of C2H continue to be of interest to us, and we plan to detect ground-state products (CH, OH, C20) by LIF and to optically pump the 02C2Hintermediate, thereby altering the product channels. We would like ultimately to determine the potential energy surface in the regions near the barriers and to apply our experimental techniques to other prototypical systems germane to combustion. Acknowledgment. We have benefited from discussions with J. Tiee, J. McDonald, Y. Haas, and S. Filseth and from the expert technical assistance of R. Senaha. This research was supported by the Air Force Office of Scientific Research and the Office of Naval Research. We acknowledge the generous loan of equipment by the San Francisco Regional Laser Center.
Kinetic Study by Electron Paramagnetic Resonance and Mass Spectrometry of the Elementary Reactions of Phosphorus Tribromide with H, 0, and OH Radicals J. L. Jourdaln, 0. Le Bras,’ and J. Combourleu Centre de Recherches sur la Chimie de la Combustion et des Hsutes Temphratures (C.N.R.S.), 45045 Orlhans Cedex, France (Received: February 10, 1982; In Final Form: June 21, 1982)
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The reactions of phosphorus tribromide with H, 0, and OH were studied in a discharge flow reactor coupled to an EPR and a mass spectrometer. The following rate constants were obtained at 295 K: (1)H + PBr3 products, kl = (1.7 f 0.3) X lo-”; (2) 0 + PBr3 products, k 2 = (3.6 f 0.5) X (3) OH + PBr3 products, k 3 = (8.5 f 0.5) X (in units of cm3 molecule-’ s-l). The products were analyzed by EPR and mass spectrometry. Br atoms were detected in reactions 1-3 and P atoms in reaction 1. The mechanism of the initial step of each reaction, deduced from the analysis of the reactants and products curves, showed that reaction 1 proceeds via an abstraction mechanism while reactions 2 and 3 proceed via a mechanism of atom exchange.
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Introduction Elementary reactions of H,0, and OH with some halogenated compounds of boron and $”rus, BC13,BBr, and PC13 have recently been studied in our laboratory1-3 as these compounds, and also the presently considered compound PBr3, are potential flame inhibitors.4 Also, (1) J. L. Jourdain, G. Laverdet, G. Le Bras, and J. Combourieu, J. Chim. Phys., 77, 9, 80. (1980). (2) J. L. Jourdain, G. Laverdet, G. Le Bras, and J. Combourieu, J . Chim.Phys., 78, 3, 253 (1981). (3) J. L. Jourdain, G. Le Bras, and J. Combourieu, J.Phys. Chem.. 85, 655 (1981).
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these reactions might be related to flame retardant processes since it is now suggested that the inhibiting effect of several flame retardants added to flammable materials takes place at least in part in the gas phase by reactions involving the flame Propagating radicals H, 0, and O K 5 Then the rate constants and pathways of these reactions need to be known for modeling purposes in order to estimate the potential inhibiting effect of such compounds. (4) G. Lask and H. Gg- Wagner, Symp. (Znt.) Combust., [Proc.], 12th, 8, 432 (1960). ( 5 ) J. W. Hastie and C. L. Mc Bee, ACS S y m p . Ser., No. 16, 118 (1975).
0 1982 American Chemical Society
