J. Phys. Chem. 1984,88, 66-68
66
then it will interfere with “plateau absorbance readings” (implied by both their and the present treatment) by turning the plateaus, gradually, into true maxima. The analysis of the system requires then the consideration of the full set of reactions, including those responsible for the disappearance of the intermediate. Little is known about the nature of the decomposition reactions. Catalatic decomposition of the oxidant, however, does not contribute to them
as such a reaction does not cause any decrease of the concentration of the
Acknowledgment. We are indebted to Mr. Norman A. Marlow from the American Telephone and Telegraph Company for helpful discussions. Registry No. Chlorite, 14998-27-7; deuteroferriheme, 21007-21-6.
Reaction of Ethynyl Radicals with 02. Rate Constant for Formation of CO A. H. Laufer* and R. Lechleider National Bureau of Standards, Chemical Kinetics Division, Center f o r Chemical Physics, Washington, D.C. 20234 (Received: December 10, 1982; In Final Form: June 1 , 1983)
Absolute rate constants and branching ratios for reactions of the C2H-02 system have been obtained from observation of the CO product buildup. The reactions and rate constants of the primary processes derived from a best fit of the CO temporal history are as follows: C2H + 0, C 2 H 0 + 0 (3a), k3a= 1.0 X cm3 molecule-’ s-l; C2H + O2 CO + HCO (3e), k3e= 4.0 X cm3 molecule-’ s-l. In the system, C 2 H 0 produced in reaction 3a subsequently reacts with O2 to produce CO. The relationship of the C2H + 0, rate constants to those for reaction of C2H with hydrocarbons is discussed.
-
-
Introduction The primary fragment of the thermal or photolytic decomposition of C2H2is a C2H radical. Unfortunately methods available for studying the kinetics of this species are severely limited since there have been no spectral absorptions, in the gas phase, in either the visible or ultraviolet regions which can be positively attributed to the ethynyl radical. As a result, the few absolute kinetic measurements extant have utilized the appearance of to ascertain concentration profiles and hence kinetic parameters. In a single case, the C2H concentration has been monitored by mass ~pectroscopy.~ C2H is of considerable importance in all acetylene-containing systems, which range from the atmosphere of Jupiter and Titan to flames. In the latter, the reaction of C2Hwith O2 is of major importance. It is interesting, therefore, to note that the two reported values of the rate constant for the C2H-02 reaction differ by a factor of 4.3,4 Using the very sensitive and rapid technique of emission spectroscopy, Renlund et al.3 were able to identify unambiguously the infrared emission from vibrationally excited C 0 2 as well as the electronic emission from both CH(A2A) and the Asundi CO(a’3X+-a311) system. The profusion of excited products must represent several different reaction pathways, viz. C,H
-
+ o2
-
+ HCO
(1)
+ co2(v’3i)
(2)
c0(~’32+)
CH(A~A)
plus also the reactions where all the products are in their ground electronic and vibrational states. The independent mass-spectrometric study4 positively identified another reaction channel, i.e. CZH
+0 2
-
C2HO
+0
(3)
by the unique m / e 41 signature of the ketyl radical (C2HO). However, reactions 1 and 2 were specifically excluded since neither H C O nor C 0 2 appeared in the reaction mixture. A positive identification of CO could not be obtained because of the presence of a background spectrum at m / e 28 due to molecular N2. The present work was motivated, in part, to clarify the discrepancy
* Present address: Chemical Sciences Div-BES, Department of Energy, Washington, DC, 20545.
in rate measurements and, in part, to determine the mechanism of the C,H-0, reaction. The approach is similar to that used previously, Le., flash photolysis coupled with vacuum-ultraviolet kinetic spectr0scopy.l The temporal history of the product C O was monitored by its well-characterized absorption in the AIII-X’Z Fourth Positive system.6 The very strong absorption allows us to monitor directly ground-state-product formation from which rate coefficients for the C2H-02 reaction may be derived. The stoichiometry of the reaction can be obtained by titration of the initially produced C,H, (C2H),, with a reactive hydrocarbon such as propane to yield C2Hz which may be quantitatively determined by gas-chromatographic analysis.2 The equivalence of product C O to (C,H), as well as kinetic considerations place severe limits upon the allowable mechanistic interpretations. Experimental Section The flash photolysis-vacuum-ultraviolet kinetic spectroscopy apparatus and the associated facility for gas-chromatographic sampling have been previously described in detail. (ref 1 and (1) A. H. Laufer and A. M. Bass, J. Phys. Chem., 83, 310 (1979). (2) A. H. Laufer, J . Phys. Chem., 85, 3828 (1981). (3) A. M. Renlund, F. Shokoohi, H. Reisler, and C. Wittig, Chem. Phys. Lett., 84, 293 (1981). (4) W. Lange and H. G. Wagner, Ber. Bunsenges. Phys. Chem., 79, 165 (1975). ( 5 ) H. Okabe, J . Chem. Phys., 75, 2772 (1981). (6) P. H. Krupenie, “The Band Spectrum of Carbon Monoxide”, National Bureau of Standards, Washington, DC, 1966, Natl. Srand. ReJ Data Ser. ( U S . , Natl. Bur. Stand.) No. 5 . (7) The AH, were obtained from D. D. Wagman et al., NBS Tech, Note (US.), No. 270-3 (1968), except for AHf(C2H) = 127 kcal/mol from H. Okabe and V. H. Dibeler, J. Chem. Phys., 59,2430 (1973), Aifr(C20) = 80 kcal/mol derived by A. H. Laufer, J . Phys. Chem., 73, 959 (1969), and AHdC2HO) = 58 kcal/mol from ref 12. A more recent value of AHf(C2HO) = 41 kcal/mol has been derived (C. F. Melius and J. S. Binkley, private communication). (8) R. L. Brown, “A Computer Program for Solving Systems of Chemical Rate Equations”, NBS-IR 8 1-2281, National Bureau of Standards, Washington, DC. (9) Report of the CODATA Task Group on Chemical Kinetics, J. Phys. Chem. ReJ Data, 11, 327 (1982). (IO) A. H. Laufer and A. M. Bass, Chem. Phys. Lett, 46, 151 (1977). (11) K. Schofield, J . Phys. Chem. ReJ Data, 8, 723 (1979). (12) A. M. Renlund, F. Shokoohi, H. Reisler, and C. Wittig, J . Phys. Chem., 86, 4165 (1982).
This article not subject to U S . Copyright. Published 1984 by the American Chemical Society
Reaction of Ethynyl Radicals with O2 references therein). Briefly, a S ~ p r a s i lreaction '~ vessel was placed inside a chamber in which a photolysis flash through N2 could dissipate 4850 J in 5 ps. The photolysis flash profile was monitored optically. Hydrocarbon products were rapidly analyzed by withdrawal of a sample, following the flash, from the center of the reaction cell through stainless steel tubing onto a 7 m long, 6 mm 0.d. stainless steel chromatograph column packed with 30% (w/w) squalane on Chromosorb P. The flame detector was calibrated with known quantities of products. Spectroscopic analysis was performed with a Garton-type analysis flash of 2-ps duration triggered at preset delay times, with a photomultiplier-oscilloscope delay circuit. The vacuum-ultraviolet output was focused, through LiF optics, upon the slit of a 2-m Eagle mount spectrograph whose dispersion was 2.8 A mm-' in first order. Spectra were recorded on Kodak SWR plates. A single flash, through a 50-pm slit, produced adequate plate darkening for densitometric analysis. CO concentrations were determined by using the 0-0 transition of the A1n-XIZ+ Fourth Positive system at 154.42 nm.6 Concentrations were calibrated against known quantities of CO. The stronger absorption lines represented by the (l,O), (2,0), or (3,O) transitions could not be used for kinetic measurements since they were partially obscured by the substrate mixture. The presence of vibrationally excited CO product could be determined from observation of the (0,l) or (0,2) transitions of the C O Fourth Positive system which were not obscured by substrate absorption. The minimum detectability for CO(XIZ+)r,u>o in our system was 15-20% of the total C O present. At longer delays, where the detectability is improved, any CO(XIB+)ttu>o will be quenched to CO(X'B+)",O. The source of C2H in these experiments was photolysis of CF,C2H. The 3,3,3-trifluoropropyne obtained from PCR Inc. was used following purification by gas chromatography. No C2H2was detected following photolysis of the purified substituted propyne. Ultrapure O,, He, and N2 (typically 99.99%) were used without further purification. In a typical experiment, samples of CF3C2H and 0, were prepared as mixtures in either He or N,; aliquots were taken such that the desired substrate pressures were present in the reaction vessel. He or N, was added to the samples to the desired pressure. In all cases, adequate mixing time was alloted to permit sample uniformity prior to photolysis. The experiments were performed at 295 K since heating of the sample by the flash is minimal in the presence of a large excess of inert gas.
Results and Discussion Initial experiments were performed to determine the yield of (C2H), obtained from the photolysis of a fixed pressure of CF3C2H. The technique used involved the reaction of the C2H with added propane until the yield of product C2H2,determined chromatographically, reached a plateau value with respect to further addition of hydrocarbon. At the plateau the yield of C2Hz is equivalent to (C,H),. Although propane undergoes some photolysis, the yield of propylene always was less than 0.1% of the propane. In addition, the propylene yield did not change during the cophotolysis of CF3C2H-C,Hs as compared to the neat photolysis of C3Hs, indicating no reaction between C2H radicals and propylene product. C2H2was the only other hydrocarbon product detected. A similar series of experiments were performed with respect to added O2 to the CF,C,H photolysate. Here a plateau for C O production, monitored spectroscopically at 500 ps after the flash, was determined. These experiments indicated that the sole reaction of C2H in the 0, system was with O2 to produce CO. With a sample of 100 mtorr of CF3C2H2,the oxygen concentration necessary to achieve the plateau was less than 100 mtorr of added 0,. On the basis of this analysis the photodecomposition of CF3C2Hwas less than 2%. Within experimental (13) Certain commercial instruments and materials are identified in this paper in order to adequately specify the experimental procedure. In no case does such identification imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the instruments or materials identified are necessarily the best available for the purpose.
The Journal of Physical Chemistry, Vol. 88, No. 1, 1984 61 4.0
8
r
1.61
/
x%
ow 0
I
I
I
200 300 Time, (ps)
100
I
I
400
500
Figure 1. Experimental conditions: 100 mtorr of CF,C2H + 100 mtorr of O2 + 50 torr of N2 [O], 600 torr of N2 [XI, SO torr of He [O], and 600 torr of He [A].Curve A: Calculated CO concentration assuming single reaction channel to produce C 2 H 0 plus subsequent reaction with O2 2CO. Curve B: Best fit assuming C2H + O2 C 2 H 0 + 0 and C2H + O 2 CO + HCO in the ratio of with an overall k = 5.5 X cm3 molecule-' s-I.
-
-
-.
error, the stoichiometry of the reaction as determined from the ratio of the (CO)produd determined spectroscopically to the (C2H), determined by reaction with C3Hs is equal to 2.0 f 0.3. The stoichiometry is constant over the complete pressure range from 50 to 600 torr independent of added He or N2. The presence of a C O plateau indicates that the C,H reacts exclusively with O2 and, therefore, the temporal history of CO production is a measure of the total thermal C2H O2 reaction. The temporal history of C O production is shown in Figure 1. At a fixed concentration of C2H and 02,the C O buildup was independent of both pressure from 50 to 600 torr and inert gas, either He or N2. If the CO increase and the C2H removal can be represented through a pseudo-first-order process with 0, by a single process such as reaction 4, then the value of k4 may be C2H 0 2 2CO + H (4)
+
+
-+
determined from the half-life of the reaction or, preferably, from the slope of a In [(CO),/[(CO), - (CO),] vs. time plot where k4 is defined by d[CO]/dt = 2k4[C2H][02].The preferred value cm3 molecule-' SKIwhere the unfor k4 is (4.0 f 0.5) X certainty represents one standard deviation of the slope of the (In) plot forced through the origin as indicated by the kinetic equation. Now, if the rate constant represents the loss of C2H as well as the appearance of CO, the present value of k4 compares favorably with the mass-spectral determination of k4 = 5.5 X cm3 molecule-' s-', If one neglects the production of either vibrational or electronic excitation in the various reaction products, there are five possible pathways to be considered in the overall mechanism for the C2H-0, reaction. C2H
---
+ 0,
+0
AH = -10 kcal/mo17
(3a)
C20+ OH
AH = -38 kcal/mol
(3b)
+ CO, 2CO + H CO + HCO
AH = -79 kcal/mol
(3c)
AH = -129 kcal/mol
(3d)
C2H0
CH
AH = -158 kcal/mol
(3e)
In the discharge flow-mass-spectrometric study: the ketyl radical C,HO was directly observed in the reaction products in concentrations leading those investigators to suggest that the major reaction pathway is reaction 3a although that path is the least exothermic. In the system of Lange and Wagner,4 the removal of C 2 H 0 presumably is by further reaction with the excess O2 to produce 2CO as required by the stoichiometry of the system as measured
68
The Journal of Physical Chemistry, Vol. 88, No. 1, 1984
in the present work. The rate constant for CzHO reaction is approximately 40% of the C2H removal rate or 2.2 X lo-’, cm3 molecule-’ s-I (ref 4) so that the total mechanism, based upon the kinetic determinations of Lange and Wagner,4 for production of CO would be
+0 2
C2H
k3a = 5 . 5
X
C2HO
-
+0
lo-’, cm3 molecule-’
+0 2
C2HO
--.+
k5 = 2.2 X
2CO
s-l
+ OH
cm3 molecule-’
(5) s-I
This set of reactions was modeled by using the concentration conditions of our experiments.* The expected sigmoid-shaped curve of CO production is superimposed on Figure 1. The increase in CO concentration with time suggested by this mechanistic approach constrained by the measured rate constants is incompatible with the present experimental data. Within the limitations of observed C 2 H 0 production, it is clear that an alternate source of “prompt” C O is required. The most exothermic CO-producing channel, reaction 3e, also results in H C O which previous workers did not observe by mass-spectrometric analysis of the reaction products. However, in that system the H C O concentration may be quite small due to subsequent reaction with 02: HCO 0 2 CO + HO2
+
k6 = 5.5 x
-
cm3 molecule-l s-l (ref 9 )
A model calculation using reaction 3e in conjunction with reaction 6 shows that the HCO profile increases at short times but is always less than the concentration of C2H. The mass-spectral detection limit for a free radical strongly depends upon the cross section for ionization and, if that for HCO is less than that for C2H, the HCO radical will not be observed. Further, the value of the rate constant for reaction 6 is for a thermal procesi. The exothermicity of reaction 3e and consideration of linear momentum conservation lead one to expect the H C O fragment to possess -75 kcal/mol internal energy. If so, the H C O fragment may then either react more rapidly with O2 or, with sufficient internal energy, rapidly decompose into CO. In either case, the H C O concentration becomes vanishingly small and the overall chemistry becomes indistinguishable from reaction 3d. The best fit to the present CO time history requires two primary reactions, reaction 3a followed by reaction 5 , and reaction 3e followed by the decomposition of HCO or its rapid reaction with 0,. The question now becomes the correct branching ratio, Le., reactions 3a/3e. The optimum fit is obtained when reaction 3a represents 20% of the primary process and is shown in Figure 1. A fit assuming a ratio of 30/70 is tolerable and within the error limits of our experimental data. We were unable to observe any vibrational excitation in the ground-state CO product. In a previous study,I0 with the same
Laufer and Lechleider experimental arrangement, we have been able to observe less than 15% of the total C O produced as the result of reaction to be vibrationally excited. If, alternatively, CO were produced in either the a311or a’32+ state, 6.01 and 6.86 eV6 above the ground state, respectively, the presence of 100 mtorr of O2with a rate constant for quenching of a311-X111 of 1.7 X lo-’’ cm3 molecule-’ s-l (ref 11) would result in ground-state CO within 2 ys following the flash. The minimum observation time in the present work is about 10 ys so that any effect of the inert gas, N2 or He, upon the quenching would be masked by 02.These results suggest that production of electronically excited CO is not important in our system. Stoichiometric considerations indicate that the products observed by emission spectroscopy represented, in part, by reaction 3c plus those due to reaction 3b are, in our system, probably minor reaction pathways. The value of k3 suggested by Renlund et al.,3 if representative of reaction of C,H(X), is clearly in disagreement with the present results. While there can be no doubt that the various energetic fragments were observed in the laser experiments, it is possible thatjhe precursor was not ground-state, thermal C2H but rather the AZII(C2H)which lies 0.47 eV above the ground state.12 The extra energy might be expected to enhance the reaction rate of the chemiluminescent reaction to produce CO(a’3X+)tu>0.The thermal process to produce CO(a’3Zt)u>0 is slightly endothermic and a rapid rate constant of the order of lo-” cm3 molecule-’ s-’ (ref 12) is unusual. The additional 0.47 eV makes the overall process exothermic. We agree with the suggestion, therefore, that the emissions from both electronically excited CH(A2A-X211) and the Awn$ CO(a’32+-a-311) probably are the result of reaction by C2H(A211)or C2H(X28+)u,o. If so, then the previously reported rate constants for C2H reaction with both H, and CH43 does not represent the chemistry of “cold” C2H(R2Xt). In summary, a rate constant for production of C O by reaction 4, Le., C2H O2 2CO H, has been determined to be (4.0 f 0.5) X cm3 molecule-’ SKI.A branching ratio represented by processes 3a and 3e has been deduced by a “best” fit to the CO temporal profile. We estimate that at the shortest delay times less than 15% of the CO present is vibrationally excited. The rate constant was measured at a single O2 concentration. While it would have been desirable to determine the rate parameters at other 0, concentrations, the experimental technique imposed severe limitations upon the range over which the O2concentration could be varied. These limitations were (a) the stoichiometric requirement of exclusive C2H reaction with O,, (b) adequate absorption by product CO, and (c) minimal absorption overlap or interference by the substrate mixture in the spectral region of the CO(A’II-X’Z+) system and they posed particularly stringent experimental requirements.
+
-
+
Acknowledgment. This work was supported, in part, by the NASA Planetary Atmospheres Program. Registry No. CF,C,H, 661-54-1; ethynyl, 2122-48-7.
