310
The Journal of Physical C h e m M y , Vol. 83, No. 3, 1979
A. H. Laufer and A. M. Bass
Photochemistry of Acetylene. Bimolecular Rate Constant for the Formation of Butadiyne and Reactions of Ethynyl Radicals Allan H. Laufer" and Arnold M. Bass Chemical Kinetics Division, Center for Thermodynamics and Molecular Science, Nafional Bureau of Standards, Washington,D.C. 20234 (Received August 16, 1978) Publication costs assisted by the National Bureau of Standards
The vacuum-ultravioletflash photolysis of acetylene has been investigated and the mechanisms and rate constant for butadiyne production have been measured. The major photolytic decomposition path deduced is CzH2+ hv C2H + H. A minor channel correspondingto C2Hz C(3P)+ CH2(g3CJ (6) was also suggested. Evidence suggests that butadiyne is formed in a first-order process: CzH + CzHz C4Hz+ H (2). The rate of (2) was determined by monitoring butadiyne concentration spectroscopically and a rate constant, k 2 = 3.1 f 0.2 X IO-" cm3 molecule-' s-l, determined. Addition of Hz reduces the yield of C4Hz by the reaction C2H + H2 CzHz + H (8) and a ratio k8/k2 = 4.9 X was obtained. The photolysis of CF3-C2H was briefly investigated.
-
-
Introduction Butadiyne (C4H2),also known as diacetylene, has been observed as a product in high temperature pyrolyses and photochemical systems containing acetylenic type compounds. The mechanism invoked to explain its formation depends on the precursor and excitation mode, thermal or photochemical. Even in photochemical systems, which are often more tractable than those involved in pyrolysis, the mechanism of butadiyne formation has not been verified. In the vacuum-ultraviolet photolysis of acetylene, for example, Takita et al.' suggest two modes of butadiyne formation, i.e. C2H2*+ C2H2 C4H2 + (2H or H2) (1) C4Hz + H C2H + CZH, (2) Specifically excluded is the combination reaction of two ethynyl radicals C2H C2H C4H2 (3) The exclusion of (3) is based upon previous shock tube studies2 in which rapid formation of butadiyne from acetylene was presumed to occur via ( 2 ) . If ( 2 ) is rapid, then the relative concentrations of C2H and C2H2would preclude (3) from contributing significantly to C4Hz formation in a low-intensity photolysis experiment. In a previous study of the acetylene phot~lysis,~ in which examination of product formation was limited to a determination of the hydrogen yield and acetylene disappearance rates, a solid polymer was also noted. It was conjectured that reactions leading to higher hydrocarbon products proceeded through excited acetylene and/or combination of ethynyl radicals (3). Ethynyl as well as vinyl radicals have been directly observed from their ESR spectra as the result of the photolysis of CzHz during its deposition from the gas phase4 a t 4 K. Since the radical spectra were completely suppressed when solid acetylene was photolyzed and there was no evidence for H atom production, these workers assumed an excited state mechanism for C2H formation: CZH2" + CzHz C2H3 + CZH (4) More recently, Milligan et ala5photolyzed matrix-isolated acetylene in the vacuum ultraviolet and attributed the strong absorption feature a t 1848 cm-l to C2H. Evidence for the assignment was based upon isotopic substitutions. No absorption was observed in the ultraviolet. Graham et a1.6a assigned to C2H the observed absorptions in a
--
+
-+
0022-3654/7~/2083-0310$01 .OO/O
-
-
matrix at both 3000 and 10000 b, following the gas-phase photolysis of acetylene. It appears that the system a t 10000 A really is part of the Phillips systems of C2.6bThe assignment of the 3000-b, transition remains tenuous. Nevertheless there appears to be some evidence that C2H is indeed formed during acetylene photolysis but its role in the gas-phase chemistry of acetylene has not been completely determined. The formation of butadiyne is not limited, however, to systems containing acetylene. For example, the nearultraviolet photolysis of both neat mono halo acetylene^^^^ and 3,3,3-trifluoropropyne (CF3CzH)9in the presence of NO produce butadiyne as a major product. Analogous to the case of acetylene, the production of butadiyne from the photolysis of bromoacetylene (C211Br)is considered to arise by a displacement reaction (eq 5 ) rather than by C2H + CzHBr C4H2 + Br (5)
-
reaction 3, the combination of ethynyl radicals. At very high intensities, some contribution from (3) may be pre~ent.~ Similarly, in the case of 3,3,3-trifluoropropyne, production of butadiyne is speculated to arise from a reaction similar t o (5) with the concomitant production of CF3. To elucidate the mechanism of butadiyne formation and to help clarify the acetylene photolysis, we have measured the rate of butadiyne formation in a vacuum ultraviolet flash photolysis experiment. The parameters include flash energy, and hence precursor concentration, total pressure in the system, and precursor molecule. We have obtained evidence for the occurrence of reaction 2 and determined its rate constant a t room temperature. Experimental Section The vacuum-ultraviolet flash photolysis apparatus used in conjunction with kinetic spectroscopy and gas chromatographic sampling has been previously described in detail.1° Briefly, a reaction cell fabricated with either LiF windows or entirely from Suprasil was placed inside a chamber in which a photolysis flash through Nz could dissipate 3000 J in 5 ys. Analysis of hydrocarbon reaction products through C4 was done, following the flash, by the rapid withdrawal of a sample from the center of the reaction cell through a small bore stainless steel tube and injection 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 0 1979 American Chemical Society
Photochemistry of Acetylene
The Journal of Physical Chemistry, Vol. 83, No. 3, 1979 31 1
known samples. All photolyses were performed a t room temperature. Spectroscopic analysis was performed using a Gartontype analysis flash of 2-ps duration triggered a t preset delays;. The flash was focussed, through LiF optics, upon the slit of a 2-m Eagle vacuum spectrograph. The dispersion of the spectrograph was 2.8 A/mm in first order and spectra were recorded on Kodak SWR plates. A single flash, through 40-pm slits, produced adequate plate darkening for densitometric analysis. Acetylene, obtained commercially, was used following repeated trap-to-trap distillation from 196 to 77 K. Chrornatographic analysis indicated the abseince of impurities and photolysis gave no indication of chemistry attributable to methyl radicals from acetone, which is often used to stabilize acetylene. Butadiyne was prepared from 1,4-dichlorobutyne-2 by reaction with KOH in aqueous solutilon’l and purified by distillation from 250 to 77 K. Mass spectrometric analysis of the butadiyne with 70-eV electrons produced the anticipated pattern [mle 50(100), 49(40.8), and 48(9.2)]. An acetylene impurity, amounting to about 5%, was detected chromatographically. 3,3,3Trifluoropropyne was obtained from Peninsular Chemical Co. aind used without further purification. A few experiments were done using bromoacetylene (C,HBr), prepared by the method of Bashford et a1.,12 as a photochemical precursor. Acetylene was a persistent impurity and could not be entirely removed even after repeated distillations. CzHBr did not elute from the squalane column. In ,a typical experiment, samples of substrate were prepared as mixtures in ultra-pure helium; aliquots were taken such that substrate pressures of between LO and 100 mtorr were present in the reaction cell and the samples pressurized with helium to the desired pressure. After allowing the samples to mix, they were flashed and analysis was performed as discussed above.
Results and Discussion The major product observed by gas chromatography following the vacuum-ultraviolet flash photolysis of CzHz was butadiyne, but its yields were quite low. Based upon the C4H2 yield, approximately 5% of the acetylene was consumed by the photoflash and subsequent chemistry. We did not search for hydrogen. Iit should be noted that very small quantities of methylacetylene and allene were also formed as products. These have not, to our knowledge, been previously observed as products of the acetylene photolysis. A possible precursor for the C3H4products is CH2 for which reactions with C2Hr2has been well characterized:13
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C2H2
C(3P) + CH2(R3Bl)
(6)
The tliermochemical onset for (6) occurs a t 135!3 A (Table I), well within the transmission window of LiF. Decomposition into this channel was sufficiently small (less than 5%) such that we were unable to observe either the production of C atoms spectroscopically nor were we able to discern any unique chemistry attributable to C atoms in this system. The absorption spectrum of 3,3,3-trifluoropropyne has been briefly reported in the ultraviolet regiong a t 200 nm but another, much stronger, absorption band begins at about 135 nm and increases toward the limit of our optical system, 105 nm. Photolysis of the substituted propyne in the vacuum ultraviolet was notable in that, while photodecomposition undoubtedly occurred, no hydrocarbon products through C4were observable. This is in agreement with studies a t longer wavelength^.^ Polymer formation
TABLE I: Thermochemical Values AH,”,,
species
-
kcal/mol
ref
C CHZ C*H CZH, C4H2
171.3 93.4 127 54.3 113 198.2 52.1
a b
c2
H
-
C
a d e a
a D. D. Wagman, W. H. Evans, V. B. Parker, I. Halow, S. M. Bailey, and R. H. Schumm, Natl. Bur. Stand., Tech. K. E. McCulloh and V. H. Note No, 270-3 (1968). H. Okabe Dibeler, J. Chem. Phys., 64, 4445 (1976). D. and V. H. Dibeler, J. Chem. Phys., 59, 2430 (1973). R. Stull, E. %. Westrum, Jr., and G. C. Sinke, “The Chemical Thermodynamics of Organic Compounds”, Wiley, New Yorlc, 1969. e “JANAF Thermochemical Tables”, 2nd ed., Natl. Stand. R e f , Data Ser., Natl. Bw. Stand, No. 37 (1971).
TABLE 11: Yield of Butadiyne-LiF Cell
mtorr 75 75 50 50 25 25
mtorr
torr
units)
50
700 700 700
100
700
100 100
700 700 700
13 30 11 28 5 28
-0
is not observable in our system. The effect of added acetylene is shown in Table 11. It is clear that although CF3C2H itself does not produce butadiyne, addition of C2H2 to the photolysis mircture greatly enhances the production of C,H2 indicating that CF3C2Hprovides a precursor for the formation of butadiyne. The identity of the precursor is made evident when a mixture of propane and CF3C2H is photolyzed iin the suprasil cell. Under these conditions (A I-160 nm), the direct photolysie of C3H8is miminal and the only products from its decomposition are CH4 and propylene amounting to less than 1% conversion. When CF3C2H is added, copious quantities of C2H2are produced by reaction 7. C2H + RH C2H2 R (7) This clearly indicates the presence of C2H radicals, in agreement with previous investigation^.^-^ These results are further evidence that, though ethynyl radicals are necessary precursors to butadiyne formation in these systems, the presence of C2H2is also required and the probable mode of formation is via reaction 2. The photolysis of neat CF3C2Hin 700 torr of helium is a further indication that. at least in this system, butadiyne is not formed by the combination reaction (eq 3). This is somewhat surprising since the reaction is exothermic by 141 kcal/mol (Table I) and a few collisions would suffice to remove the energy required for redissociation into fragments. An alternative view is that the rate constant, for the addition of C2H to the parent CF3C2H,is siimilar to that for combination of ethynyl radicals. In the presence of a high concentration of CF3C2H,the chance of radical combination is minimized. The above interpretation is a t variance with that of Takita et a1.l who prefer the excited acetylene path to butadiyne formation, reaction 1,rather than the C2H path. The experiments of Takital involved comparatively high pressures of C2H2in the absence of inert gas. The role of +
+
312
The Journal of Physical Chemistry, Vol. 83, No. 3, 1979
W
=
1
>P
-
1
U
1
= m I-
I
1
0
0
0
10
20 30 40 HYDROGEN (torr)
Figure 1. Yield of butadiyne as function of added mtorr; total pressure = 50 torr (H2 4- He).
0
10
20
30
40
50
H,.
60
[C,H,]
10
20
30
40
50
60
TIME ips)
= 75
50
HYDROGEN ( t o r r )
Flgure 2. Plot of eq A (see text).
excited C2H2may contribute more than in the present work. The experiments with CF3C2H,however, clearly show that CzH and not C2H2*is the precursor responsible for butadiyne formation. Further, in the presence of Hz, no new product was observed; the net result was a decrease in butadiyne formation. There is no evidence for the formation of vinyl radicals. The decrease is shown in Figure 1. The curvature is expected if we assume the only paths for ethynyl reactions are ( 2 ) and (8). In the absence C2H + Hz CzHz + H (8) of H,, reaction 2 is the only operative channel. As H2 is added, reaction 8 assumes greater importance. The yield of C4H2 C2H2is a constant as the Hz pressure increases. If we let Co = the concentration of butadiyne in the absence of H2 and C = the concentration of butadiyne in the presence of Hz, then Co - C is equivalent to product acetylene formed by reaction 8. Writing the usual kinetic expression we arrive a t +
+
where [C2Hz]is reactant acetylene. A plot of (A) is shown in Figure 2. From the least-squares determined slope and intercept, the ratio of k8/kz = 4.9 X Further evidence that the production of butadiyne is first order in C2H was obtained by measuring the rate of
Figure 3. Yield of butadiyne with time: lines are calculated values of k,: (-) 3.5 X lo-” cm3 molecule-’ s-’: (- -) 4.0 X I O - ” cm3 molecule-’ si;(---) 3.0 X lo-” cm3 molecule-‘ s-l. [C2H2] = 75 mTorr. (+) two electrodes, 20 torr of helium; (V)six electrodes, 20 torr of helium; (0)three electrodes, 700 torr of helium: (@)two electrodes, 700 torr of helium; ( 0 )six electrodes, 20 torr of helium: ( 0 ) two electrodes, 20 torr of helium: (A)four electrodes, 20 torr of helium.
butadiyne formation as a function of time and flash intensity. The absorption spectrum of butadiyne in the vacuum ultraviolet has been previously examined.14 The strong band a t 144.6 nm provides enough absorption and is more than adequate for plate densitometry. Interference due to the C2H2band a t 143.9 nm is minimal. In our apparatus, we are able to observe less than 1mtorr of C4H2. The results are shown in Figure 3 in which the yield of butadiyne is plotted vs. time. Since the optimum source of CzH precursor for C4H2 formation was the photolysis of acetylene itself, the usual method of determining rates of formation for various reactant pressures was not appropriate, since any change in C2H2pressure also resulted in a change in the initial C2H Concentration. We therefore varied the ethynyl concentration by changing the number of capacitors in the charging circuit and hence the flash energy while keeping the acetylene pressure constant. If the reaction to produce butadiyne occurs under pseudo-first-order conditions, Le., reaction 2, the rate of butadiyne formation should increase linearly with the flash energy and the time required to build up to one-half of the final value (of butadiyne concentration) should be independent of the initial concentration of ethynyl. This is shown in Figure 3 where the maximum yield has been adjusted to a value of 3 mtorr. Although the flash energy differed by a factor of 3, there is no evidence for differing production rates as a function of the initial radical concentration (i.e., flash energy). Further, over the range of 20-700 torr of added helium the data indicate the absence of a pressure effect. The latter may indicate a second-order reaction such as (3) which is in the high-pressure limit but taken in conjunction with other observations, it offers support for reaction 2 as the operative channel. Values of the rate constant were fitted to the data as shown in Figure 3. The best fit appears to be 3.5 X cm3 molecule-I s-l. Calculated lines for both 3.0 X as well as 4.0 x cm3 molecule-I s-l are also indicated. Considering the concentration determination has an error associated with each data point of at least 10% as shown in a few cases, the fit of the data is quite good. The rate cm3 constant, kz, can be expressed as 3.5 f 1.0 X molecule-I s-l. The data may be treated in another manner. If C4H2 is formed solely by reaction 2 and the only fate of ethynyl radicals is to produce butadiyne, then [C2Hl = [C2HIoexp(-k2[C2H2lt)
(B)
The Journal of Physical Chemistry, Vol. 83, No. 3, 1979 313
Photochemistry of Acetylene 4
The rate constant for H atom addition to butadiyne, reaction 9, is in its high pressure region above 1torr and H + C4Hz C4H3* (9) has a value of 2 X cm3 molecule-l s-l.16 The model shows that a t all times up to 100 p s , the yield of C41& is barely affected by (9), but a t 250 IS, the yield has lbeen reduced by -89’0 from its maximum value. By 1 ms, the yield has been reduced by -25%. There is no effect if we add alternate channels for H atom removal, e.g., the combination of 2H to produce Hz. We observe a net reduction of C4Hzof approximately 30% which is smaller than the calculated reduction of about 50%. If we change the value of h9 to 1 X 10-l2 cm3 molecule-l s-l, the calculation is in better agreement with experiment but still suggests a larger decrease than observed. This may suggest secondary chemistry which can produce butadiyne but speculation is nlot warranted a t this time. The value of h2 suggested by Cullis et alasof about cm3 molecule-l sir1 a t room temperature is clearly in disagreement with ithe present work. We cannot account for the large discrepancy except that the methodology of Cullis requires the use of several rate constant ratios and estimations of certain activation energies which may produce significant errors. Note Added in Proof. Since the submission of this manuscript, the authors have become aware of another rate constant measurement of CzH radicals with H2 and CzH2 utilizing the flow discharge-mass spectroscopic technique.17 The values are iin excellent agreement with the present work. -+
3
1
0
10
20 Time
30
40
50
(ps)
Figure 4. Plot of e q E. Data points from Figure 3.
From a standard treatment of the suggested mechanism, it may be shown that
A plot of (B) is shown Figure 4. The slope determined by least squares, k,[CzH2],yields the preferred value of k2 = 3.1 4: 0.3 X cm3molecule-l s-l. Error bars for a few typical data points are indicated. Data at long times have been omitted since they represent small differences between similar numbers and are subject to large uncertainties. Efforts were made to obtain rate data information using 3,3,3-trifluoropropyne as the source of ethynyl radicals. As evidenlced in Table 11, however, butadiyne formation from acetylene itself could not easily be eliminated. A spectroscopic rate measurement, therefore, involveiv a determination of the time-dependent yield of C4Hz from the (CF3C,H)-(C2H2)system from which the time-dependent yield from CzHzalone has to be subtracted. This type of procedure may lead to severe inaccuracies and, in fact, the data were not good. At best, the results were not in disagreement, with the value determined from C2Hzalone. At long times in the millisecond region, it was noted that the butadiyne concentration decreased with respect to that a t 60-100 ps. This was observed spectroscopically; chromatographically, the yield of C4H2 was always smaller than that indicated spectroscopicallly a t 60 ps. The decrease was not dependent on pressure and was presumably due to chemical removal of the C4H2. The system was modeled assuming that H atoms, formed in the primary process and as the result of reaction 2, were the reactive species. H atoms are removed in this system by reaction with CzHz whose rate constant is pressure dependent.15
Acknowledgment. This research was supported, in part, by the P1anetar:y Atmospheres program of the National Aeronautics and Space Administration under Contract W-13, 454.
References and Notes (1) (a) S. Taka, Y. Mori, and I. Tanaka, J. Phys. Chem.,72,4360 (1968). (b) S. Tal&, Y. Mori, and I. Tanaka, J. Phys. Chem., 73, 2929 (1969). (2) J. N. Bradley arid G. 8. Kistiakowsky,J. Chem. Phys., 35, 264 (1961). (3) L. J. Stief, V. &I.DeCarlo, and R. J. Mataloni, J . Chem. Phys., 42, 3113 (1965). (4) A. L. Cochran F. J. Adrian, and V. A. Bowens, J . Chem. F’hys., 40, 213 (1964). (5) D. E. Milligan, \A. E. Jacox, and L. Abouaf-Marguln, J . Chem. F’hys., 46, 4562 (1967). (6) (a) W. R. M. Graham, K. I. Dismuke, and W. Weltner, Jr., J. Chem. Phys., 60, 3817 (1974); (b) D. P. Gilra, ibid., 63, 7163 (1975). (7) A. M. Tarr, 0. P. Strausz, and H. E. Gunning, Trans. Faraday8oc., 61. 1946 (1965). (8) C. F. Cullis: D. ,I.’Hucknell, and J. V. Shepard, Proc. R. SOC.London, Ser. A , 935, !525 (1973). (9) D. F. Howarth and A. G. Sherwood, Can. J. Chem., 51, 1655 (1973). (10) A. M. Bass and A. H. Laufer, Int. J . Chem. Kinef., 5, 1053 (1973). (11) J. B. Armitage, E. R. H. Jones, and M. C. Whiting, J . Chem. :Sot., 44 (1951). (12) L. A. Bashford, H. J. Emeleus, and H. U. A. Briscoe, J , Chem. Soc., 1358 (1938). (13) A. H. Laufer and A. M. Bass, J . Phys. Chem., 78, 1344 (1974). (14) W. C. Price and A. D.Walsh, Trans. Faraday Soc., 41, 381 (1945). (15) W. A. Payne and L. J. Stief, J . Chem. Phys., 64, 1150 (1976). (16) W. Schwanebalck and J. Warnetz, Ber. Bunsenges. Phys. Chem., 79, 530 (1975). (17) W. Lange and 14. Gg. Wagner, Ber. Bunsenges. Phys. Chem , 79, 165 (1975).
