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J. Phys. Chem. 1995,99, 9408-9415
Ultraviolet Photochemistry of Diacetylene: Metastable C4H2* and Nitrogen
+ C2H2 Reaction in Helium
Rex K. Frost, Gary S. Zavarin, and Timothy S . Zwier* Department of Chemistry, Purdue University, West Lafayette, Indiana 47907-1393 Received: December 14, 1994; In Final Form: March 28, 1995@
The collisional photochemistry of diacetylene triplet metastable state (C4H2*) with acetylene in nitrogen and helium buffers is studied in a reaction tube attached to a pulsed nozzle. The photolysis laser counterpropagates the molecular expansion through the reaction tube exciting the C4H2 'Au I&+ 2'06'0 transition at 231.4 nm. Efficient intersystem crossing forms the triplet state from which reaction occurs. The finite 8 mm length of the tube serves to quench the reaction after 10-30 ps so that primary products and not polymer are formed. Upon exiting the reaction tube, the photochemical products are soft ionized with 118 nm vacuumultraviolet light and mass analyzed in a linear time-of-flight mass spectrometer. The primary photochemical processes are C4H2* C2Hz C6H2 2H (H2) and C4H2* C2H2 C6H3 -I- H. Under all conditions examined, C6H2 dominates the observed products, with C6H3 present above the I3CCsH2 background only at high nitrogen concentrations which deactivate diacetylene within its metastable vibrational manifold more efficiently than does helium. C6DH is the major product in the C4H2* C2D2 reaction, indicating that both C4H2* and C2H2 contribute one hydrogen to the C6H2 product. CS& is observed as a new minor product of the reaction of excited diacetylene with C a 2 , but only at long reaction times. Formation of c8H4 is significantly enhanced by the presence of C2H2. A simple mechanism for the reaction of diacetylene and acetylene with metastable excited diacetylene is formulated. Assuming equal photoionization cross sections for the products, concentration data are used to determine that k(C4H~)/k(C2H2)= 11 f 2 in helium buffer and 7 f 1 in nitrogen buffer, where k(i) is the total rate constant for the reaction of C4H2* with species i.
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C4H2* C4H2
Diacetylene (C4H2), the first member of the family of polyacetylenes, shows unusual photochemical reactivity even when excitation occurs well below any dissociation thresholds for the molecule. Glicker and Okabe' have determined a photochemical quantum yield of C#J = 2 for the disappearance of diacetylene excited throughout the wavelength range 147254 nm. Reaction is known to occur primarily out of a metastable excited state of diacetylene (C&*), either the 32,,uf or 3A, state, whose origins are at 2.7 and 3.2 eV above the ground state.2 Free radical formation is not observed over the range from 254 to 184 nm, with an upper limit on the quantum yield for C4H formation of 0.06 at 228 nm. At the same time, the metastable state(s) of C4H2 are highly resistant to electronic quenching by rare gases, nitrogen, and hydrogen, with a photochemical quantum yield of 2 for C a 2 * reaction even under conditions in which C4H2* suffers thousands of collisions with nonreactive buffer gases.' The spectra provided by the Voyager missions have enabled the unambiguous identification of diacetylene in the atmosphere of Titan, a moon of Titan is noted for being the only body in our solar system besides earth which has a predominantly nitrogen atmosphere. Due to its high photochemical reactivity in the ultraviolet, C4H2 is a natural precursor to yet larger hydrocarbons in Titan's atmosphere4and probably those of U r a n ~ s , ~ - ~ Pluto,Io and Triton" as well. In our recent study,'2,'3three primary reaction pathways for C4H2* C4H2 were identified:
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* To whom correspondence @
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I. Introduction
should be addressed. Abstract published in Advance ACS Abstracts, May 15, 1995.
0022-365419512099-9408$09.00/0
C,H,
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C6H2 C2H2 (IC) Thus, these studies gave a first indication that C4H2* reacts efficiently to form larger hydrocarbon products, serving as a potential route to such species in planetary atmospheres and in flames. The lack of a dependence of the product distribution on the vibronic level excited (varying from 118 to 128 kcaY mol energy) provided further evidence that C4H formation was insignificant under the conditions of our experiment. In the present study, the reactivity of C4H2* toward C2H2 is examined in nitrogen and helium buffers. The method used in our previous C&* C4H2 workI3 is implemented in a new chamber which incorporates differential pumping. C4H2* C2H2 chemistry has been chosen for study because acetylene concentrations are about 10- 100 times that of diacetylene in Titan's atmo~phere,'~ so that the reaction may be another significant route to more complex hydrocarbons. This reaction also provides an important comparison with acetylene's photochemistry. The studies of Seki et al.I5 and Seki and OkabeI6 have found that C2H2 reacts to form C4H2 following 193 nm excitation via two pathways: (i) a prompt route involving the C2H radical (C2H C2H2 C4H2 H) and (ii) a metastable reaction ascribed to a triplet state species (CzHz* C2H2 C4H4+ C4H2 H2). Thus, both C4H2 and C2H2 are known to react out of metastable triplet states. The possibility exists, then, for the cross reactions C4H2* C2H2 and C2H2* C4H2 also to occur. In this paper we examine the former reaction. Figure 1 depicts an energy level diagram of the reaction paths of both C4Hz* C2H2 and C4H2* C4H2 chemistry based on
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J. Phys. Chem., Vol. 99, No. 23, 1995 9409
Ultraviolet Photochemistry of Diacetylene
ION SOURCE CHAMBER
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TOF CHAMBER
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4 Products
Reactants
Products
Figure 1. Schematic energy level diagram of the ultraviolet photochemistry of a diacetylene/acetylene mixture. The 3Auand 3&+ states are metastable excited states of C4H2. Reaction takes place from a distribution of energies determined by the extent of collisional deactivation of C4H2* toward the metastable state origins. Heats of reaction are estimated using heats of formation for CnH2and CnHafrom ref 23. The 2 1 0 6photon 1 ~ energy is 123.5 kcaUmo1. The and 3Z,,+ state energies (74 and 63 kcal/mol, respectively) are taken from ref 2. thermochemical estimates. Not surprisingly, the energetics of the C4H2* C2H2 reactions are estimated to be quite similar to their C4H2* C4H2 counterparts. As we will see, the dominant reaction product observed from C4H2* C2H2 is C&, with c6H3 appearing as a minor product only under strongly vibrationally deactivating conditions. The major reaction pathways are thus
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By necessity the C4H2* C2H2 reactions 2a and 2b compete with C4H2* C4H2 reactions la-lc, with C6H2 a product of both reactions. Besides the identification of the primary products, isotopic studies are used to test the mechanism for the reaction. A method for estimating the rate constant for the C4H2* C2H2 reaction relative to that for C4H2* C4H2 is also introduced.
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H. Experimental Section Figure 2 is a schematic diagram of the new chamber used in
this study. The instrument consists of two differentially pumped vacuum chambers: the ion source chamber and the time-offlight (TOF) tube. The two chambers are connected by a 9 mm circular orifice through which ions are extracted from the ion source chamber into the TOF tube. A piezoelectric pulsed valve (PV) is used to inject the reaction mixture into the ion source chamber. The pulsed valve points directly into the source chamber diffusion pump, increasing the pumping efficiency of the system. The higher flow rates allowed by this geometry increase the attainable signal levels compared to our previous work13 and enable a larger range of flow conditions and reaction tube pressures to be examined. Diacetylene is excited to various vibronic levels of the 'Austate using the doubled output of an excimer-pumped dye laser (-0.5 &/pulse doubled output). This photoexcitation laser (PL) counterpropagates the molecular expansion through a ceramic reaction tube attached to the end of the pulsed valve. The PL beam is directed into the tube by a turning prism located inside the ion source vacuum chamber.
J
6" Diffusion Pump
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Figure 2. Schematic of the new instrument used to study the photochemistry of diacetylene. An expanded view of the reaction tube is included as an insert in the upper right-hand comer. The SI-UV prism shown in the schematic is inside the vacuum chamber and directs the photoexcitation laser upward, counterpropagating the molecular expansion.
The metastable C4H2* produced by the PL reacts with C4H2 and C2H2 during its traversal of the 2 mm i.d. x 8 mm long reaction tube. The tube is long enough to provide sufficient collisions to produce detectable photochemical products, yet short enough to quench the reaction after only 10-20 ,us so that primary products and not polymer are observed. Typical operating conditions employ a total gas throughput of 2 standard cm3/min,a pulsed valve pulse width of 300 ,us, and a repetition rate of 20 Hz. The estimated total pressure in the reaction tube during the gas pulse is about 2 Torr. A ground plate with a 2 cm circular hole located between the ion acceleration plates and the reaction tube is necessary to avoid arcing from the reaction tube to the ion source plates at the shortest nozzle-to-ion source distances. Upon exiting the reaction tube, the photochemical products are soft ionized with vacuum-ultraviolet ( V W ) light 8 cm downstream between the first stage ion source acceleration plates, approximately 80 ,us after photoexcitation. The xenon gas cell used for tripling and the details of the ionization scheme have been described previously.13 The ion acceleration plates extract the ions into the TOF chamber perpendicular to the molecular expansion. The two-stage Wiley-McLaren ion acceleration region has acceleration plate spacings of 2.54 cm (first stage) and 1.27 cm (second stage). This open design reduces interference of the plates with the gas pulse. The ions are mass analyzed in a linear time-of-flight mass spectrometer and detected using a microchannel plate (MCP) ion detector. An einzel lens is used to focus the ion beam onto the MCP. A 1.0 ,us long, 800 V pulse is applied to a plate in the flight tube to pulse away most of the C4H2+ ions before they reach the MCP detector. This signal, roughly- 200 times larger than photoproduct signal, would saturate the MCP detector, interfering with the detection of the small product ion signals arriving at later times. Several types of spectra are reported in this study. Difference mass spectra highlight photochemical products by subtracting background mass spectra from mass spectra obtained with the photoexcitation laser present. Action spectra are recorded by monitoring photoproduct ion intensities as a function of photoexcitation wavelength. The delays between the pulsed valve and the photoexcitation and ionization lasers are set to maximize product signal and are held constant throughout action spectra. Reaction time scans of photoproducts are recorded by maintain-
9410 J. Phys. Chem., Vol. 99, No. 23, 1995
Frost et al.
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Figure 4. Difference time-of-flight mass spectra highlighting the C4H2* C4H2 photoproducts of a 5% C4H2 in helium mixture (Le., without C2H2 present). A comparison with Figure 3c demonstrates that the C6H2 product intensity increases dramatically relative to C8H2 and C8H3 in mixtures containing acetylene.
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c) Difference Mass Spectrum
Two differences between the photoproduct mass spectra in Figures 3c and 4 are observed. First, the C6H2 peak is much more intense relative to C8H2 and CsH3 when acetylene is present, with its relative size depending on acetylene concentration. Thus, C6H2 is the dominant product of C4H2* C2H2 chemistry. Second, mass 100 (C&) is enhanced significantly by the presence of C2H2. The source of this product will be discussed further in section C. Imperfect subtraction of reactant and impurity ion signals in the difference mass spectra leave negative peaks, most notably in the 2-chloro-1-buten-3-yne mass channels (masses 86 and 88). Fortunately, this impurity loss is not involved in any way in the observed diacetylene photochemistry. The masses and relative intensities of the photochemical product signals are unaffected by the amount of 86/88 impurity over impurity signal levels ranging from 10 times larger to 5 times smaller than the observed products. In addition, collisions of C4H2* with impurity molecules are 50- 100 times less frequent than those with C4H2. Furthermore, the action spectra of masses 86 and 88 are not functions of excitation wavelength within our signalto-noise. The reason for the decrease in impurity ion signal with PL present is unclear, though several possibilities can be put forward. The most likely of these invokes changes in transport efficiency of the gas mixture to the ion source under heating by the PL. Laser-produced electron scattering or photodissociation of the 2-chloro- 1-buten-3-yne are also possibilities. B. Action Spectra. Figure 5a is the R2PI spectrum of the reactant C4H2, while Figure 5, b and c, are the action spectra of the C& and c8H4 photoproducts, respectively. Action spectra are produced by tuning the PL through the 2 1 ~ 6transition 1~ of C4H2 at 231.4 nm while monitoring product ion signals. The reproduction of the diacetylene absorption spectrum by the photoproducts confirms that formation of the C6H2 and CsH4 products follows exclusively from absorption of an ultraviolet photon by gas-phase C4H2. In the following isotope study, the C6H2 ion intensity is shown to arise from both C4H2* f C4H2 and C4H2* C2H2 chemistry. However, the spectrum shown in Figure 5b was taken with [C~H~]/[C~HZ] = 8 so that interference from C4H2* C4H2 chemistry is minimized. C. Isotope Study. Figure 6 is a difference mass spectrum of the photoproducts formed in a mixture of 4% diacetylene, 37% C2D2, and 59% helium. The dominant mass 75 product is ascribable almost entirely to C6DH from the C4H2* f C2D2 reaction, since no interfering C6H3 products are produced in the C4H2* C4H2 reaction (Figure 3c), and the C6H2 contamination from I3CC~H2(6%) is negligible under the conditions of Figure 6. The C4H2* C2D2 reaction therefore proceeds
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Figure 3. Time-of-flightmass spectra (a) with the photoexcitation laser on and (b) with the photoexcitation laser off, showing the impurities present in the gaseous mixture, and (c) the difference mass spectrum highlighting the excited C4H2 photoproducts in a gaseous mixture of 3% diacetylene, 29% acetylene, and 68% helium. The C4H2+ peak (not shown) is 200 times the size of the product peaks. ing a constant delay between the pulsed valve opening and the ionization laser while varying the delay between the photoexcitation laser and the VUV laser. The synthesis and handling of diacetylene have been described previou~ly.'~ Variable concentration reactant mixtures are prepared in a gas handling system by combining metered flows of 2%-6% C4H2/He (or C4H2/N2) and C2H2 (or CzD2). C2H2 (99.6% pure, Airco), C2D2 (99.6 atom % pure, C/D/N Isotopes), N2 (99.995%, Airco), and helium (99.995%, Airco) were used as purchased. 111. Results and Analysis A. Difference Mass Spectra. Figure 3a is the time-of-flight (TOF) mass spectrum obtained by tuning the PL to the 2l06'0 'A, I&+ transition of C4H2 at 231.4 nm with a gaseous mixture of 3% C4H2, 68% He, and 29% C2H2. At 231.4 nm, absorption by C4H2 is predominant even though the C2H2 concentration is almost 10 times that of C4H2. Figure 3b is the corresponding TOF mass spectrum obtained without the PL present. The difference mass spectrum (Figure 3c), obtained by subtracting the PL-off mass spectrum from the PL-on mass spectrum, clearly shows the photochemical products resulting from laser excitation of C4H2. For comparison, Figure 4 presents the difference mass spectrum of a mixture of 5% diacetylene in helium without C2H2 present. In Figures 3c and 4, the peaks at masses 74, 98, and 99 correspond to photoproducts C6H2, C&, and C&. The mass 100 peak in Figure 3c corresponds to CSH4. Masses 86 and 88, present here at 3% of the yield of C4H2, are 2-chloro-1-buten-3-yne side products of the synthesis of C4H2. The mass 86 peak is set at an arbitrary value of 100 in Figure 3. The mass 78 impurity arises from residual benzene in the gas handling system from previous experiments.
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J. Phys. Chem., Vol. 99, No. 23, 1995 9411
Ultraviolet Photochemistry of Diacetylene
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Figure 7. Reaction time profiles of the C6H2 (a) and c8H4 (b) products , C2H2, and 63% helium mixture. In these scans, the of a 4% C H ~33% delay between the pulsed valve and the ionization laser is held constant while the delay between the photoexcitation and ionization lasers is varied.
attained, i.e. 43,200 43 300 43,460 i3xcitation Freciuency (cm")
Figure 5. Resonant two-photon ionization spectrum of C4H2 (a) compared to the action spectra of the photochemical products C6H2 (b) and cgH4 (c) in the 2I06Io region of C4H2. Action spectra are taken by tuning the photoexcitation laser while photoproduct ion signal produced by VUV ionization is monitored. In (b), C ~ H ~ / C ~=H8Zso that contamination of CsH2 ion intensity from C4H2* f C4H2 chemistry is minimized. The presented ion intensities are offset and are not on the same scale.
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Figure 6. Difference time-of-flight mass spectra of photoproducts of
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almost exclusively to form C6DH, i.e.
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C6DH 4- H 4- D (HD)
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The C a 2 , C8H2, and C8H3 products arise from C4H2* C4H2 reactions (eqs la-lc) and are present in the expected relative abundances based on Figure 4. A comparison of Figure 3c taken with C2H2, with Figure 6 shows no isotopic shift in the mass 100 peak in the presence of C2D2. We conclude on this basis that the mass 100 peak in both spectra is c & $ , resulting from C4H2* C4H2 reaction, and does not directly involve C2H2 or C2D2. The photoproduct c8H4 is observable as a very minor product in the absence of acetylene but grows significantly in the presence of acetylene. The effect of acetylene is probably to vibrational deactivate C4H2* to an extent not previously
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Here the is used to distinguish the CsH4-producing reactant from that present in the absence of C2H2 (denoted by *). Under these highly deactivating conditions, the C 8 h reaction complex must be long-lived enough to be stabilized by collisions prior to dissociation back to reactants. We will consider the nature of C4H2' further in the discussion section. D. Reaction Time Study. The reaction time profiles of C6H2 and c8H4 in a mixture of 4% C4H2, 33% C2H2, and 63% He are presented in Figure 7. Reaction time scans are produced by monitoring product ion signal while varying the delay between the firing of the photoexcitation laser (PL) and the VW laser at constant VUV-pulsed valve delay. Qualitatively, increasing (VW-PL) at constant (VUV-PV) is equivalent to increasing the average reaction time of the C4H2* by changing the average position in the reaction tube of the C&* whose products are being monitored. At the high [C2H2]/[C4H2] concentration ratio of Figure 7, product contamination of the C6H2 profile from C4H2* C4H2 chemistry is small. Unfortunately, the high C2H2 concentration also produces poorer signal-to-noise reaction time scans. Nevertheless, the C6H2 profile is seen to have the same shape as that of C8H2 and C8H3 (not shown) which are primary products of the C4H2* C4H2 chemistry. The width of the product signals confirms that the products are formed in reaction times of 10-20 ps, consistent with the expected flow velocity down the reaction tube (-1 x lo5 c d s ) . By contrast, a clear delay in the onset of the CgH4 product relative to the other products is observed. Notably, the C6H2: C&C& intensity distribution does not change as the c8H4 ion intensity grows in at longer reaction times, suggesting that C8H4 formation is decoupled from these reaction channels. The relative shift in the CSH4 onset may correspond to an induction period for its formation, as could occur for a secondary product produced via reactions 4a and 4b. However, it should also be noted that the C10H3 secondary product from C4H2* C4H2 is not shifted relative to the primary products, so an induction
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9412 J. Phys. Chem., Vol. 99,No. 23, 1995
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Figure 8. Integrated C6H2. CsH2, and C8H3 ion signal, expressed as percent of total ion intensity, as a function of photoexcitation laser power. C2H&H2 = 7 so that contamination of c6H2 ion intensity from C4H2* + C4H2 chemistry is minimized.
period in the reaction time scan cannot be taken as a foolproof signature for a secondary product. E. Photoexcitation Laser Power Study. Figure 8 presents a study of the dependence of the relative product yields on photoexcitation laser power. Both C4H2* C2H2 and C4H2* C4H2 reactions contribute to the C6H2 intensity, but again, the latter reaction contributes only 25% of the intensity and is linear in laser powerI3 under the conditions of Figure 8. Due to instabilities in the V W source, absolute PL power studies are difficult to conduct over extended time periods. Instead, Figure 8 plots relative product yields as a function of PL power for a loosely focused beam (using a 50 cm focal length lens). Changes in the product distribution with laser power are small and can be readily extrapolated back to zero laser power. The highest laser powers slightly skew the distribution away from C8H3 toward C a 2 . This dependence on laser power is probably due to laser heating of the mixture, but multiphoton effects may also contribute weakly. As a result, we have restricted our studies to the lower photoexcitation laser energies ((0.5 mJ/ pulse). F. Reaction in the Presence of N2. The effect of N2 on C4H2* CzH2 chemistry has also been studied. N2 is known from the work of Glicker and Okabe' to be a very inefficient electronic quencher of metastable C4H2*. However, by comparison to helium, nitrogen vibrationally deactivates metastable C4H2* much more efficiently, thereby shifting the internal energy distribution of the reactant C4H2* to lower energies nearer the origin of the metastable state(s) (Figure 1). In the prior studyI3 of C4H2* C4H2, this vibrational deactivation manifested itself in a transfer of product intensity from C8H2 to C8H3 products. Figure 9 presents a difference mass spectrum of the photoproducts formed in a mixture of 1% C4H2, 3% C2H2,81% N2, and 15% He. Comparing Figure 9 to Figure 3 demonstrates that nitrogen has the expected effect on the ratio of C8H3 to C8H2 even with C2H2 present in the mixture. At the same time, the vibrational deactivation facilitated by N2 produces the first observable intensity in the C6H3 product. This C6H3 intensity must result from C4H2* C2H2 chemistry because the C6H3 product is not produced in the C4H2* C4H2 reaction under any conditions.I3 Furthermore, the C6H3 signal is too large to be ascribed entirely to a I3C isotope of C6H2. which should be present at 6% of the C6H2 intensity. Note that even under these high N2 conditions, the C&/C6H2 product ratio formed in the C4H2* C2H2 reaction is about 10 times smaller than the C&/ C8H2 ratio formed in C4H2* C4H2. Thus, N2 has the same qualitative effect on C4H2* CzH2 chemistry as it does on C4H2* C4H2; namely, it enhances the
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Figure 9. Difference time-of-flight mass spectra of photoproducts of excited diacetylene in a mixture containing a high concentration of N2 (1% C&, 3% C2H2, 81% Nz, and 15% helium). Note the increased C B H and ~ C6H3 intensities by comparison to Figure 3.
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Figure 10. Plot of the ion intensity ratio I(C&)/(I(CsHz) I(C8H3)) versus the concentration ratio [C~H~]/[C~HZ] in photoexcited, C4H2/ C2Hzreaction mixtures employing helium ( 0 )and nitrogen (W) buffers.
C,H3 product channel relative to C,H2. This may result from a suppression of the C,H2 2H product channel as C4H2* is vibrationally deactivated below the threshold for C,H2 2H formation, while the more exothermic C,H3 H channel remains open (Figure 1). Alternatively, internally excited C,H3 products may be collisionally stabilized by N2 in competition with decay into C,H2 H. It is worth noting that, under the low C2H2/high N2 concentrations of Figure 9, the c&$product is quite small. It thus appears that the C4H2+ produced in deactivation by C2H2 (reaction 4)is different than the vibrationally deactivated CJ+* produced by N2. G . Concentration Study. In Figure 10, the integrated intensity of C&, product signals relative to that for cg products, Z(C6H2)/[Z(CsH2) I(C&)], is plotted as a function of the acetylene to diacetylene concentration ratio, [C2H21/[C4H21.Both helium and nitrogen buffers are examined. In the He buffer study a mixture of 3% C4H2 in He is mixed with pure C2H2, while in the N2 study, a mixture of 3% C4H2 in N2 is used. As seen from the figure, over a wide range of concentrations the intensity ratio varies linearly with [C2H2]/[C4H2] in both He and N2. In the discussion section, these data are used within a simple reaction scheme to estimate the ratio of rate constants for C4H2* C2H2 relative to C4H2* C4H2.
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IV. Discussion A. The Nature of the Primary Products. The major qualitative conclusion of the present study is that the single, dominant product of the C4H2* C2H2 reaction under all
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Ultraviolet Photochemistry of Diacetylene
J. Phys. Chem., Vol. 99, No. 23, 1995 9413
conditions explored is C6H2 (+2H, H2). Furthermore, the dominance of the C6HD product in the C4H2* C2D2 reaction indicates that C4H2 and C2H2 each contribute one hydrogen to C6H2. Our results do not provide spectroscopic proof that the C6H2 (C6HD) product is triacetylene; however, this product is clearly energetically favored over a diradical or carbene product in which one or more hydrogens are located along the interior of the carbon chain. The exclusive formation of the c6HD product further points to significant barriers to WD exchange within the C&D2 reaction complex. This is consistent with the large barriers to H atom migration (-50 kcdmol) calculated for triplet alkyl radicals by Harding.22 Dominance of the c6HD product also indicates that cyclization is unlikely. As a result, a mechanism analogous to that proposedI3 for the C8H2 product from C4H2* f C4H2 seems quite likely. As shown in Figure 11, this mechanism views metastable C4H2* as a cumulene diradical, as calculated by Karpfen and L i ~ c h k a , 'in~ which attack on C2H2 occurs from one of the radical centers. Loss of H and D, either as free atoms or as HD, from the interior of the C6H2D2 diradical intermediate would then produce the C6HD product observed. A surprising feature of the present results is that, under all conditions explored, the C6H3 product in the C4H2* C2Hz reaction is at best a minor product, while the analogous C8H3 product in the C4H2* C4H2 reaction can dominate at high N2 concentrations. One possible reason for the different efficiencies of the CsH2/C&3 and C8H2/&H3 counterparts would be differences in the energetic asymptotes for C8H2 2H formation from C4H2* f C4H2 relative to C&2 f 2H from C4H2* C2H2. However, the heats of formation for C6H2 and C8H2 reported recently by Kiefer et al.23would suggest that this effect would not be large. Alternatively, the larger c8 backbone product may be longerlived relative to cteavage of a second C-H bond, allowing C8H3 to be collisionally stabilized when C6H3 is not. The competition between the C&2/C&I3 and CsH&H3 counterparts is addressed in further detail in the next section in relation to the proposed mechanism. B. Relative Rate Constant for C4H2* CZHZ.The results of section I11 have highlighted the interplay between the C4H2* C4H2 and C4H2* C2H2 chemistry occurring in the reaction tube. In seeking experimental conditions under which the primary C4H2* reaction products can be observed, full thermal equilibration of the C4H2* is not guaranteed. As a result, vibrational deactivation of C4H2* occurs in parallel with reaction, and the competition and distribution of products may be affected by the internal energy of the C4H2*; Le., the reaction rate constants are functions of C4H2* internal energy. Despite this, the range of observations allows an estimate of the rate constant of the C4H2* C2H2 reaction relative to that for C4H2* C4H2. In seeking such an estimate, we propose a simple reaction scheme which accounts for three key observations: (i) The quantity Z(C~H~)/[Z(CBH~) I(C&)] is found to vary linearly with [C2H2]/[C4H2] in both He and N2 buffers. The slopes of the functions are different in the two buffers. (ii) c!3&is observed only at long reaction times, indicating an induction period for its formation. The distribution of products between C6H2, C6H3. C8H2, and C8H3 is stable as c8H4 grows in at longer reaction times, suggesting that c8H4 is formed from a different reaction pathway than the primary products. (iii) C8& formation is enhanced dramatically by the presence of C2H2, even though c& is formed solely from the reaction of two diacetylene molecules. We propose a simple mechanism for formation of C a 2 , Ca3, C8H2, and C8H3 products. Symbols used for the rate constants
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