J . Phys. Chem. 1992,96, 5875-5880
580 nm. Because the 9-10-nm bandpass of the 580-nm interference filter is not negligible in comparison with the width of the 580-nm absorption band, this is an "effective" value of eh,-. By use of a value of 5 X lo4 M-' cm-1 for C ~ . - , ~ O a value of 0.18 is obtained for Gk.-. This can be compared with G(free ions) in cyclohexane ~ 0 . 1 5 . 2A~ value of Gh.- greater than G(free ions) is expected because on a time scale of a few hundred nanoseconds the contribution of C02'-(geminate) Pe Pee-(geminate) + C02should be significant. Combmed with the uncertainty in h-, this preclude using the value of GPc.- derived here as an accurate measure of G(free ions).
+
-
COrtC~USiOM
The experiments done here have shown that CO2*-transfers an electron to an aromatic system. In other words, it lengthens the lifetime of the electron that is formed in hydrocarbon radiolysis and can transfer it to electron-affinic species. Tberefore, C02 would not be an appropriate electron-scavenging species in hydrocarbon radiolysis if its purpose were, for example, to prevent production of radical anions of an aromatic solute. However, if the aim were to slow down the geminate recombination while making the electron available for transfer to an aromatic molecule, C02 would be appropriate as an electron scavenger. Other information regarding C02as an electron scavenger is that reaction of C02'- with an aromatic radical cation has been observed to result in formation of the fluorescent state of the aromatic.24 The C02* ion is readily produced by pulse radiolysis, and the technique reported here can be used to make similar measurements for other solvents and solutes which exhibit a measurable radical anion absorption. A detailed study of this sort to determine the effects of electron affinity and solvent polarity on the transfer rate would be of interest.
Acknowledgment. We thank Alexander Trifunac and David Werst for discussions. The experimental assistance of Nong L m g , Piotr Piotrowiak, and John Miller is gratefully acknowledged. None of these experiments would have been possible without the assistance of Don Ficht and George Cox, whose skill in running the accelerator made the experiments possible.
5875
Registry No. Perylene, 198-55-0 carbon dioxide, 124-38-9; perylene radical anion, 34505-65-2; carbon dioxide radical anion, 14485-07-5.
References and Notes (1) Work performed under the auspices of Basic Energy Sciences, Division of Chemical Science, US-DOE, under Contract no. W-31-109-ENG-38. (2) Holroyd, R. A.; Gangwer, T. E.; Allen, A. 0. Chem. Phys. Lett. 1975, 31, 520. (3) Nishikawa, M.; Itoh, K.; Holroyd, R. A. J. Phys. Chem. 1988, 92,
5262.
(4) Baxendale, J. H.; Keene, J. P.; Rasburn, E. J. J . Chem. Soc., Faraday Trans. I 1974, 70, 718. ( 5 ) Dainton, F. S.;ONeill, P.; Salmon, G. A. J . Chem. SOC.,Chem. Commun. 1972, 1001. (6) Sowada, U.; Holroyd, R. A. J . Chem. Phys. 1979,70, 3586. (7) Frank, A. J.; Gritzel, M.; Henglein, A.; Janata, E. Eer. Bunsen-Ges. Phvs. Chem. 1980.80. 294. 18) D'Angelantonio, M.; Mulazzani, Q. G.; Venturi, M.; Ciano, M.; Hoffman, M. 2.J . Phys. Chem. 1991, 95, 5121. (9) Venturi, M.; Mulazzani, Q. G.; D'Angelantonio, M.; Ciano, M.; Hoffman, M. 2. Radiat. Phys. Chem. 1991, 37, 449. (10) Neshvad, G.; Hoffman, M. Z.; Mulazzani, Q.G.; Venturi, M.; Ciano, M.; D'Angelantonio, M. J. Phys. Chem. 1989, 93, 6080. (11) Neshvad, G.; Hoffman, M. 2.J . Phys. Chem. 1989,93,2445. (12) Whitburn, K. D.; Hoffman, M. Z.; Brezniak, N. V.; Simic, M. G. Inorg. Chem. 1989, 25, 3037. (13) Farver, 0.;Pecht. I. Inorg. Chem. 1990, 29,4855. (14) Naik, D. B.; Moorthy, P. N. J . Chem. Soc., Perkin Trans. 2 1990, 705. (15) Jackman, M. P.; Lim, M. C.; Sykes, A. G.; Salmon, G. A. J . Chem. Soc., Dalton Trans. 1988, 2843. (16) Isied, S.S.;Kuehn, C.; Worosila, G. J . Am. Chem. Soc. 1984,106,
1722.
(17) Michael, B. D.; Hart, E. J.; Schmidt, K. H. J. Phys. Chem. 1971,75,
2798.
(18) Jonah, C. D.; Hart, E. J.; Matheson, M. S.J. Phys. Chem. 1973, 77,
1838.
(19) Hart, E. J.; Anbar, M. The Hydrated Electron; Interscience: New York, 1970; Chapter IX. (20) uv Atlas of Organic Compounds; Verlag Chemie: Weinheim, and
Butterworths: London; 1968; Voi. 4. (21) Shida, T. Electronic Absorption Spectra of Radical Ions; Physical
Sciences Data 34; Elsevier: New York, 1988. (22) Sowada, U.; Holroyd, R. A. J . Phys. Chem. 1981,85, 541. (23) Allen, A. 0. Yields of Free Ions Formed in Liquids by Radiation; NSRDS-NBS 57; US. Department of Commerce, National Bureau of Standards: Washington, DC, 1976. (24) Sauer, M. C., Jr.; Jonah, C. D. Unpublished results.
Kinetics of the Reactlons of C2H wlth C2H2,H,, and D, M h o Koshi,* NobuhIro Nishida, and Hiroyuki Matsui Department of Reaction Chemistry, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan (Received: January 24, 1992; In Final Form: March 27, 1992)
-.
-
-.
Kinetics of the reactions C2H + C2H2 C4H2 + H (l), C2H + H2 C2H2 + H (2), and C2H + D2 C2HD + D (3) have been studied at T = 293 K with two time-resolved diagnostics, laser-induced fluorescence (LIF) detection of H (or D) atoms at the Lyman-a wavelength and mass spectrometric detection of C4H2 The C2H radical was prepared by the ArF (193 nm) laser photolysis of C2H2. Measurements of the yield of H (or D) atom indicated that the C2H radicals produced by the photolysis were converted effectively to H (or D) atoms; Le., formation of the stable intermediates such as C4H3, C2H3, or C2HD2was negligible under the present experimental conditions (p = 30 Torr). Rate constants of kl = (1.6 f 0.1) X 1O-Io, k2 = (7.1 f 1.1) X lo-", and k3 = (2.0 f 0.3) X lo-*'cm3molecule-' s-' were derived from the pseudo-first-order rise rates of the H (or D) atom. Rate constant of kl = (1.4 f 0.3) X cm3 molecule-' s-I was also obtained at p = 5 Torr by measuring the appearance rate of C4H2. The ratios of the rate constants k 2 / k l = (3.4 f 0.2) X lo-' and k 3 / k , = (1.4 f 0.5) X lo-' were deduced by measuring the dependence of the C4H2production on the partial pressure of added H2or D2. Transition state theory calculations on the basis of ab initio transition-state properties demonstrated the importance of the tunnel effect for reaction 2 at room temperature.
1. Introduction
The reactions of the ethynyl radical, C2H,are of considerable interest because of their importance in several chemical systems. The C2H radical is known to be an important precursor to soot formation in the pyrolysis of C2H2.1 It was also detected in interstellar spaces2 0022-3654/92/2096-S815$03.00/0
The rate constants for the C2H reactions with C2H2and H2 C2H + C2H2 -w C4H2 + H (1) C2H + H2 C2H2 + H (2) have previously been measured at room temperature by several groups. However, there exist serious discrepancies among the
-
0 1992 American Chemical Society
5876 The Journal of Physical Chemistry, Vol. 96, No. 14, 1992
reported values of kl and k2. Laufer and Bass3 determined the rate constant of reaction 1 by observing the appearance rate of C4H2 in the flash photolysis of C2H2. The rate constant of reaction 2 was deduced by measuring the dependence of the rate of C4H2 production on the partial pressure of added H2. Renlund et measured the rate of reaction 2 by monitoring time-resolved chemiluminescence from CH(A2A). Their rate constant is about 2 orders of magnitude greater than that of Laufer and Bass. Renlund et al. suggested that their rate constant was largely affected by the reactions of vibrationally or electronically excited radicalsq6 Lange and Wagner' detected the C2H radical by mass spectrometry in a fast discharge-flow system. Their rate constants of reactions 1 and 2 are slightly higher than those of Laufer and Bass, but the authors stated that their rate constants were the lower limits because of incomplete mixing of reactant gases in the observation zone. In the work of Stephens et a1.,8 the decay rates of C2H were measured by using color center laser absorption spectroscopy. Their values of kl and k2 are about 5 and 3 times larger than those of Laufer and Bass. They claimed that the rate of appearance of product was not necessary to be as fast as the rate of disappearance of reactants because the overall reaction could involve a long-lived intermediate (such as C4H2 in a triplet excited state or C4H3radical in reaction 1. However, this suggestion may be questionable because the value of k , derived by Stephens et al. (from the decay rate of C2H) is in good agreement with the most recent value of the Shin and Michael9 who monitored the production rates of H atoms detected by resonant absorption at the Lyman-a wavelength. To establish the rate constants of these reactions, measurements on the different species are desirable so that the validity of the measurements could be cross-checked. In the present work, the rate constants of reactions 1 and 2 were determined by monitoring appearance rates of products in two separate experiments. In the first experiment, the rise rates of H atoms were measured by using the laser photolysis Lyman-a laser induced fluorescence (LP/ WV-LIF) technique. In the second experiment, the C4H2 radical was detected by using laser photolysis time-resolved mass spectrometry (LP/MS). Harding et al.l0 have performed TST (transition-state-theory) calculations of reaction 2 on the basis of the ab initio molecular orbital calculations of the transition state. Their calculations predicted the linear transition state and the importance of tunnel effect at room temperature. In order to investigate the isotope effects on reaction 2, the rate constant for the reaction C2H
+ D2
-
C2HD
+D
(3)
was also determined in the present experiments.
2. Experimental Section Details of the apparatus used in the LP/VUV-LIF experiment were described elsewhere.12 The C2Hradical was generated by ArF laser photolysis of C2H2with a typical laser fluence of 8 mJ/cm2. Sample gas mixtures (1-10 mTorr C2H2and 0-3.2 Torr H2or D, diluted in He) were slowly flowed in a Pyrex reaction cell of 1.5 cm inner diameter. Unfocused output of an ArF laser (Questek 2220) was directed along the axis of the reaction cell. The He buffer gas (30 Torr) ensured the complete thermalization of the hot H atom produced by the 193-nm photolysis of C2H,. The rates of H- or D-atom production by reactions 1-3 were measured by monitoring laser-induced fluorescence at the Lyman-a wavelength (121.567 nm for H atoms and 121.533 nm for D atoms). Tunable VUV radiation around 121.6 nm was generated by frequency tripling in Kr (60 Torr). The output of an UV dye laser (Lambda Physik LPD3002, 6-10 mJ/pulse at the wavelength around 364.6 nm with DMQ dye) was focused by a quartz lens off = 7 cm into a tripling cell that was attached to the reaction cell through a LiF window. The Lyman-a resonant fluorescence was detected by a solar-blind photomultiplier tube (Hamamatsu Photonics R972) at right angles to both the photolysis laser and the probe VUV radiation. The VUV-LIF intensity was normalized by the intensity of the VUV radiation
Koshi et al. monitored behind the reaction cell by using an ionization cell filled with 1% NO in H e at 30 Torr. The apparatus used in the LP/MS experiments was identical to that used in the kinetic studies of SiH3 radical.I3J4 A Pyrex flow cell of 1.6 cm inner diameter was installed in a vacuum chamber of a quadrupole mass spectrometer (Anelva TEdOOS). Gases in the cell were sampled continuously into an electron-impact ionization chamber of the mass spectrometer through a 100-rm pinhole located at the center of the cell. The ion signals from a secondary electron multiplier were recorded with a gated counter following pulse amplification and discrimination. The time profiles of the mass-selected signals were obtained by scanning the gate delay with a fixed gate width of 50 or 100 ps. The C2H radical was also generated by ArF laser photolysis of C2H2. The ArF laser (Questek 2220) with fluence of 6-25 mJ/cm2 was operated at a repetition rate of 9 Hz so that gases sampled through the pinhole were irradiated by only one pulse of the photolysis laser. Since the time response of the LP/MS apparatus is much slower (0.2 ms rise time) than that of the LP/VUV-LIF, lower concentrations (0.1-0.3 mTorr C2H2,0-0.3 Torr H2or 0-0.7 Torr D2 at a total pressure of 5 Torr in He) were required for the time-resolved measurements. Heterogeneous loss of C2H on the wall of the Pyrex cell was a significant process in these experiments. This loss could be reduced by coating the wall of the Pyrex reactor with Teflon film. The rate of the loss without coating was measured to be =600 s-I, whereas it was reduced to 78 S-I with Teflon coating. Mass spectra have been searched for the identification of the products of reactions 1-3. Only the C4H2 radical could be detected as the reaction product. The C4H2 ( m / z = 50) radical was detected with an ionizer electron energy of 16 eV. An attempt to detect C2H was unsuccessful because of the strong background signal at m / z = 25 caused by the dissociative ionization of C2H2 even with very low ionizer electron energy. All sample gases (C2H2,99.8%; H,, 99.995%; D2, 98%; He, 99.9995%) were obtained from Nihon-Sanso Co. and were used without further purification. 3. Results and Discussion 3.1 Results of the LP/WV-LIF Experiments. In addition to the V W - L I F detection of H atoms, the Lyman-a absorption after the ArF laser photolysis of C2H2also could be observed by monitoring attenuation of the VUV radiation using the ionization cell. From the intensity of this absorption, absolute concentration of H atoms was estimated with the effective absorption cross section that was calculated from the known oscillator strength,ls the line width of the probe VUV laser (1.1 cm-I fwhm), and the Doppler line width of H atoms (1.02 cm-I fwhm). The yield of H-atom production (and hence C2H production) in the 193-nm photolysis of C2H2,[H],/[C2H,],, was determined to be 4.3 X lo4 with a laser fluence of 6.6 mJ/cm2. From this value and the C2H2absorption cross section at 193 nm (1.35 X cm2),I6 the quantum yield for H atoms was calculated to be 0.25. This value is consistent with the quantum yield of 0.21 reported by Shin and M i ~ h a e l .Because ~ of this low yield of H atoms and C2H, pseudo-first-order conditions (C2H2or H2or D2 in large excess) were always maintained in the present experiments. The Lyman-a absorption could not be observed in the kinetic measurements in which lower concentrations of C2H2were employed in order to avoid possible complications caused by secondary reactions. It is well recognized that the photochemical primary process in C2H2is fairly complex. Not only H atom and C2H(X2Z+), but also CH, C2,C2H(A211),and triplet (3B2)vinylidene radicals can be produced in ArF laser photolysis of C2H2.6J7-23Possible complicating effects of these radicals on the kinetic measurements of the C,H reactions have been discussed by Shin and Michael9 and Laufer et a1.2'~22Shin and Michaelg found that the yields of H atom in the ArF laser photolysis of C2H2were proportional to the laser fluence up to 45 mJ/cm2. Since laser fluence in the present experiment was kept lower than 25 mJ/cm2, production of C H or C2radicals by multiphoton proces~es~+~ was unimportant. In addition, the time scale for the kinetic measurements was much
Kinetics of the Reactions of C2H with C2H2,H2, and D2
The Journal of Physical Chemistry, Vol. 96, No. 14, 1992 5877
F 8
Y 7 ..
.. .. . .
* . . ? :
3
-50
50
0
Time
100
I
. . . # . .
150
200
, . . . ,
OO
/ @s
Figure 1. LIF intensities of H atoms as a function of time after ArF laser photolysis of C,H,: (A) \C,H,] = 1.0 X lOI4 molecules/cm3 in 30 Torr He; (B) [C,H,] = 5.4 X [H,] = 3.4 X 10l6 molecules/cm3 in 30
Torr He.
Curves are the result of the nonlinear least-squares fit of the data to an exponential function of q (i).
I
t
T
2
4
6
8
10
[Hz] or [Dz] / 10l6 molecules cmV3
Figure 3. Pseudo-first-order rise rates of H atoms as a function of [H2] (circles) and of D atoms as a function of [D,] (squares) obtained in the ArF laser photolysis of C2Hz/H2and CzHz/D2mixtures. [C,H,] = 5.4 X lOI3 molecules/cm3. Error bar represents two standard deviation of the nonlinear least-squares analysis of the time profile. Lines are results of the linear least-squares fit of the data.
1
L
(A) H-atom
~b~'""""'""~""' 3
2
1
[CZHZ]
/
4
1014 molecules cm-3
Figure 2. Pscudc-fmt-order rise rates of H atoms as a function of [CzHz] obtained in the LP/VUV-LIF experiments. Error bar represents two standard deviation of the least-squares analysis of the time profile. The line is a result of the linear least-squares fit of the data.
-
longer than that for the collisional deactivation of C2H(AzII).6 The rate of H-atom abstraction reaction, H C2H2 H2 C2H, is also very under the present experimental conditions. Therefore, the thermalizad ground-state C2H radical was the only reactive species in the present experiments. A typical example of the VUV-LIF signal after the 193-nm photolysis of C2H2is shown in Figure 1A. Following the initial H-atom production by the photolysis, the LIF intensity is gradually increasing with time and then decays very slowly at long times ( r > 150 rs). Since the rate of this slow decay ( 7 1.8 ms at p = 30 Torr) was independent of the concentrations of C2HZ,the heterogeneous loss on the cell wall might be responsible for the decay. Neglecting the sharp spike at t = 0 caused by the scattered light of the ArF laser, the time profile at r < 150 rs was found to be well fitted to the following function: z(r) = Io Zr(1 - exp(-k,t)) (i)
+
+
-
+
In Figure 2, the pseudo-first-order rate constants k, are plotted against the concentrations of C2Hz. The bimolecular rate constant for reaction 1 determined from the slope of this plot was equal to (1.6 f 0.1) X an3molecule-' s-'. (The error is given by two standard deviation of the least-squares fit.) The quantities Zo and Zf in eq i represent the initial amount of H atoms produced by the photolysis and the amount of H atoms generated by reaction 1, respectively. Although the scattered light of the ArFlaser at t = 0 prevented the direct determination of
-50
0
50
150
100
Time
/
200
ps
Figure 4. LIF intensities of H atoms (A) and D atoms (B) observed after ArF lascr photolysis of the C1H2/DZmixture. A curve in (B) is the result of nonlinear least-squares fit of the data to a single exponential function of time. [C,H,] = 5.4 X 1013, and ID2] = 4.2 X 10l6molecules/cm3.
the value of Zb the ratio Zf/Zo was estimated to be 1.01 f 0.3 (f2u). Because the initial equal amount of C2H and H atoms was produced by the photolysis of C2H2,the ratio of Zf/Zo = 1 implies that all of the C2H radicals generated by the photolysis reacted with C2H2to yield H atoms. The rise rate of H atoms was accelerated by the addition of H2, as shown in Figure 1B. The time profile of the LIF signal also could be represented by eq i. The pseudo-first-order rate constants, which might be expressed as k, = kl[C2H2] k2[H2], are plotted in Figure 3 as a function of [H2]. The slope of the cm3 molecule-' plot gives a value of k2 = (7.1 f 1.1) X The ratio If/Zo was also found to be very close to unity (1 -05 f 0.21), indicating that reactions 1 and 2 were the only major processes for the consumption of C2H. Figure 4 shows the time profiles of H and D atoms after the photolysis of the C2H2/D2mixture. The time profile of D atoms could be fitted to the single exponential function of time. Equation i also could adopted to the H-atom profile, but the value of If/Zo was less than unity because the C2H radical was partly consumed by reaction 3. In the C2H2/D2system, an additional pathway for the H-atom production
+
C2H
+ D2
-
C2D2
+H
(4)
5878 The Journal of Physical Chemistry, Vol. 96, No. 14, 1992
,
1.8 t
-9 -r=, c
0
5: u
I
1.4 1.2 1.0
-
0.8-
bC
c
3: 0.6 -
Y v
Koshi et al.
0.2 0.4
0.0
I
,
I
,
I
,
,
.
,
0.0
-2.5
2.5
Time
5.0
7.5
/ ms
Figure 6. Time profiles of the mass signal at m / z = 50 (C4H2)after the photolysis of C2H2(A) and C2H2/H2(B) mixtures: (A) [C2H2]= 7.9 X 10I2molecules/cm3 in 5 Torr He; (B) [C2H2] = 9.6 X 10I2, [H2] = 5.6 X 10" molecules/cm' in 5 Torr He. A curve in part A is the nonlinear least-squares fit of the data.
:::I/__ 0
0
I
4
2
[CZHZ]
/
,
, 6
,
I
8
,
,
10
1OI2 molecule c r r ~ - ~
Figure 7. Pseudo-first-order rise rates of C4H2as a function of [C2H2] after the ArF laser photolysis of C2H2in 5 Torr He. Error bar represents two standard deviation of the nonlinear least-squares analysis of the time profiles at m / z = 50. A line is the linear least-squares fit of the data.
is in good agreement with the rate constant of kl derived in the present LP/VUV-LIF experiments. The time profile of the C4Hz signal after the photolysis of C2H2/H, mixture is shown in Figure 6B. The rate of the C4H2 production was largely accelerated and the signal intensity was reduced by the addition of H2. This decrease in the intensity was obviously caused by reaction 2. Because the production rates of C,H, in Figure 6B were faster than the response time of the present detection system, lower concentration of C2H, was required to observe the acceleration of the H-atom production by the addition of H,. However, the signal intensity with such low concentration of C,H, was not strong enough for kinetic analysis. The rate constants of reactions 2 and 3 were determined by measuring the dependence of the C4H2 yields on the partial pressures of H, or Dz. Neglecting the contribution from reaction 4, the time variation of [C4H2]is expressed as [C,H21 = k,[C2H21[C2Hlo(1- e-"')/a
(4
Here, a = k,[C,H,] + k2[H2]or kl[C2H2]+ k3[D2]. From this equation, following relation can be derived:
Kinetics of the Reactions of C2H with C2H2,H2, and D2
[Hz]or [Dz] /
molecules cm-3
Figwe 8. Plot of ([C2H]o/[C4H2]f-l)[C2H2]versus [H2] (circles) and [D2] (squares) obtained in the LP/MS experiments. Partial pressures of C2H2 were ranging from 0.3 to 0.7 mTorr. Lines are the linear least-squares fit of the data. TABLE I: Comparison of Rate Constants for the C2H Reactions with C,H, H, a d D,‘ C2H + C2H2 C2H + H2 C2H + D2 ref methodb k,/10-10 k2/ 10-l~ k3/ io-” 7.1 f 1.1 2.0 f 0.3 this work LP/LIF (H, D) 1.6 f 0.1 1.4 f 0.3 5.1 f 2.4 1.3 f 0.5c this work LP/MS (C.Pz)
9 1.3 f 0.3 LP/RA (H) 8 4.8 f 0.3 LP/ABS (CzH) 1.5 f 0.1 2.3 f 0.3 11 4.4 f 0.4 LP/ABS (CzH) 3 m/ABS (C4H2) 0.31 f 0.02 1.5 >0.5 >1.7 7 DF/MS (C2H) ‘In unit of cm3 molecule-’ s-l. bLP = laser photolysis, FP = flash photolysis, DF = discharge flow, MS = mass spectrometry, LIF = laser-induced fluorescence, RA = resonant absorption, ABS = absorption. Species monitored is indicated in parentheses. ‘Calculated from with k, = 1.5 the measured ratio of the rate constants k 2 / k land x 10-10 cm3 molecule-’ s-I.
It has been confirmed in the LP/VUV-LIF experiments that all of the C2H radicals produced by the photolysis of C2H2/He mixtures were converted to H atom (and hence to C4H2). This means that [C2HIois equal to [C4H21fwithout the addition of H 2 Therefore, the ratio [C2H]o/[C4H2]fis simply equal to the ratio of the signal intensities of m / z = 50 (C4H2)at t = 03 without and with the addition of H2or D1.The quantities of the left-hand side of eq vii thus obtained are plotted in Figure 8 as a function of the partial pressures of H 2 or D1. From the slope of this plot, the ratios of rate constants were evaluated to be k2/kl = (3.4 f and k 3 / k , = (8.7 i 0.3) X 10”. 0.2) X 3.3. Comparison of the Rate Constants. The rate constants for reactions 1, 2, and 3 obtained in the present LP/VUV-LIF and LP/MS experiments are summarized in Table I with the previous literature values. The rate constants of reaction 1 obtained in the present work are in good agreement with the results of Shin and Michael9 and Stephens et ale8 These values overlapped each other within the combined experimental errors. The lower value of Laufer and Bass3 was derived from the appearance rates of C4H2. Stephens et al. have suggested two possible mechanism for the reaction to explain the discrepancy between their value and that of Laufer and Bass: (a) an isomer of diacetylene might be formed that could not be detected in the absorption measurement of Laufer and Bass, and (b) C4H3radical was a possible intermediate in the reaction. The formation of C4H3from C2H precursor in the thermal decomposition of C2H2was also suggested by some researcher^.^^,^^ Mechanism b was ruled out by Shin and Michael because of the agreement of their production rates of H atom with the depletion rate of C2H derived by Stephens et al. The present results further confirm their conclusion. Complete conversion of C2H to H atom
The Journal of Physical Chemistry, Vol. 96, No. 14, 1992 5879 observed in the LP/VUV-LIF experiments clearly indicated that a stable C4H3radical could not be formed in the reaction. The rate constants of k2 and k3 in Table I for the LP/MS experiments were derived from an average value of kl obtained in the present LP/WV-LIF and LP/MS experiments (1.5 X W0 cm3 molecule-’ s-l). Their relative errors were estimated from the sum of the relative errors in the measured ratios of the rate constants (k2/kl and k 3 / k , )and the relative error in kl. The rate constants k2 and k3 derived from the VUV-LIF experiments are higher than the LP/MS results, but these values agree with each other within the combined uncertainties. The LP/MS value of k2 is also in agreement with the rate constants of Stephens et ala8 and Lander et al.,” and the VUV-LIF value of k3 agrees well with the results of Lander et al. Because the rate constants of Stephens et al. and Lander et al. were derived from the depletion rates of C2H, the agreements with the present results indicated that the decay rate of the reactant and the rise rates of the products were the same. Therefore, the formation of C2H3 radical as a stable intermediate in the reaction of C2H with H2 is ruled out. This conclusion is also consistent with the ab initio calculation of Harding et a1.I0 for the transition state of reaction 3. Their calculations indicated that the activated complex had a linear H-He
