J. Phys. Chem. 1988, 92, 131-134
131
Rate Constants for the Gas-Phase Reactions of OH Radicals with 1,3-Butadiene and Allene at 1 atm in Ar and over the Temperature Range 305-1 173 Kt Andong Liu,t William A. Mulac, and Charles D. Jonah* Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439 (Received: April 21, 1987;
In Final Form: July 22, 1987)
Absolute rate constants for the gas-phase reaction of the OH radical with 1,3-butadiene and allene in an argon atmosphere were measured at 1 atm over the temperature range 305-1 173 K. It was not possible to determine the rate of the H abstraction reaction from either allene or 1,3-butadiene; an upper limit for these rates is about twice that which one would predict from the OH + ethylene reaction. At temperatures below 600 K, k(OH + 1,3-butadiene) = (1.4 h 0.1) X lo-" exp[(440 4 0 ) / g exp[(100 f 5 0 ) / 7 cm3/(molecule.s). Above 600 K the rate cm3/(molecule.s) and k(OH + allene) = (6.7 f 0.9) X constant of the allene reaction decreases weakly while the rate constant for the 1,3-butadiene reaction decreases markedly.
*
Introduction It is widely accepted that the hydroxyl radical plays an important role in both atmospheric and combustion chemistry. Atkinson has recently reviewed the reactions of O H with organic compounds under a variety of conditions.' However, there is still a lack of data at high temperatures and at pressures around 1 atm, the region that is most relevant to the studies of combustion. In addition to being useful to the study of combustion and the modeling of flames and of air pollution, an understanding of the basic reaction mechanisms can be obtained from a knowledge of the rate constants over a wide temperature and pressure range. Recently, experimental measurements have shown that reaction mechanisms can change as a function of temperature. For example, a change in reaction mechanism has been seen in the reaction of OH with p r ~ p e n e . ~In, ~those studies a sharp increase in the rate constant was seen at higher temperatures. This increase was ascribed to an H abstraction reaction, a reaction that has a large activation energy. In our previous studies of the reaction , ~ observed the abstraction of the (vinyl) of OH with e t h ~ l e n ewe hydrogen from ethylene. This was the first direct measurement of the rate of a vinyl hydrogen abstraction. It thus seemed of interest to measure other rates for non-alkane hydrogen abstraction by OH. The requirements for such a study are that there can be no alkane hydrogens in the compound to be studied. The alkane hydrogens would be abstracted preferentially to the non-alkane hydrogens, since the non-alkane hydrogens would be more strongly bonded. Three obvious candidates exist for this study: acetylene, 1,3-butadiene, and allene. We shall report on our studies of the reactions OH + 1,3-butadiene and O H + allene in this paper. The results of OH + acetylene will be published ~eparately.~ The reactions of allene and 1,3-butadiene with OH have been measured previously from 300 to 425 K where the addition reaction was found to be at the high-pressure limit in 50-100 Torr of argon.6 Experimental Section A complete description of the experimental technique has been given p r e v i o u ~ l y , 4and ~ ~ only ~ ~ the basic principles and changes to the system will be described here. A flow system was used for these measurements, and the composition of the sample was controlled by varying the flow rates of the reactants and buffer gas. Argon (flowing at 1 standard liter per minute (slm)) which contained approximately 6 Torr of water and a reactant gas mixture (flowing at 0.0005-0.005 slm) were flowed continuously through the reaction cell at ambient pressure so that the pressure in the cell was approximately 750 Torr. The reactant mixtures were 10.35% allene (purity greater than 98%) in argon and 2.0% 1,3-butadiene (99.8% purity) or 0.5% 1,3-butadiene in argon. The 'Work performed under the auspices of the Office of Basic Energy Sciences, Division of Chemical Science, US.-DOE, under Contract No. W-31109-ENG-38. *On leave from The Institute of Low Energy Nuclear Physics, Beijing Normal University, Beijing, China.
concentrations of reactants in the cell at 0.001 slm flow rate and 25 "C were 10.35% allene, 3 X lOI5 molecules/cm3; 0.5% 1,3butadiene, 1.7 X lOI4 molecules/cm3; and 2.0% 1,3-butadiene, 6.6 X lOI4 molecules/cm3. The gas mixture in the cell was irradiated with an electron beam (1 5 MeV, 0.25-3 ps, peak current = 1.5 which ionizes the argon. The ion-recombination reaction and further reactions give Ar* and Ar**. These species transfer energy to the water, decomposing it to form H and OH. From the maximum absorption of the O H radical and from the rate of disappearance of O H through a second-order reaction with itself, the Concentration of O H was estimated as 2 X 1Ol2molecules/cm3, which was at least 2 orders of magnitude less than the concentrations of the reactants. Resonance absorption was used to measure the OH concentration. A microwave discharge in water-helium was used to generate the OH resonance line (308 nm). The resonance line was isolated by using an interference filter and detected by using a photomultiplier (Hamamatsu R928 using only five dynodes). The output of the photomultiplier was digitized by using a Biomation 8 100 transient recorder and accumulated and averaged in a computer (DEC 11/23). See ref 4 and 8 for details. Exponential decays were observed over the entire temperature range. A typical O H decay curve is shown in Figure 1. The Beer-Lambert law does not hold for OH absorption under our conditions and the concentration of O H radicals were determined from the expression OD = (ecl)" where e is the extinction coefficient and c and I are the O H concentration and cell path length, respectively. The data were analyzed with a nonlinear leastsquares program. The expression used to analyze the data was
where the pseudo-first-order rate constant is
k ' = kb[reactants] + kd
(11)
The bimolecular rate constant for OH reacting with the reactants is kb and kd is the rate for O H disappearance due to reaction with all other species (which includes impurities in the gas mixtures, pyrolysis products, and reaction with O H itself). The exponent (1) Atkinson, R. Chem. Reu. 1986, 86, 69. (2) Tully, F. P.; Goldsmith, J. E. M. Chem. Phys. Let?. 1985, 116, 345. (3) Smith, G. P.; Fairchild, P. W.; Jeffries, J. B.; Crosley, D. R. J . Phys. Chem. 1985, 89, 1269. (4) Liu, A.; Mulac, W. A.; Jonah, C. D. Znt. J . Chem. Kinel. 1987, 19, 25. (5) Liu, A.; Mulac, W. A.; Jonah, C. D., et al., unpublished results. (6) Atkinson, R.; Perry, R. A.; Pitts, J. N., Jr. J . Chem. Phys. 1977, 67, 3 170. (7) Jonah, C. D.; Mulac, W. A,; Zeglinski, P. J. Phys. Chem. 1984, 88, 4100. (8) Beno, M. F.; Jonah, C. D.; Mulac, W. A. In?. J . Chem. Kinet. 1985, 17, 1091.
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Liu et al.
132 The Journal of Physical Chemistry, Vol. 92, No. 1, 1988
1
O-Oo6
TABLE I: Rate Constants for OH
OH
+ 1,3-butadiene
d
313 333 338 373 393 408 438 483 563 623 673 723 773 873 923 1023 1053 1153 '1173 1203
0.003
0.000
0.2
0
0.4
Time
0.6
( ms )
Figure 1. Typical OH decay profile for OH + allene (853 K, 1 atm of argon; concentration of allene, 1.05 X 1015molecule/cm3). The solid line is a nonlinear least-squares fit to a pseudo-first-order reaction.
40000-J
313 K
allene k X 10l2, cm3/(molecule-s) 9.0 f 1.0 8.7 f 0.9 8.2 f 0.8 8.8 f 0.9 8.0 f 0.9 7.8 f 0.8 7.2 f 0.7 7.3 f 0.7 7.6 f 0.8 7.9 f 0.8 ' 8.2 f 0.8 7.8 f 0.8 6.7 f 0.7 6.5 f 0.7 5.6 f 0.6
OH
k X lo", cm3/(molecule-s) 6.1 f 0.6 5.0 f 0.5 5.1 f 0.5 4.6 f 0.5 4.1 f 0.4 4.7 f 0.5 4.2 f 0.4 3.5 f 0.4 2.9 f 0.3 3.0 f 0.3 3.0 f 0.3 2.4 f 0.3 2.0 f 0.2 1.7 f 0.2 1.5 f 0.2 1.1 f 0.1 1.0 f 0.1 0.65 f 0.06 0.69 f 0.07 0.77 f 0.08
T, K
0
+ 1,3-Butadiene and OH + Allene T, K
305 373 398 478 543 613 673 773 808 853 873 888 973 1073 1173
$.
10-107 30000
-
3 0
v)
623 K
20000
i
0
n00
%3 O X
oooo O
8
10-l1 Y
10000
1153 K
0 0
1
2
3
14
X 10
5
4
6
3
molecule/cm
0.5
Figure 2. Observed pseudo-first-order rate constant of OH + 1,3-butadiene vs concentration of added 1,3-butadiene at 313,623, and 1153 K.
8ooool
373 K
1.0
1.5
2.0
2.5
3.0
3.5
1 0 0 0 / T (K )
Figure 4. Arrhenius plot of overall rate constants of OH (0)this work; (X)
ref 6.
+ 1,3-butadiene:
7
-
0 v)
Y
X 10
15
molecule I c m
3
Figure 3. Observed pseudo-first-order rate constant of OH concentration of added allene at 373, 613, and 1173 K.
+ allene vs
n ranged from 0.81 at room temperature to 0.95 at temperatures
0.5
1.0
2.0
1.5
2.5
3.0
3.5
1000/T(K)
Figure 5. Arrhenius plot of overall rate constants of OH
(shaded circles) this work; (X) ref 6.
+ allene:
above 1100 K.'
Results and Discussion The bimolecular rate constants were obtained from the slope of the plot of the pseudo-first-order rate constants vs concentration of reactants. Typical results are shown in Figures 2 (1,3-butadiene) and 3 (allene). The reaction of OH with radiolysis products was tested by changing dose entering the cell by a factor of 10.
The intercept kd decreased at lower doses; however the bimolecular rate constant was independent of dose. The bimolecular rate constants are listed in Table I. The indicated errors (about &lo%) are the estimated overall experimental errors. These include the errors in the nonlinear fit (1-2%), the linear fitting of the pseudo-first-order rate constants vs concentrations (3-8%), and inaccuracies in the flow controllers.
Gas-Phase Reactions of O H Radicals
The Journal of Physical Chemistry, Vol. 92, No. I, 1988 133
.-
10-10j
TABLE II: Comparison of Simulated Rate Constants in the Reactions of OH with Ethylene, Allene, and 1,3-Butadiene kl, cm3/ (molecu1e.s)
k3,0 cm3/ (molecules)
k-l/k,[MI
9
S-'
1.2 X 2.4 x 2 x 109. exp(-2 loo/ 7') exp(-l4000/ T ) exp(600/ 7') allene 5.0 X 2.4 x lo-". 1 x 104. exp(2100/7') exp(-lOOOO/T)b exp(200/7') 5 x 104. 1,3-buta- 1.2,X 3.6 x diene exp(500/T) exp(-2100/7') exp(-l0000/T) ethylene
......' 4.
"Determined only for ethylene-see text for discussion. k2[MI).
k-ll(k4
+
The competition between the unimolecular decomposition k-l and the bimolecular collision stabilization k2[MI is temperature and pressure dependent. A complete study of these effects would require further experiments as a function both of pressure and composition of the buffer gas. In addition, more theoretical work needs to be done.1° In this paper, only a simple comparison of the experimental results and an estimate of the magnitudes of the different elementary reactions will be given. The results for the allene and 1,3-butadiene will be discussed separately. I ,3-Butadiene. To compare the temperature dependence of the different olefins in 1 atm of argon, the Arrhenius plots of O H with ethylene, allene, and 1,3-butadiene are shown in Figure 6. It appears as if the overall rate constants of 1,3-butadiene with OH are at the high-pressure limit for temperatures less than 600 K. There are several observations that lead us to this conclusion. One would expect the rate to decrease when the reaction ceases to be in the high-pressure limit, and the lack of decrease below 600 K is consistent with that assumption. Second, from the data in ref
4 and papers cited therein, it appears as if the reaction of O H with ethylene at 1 atm of pressure is in the high-pressure limit at temperatures below 560 K. The 12 additional internal degrees of freedom that exist in butadiene in comparison to ethylene would enable more energy to be accommodated within the OH-butadiene adduct. This would then allow more time for collisions to deactivate the er'ergy-rich adduct and form a stable addition compound. The decrease in rate constant as a function of temperature is consistent, since it occurs at higher temperatures in 1,3-butadiene than in ethylene. The kinetic data do not give strong evidence for an H-atom abstraction in the reaction of O H with 1,3-butadiene (however, see below). There are two reasons for this. If we assume that the H-atom abstraction rate constant from 1,3-butadiene is about 1.5 times that from ethylene4 (the number of H atoms in 1,3butadiene to the number of H atoms in ethylene), then we would predict a value of 6 X cm3/(molecule.s) at 1173 K and considerably lower at lower temperatures. The hydrogen abstraction rate is masked by the rate of O H addition to 1,3-butadiene. This rate is considerably faster for 1,3-butadiene than it is for ethylene, and more importantly, the rate of O H addition to butadiene does not decrease sharply at higher temperatures as it does for the ethylene reaction. Thus we cannot observe the increase in the rate constant as we did in ethylene. However the small increase in the rate constant that appear between 1150 and 1200 is consistent with the'onset of the hydrogen abstraction reaction. The magnitudes for kl and k-l/k2were estimated by assuming that they were of the form A exp(-E/T), k3 was derived from ethylene as described above, and the experimental data were fitted. The results are plotted in Figure 6, and the appropriate parameters are given in Table 11. The results for ethylene4 and allene are also shown for comparison. Simple comparison of the experimental data with the assumed mechanism will give an upper limit to the H abstraction rate of about 12 X cm3/(molecule.s) at 1173 K.4 It is impossible to obtain agreement between predicted and measured rates if a larger rate constant is used. From Table I1 we see that the addition rate constant for 1,3-butadiene has a large A factor (about 10 times that of ethylene and 2 times that of allene) and an apparent activation energy that is similar to ethylene and larger than allene. For some diolefins the overall rate constants for the OH addition can be estimated from the sum of the rate constants for each of the groups.' This simple summation does not seem to work for these three molecules. This is not surprising since the electronic structures of the two double bonds in allene and 1,3-butadiene will interact. The negative activation energies are often observed in the addition reaction of O H to olefins and are explained as the contribution from the temperature dependence of the A factors.12 Allene. The temperature dependence for the reaction of O H plus allene is considerably different than for the other olefins in that the reaction rate constant was almost constant over the entire temperature region. The lack of decrease in rate at higher temperatures suggests that there are additional reaction channels for the OH adduct (C3H40H)*other than the back reaction to OH
(9) Laidler, K.J. Chemical Kinetics, 2nd ed.; McGraw-Hill: New York, 1965; pp 143 ff and 175-178. (10) See, for example, Dean, A. M. J. Phys. Chem. 1985, 89, 4600.
(11) Ohta, T. J. Phys. Chem. 1983,87, 1209. (12) Atkinson, R.; Perry, R. A.; Pitts, J. N., Jr. Chem. Phys. Lett. 1976, 38, 607.
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
1000/T(K)
Figure 6. Comparison of temperature dependence of overall rate constants a t 1 atm of argon for OH reaction with ethylene (solid circles), allene (shaded circles), and 1,3-butadiene (open circles).
The Arrhenius plots are given in Figures 4 and 5. Experimental results from ref 6 are included on both figures, and the good agreement is clear for temperatures below 424 K. Arrhenius parameters at lower temperatures are k = (1.4 h 0.1) X lo-" exp[(440 k 40)/7'l cm3/(molecule~s)for O H + 1,3-butadiene (313-623 K) and k = (6.7 h 0.9) X 10-l2 exp[(100 f 50)/TJ cm3/(molecule.s) for OH + allene (305-613 K). We assume that the reaction mechanism for these reactions is that of O H addition to the double bond followed by decomposition or collisional stabilization. This mechanism is similar to that for many o l e f i n ~ ' *and ~ . ~ can be described by a simple Lindemann me~hanism.~The reaction mechanism is then
OH
+ olefin
(olefin-OH)+ + M
k-1
(olefin-OH)?
-
(1)
(olefin-OH) + M (2) The pressure-independent reaction for H atom abstraction by the OH radical may also take place: OH
+ olefin
k3
k2
olefin radical
+ H20
(3) For this chemical mechanism we get an expression for the observed rate constant (using a steady-state approximation):
1+k2MI
J . Phys. Chem. 1988, 92, 134-137
134
and starting material. This reaction can be expressed as (olefin-OH)*
k4 -C
product
(4)
The activation energy of this reaction should be less than or equal to that of the back-reaction of (1). The reaction of the energy-rich adduct would then not reform OH, and thus we would not see a decrease in the rate. We would then get the kinetic expression for the apparent rate constant from reactions 1-4.
Data in the literature strongly suggest the existence of such a reaction for allene.I3 By use of high-intensity, crossed molecular beams and detecting fragments with a photoionization mass spectrometer, C3H3,CH,, and H 2 C 2 0were measured. C3H3is evidence for reaction 3, the H-atom abstraction. It was proposed that CH, and H 2 C 2 0are products of the reaction OH
+ C3H4 + (C3HdOH)*
-+
CH3
+ H2C20
Since the work was carried out in a molecular beam, product formation requires an efficient unimolecular process. Since there is little falloff at high temperature and the rate of O H addition to allene is large, the H-atom abstraction reaction could not be seen. However, a careful analysis of the Arrhenius plot for both the experimental and computer-simulated data shows evidence that the H-atom abstraction is of probably of the same order of magnitude in allene as it is in ethylene. The curvature of this plot above 800 K could be due to the effect of this reaction. Since the same curvature occurs in all three curves as seen in Figure 6, it may indeed be a real effect. This curvature occurs because of the increased contribution of reaction 3 to the total (13) Slagle, I. R.; Gilbert, J. R.; Graham, R. E.; Gutman, D. Inr. J . Chem. Kiner. Symp. 1975, I , 317.
rate constant. The effect appears largest in allene because the back-reaction of (1) does not cause a curvature in the reverse direction. An upper limit for the reaction rate can also be determined for allene in the same fashion as for 1,3-butadiene, and the rate constant is no more than a factor of 2 greater than that which one would predict from ethylene.
Conclusions It was not possible to measure the rate of hydrogen abstraction from either allene or 1,3-butadiene because of both the larger rate of the addition reaction and the weaker decrease in the rate of the addition reaction in comparison with the reaction of ethylene. However, an upper limit of about twice the predicted rate for H abstraction from ethylene can be inferred from these data. The reactions of the three olefins ethylene, 1,3-butadiene, and allene are similar in that the reaction at low temperatures takes place through addition. At higher temperatures, differences appear. The increased stability of the intermediate 1,3-butadiene adduct may allow a longer time for stabilization of the energy-rich adduct; that is, k-l is decreased. For allene, other reaction channels may open up so that the effect of the back-reaction of the intermediate adduct to form the O H radical is diminished. Both of these effects increase the apparent rate at high temperatures which comes from the addition channel and thus make it possible to give only an upper limit to the H-atom abstraction reaction. A fuller description of these reactions require experiments as a function of buffer gas and pressure as well as temperature. Studies at lower pressures would slow the addition-initiated reactions reaction and may make it possible to observe directly the abstraction reaction. Acknowledgment. We gratefully acknowledge the assistance of the Argonne accelerator operators, George Cox, Don Ficht, and Ed Kemereit. We also gratefully acknowledge the assistance of Toni Engelkemeir in the analysis of our samples. Registry No. Hydroxyl radical, 3352-57-6; 1,3-butadiene, 106-99-0; allene, 463-49-0.
Electron Transfer from Indoles, Phenol, and Sulfite (SO,,-)
to Chlorine Dioxide (CIO,')
GBbor MerGnyi,*+ Johan Lind,l and Xinhua S h e d Department of Physical Chemistry and Department of Nuclear Chemistry, Royal Institute of Technology, S-10044 Stockholm 70, Sweden (Receiced: April 22, 1987)
With the C102/C102- couple as reference the one-electron-reduction potentials have been determined for four methylated indolyl radical cations. Their E" values are 1.23 V (N-Me), 1.10 V (2-Me), 1.07 V (3-Me), and 0.93 V (2,3-diMe). E" values were also measured for the following: tryptophylH''/trypH 1.24 V, S 0 3 ' - / S 0 3 2 -0.76 V, and phenoxy'/phenolate 0.80 V. The redox potentials were obtained from purely kinetic data (for tryptophan and 2-, 3-, and N-methylindole) or from combined kinetic and thermodynamic measurements.
Introduction
The reliability of determined one-electron-reduction potentials of transient species hinges on the confidence we can place in the accepted E o value of the reference couple. While transient measurements, unlike their stable E M F counterparts, always contain a sizeable uncertainty, failure to utilize correct reference values may play havoc with whole series of determinations. The problem of uncertain reference values is particularly pronounced in the vicinity of 1 V. Not surprisingly then, the reduction potentials of indoles and unsubstituted phenol are still far from settled. In view of the Department of Physical Chemistry, *Department of Nuclear Chemistry.
0022-3654/88/2092-0134$01.50/0
interest in the possible electron transfer between tryptophan and tyrosine in biology it would be desirable to reach consensus about the redox properties of the title compounds. One of the few stable radicals known is chlorine dioxide (C102'). The redox potential of the ClO;/ClO,- couple has been measured by several ~ o r k e r s l - ~ using standard EMF techniques. Therefore, the accepted value3 of 0.936 f 0.003 V at 25 "C has an accuracy and reliability unparalleled in transient measurements. The magnitude of the E" value of C102 makes it the ideal reference partner in transient ( I ) Holst, G. Suensk Papperstidn. 1945, 48, 23. (2) Flis, I. E. Zh. Fiz. Khim. 1958, 32, 573. (3) Troitskaya, N . V.; Mishchenko, K.P.; Flis, I . E. Russ. J . Phys. Chem. 1959, 33, 11.
0 1988 American Chemical Society
