J . Phys. Chem. 1991, 95, 1241-1244
1241
Branching Ratios in O(3P) Reactions of Terminal Olefins Studied by Kinetic Microwave Absorption Spectroscopy Seiichiro Koda,* Department of Chemical Engineering, Faculty of Engineering, University of Tokyo, Hongo, Bunkyo- ku, Tokyo 113, Japan
Yasuki Endo,+Soji Tsuchiya, Department of Pure and Applied Sciences, College of Arts and Sciences, University of Tokyo, Komaba, Meguro-ku. Tokyo 153, Japan
and Eizi Hirota Institute for Molecular Science, Okazaki 444, Japan (Received: April 9. 1990: In Final Form: August 3, 1990)
O(3P)atom reactions with ethylene, propylene, and 1 -butene were studied by use of kinetic microwave absorption spectroscopy where the atomic oxygen was supplied via ArF excimer laser photolysis of SOz. Time evolutions of vinoxy, HCO, and HzCO wcrc pursued. The fraction of vinoxy production in the propylene and I-butene reactions at 30 mTorr was found to be 0.29 f 0. I5 and 0.37 h 0.15, respectively. The fractions seemed to increase with the decrease in pressure, by comparing the present results with thosc of scvcral previous researchers. Thercforc. at least some part of the vinoxy production fraction is pressure dcpcndcnt. Thc reaction schemc explaining the pressure-dcpendentpart is suggested as follows. Initially, a triplet biradical is produced through O(3P) attack to the terminal carbon atom of the C=C double bond, which is then converted to a singlet biradical. Subsequently, a hydrogen atom migrates and then the C-C bond adjacent to the original C=C double bond dissociates to yield the vinoxy and corresponding alkyl radicals. Thus, some part of the vinoxy radical is produced via a quite different mechanism from the case of ethylene reaction, where it is produced via a direct substitution channel on a triplet surface.
Introduction
of which the reaction scheme is discussed.
Atomic oxygen (O(jP)) reactions with olefins have been of interest for a long timc from kinetics in combustion and atmospheric chemistry and also as one of important elementary reactions. In order to understand the reactions, the identity of all reaction channels, determination of their rate constants, and analysis of subsequent reaction progresses are required. Among the olefins, ethylene (C2H4)has been extensively studied in recent years.’-4 As for the other olefins, however, the identity of the primary reaction products and their relative importance are still a subject of considerable uncertainty. Several discrepancies seem to exist among the previous direct measurements of primary products; those are by Kanofsky et al.5 using photoionization mass spectroscopy, by Blumenberg et aL6 using electron-impact ionization mass spectroscopy, and by Hunziker et al.’ using modulation UV absorption spectroscopy . In the present paper, we have studied the reactions of propylene (C,H,). I-butene (I-C4HH),and ethylene by use of a kinetic microwave absorption spectroscopic method. Microwave spectroscopy permits us to observe a wide range of molecules including intcrmcdiatcs and products in a reaction system simultaneously, which is vcry important in order to identify primary reaction channels. Ground-state oxygen atoms have been produced through excimer laser photolysis of SO, at I93 nm. Sulfur dioxide is an excellent source of O(’P) for kinetic studies, which has been used for the same purpose by Slagle and co-workers.8 I n fact, the absorption cross section of SO2 at 193 nm is large (u = 6 X 10-l8 the photolysis yields cxclusively SO and O(]P), SO, is chemically inert, and also SO is significantly less reactive with hydrocarbon free radicals than is O(3P). Thus, the combination of excimer laser photolysis of SO, as an O(jP) sourcc and time-rcsolved microwave absorption spectroscopy has been applied for the measurements of the time evolutions of vinoxy. HCO, and formaldehyde (H,CO), on the basis * T o whom correspondence should be addressed. ‘Adjunct Associate Professor of the Institute for Molecular Science for 1988-1990.
0022-3654/91/2095-1241$02.50/0
Experimental Section
Apparatus and Procedure. Figure 1 shows a block diagram of the kinetic microwave spectrometer developed in the present study. A quartz tube of 148-cm length (effective length for the reaction, 104 cm) and 9-cm i.d. was used as a reaction/microwave absorption cell, which was equipped with a poly(tetrafluoroethylene) (Teflon) lens at each end. One of the Teflon lenses has a small aperture with a quartz concave lens, through which the laser light beam from an ArF excimer laser (Lambda Physik EMG 201) at 193 nm was introduced to the cell, being gradually expanded. The gaseous mixture composed of SO2 and olefin (in the ratio 1/2 or, sometimes, 2/1) was introduced to the cell from the inlet close to the concave quartz lens and flowed to the other end to be pumped out. Usually, the cell pressure was kept at 30 mTorr and the average flow rate at 500 cm s-l. All experiments were performed at room temperature (25 “C). The excimer laser was fired repetitively at a sufficiently slow repetition rate (ca. 2 Hz) to avoid any pileup error between the ( I ) Endo, Y.: Tsuchiya, S.; Yamada, C.; Hirota, E.; Koda, S. J. Chem. Phys. 1986, 85. 4446. (2) Koda, S.; Endo, Y.; Hirota, E.: Tsuchiya, S. J . Phys. Chem. 1987, 91, 5840. (3) Bley, U.;Dransfeld, P.; Himme, B.; Koch, M.; Temps, F.; Wagner,
H.
Gg. Twenty-Second Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1988, p 997. (4) Schmoltner, A. M.; Chu, P. M.; Brudzynski, R. J.; Lee, Y. T. J . Chem. Phys. 1989, 91, 6926. (5) Kanofsky, J.; Lucas, D.; Gutman, D. Fourteenth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1973; p 285. (6) Blumenberg, B.; Hoyermann, K.; Sievert, R. Sixteenth Symposium (International)on Combustion; The Combustion Institute: Pittsburgh, PA, 1977; p 841. (7) Hunziker. H. E.: Kneppe, H.: Wendt, H. R. J . Photochem. 1981.17,
377. ( 8 ) Slagle, 1. R.; Sdrzynski, D.; Gutman, D. J. Phys. Chem. 1987, 91.4375. ( 9 ) Golomb, D.; Watanabe, K.: Marmo, F. F. J. Chem. Phys. 1962. 36, 958.
0 1991 American Chemical Society
1242 The Journal of Physical Chemistry, Vol. 95, No. 3, 1991
Koda et al. 5. 0
1
U
n
'E
4.0
u
-
3, 0
0 4
\
2. 0
0
1. 0
c 0
U
0. 0 5. 0
Figure 1. Experimental setup: ArF, ArF excimer laser; M, microwave multiplicr; M W, microwavc gcncrator; RC, rcaction CCII: PG. prcssurc gauge: PC. pcrwn:il computcr: DET. lnSb detector: TD. transient digiti7cr.
0
' E u
-
.
TABLE I: Transitions and Absorption Coefficients spccics freq/MHz transition peak abs/cm2 CHICHO 170,~,-160,~6 1.28 X IO-" 34231 1.5 HCO 40.4-30.3" 6.69 X 346708.5 H?CO 321 768.6 5i,5-4',4 3.93 x 10-1' so 344 3 10.7 8-7, F, 1.33 X
4.0 3. 0
0 I
\
2. 0 1. 0
0
U
0. 0 5.
'J = 4.5-3.5. I.' = 5-4.
0
n
4.0
events induced by each laser pulse. The microwave radiation in the 300-GHz region was fcd to the reaction cell by the inlet Teflon lens and passcd along the cell axis as an approximately parallel beam of a diainctcr of around 2 cm, absorbed partially by the species and then focused by the outlet Teflon lens onto a n lnSb detcctor opcratcd at liquid helium temperature. More than 90% of thc microwave beam path in the cell was considered to be in thc rcgion irradiatcd by thc A r F cxcimer laser. The details of the microwavc spectroscopic and the data acquisition system were essentially the same as described previously.' In order to improve the S / N ratio, the time evolutions of the individual species induced by more than 100 laser pulses were accumulated. All thc samplc gascs (purity, >99%) were commercially obtained and uscd without further purification. Light /,itcvisitjsnnd lnitinl O(jP) Concentration. The incident pulse energy of the 193-nm light (ca. 100 mJ) could be estimated from thc mcasurcd intensity of the laser light at the output of the lascr apparatus and losses through the optical path. Based on the SOzconccntration and its absorption cross section ( u = 6 X ~ m ? )the , ~initial Concentration of O(3P)could be estimated. It was in thc ordcr of I X I O i 2 atoms cm-3 when the laser intensity at thc output of the apparatus was 100 mJ and the SOz pressure was I O mTorr. The concentration of O(3P)could be also estimated on thc basis of the absorption signal intensity of SO. These two diffcrent cstimations usually agreed within 2076, and thus the above avcragc value was adoptcd as thc O(3P) concentrations, whcn necessary. Absorption Coefficients. The rotational spectra of HCO,'" H2C0," and SOi2have been analyzed extensively, and the vinoxy radical has also recently been investigated in detail by microwave spectro~copy.'~The rotational transitions used in the present study are listed in Table 1. By assuming thermal equilibrium within the molcculc owing to rapid internal relaxation, the absorption coefficients havc bccn estimated as in the previous report.' The unccrtainty in thc calibration of the absorption coefficients is considered to be less than 20% as discussed previously.' Figure 2 shows typical examples of the time evolutions, after converting the relative absorption scale to individual concentration scales. The excimer laser fires a t time = 0. The typical time (IO) Blakc. C . A,: Sastry, K. V . L. N . ; De Lucia, F. C . J . Chem. Phys. 1984, 80. 95. ( II ) Cornet, R.; Winnewisser, G. J . Mol. Spectrosc. 1980. 80, 438. ( 1 2 ) Clark, W. W.; DeLueia, F. C. J . Mol. Spectrosc. 1976, 60, 332. (13) Endo. Y.;Saito, S.: Hirota. E. J . Chem. Phys. 1985, 83, 2026.
4
3.0
\
2.
: "
0
U
1. 0
5. 0
-0. 5
0. 0 0. 5 t i m e / ma
1. 0
1. 5
Figure 2. Typical time evolutions in the ethylene (a), propylene (b), and I-butcnc rcaction (c): olcfins, 20 mTorr; SO2, I O mTorr; ArF laser, 120-100 mJ. Estimated initial O('P) concentration: 1.2 X I O ' * (a), 1.1 X IO" (b), and 1.0 X 10l2 atoms (c). 5. 0 m
'fi
4.0
-
3.0
\
2. 0
I
Io
i 1. 0 U
0. 0 -5.
0
0. 0
5. 0
10. 0
15. 0
time / ma Figure 3. Typical time evolutions in the ethylene reaction over a wider tiiiic rangc: C2H4,20 mTorr; SO2, 20 mTorr; ArF laser, ca. I 2 0 mJ; cstimatcd initial O(.'P) concentration. I .2 X l0l2atoms ~ 1 1 7 . ~ .
evolutions over a wider time range are also shown for the ethylene rcaction in Figure 3. In all reactions, the yields of vinoxy, HCO, and H 2 C 0 are relatively high. The concentrations of vinoxy and H C O decrease soon after they reach the maximum values at around I ms after the initiation of the reaction, showing that these radicals react very rapidly after their production. The rapid initial rises in the concentrations of vinoxy and HCO clearly show that they are initial products. On the contrary, the H 2 C 0signal increases after :I small timc delay, which indicates that H z C O is mainly a secondary product. The more rapid increases in the vinoxy and HCO radical conccntrations in the higher olefins correspond to the higher overall reaction rate.
The Journal of Physical Chemistry, Vol. 95, No. 3, 1991 1243
O(jP) Reactions of Terminal Olefins TABLE II: Branching Ratio and Fraction CJH4
+0
-
+
+
CJH6 0
+
I-C4H,
+0
-c
branching ratio CHICHO + H HCO C H j
I
+
fraction 0.46 f 0.15
1.17 f 0.18
CH2CHO + CH, HCO + CZHS others
E
0.29 f 0. I5
1
1.39 f 0.18
CI12CH0 + othcrs HCO others others
+
Y U
e +
0.37 f 0. IS
1
0.71 f 0.19 0
"
"
"
"
"
'
'
By comparing the initial rate of production of the individual species, wc could dctcrminc the relative yield of vinoxy vs HCO. The corresponding reactions are the following: CzH4 + 0
CHzCHO HCO
+ (H)
+ (CH,)
- (very minor channels) C3H'
I-C,H,
+0
CH?CHO
+ (CH,)
- HCO + (C,H,) - (others) + 0 - C H z C H O + (others) - H C O + (others)
CI V
(Ib)
L
e
(IC) (2a)
OLb'
(2b) (2c) (3a)
(3b)
-
Discussion The important finding in the present experiments is that the yield of vinoxy is relatively high in propylene and I-butene as well as ethylene. Olefins, in particular I-butene, absorb 193-nm light to some extent and may dissociate to produce hydrocarbon radicals including vinyl. However, they are not expected to react very rapidly with SOzto contribute to the production of vinoxy via such reactions as CH?=CH + SO? C H l C H O SO considering thc thcrmochcmistry. In fact, the above reaction is 28 kJ mol-' cndothcrmic based on the reported heat of formation of vinoxy.Is Therefore, vinoxy in the present experiments comes exclusivcly from thc O(3P) reaction of the olefins. The fractions of individual species have been reported by several prcvious rcscarchcrs. The C H J C O + CH, (ca. 50%)at less than (14) Cvetanovic, R. J. J . Phys. Chem. HeJ Data 1987, 16, 261. ( 1 5 ) Rossi. M.: Golden. D. M. In/.J . Chem. Kinrt. 1979, I / . 75
6
(la)
(others) (3c) Here, the products shown in parentheses are not confirmed experimentally, though we have evidenced the production of C H 3 in parentheses are not confirmed experimentally, though we have evidenced the production of CH, in ( 1 b) by use of infrared diode laser absorption spectroscopy in the previous report.' The ratio of (ib)/(ia) ( i = 1-3) has been measured for at least four different experimental runs under the 1 / 2 or 2/1 ratio of SO?vs the olefin pressure with keeping the total pressure at 30 mTorr. The determined average values with one standard deviation are shown in Table 11. The value for ethylene is I . 17 f 0.1 8, which is in good agreement with our previous determination, 1.2. Consulting the review article by C v e t a n ~ v i c ,we ' ~ can derive a relative overall rate constant for ethylene, propylene, and 1butene as 1:5.5:5.8. At the same time we can compare the initial production rate of vinoxy from the three olefins, after correcting for the difference in the initial O('P) concentrations. From these two relative values. and based on the fact that ( l a ) and (1 b) are the exclusive reaction channels in the ethylene reaction, fractions of (ia) ( i = 1-3) channel over the total reaction can be determined as shown in Table 11. The uncertainties shown in the value of the fractions come from the two main sources; one is the standard deviation of the measurements of the branching ratio and the other is the uncertainty in the calibration of the microwave absorption coefficients.
+
0
'
'
5'0
'
'
pressure / torr Figure 4. Pressure dependence or vinoxy fraction for the propylene (a) and I -butcnc reaction (b): circles. present work: squares, Blumenberg et triangles, Hunziker, et a.'
+
7.5 Torr in the propylene reaction) and CH,CO CzHS(ca. 40% at 3.5 Torr in I-C4H, reaction) channel reported by Blumenberg et are considered to be approximately equivalent to our vinoxy-producing channels, taking into account the difficulty of identifying vinoxy by use of a mass spectrometer which was employed by Blumenberg et al. They measured the C H 3 and CzHS signal intensities, and thus their fractions can be employed as they are. The vinoxy radical was observed in the molecular beam experiment via laser-induced fluorescence measurements by Kleinermanns and Luntz,Ib but they did not quantitatively determine the fraction. Anyhow, our observation of vinoxy as one of the major primary reaction products at lower pressures seems not to be incompatible with the above previous studies. On the other hand, Hunziker et aL7 determined the vinoxy fraction by use of modulation UV absorption spectroscopy under higher pressures and suggested a pressure dependence of the vinoxy fraction. I n Figure 4, the present results and the reported values by previous researchers'.' are plotted together against the pressure. The fraction of the vinoxy channel seems to increase with lowering the pressure, different from the apparent pressure independence of the vinoxy channel in the reaction of ethylene. Probably it is composed of a pressure-dependentand a pressure-independentpart, though it is difficult to differentiate them quantitatively. According to the various studies till now including our previous one concerning the deuterium isotope effect on the branching iatio,* the reaction of ethylene t O(3P) is considered to proceed through initial production of a triplet biradical. What products are finally produced is determined by the competition between the unimolecular decomposition of the triplet biradical to yield vinoxy H (channel la) and the intersystem crossing (ISC) to produce a singlet biradical. On the singlet surface, one hydrogen atom on the attacked carbon migrates, being followed by the C - C bond rupture (channel 1 b). The ISC is considered to proceed very rapidly even under a collisionless condition, which results in the apparent pressure independence of the branching ratio.'.? Similar rcaction processes are expected in the higher olefins, though we must take into account the fact that two different olefinic carbon atoms exist in these terminal olefins. The initial attack was supposed to be directed mainly to the terminal carbon, on the basis of the stable products analysis under higher press u r e ~ . ' ~However, a recent paper has claimed that the attack
+
(16) Klcincrmanns, K.: Luntz, A. C. J . Phys. Chem. 1981, 85, 1966. (17) Cvetanovic, R. J. Ado. Phofochem. 1963. 1. 115.
Koda et al.
1244 The Journal of Physical Chemistry, Vol. 95, No, 3, I991 C H ~ - C = C H ~+ o ( ~ P )
-
H
+n
LCH~CHCHO
CH3-6-r2 (triplet) J (ISC)
~ ~ ~ - 8 -products p (singlet)
1 CH3CH2-CHO
7
CH3
+ICI12CHO]
CH3CH2
+ HCO
products
Lothers other processes Figure 5. Reaction scheme suggested for the propylene reaction.
to the intcrnal carbon may proceed to a similar extent.' At first, we will consider the case where O()P) attacks the terminal carbon, consulting the proposed reaction scheme shown in Figurc 5 . Via formation of a corresponding triplet biradical, a hydrogen atom may dissociate from the terminal carbon (substitution channcl) or the triplet biradical may be converted to a singlet biradical (ISC channel) as in the case of the ethylene reaction. Because the biradical derived from higher olefins posscsscs a largcr number of internal degrees of freedom, the substitution reaction rate may be slower compared to the case of ethylene reaction according to an ordinary consideration for unimoleculnr rate constants.*.Ix At the same time, alkyl substituents in higher olefins may cause a faster intersystem crossing. Because of the above reasons, a higher yield of the singlet compared to the substitution channel is expected. On the singlet surface, a hydrogen atom may transfer from the terminal carbon onto the adjacent inner carbon. Afterwards, either the C-C bond which was originally a double bond breaks to yield HCO and the corresponding alkyl radical or the adjacent C-C bond may break to yield vinoxy and thc corresponding alkyl radical. These two ( I 8) Steinfeld, J. 1.; Francisco, J. S.;Hase, W. L. Chemical Kinetics and Dynamicc: Prentice-Hall International Inc.: London, 1989; Chapter I I .
channels contribute to (ib) (i = 2 or 3) and (ia) (i = 2 or 3) channels, respectively. Under higher pressures, the singlet biradical may be collisionally stabilized as it is, and the yield of both (ia) and (ib) channels decreases. Thus, the relatively high yield of vinoxy and its pressure dependence can be explained at least qualitatively. I f the initial O(3P) attack is directed to the inner carbon and the alkyl substituent dissociates (substitution reaction), vinoxy radical may be also produced. In this case, the yield of vinoxy radical should be mostly pressure independent, similarly to the reaction of ethylene. The ratio of the O(3P) atom attack to the terminal carbon against that to the inner carbon is not conclusively determined on the basis of the present study alone, but it is clear that the attack to the terminal carbon contributes to some extent in order to explain the pressure dependence of the vinoxy fraction. The decrease in vinoxy and HCO radicals after they reach the maximum values in their time evolutions is mainly due to their reactions with remaining O(3P) atoms and radical species including themselves. However, the present experiments are not intended to obtain quantitative information concerning the progress of these subsequent reactions. F'or example, it is difficult to take into account the possible loss of active species on the reaction cell walls, contribution of SO radicals in later stages, and so forth. However, important conclusions are that vinoxy as well as HCO radical indeed react with other atomic and/or radical species at very high rates and that HzCO is considered to be one of the main products, in any olefin reaction. Summary
I . The O(3P) reactions with ethylene, propylene, and 1-butene have been studied by means of a novel kinetic microwave absorption spectroscopic method combined with the pulsed production of O()P) by excimer laser photolysis of SOz. 2. The branching for the production of vinoxy is relatively high, which seems to be pressure dependent in the cases of propylene and I-butene reactions. The above finding implies that the production mechanism of vinoxy is different between ethylene and the other two olefins. In the latter case, at least some part of vinoxy is produced through the C-C bond dissociation after the intcrsystcm crossing.
Acknowledgment. This work has been supported partly by the Joint Studies Program (1988-1989) of the Institute for Molecular Science, which is greatly appreciated. Registry No. 0, 17778-80-2; H2C=CHCH3, I 15-07-1; H,C=CHCH2CH3, 106-98-9; H2C=CH,, 74-85-1: H,C=CHO, 691 2-06-7: HCO, 2597-44-6; H2C0, 50-00-0.
