J . Phys. Chem. 1991, 95, 1253-1257 ment of the quinoid distortion characterized by an additional IO-cm-' increase of this frequency. In any case, the quinoid distortion is less pronounced than in the 44BPY'- and 44BPYHz*' transicnts. Partial protonation of the H-bonded radical takes place in polar media according to thc equilibrium RO-H...NC: H4-C: H jN-H RO- H-NC5H4-CSH4N-H" Our observation of thc hydrogcn adduct radical 44BPYH' is in accord with thc idcntification of this radical by Elisei et aI.l3 from lascr flash photolysis cxpcrimcnts. However, its detection by Raman spectroscopy 30 ns after the pump excitation disagrees
+
Kinetics of the Reaction OH
+ C,H,
1253
with thc rcport by these authors that the radical is formed during the first microsecond following excitation and is not detected 50 ns after the laser pulse,I3 although the experimental conditions were comparable in both cases. No kinetic measurements could be made in the microsecond time scale with the present experimental setup because of the limitation of the optical time delay arrangement (maximum pump/probe delay available, 70-80 ns). A new excitation system comprising two lasers and an electronic time delay control has been currently developed and a detailed kinetic analysis of the hydrogen atom abstraction process by simultancous transient absorption and resonance Raman measurements is planned.
in He, N2, and O2 at Low Pressure
Ching-Hui Kuo and Yuan-Pern Lee*'+ Department of Chemistry, National Tsing Hua University. 101, Sec. 2, Kuang Fu Road, Hsinchu, Taiwan 30043, Republic of China (Received: April 9, 1990; In Final Form: June 22, 1990)
The rate coefficientsof the reaction of OH and C2H4 with the diluents M = He (f& = 1-5 Torr), N, (PN. = 0.8-3.2 Torr), and 0,(PO.= 0.3-1.5 Torr) have been determined by means of the discharge flow/resonance fluorescence technique. The rate coefficients agree well with the set of smaller reported values. The termolecular rate coefficients at the low-pressure limit (all in unit of cm6 molecule-2 s-') at 300 K are k'llHe= 2.7 f 0.5, k"'o. = 5.3 A 1.1. and k"lN, = 6.1 f 1.2; the uncertaintics rcpresent 9570 confidcncc limits. The ratc cocfficicnts for M = He was studied over the temperature range 25 1-430 K . A ncgativc activation cncrgy ( - E , / R ) in the range 1290-1 560 K, much greater than the previously recommended valuc, has bccn observed.
Introduction The reaction between C2H4and O H plays an important role in atmospheric and combustion chemistry. It is now accepted that at low temperatures the reaction mechanism is dominated by electrophilic addition of O H to the double bond of C2H4 (reaction I ) , whereas at temperatures above 750 K the H-atom abstraction (reaction 2) is likely the major channel.' O H + C2H4 + M C2HjOH + M (1) H2O + C2H3 O H + CZH, (2) There have been many measurements of the rate coefficient of reaction 1 at high pressures using various techniques.'-'s According to the current evaluation based on the reported studies, the value recommended for k,,,,the bimolecular rate coefficient cm3 molecule-' s-I at thc high-prcssurc limit is (9 f I ) X at 300 K.'h." A small negative ( E , , / R -400 K ) or zero activation energy for reaction I at high pressure is also recommcndcd. However, studics of this reaction at low pressure have been few. Most investigations were performed at a fixed pressure,'x.z'and hence the termolecular rate coefficient for reaction I could not be accurately evaluated. Howard employed the discharge flow/laser magnetic resonance technique to study the reaction at 296 K under 1-7 Torr of helium and reported a prcssurc dcpcndencc of the bimolecular rate coefficients; a value k, = 4 X cm3 molecule-' s-I was estimated in this limited range of pressure, but no explicit termolecular rate coefficient was given.22 The bimolecular rate coefficients reported by Howard arc approximatcly 60-70'X of thosc (under similar prcssurcs) determined by Davis and co-workers,s who employed the flash photolysis/resonance fluorescence technique to study reaction 1 at 300 K under 3-300 Torr of helium. The bimolecular rate cocfficicnts dctcrmincd under I Torr of helium by Morris et al.Ix and by Pastrana and CamZowere consistent with the values re-
-
+
'Also affiliated with the Institute of Atomic and Molecular Sciences, Acadcmia Sinica. Taiwan. R.O.C.
ported by Davis and co-workers, whereas that determined at 2.9 Torr by Bradley et al.IYwas in excellent agreement with those reported by Howard. The only reported termolecular rate
( I ) Liu. A . D.: Mulac, W. A.; Jonah, C. D. Int. J . Chem. Kinet. 1987, 19, 25. (2) Greiner, N . R. J . Chem. Phys. 1979, 53, 1284. (3) Smith, I . W. M.; Zellner, R. J . Chem. Soc.. Faraday Trans. 1973, 2, 1617. (4) Stuhl, F. Ber. Bunsen-Ges. Phys. Chem. 1973, 77, 674. (5) Davis, D. D.; Fischer, S.; Schiff, R.; Watson, R. T.; Bollinger, W. J . Chem. Phys. 1975, 63, 1707. ( 6 ) Gordon, S.; Mulac, W. A . Int. J . Chem. Kinet. 1975, Symp. I , 289. (7) Lloyd, A . C.; Darnall, K. R.: Winer. A . M.; Pitts, J . N., Jr. J . Phys. Chem. 1976. 80, 789. (8) Atkinson, R . ; Perry, R. A.; Pelts, J . N . , Jr. J . Chem. Phys. 1977, 66, 1197. (9) Overend, R.; Paraskevopoulos, G.J . Chem. Phys. 1977, 67, 674. (IO) (a) Atkinson, R.; Aschmann, S. M.; Winer, A . M.; Pitts, J . N . , Jr. In!. J . Chem. Kinet. 1982, 14, 507. (b) Atkinson, R.; Aschmann. S. M. Int. J . Chem. Kinef. 1984, 16, 1175. ( I I ) Klein, Th.; Barnes, 1.; Becker, K. H.; Fink, E. H.; Zabel, F. J . Phys. Chem. 1984, 88, 5020. (12) Zellner. R.: Lorenz. K. J . Phvs. Chem. 1984. 88. 984. ( I 3) Schmidt, V.; Zhu, G: Y.;Beck&, K. H.; Fink, E. H: Ber. Bunsen-Ges. Phys. Chem. 1985, 89, 321. (14) Tully. F. P. Chem. Phys. Lett. 1983. 96, 148. (15) Cox; R. A.; Derwent,.R. G . Enoiron. Sci. Techno/. 1980, 14, 57. (16) DeMore, W. B.; Molina, M. J.; Sander, S. P.; Hampson, R. F.; Kurylo, M. J.; Golden, D. M.; Howard, C. J.; Ravishankara, A. R. Chemical Kinetics and Pho!ochemical Data for Use in Stratospheric Modeling; Publication 87-41. Jet Propulsion Laboratory: Pasadena. C A , 1987. ( 1 7 ) Baulch, D. L.; Cox, R. A.; Hampson, R. F., Jr.; Kerr, J . A.; Troe, J.; Watson, R. T. J . Phys. Chem. ReJ Data 1984, 13, 1310. ( 1 8 ) Morris, E. D., Jr.; Stcdman, D. H.; Niki, H. J . Am. Chem. Soc. 1971, 93, 3570. (19) Bradley, J . N.; Hack, W.: Hoyermann, K.: Wagner, H. Gg. J . Chem. Soc., Faraday Trans. 1973, 69. 1889. (20) Pastrana, A. V.; Carr, R. W., Jr. J . Phys. Chem. 1975, 79, 765. (21) Farquharson, G.K.; Smith, R. H . Aust. J . Chem. 1980, 33, 1425. (22) Howard, C. J. J . Chum. Phys. 1976.65, 4771.
0022-3654/91/2095-l253%02.50/0 0 I991 American Chemical Society
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Kuo and Lee
The Journal of Physical Chemistry, Vol. 95, No. 3, 1991
coefficient at the low-pressure limit for reaction 1 in helium at 300 K, (3.1 f 0.5) X cm6 molecule-2 s-l, was determined by Farquharson and Smith using the discharge flow/resonance fluorcsccncc tcchniquc.2' However, the study was carried out with ~ ) a narrow largc concentrations of O H (> l0l3 molecules ~ m - and rangc (0.54-0.68Torr) of pressure. At low pressures, the reaction has also bccn studied under N, (3 Torr) by Davis et al.' and Ar (3 and IO Torr) by Zcllncr and Lorcnz.'? The latter cstimated a termolecular rate coefficient (at the low-pressure limit in Ar) cmh molecule-2 s-' from their measurements at 3-97 of 5.5 x Torr. Finally. there has been no study of the temperature dependence of the rate coefficient under low pressure. In view of the unccrtaintics in thc reported rate coefficients and thc nccd for thc study of the tcmperature depcndence of the termolccular ratc coefficient of reaction I , we have made careful mcnsurements under 1-5 Torr of helium in the temperature range 25 1-430 K by means of the discharge flow/resonance fluorescence technique. We have also studied the rate coefficients for M = Oz and N, at 300 K. Experimental Section The experimental technique for the discharge flow/resonance fluorescence system has bcen described in previous report^;?^.^^ hence only a brief description is given here. The OH radicals wcrc produccd by rcacting excess NO? with H atoms that wcrc gcncratcd by a microwave discharge of a gaseous mixture of H, and Hc. It was found that a ratio [NO,]/[OH],, > 15 was needed to ensure the determination of a rate Coefficient indcpcndent of [NO,]. The excessive NO2 presumably removed most organic radicals produced from reaction 1 ; these radicals have large reaction coefficients for their reactions with OH. When [NO,]/[OH], < IO, the rate coefficient incrcascd rapidly. C2H, was added through a movable injector downstrcain to whcrc OH was formed. The resonance fluorescence technique was employed to detect OH downstream from the flow reactor. The emission -309 nm from a microwave-discharged H 2 0 / H e mixture served as an excitation source. After passing a 309.6-nm interference filter, the fluorescence of OH was detected with a photon-counting system. Usually the flow tube and the injector were coated with a thin film of halocarbon wax to minimize the loss of OH due to wall reaction. For some experiments at higher temperature, a Teflon tube (22mm i.d.) was inserted inside the flow tube to replace the halocarbon wax. No difference in the rate coefficients has been observed. The value of k',, for the injector, typically -(l-3) s-l, was measured by producing O H in the sidearm of the flow tube and observing the OH decay with no C2H4 in the system as the injector was pullcd out. Thc valuc of k , for the flow tube, determined by passing NO, through the injector to react with H atoms in the flow tube and observing the O H decay as the injector was pulled out. was typically less than 3 s-'. These rate coefficients were determined at the beginning and at the end of each day's experiments. The C?H, has a stated purity of 99.5% and was used without purification except degassing at 77 K. The IR absorption spectrum of the gas sample revealed no measurable absorption due to impurities. Usually a diluted gas sample C2H4(approximately 2% in He), prepared with standard gas-handling techniques, was used in the experiments because of the small C2H4 concentration required in the experiments. The concentration of the C,H4/He mixture was dctcrmincd by comparison of the integrated IR absorption at I9 16.4,1914.7, 191 1.3,and I88 I .6 cm-' (determined with a Bomcm DA3.002 FTIR, resolution 0.1 cm-I) to those samplcs with known concentration. The accuracy of the concentration measurement was estimated to be approximately &3%. The NOz was prepared by slowly reacting N O (99%) with excess 0,(99.97%) and stored under O2for at least 24 h before use. The He (99.9995%) and H z (0.3% in He) were passed through a 77 (23) Lin. Y.-L.; Wang. N.-S.; Lee, 1201.
(24) Wang. N.-S.; Lee, 87.
Y.-P. Int. J . Chem. Kiner. 1985. 17,
Y.-P. Proc. Natl. Sci. Coun., Rep. China 1985. 9,
60
50
40
10 0' 0
20
10
30
40
50
I
60
molecule cm-3 Figure 1. Plot of k' vs [C,H,] at 300 K for M = He at 1-5 Torr. [C,H,]
-1
-g
-
h
/ lo',
251K
4
0
8
B
3
z ' 1 -Y
n 0
5
10
15
20
[He] / 10l6 molecule ~ m - ~ ' Figure 2. Plot or k" vs [Hc] at six tempcraturcs in the range 251-430
K.
K trap before being discharged. 0, (99.97%)and N 2 (99.999%) were used without further purification. Typical experimental conditions were as follows: flow velocity B = 850-1 600 cm s-l; total pressure P = 1.0-5.0Torr; [OH], = (0.8-2.0)X IO" molecule cm-3; [NO,] = (4-8) X 10l2molecule ~ m - [C2H4] ~; = (5-70)X 10l2molecule ~ m - total ~ ; flow rate FT = 4-20 S T P cm3 s-'; temperature T = 251-430 K. Results and Discussion The reaction was studied under pseudo-first-order conditions with [C2H4]at least 50 times larger than [OH],, the initial concentration of OH. As described p r e v i ~ u s l y ,the ~ ~ decay * ~ ~ of the OH fluorescence signal as a function of the reaction distance was determined to give the pseudo-first-order rate coefficient. The rate coefficient was then corrected for the removal of O H by the injector wall ( k f w )and the axial diffusion (usually
