J. Phys. Chem. 1989, 93, 1035-1042
+
1035
series, the OH HOz abstraction channel shows a very early transition state. The calculations also show essentially no barrier to reaction, and the H-bond minimum of ref 23 is also apparent. These calculations are therefore quite consistent with the picture presented so far; that is, the early transition state and a slight minimum in the potential energy surface coupled with essentially no barrier lead to a large cross section for the abstraction channel. In summary, the "Holy Grail" reaction is still incompletely understood, but a more consistent picture seems to emerge, particularly if one concedes that the pressure dependence is small or negligible. In this picture the dominant mechanism is H-atom
abstraction over a loose transition state and the small negative temperature dependence may be explained by an attractive long-range hydrogen-bond interaction between the HO and HOO molecules. The picture is by no means complete, and further study of this fascinating reaction is clearly warranted. Studies which would shed further light on this reaction include: (a) measurement of the products, including the energetically accessible 02(A); (b) additional isotope studies, including H2I80if possible; (c) single studies ranging over large temperature and pressure ranges to observe the onset and falloff of any pressure dependent channels; and (d) studies of decays of HO, in excess OH.
(27) Hampson, R. F. "Chemical Kinetic and Photochemical Data Sheets for Atmospheric Reactions"; Report No. FAA-EE-80-17, U.S. Federal Aviation Administration, 1980. (28) Kurylo, M. J. J. Phys. Chem. 1972, 76, 3518. (29) Wong, W.; Davis, D. D. Int. J . Chem. Kinet. 1974, 6, 401.
Acknowledgment. We acknowledge useful discussions with Darin Toohey and Jon Abbatt. This research was supported the National Science Foundation, Grants ATM-8601126 and CHE8601431. Registry No. OH,3352-57-6; H02, 3170-83-0.
'
Oxygen Atom Exchange in the Interaction of ''OH with Several Small Molecules Gary D. Greenblatt and Carleton J. Howard* Aeronomy Laboratory, National Oceanic and Atmospheric Administration, Boulder, Colorado 80303, and Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado (Received: March 15, 1988)
Oxygen atom exchange between 180H and several oxygen-containing molecules was studied in a flow tube by using laser magnetic resonance detection of the reagent l80H and product I60H. No significant exchange was observed for 02,H20, CO, C02, NzO, OCS, and SO2at 298 and 400 K, and upper limits to the exchange rate coefficients are reported. The rate and (1.44 f 0.15) X coefficients for the reactions of l80H and 160H with CO were found to be (1.49 & 0.15) X cm3 molecule-' s-], respectively, at 298 K. NO and NO2 were found to exchange rapidly with k,, = (1.8 0.6) X IO-'' and (1.0 h 0.4) X lo-" cm3 molecule-l s-l, respectively, at 298 K. On the basis of a simple model of adduct formation k , values for the OH NO and NO2 association reactions were estimated to be Z(3.6 f 1.2) X lo-" and Z(1.5 f 0.6) X lo-" cm3 molecule-' s-l, respectively. Error limits are 95% confidence limits.
+
Introduction The application of isotopes to kinetics has aided in the analysis of mechanisms,' provided information about the geometries of reaction intermediates and potential energy surface^,^^^ and has been used to eliminate interference from secondary reactions that regenerate reactant^.^ The effect of quantum mechanical tunneling has been demonstrated by substitution of deuterium for hydrogen atomse5 Also the dynamics of reactions that occur through the formation of bound complexes including association reactions and radical-radical reactions can be probed by using isotopically labeled reactant^.^^' Oxygen atom exchange has been studied in reactions of isoand H0210 tope-labeled oxygen atoms with to measure the rate coefficients for the formation of the reaction intermediates. More recently, Van Doren et al." have studied ion-molecule reactions of 0- and have observed rapid isotope exchange between lSO-and CO, SO2,NO, N20,HzO, COz, and 02.A potentially fertile area of kinetics is the study of oxygen isotope exchange between hydroxyl radicals and oxygen-containing diatomics and triatomics. Hydroxyl radical reactions are fundamental to many combustion processes12 and to atmospheric chemistry13 and have been the focus of theoretical studies.14-17 Examples are the reactions of hydroxyl with nitric oxide and nitrogen dioxide: OH + NO + M -,H O N O M (1)
The rate constants for these reactions have been studied by several techniques over a broad range of pressures and temperature^.'^-^^ ~
0 2 6 . 8 9 9
OH
+ NO2 + M
-
+ HON02 + M
(2)
Author to whom correspondence should be addressed at NOAA, R/E/ AL2, 325 Broadway, Boulder, CO 80303.
0022-3654 ,/89 ,12093- 1035SO1.50/0 I
~
(1) Greenblatt, G. D.; Zuckermann, H.; Haas, Y. Chem. Phys. Lett. 1987, 134, 593. (2) Gericke, K.-H.; Comes, F. J.; Levine, R. D. J . Chem. Phys. 1981, 74, 6106. (3) Butler, J. E.; Jursich, G. M.; Watson, I. A.; Wiesenfeld, J. R. J. Chem. Phys. 1986, 84, 5365. Cleveland, C. B.; Jursich, G. M.; Trolier, M.; Wiesenfeld, J. R. J. Chem. Phys. 1987, 86, 3253. (4) Sinha, A.; Lovejoy, E. R.; Howard, C. J. J . Chem. Phys. 1987, 87, 2 122. ( 5 ) Smith, I. W. M. Kinetics and Dynamics of Elementary Gas Reactions; Butterworths: London, 1980; p 195 and references therein. (6) Anderson, S. M.; Klein, F. S.; Kaufman, F. J . Chem. Phys. 1985,83, 1648. (7) Dransfeld, P.; Lukacs, J.; Wagner, H. Gg. Z . Naturforsch., A , : Phys., Phys. Chem., Kosmophys. 1986, 41, 1283. (8) Herron, J. T.; Klein, F. S. J. Chem. Phys. 1964, 40, 2731. (9) Brennen, W.; Niki, H. J. Chem. Phys. 1965, 42, 3725. (10) Sridharan, U. C.; Klein, F. S.; Kaufman, F. J . Chem. Phys. 1985,82, 592. (1 1) Van Doren, J . M.; Barlow, S. E.; DePuy, C. H.; Bierbaum, V.M. J. Am. Chem. SOC.1987, 109,4412. (12) Atkinson, R. Chem. Reu. 1986, 86, 69. (1 3) Graedel, T. E. In The Photochemistry of Atmospheres; Levine, J. S., Ed.; Academic: Orlando, FL, 1985. (14) Smith, I. W. M. Chem. Phys. Lett. 1977, 49, 112. Smith, I. W. M.; Zellner, R. J. Chem. SOC.,Faraday Trans. 2 1973, 69, 1617. (1 5 ) Zellner, R. J . Phys. Chem. 1979, 83, 18. (16) Golden, D. M. J . Phys. Chem. 1979,83, 108. (17) Mozurkewich, M.; Benson, S. W. J . Phys. Chem. 1984, 88, 6429. Mozurkewich, M.; Lamb, J. L.; Benson, S. W. J . Phys. Chem. 1984,88,6435. Lamb, J. L.; Mozurkewich, M.; Benson, S. W. J . Phys. Chem. 1984,88,6441. (18) Anderson, J. G.; Margitan, J. J.; Kaufman, F. J . Chem. Phys. 1974, 60. 3310.
D 1989 American Chemical Societv -
Greenblatt and Howard
1036 The Journal of Physical Chemistry, Vol. 93, No. 3, 1989
TABLE I: Transitions Used To Detect I6OH and '*OH laser wavelength,
magn field,
Lac,m
kG
detection limit," molecules cm-l
118.6 79.1 79.1
14.4 6.6 3.8
3 x 107 3 x 109 3 x 109
molecule
160H 160H l80H
--
transition
211,/2,J = M , = -3/2 2&129 J = j 2 , Ml = -'I2 2n1p,J = ' 1 2 , MJ = -'I2
'
-
2n,,2,J 2113/2, J 'II312, J
= 3j2, M ] = -3j2 = ' j 2 , MJ = -'I2 = 3 j 2 , M I= -Ij 2
'Signal/noise = 1 with a time constant of 0.3 s.
H O N O is a temporary nighttime sink for O H and N O radicals in the polluted trophosphere. HONOz serves as a reservoir for both hydrogen oxides and nitrogen oxides in the troposphere. Dransfeld et aL7 have recently observed oxygen isotope exchange in reactions of l80H with NO and NOz at room temperature. For isotope scrambling to occur, the hydrogen atoms in the hot HONOt and HONOJ complexes have to be labile. Exchange-rate constants were obtained that were comparable to the values of k_,7 the high-pressure limit of the rate coefficients for the association reactions. Another important system is the reaction of hydroxyl with carbon monoxide, which has a central role in such diverse environments as in the atmosphere and in hydrocarbon c o m b u s t i ~ n : ~ ~
OH
+ CO
-+
C02
+H
(3)
In the troposphere, this is a major O H radical removal pathway and also determines the lifetime of carbon monoxide. Equally important, reaction 3 is a mechanism by which carbon dioxide is produced in combustion conditions. The temperature and pressure dependencies of the reaction have been extensively studied to encompass the wide range of conditions over which this reaction occurs, but there is considerable disagreement among these studies. Only recently have many of the discrepancies been resolved in the pressure and temperature dependencies.zs-26 A mechanism based on transition-state theory was proposed by Smith and Zellner to explain quantitatively the observed temperature and pressure b e h a ~ i 0 r . lAccording ~ to their model, the hydroxyl and carbon monoxide react to form a hot intermediate complex that has a lifetime of at least several vibrational periods:
(6 Torr). With COz, both C'60180and Cl8O2exchange products were detected at all pressures up to 760 Torr. The results were interpreted to suggest the existence of "long-lived" HOCO and HOCOZcomplexes with hydrogen movement leading to isotope exchange. A second study by Stevens and c o - w ~ r k e r susing ,~~ similar experimental conditions and detection, examined the kinetic isotope effect of I60H with isotopically enriched C O in 140-760 Torr of air, Oz, Ar, and He. The ratio of the rate constants for the carbon- 12 and -1 3 isotopes and for the oxygen- 16 and - 18 isotopes were close to unity, indicating only a minor kinetic isotope effect for reaction 3. They did not observe any exchange and were able to place an upper limit to the rate constant for the exchange cm3 molecule-' s-l. In a third study of oxygen of k , C 3 X of reaction 3 by Niki and ~ o - w o r k e r sFourier , ~ ~ transform infrared spectrometry was used to measure the rate coefficient for the reaction of O H with l3CI6Oand l2C'*0 in 700 Torr of air. To eliminate possible complications from isotope scrambling with the COz product, an attempt was made to observe 1zC'60180from the I60H CI8O2reaction. N o exchange product was detected, in contrast to the earlier results of Kurylo and Laufer. In this paper, we report the results of a study of the oxygen atom isotope exchange between I80H and normal NO, NOz, NzO, CO, C 0 2 , OCS, SO2, HzO, and Oz. The objective of this experiment was to learn about the dynamics of the collision complexes and the mobility of the H atom in these complexes. This information is useful to understanding the mechanisms of gasphase reactions.
+
Experimental Section The oxygen atom exchange reactions were studied in a discharge-flow apparatus in an experiment similar to that reported OH + CO HOCO' CO, + H (4) by Dransfeld et al.' The I60H and l 8 0 H concentrations were measured by laser magnetic resonance detection. The flow tube, the detection system, and the measurement techniques were deHOCO scribed in detail p r e v i ~ u s l y . ~ ~ Paramagnetic species were detected by using a laser line in near The complex can dissociate either by way of the product channel resonance with a rotational transtion. A magnetic field was tuned or to re-form the reagents. At high pressures, the hot complex until an O H rotational transition was in resonance with the laser HOC07 can be stabilized by collisions. Cis and trans isomers of HOCO have been observed in low-temperature m a t r i ~ e s . ~ ~ J ~and absorption occurred. The laser frequencies and magnetic fields used to detect l80H and I60H were confirmed by using the OH Oxygen isotope exchange between hydroxyl and CO and COz has rotational frequencies measured by Evenson and c o - ~ o r k e r sand ~~ been reported by Kurylo and L a ~ f e r . 2In ~ their experiment, H2'*0 ~ I lists the the g values for O H reported by R a d f ~ r d . ~Table was photolyzed in the presence of C I 6 0 (or COz) and an inert transitions used in our experiments. The detection of O H at the buffer gas in a static cell. After a period of irradiation, the 79.1-pm laser line was 100 times less sensitive than at the 118.6-pm products were analyzed by mass spectrometry. Product CI8Owas line because the transitions are between the O H 2113/2 and zI11,2 detected, but only after irradiation at low pressures of buffer gas spin states are electric dipole forbidden to a first-order approximation. The magnetic field required to detect 180H at 118.6 pm was beyond the capability of our electromagnet. (19) Anastasi, C.; Bemand, P. P.; Smith, I. W. M. Chem. Phys. Lett. 1976, A 2.54-cm i.d. Pyrex reactor was fitted with a 0.95-cm 0.d. 37, 370. (20) Anastasi, C.; Smith, I. W. M. J. Chem. Soc., Faraday Trans. 2 1976, movable inlet to vary the reactant contact distance over a length 72, 1459. of 3 5 cm. With the inlet fully inserted, there was a distance of (21) Anastasi, C.; Smith, I. W. M. J. Chem. Soc., Faraday Trans. 2 1978, 5 cm from its tip to the detection region. The O H radicals were 74, 1056.
-
(22) Robertshaw, J. S.; Smith, I. W. M. J . Phys. Chem. 1982,86, 785. (23) Burkholder, J. B.; Hammer, P. D.; Howard, C. J. J. Phys. Chem. 1987, 91, 2136. (24) Cox, R. A,; Derwent, R. G.; Holt, P. H. J. Chem. SOC.,Faraday Trans. 1 1976, 72, 2031. (25) Ravishankara, A. R.; Thompson, R. L. Chem. Phys. Lett. 1983,99, 377. (26) Hynes, A . J.; Wine, P. H.; Ravishankara, A. R. J. Geophys. . . Res. 1986, 91, i1815. (27) Milligan, D. E.; Jacox, M. E. J . Chem. Phys. 1971, 54, 927. (28) Tevault, D. E.: Lin. M. C.; Umstead. M. E.; Smardzewski. R. R. Inf. J . Chem. Kinet. 1979, 1 1 , 445. (29) Kurylo, M. J.; Laufer, A. H . J. Chem. Phys. 1979, 70, 2032.
(30) Stevens, C.M.; Kaplan, L.; Gorse, R.; Durkee, S.; Compton, M.; Cohen, S.; Bielling, K. Int. J. Chem. Kine?. 1980, 12, 935. (31) Niki, H.; Maker, P. D.; Savage, C. M.; Breitenbach, L. P. J. Phys. Chem. 1984, 88, 2116. (32) Howard, C. J.; Evenson, K. M. J . Chem. Phys. 1974, 61, 1943. J . Chem. Phys. 1976, 64, 197. Howard, C. J. J. Phys. Chem. 1979, 83, 3. (33) Brown, J. M.; Schubert, J. E.; Evenson, K. M.; Radford, H. E. Astrophys. J . 1982, 258, 899. Comben, E. R.; Brown, J. M.; Steimle, T. C.; Leopold, K. R.; Evenson, K. M. Astrophys. J. 1986, 305, 513. (34) Quoted from: Carrington, A. Microwaue Spectroscopy of Free Radicals; Academic: London, 1974; p 190.
The Journal of Physical Chemistry, Vol. 93, No. 3, 1989 1037
Interaction of I8OH with Several Molecules generated in a sidearm at the top of the reaction zone by either of two methods: OH NO H NO2 k = (1.3 f 0.2) X lo-', cm3 molecule-' s-l (ref 35) (5)
+
+
F
+ Hz0+
OH + HF k = (1.3 f 0.1) X lo-" cm3 molecule-'
s-I
(ref 36) (6)
The H or F atoms were generated from dilute mixtures of HZor CF4 in helium, in a 2450-MHz microwave discharge. The sidearm had separate discharge tubes for H and F atom generation as it was necessary to condition the surface of the F atom discharge region. This minimized the production of impurities from the walls in the discharge. The walls of the flow tube, the injector, and the sidearm (except for the discharge regions) were coated with halocarbon wax to minimize the O H wall loss rate. Typical loss rates in the flow tube and the injector were k, = 10 s-l and kI < 5 s-l, respectively. Any vibrationally excited O H from the source was rapidly deactivated by collisions with the reagent species. Rate constants for vibrational quenching of OH(v"= 1) by H, NOz, NO, and HzO are 3 X 10-10,37 (4.8 f 0.8) X and 1.4 X lo-" cm3 molecule-' re(3.8 f 0.6) X spectively. Experiments were conducted at 296 f 2 K and at pressures of 135-335 Pa (1-2.5 Torr). Several experiments were conducted a t 400 f 3 K to examine temperature effects. Average flow velocities were 500-1500 cm s-l. The carrier gas was helium in all experiments. Pressures were measured with capacitance manometers, which were calibrated against a water manometer and found to be accurate to f2%. Flow rates for He, CO, H20, and the CF4/He mixture were measured by calibrated thermal conductivity mass flowmeters. The flowmeters were calibrated, and the flow rates for all other gases were determined by measuring the pressure rate of change in a calibrated volume. All the flow rates were accurate to f5% with the 95%confidence level. The reagent concentrations used in the rate constant measurments were [OH], = (0.8-1) X lo", [NO] = (0.5-5) X 1OIz, [NO21 = (0.2-1) X 1013,[CO] = (1-6) X [C2F3Cl]= (0.3-3) X [HzO] = (0.2-1.6) X 1014, and [He] = (3-6) X 10l6 molecules ~ m - ~The . maximum concentration for SO2,N 2 0 , Oz, COz, OCS, and NZ, was 1 X 10l6 molecules ~ r n - ~Since . large amounts of these reactant gases were required to set upper limits on k,, blank runs with N z gas were made to simulate the effects of the large reactant gas flow rates. The I60H signal was calibrated by adding known amounts of NOz (3 X 1OIo-1 X 10l2 molecules ~ m - through ~) the movable inlet to flows of excess hydrogen atoms (0.5 X 1013-3.0 X lOI3 molecules ~ m - that ~ ) were generated in a sidearm and measuring the I60H laser magnetic resonance (LMR) signal. With these initial concentrations, reaction 5 was fast and pseudo-first-order in NOz. With a velocity of 1200 cm s-l, the reaction distance was set at 1 5 cm to ensure complete conversion (>99%) of the NO2 to O H and to allow for the relaxation of any vibrationally excited O H by hydrogen atoms. The apparatus was calibrated for I60H by plotting the LMR signal versus [NO2], (=[I60H]). The plot was linear, and the slope was the calibration factor. Afterward the source was changed to the F + H2I60reaction, and a known [I6OH] was established from the LMR signal. The water source was then changed to the isotopically labeled HZl80 to generate I80H under identical conditions. By assuming that the same amount of radicals was present from the labeled water, it was thus possible to calibrate the strength of the I 8 0 H signal relative to the 160Hsignal. A second calibration method employed the stoichiometric conversion of the l80H to I60H by adding (35) Clyne, M. A. A.; Monkhouse, P. B. J . Chem. SOC.,Faraday Trans. 2 1977, 73, 298. ( 3 6 ) Frost, R. J.; Green, D. S.; Osborn, M. K.; Smith, I. W. M. Int. J . Chem. Kinet.1986, 18, 885. (37) Spencer, J. E.; Glass, G. P. Chem. Phys. 1976, 15, 35. (38) Smith, I. W. M.; Williams, M. D. J. Chem. SOC.,Faraday Trans. 2 1985, 81, 1849. (39) Glass, G. P.; Endo, H.; Chaturvedi, B. K. J . Chem. Phys. 1982, 77, 5450.
excess N O to the flow tube. As discussed below, the exchange was rapid and allowed for the complete conversion between isotopic species. The results of the two calibration methods agreed to within 10%. The purities of the gases, as stated by the manufacturers, were as follows: CO, > 99.995%,N z > 99.999%,N 2 0 > 99.99%,Oz > 99.999%,He > 99.999%,C O > 99.99%,SO2(anhydrous grade) > 99.98%, OCS > 97.5%,and N O > 99.0%. C O was further purified by passing it through an oven at 820 "C packed with quartz wool to remove iron carbonyl compounds. N O was passed through a trap containing silica gel at 196 K to remove NO2 and other impurities. H e was passed through a trap a t 77 K packed with molecular sieve. The HZl80reagent had a minimum enrichment of 95%I8O. Both HZl80and H2160were degassed prior to use. NO2 was synthesized by reacting N O with Oz. The product NO2was colIected and purified by trap-to-trap distillation in excess O2until a white solid was obtained. The NOz was stored in a metal cylinder under 300 Torr of Oz. Prior to use, the NO2 was frozen at 196 K, and the Oz was pumped away. The NO2 reservoir was kept in an icewater bath while the NO2 was being removed, to maintain a stable NOz vapor pressure and flow rate. The reaction of I60H with CzF3Cl, a known O H scavenger, was studied to test the OH source (F HzO). From three measurements at a pressure of 2.0 Torr, a rate coefficient of (6.1 f 1.2) X cm3 molecule-' s-l was obtained, in close agreement with a previous result of (6.0 f 1.2) X 10-12.40 This indicated that the source was operating cleanly with no interferences from secondary processes and also determined that accurate kinetic measurements were obtained. The flow tube kinetics including the effects of O H loss on the flow tube and injector surfaces in the I 8 0 H + N I 6 0 exchange reaction were examined by a computer model of the reaction scheme. The model FORTRAN program solves a system of equations using a GEAR differential equation a1gorithm4l and has been described previously!z Axial diffusion was not included in the model as the correction was calculated to be less than 5%.
+
Data Analysis Oxygen atom exchange between I80H and N I 6 0 proceeds with a bimolecular rate constant, k,, (=k7a= k7b):
I80H + N I 6 0 I60H + NI80
-+
+
+ N180 180H + N I 6 0
I60H
(7a)
(7b)
The kinetic analysis of a similar reaction system proceeding to equilibrium has been described by Anderson et al.6*43 in their study of I8O exchange reactions with NO. From an initial set of concentrations for the reagents and products, ['*OHIO, [160Hlo, [Nl60Io, and [NI80lo, the system proceeds to equilibrium: ['80H]e[N'60],
-- K , = - k7b
['60H]e[N'80],
k7a
1
(8)
The equilibrium constant, K,, is unity if the primary isotope effects are negligible. This assumption is justified because the changes in the total partition functions and the zero-point energies nearly cancel in (8), giving a value for K, close to unity (K,= 0.94) at 298 K. Using the relationships that the total N O and O H concentrations are conserved
+ [NI8O], [OHIT = [I60H], + [l80HIo= [l60H], + [l8OH],
[NOIT = [NL60],+ [N'80]o = [N160],
(sa)
(9b)
(40) Howard, C. J. J . Chem. Phys. 1976, 65, 4771. (41) Hindmarsh, A. C., Report UCID-30001, Lawrence Livermore Laboratory Publication, 1972. (42) Hills, A. J.; Howard, C. J. J . Chem. Phys. 1984,81,4458. (43) There is a misprint for the general expression in footnote 30 in ref 6 . The correct expression is
['*0], + [N'*O], [NOIT + [OIT
1
1038
Greenblatt and Howard
The Journal of Physical Chemistry, Vol. 93, No. 3, 1989 75
TABLE II: lsOH Exchange Rate Coefficients (in cm3 molecule-' s-') at 298 K
k." reactant
this work (1.8 i 0.6) X (1.0 i 0.4) X lo-"" < i x 10-17 < I x 10-1s" 8.0 X 1.8 X IO-'' (3.8 f 0.6) X IO-" (1.5 f 1.0) X lo-"
isotope exchange isotope exchange pressure dependence pressure dependence pressure dependence vibr relax evaluation
(21.5 f 0.6) X IO-" (1.1 f 0.2) x 10-11 1.6 X lo-" >1.6 X lo-" >3.0 X lo-" (4.8 f 0.8) X lo-" (2.4 f 1.2) X IO-"
OH NO2 isotope exchange isotope exchange pressure dependence pressure dependence pressure dependence vibr relax evaluation
this work Dransfeld et ai.' Anastasi and Smith2] Atkinson et aLS8 Overend et aLS9 Smith and Williams38 DeMore et
+
this work Dransfeld et ai.' Anastasi and Smith" Wine et Robertshaw and Smithz2 Smith and Williams3* DeMore et al."
distributed among the oxygen atoms of the adduct, these values represent lower limits to k,. Table V is a compilation of k, values determined by various experimental methods. Included in the table are the values recommended from an evaluation of kinetic data.44 These values are weighted averages of different experimental studies. Direct determinations of k, for OH reactions with N O and NO, are not available because it is difficult to make measurements at sufficiently high pressures to obtain an accurate extrapolation. Values of k , have been extrapolated from measurements of the effective bimolecular rate constant as a function of pressure (see Table V). The extrapolation is expected to be more reliable for NO2 than for N O because only for NO, did the data reach the falloff region, covering a wider range of the effective bimolecular rate constant. Another method of estimating k , has been developed by Smith and c o - ~ o r k e r swho , ~ ~ used vibrational energy-transfer reactions to probe the adduct formation rate constant. Their method involves marking the reagent OH with a quantum of vibration to distinguish it from the ground-state OH. Since vibrational relaxation of OH(v=l) by collisions with NO and NO, proceeds via the adduct, the rate coefficient for this process should be a measure of k,, unless the added quantum of vibration has an effect on the adduct formation rate or the relaxation process is not efficient. For NO, the values of k, determined from isotope exchange (23.6 X 10-I' cm3 molecule-' s-') and vibrational relaxation experiments (3.8 X lo-'' cm3 molecule-' s-') agree remarkably well. These values are significantly larger than values from the pressure-dependence experiments ((0.8-1.8) X lo-'' cm3 molecule-' s-') and suggest a need to extend the measurements to higher pressures. For NO,, our isotope exchange experiment yields a value (11.5 X IO-'' cm3 molecule-' s-') in reasonable agreement with pressure-dependence studies (21.6 X lo-" cm3 The value of k , from the vibrational relaxation molecule-' SI). study is notably higher (4.8 X lo-'' cm3 molecule-' s-'). A likely explanation for the efficient quenching of OH(u=l) by NO2 is that this reaction could p r d by both a H O N 0 2 and a HOONO complex. The HOONO complex formed from vibrationally excited OH also has an exothermic channel leading to HOz N O products which is not significant for ground-state reactants at room temperature. The HOONO mechanism proceeds via a loose transition state with a collision rate coefficient of -3 X lo-'' cm3 molecule-' s - ' . ~ ' Thus the vibrational deactivation study may greatly overestimate k,. If the vibrational deactivation were via the HONOz complex only, the measured rate constant for the process is applicable to k,. The HOONO isomer is unlikely to
+
(58) Atkinson, R.; Hansen, D.A,; Pitts, Jr., J. N. J . Chem. Phys. 1975, 62, 3284. ( 5 9 ) Overend, R.; Paraskevopoulos, G.; Black, G. J. Chem. Phys. 1976,
4149. (60) Wine, P. H.; Kreutter, N. M.; Ravishankara, A. R. J . Phys. Chem. 1979, 83, 3191. (61) Howard, C. J. J . Am. Chem. SOC.1980, 102, 6937.
64.
The Journal of Physical Chemistry, Vol. 93, No. 3, 1989 1041
Interaction of '*OH with Several Molecules
X
I dovoble Reactant Inlet
X
qadical Source Inlet c- Source
Gas
I
T
Reaction
n Region
zone
Figure 3. Schematic diagram of a flow tube with a movable inlet and a sidearm radical source reactor. T is the fixed reaction time between the sidearm and the detection region. t represents the variable reaction time from the end of the movable inlet to the detection region. [XI,is the initial concentration of X, the radical reagent, in the flow tube.
undergo isotope exchange, so this channel would not be observed in the present measurement. Although it is possible that HOONO, which is more weakly bound than HONOZ,may isomerize and be stabilized as HON02.23 Recently, Smith and co-workers6z have measured the rate constant for the deactivation of OH(u=l) by CO. As discussed above for the analogous OH(u=l) N O reaction, the reaction is expected to proceed via a hot H O C 0 complex. The relaxation cm3 rate constant by CO was measured to be (1.0 f 0.2) X molecule-' s-l and presently is the only experimental estimate of k , for the O H + CO reaction. It is interesting to note that this k , value is an order of magnitude slower than the k , values for OH N O and NOz. The results of the present study, that O H undergoes rapid oxygen atom exchange with N O and NOz but not with CO and SO2,should serve as a motivation for new theoretical studies. It would be interesting to understand these results based on the potential energy surfaces of the O H interactions with NO, NOz, CO, and SOz.
+
+
Appendix: Kinetic Analysis of a Product in a Flow Tube Reactor The analysis of the kinetics in a flow tube with a movable reactant inlet and including wall losses was described previously by Westenberg and de H a a ~ .In~this ~ appendix, we present an alternate treatment of this analysis. Figure 3 is a schematic diagram of a flow tube. An inert gas enters the flow tube at the left and establishes a gas flow velocity, u. A radical reagent X is introduced into the flow tube through a sidearm and has an initial concentration [XI, in the flow tube. The second reactant R enters the center of the flow tube through a movable inlet. The reaction between X and R occurs along the reaction zone, z, which is the distance from the mixing point of X and R to the detector. Kinetic measurements are made by measuring [XI in the detection region for several positions of the movable inlet. The reaction distances z are transformed to reaction times by using the relationship t = Z/U
(A. 1)
The data ([XI vs z) can then be analyzed to yield the bimolecular rate coefficient for reaction of X with R. In the flow tube, the radical is removed by three processes: (62) Brunning, J.; Derbyshire, D. W.; Smith, I. W. M.; Williams, M. D. J . Chem. Soc., Faraday Trans. 2 1988, 84, 105. (63) Westenberg, A. A.; de Haas, N. J. Chem. Phys. 1967,46,490; 1968, 48, 4405; 1969, 50, 707.
wall
Reactant R
+R
-+ P
X-
inlet
k,
('4.2)
kI
other products
(A.3)
k' = k[R]
(A.4)
Reaction A.2 is the first-order loss of X on the surface of the flow tube. This occurs over the fixed distance from the point at which X enters the flow tube from the sidearm source to the detector and has a fixed reaction time T . Reaction A.3 is the first-order loss of X on the inlet surface. As the inlet is retracted to give longer reaction times, less inlet surface area is exposed to X and consequently less X is lost via (A.3). This wall loss occurs from the sidearm to the end of the inlet and has a reaction time ( T t ) . Reaction A.4 is a reaction of X with a second reagent, R, which is introduced through the inlet and has a second-order rate constant k. With [R] >> [XI, reaction A.4 is pseudo-first-order in X, where k' = k[R] is the first-order rate constant. Reaction A.4 has a reaction time of t . The concentration of X measured at the detector, [XI,, is described by [XI, = [XI, exp(-k,T)
exp(-k,(T - t ) ) exp(-k[R]t)
(AS)
In an experiment, the inlet position is varied and [XI, is measured as a function o f t . By changing eq A S to logarithmic form and differentiating with respect to t, one obtains the following: d(ln [X],)/dt = kI - k[R) (-4.6) The slope of the plot of In [XI, versus reaction time is equal to the effective first order rate constant ( k , - k[R]). In a flow tube experiment, the experimental conditions are established so that kI is very small or else one may correct for it. In most experiments kI is determined by measuring the change in [XI vs z (or t ) with [R] = 0. The bimolecular reaction rate constant, k, is determined from the slope of a plot of ( k , - k[R]) versus [R] and the intercept is kI. A movable inlet experiment with a fixed radical source does not yield the wall loss rate constant for the radical on the flow Thus the loss processes tube wall, k,, as suggested e1sewhe1-e.~~ in a flow tube are separable when the decay of a reagent is monitored. This analysis also shows that when using the movable inlet method, the absolute t or z values are not needed. The rate coefficient k for reaction A.4 can also be measured from the rise in the product concentration [PI. However, the analysis is complicated when wall reactions are present.63 The product [PI is generated only in the reaction region and during the reaction time t . In some cases, it is also lost on the flow tube surface
P-
wall
k,P
where k,P is the first-order loss rate constant for P. Assuming P is formed only by reaction A.4 and lost only by reaction A.7, the rate equation for P is d[P]/dt' = k[R] [XI - k,P[P] ('4.8) where i r i s a time between the end of the inlet and the detector. To solve eq A.8, it is necessary to insert an expression for [XI at each time t'in the reaction zone. The [XI a t the beginning of the reaction zone, [XI,, varies with the position of the inlet. For a fixed value of t [XI1 = [XI, expHk,
+ k d ( -~ t))
(A.9)
and [XI for some point t' after the inlet [XI = [XI, exPHkw + k[Rl)t?
(A.lO)
Equation A.8 is solved by substituting for [XI, with eq A.9 and A.lO, to obtain d[P] /dt' = -k,P[P] k[RI [XI, exp(-(kw+ kIX7 - t ) ) exp(-(kw + k[Rl)t? 64.1 1)
+
(64) Finlayson-Pitts, B. J.; Pitts, Jr., J. N. Atmospheric Chemistry; Wiley: New York, 1986; pp 240-242.
J . Phys. Chem. 1989, 93, 1042-1048
1042
This expression is integrated from t' = 0 to t with the boundary condition that at t' = 0, [PI = 0 and t' = t , [PI = [PId at the detector. The final result is k[RI [XI, exp{-(kw + kI)(7 - t N [exp{-kwPt)[Pld = (kW1 + k , - kwp) k[R1)t'l (A'12) In contrast to eq A.5, the wall loss terms do not separate from +
the product formation data. To solve for k using product data, it is necessary to use a computer model and to know accurately kwP, k,, and k I . A similar analysis for reversible reactions such as I 8 0 H N O and NO, is considerably more complicated.
+
Registry No. OH, 3352-57-6; 02,7782-44-7; H20, 7732-18-5; CO, 630-08-0; C02, 124-38-9; N20, 10024-97-2; COS, 463-58-1; S02, 7446-09-5; NO, 10102-43-9; NO2, 10102-44-0.
The CH -tCO Reaction: Rate Coefficient for Carbon Atom Exchange at 294 K S. M. Anderson,*,$K. E. McCurdy,+ and C. E. Kolb Center for Chemical and Environmental Physics, Aerodyne Research, Inc., 45 Manning Road, Billerica, Massachusetts 01821 -3976 (Received: March 3, 1988)
A fast-flow reactor equipped with isotope-specific laser-excited fluorescence detection of CH radicals has been used to study carbon atom exchange in the reaction between CH and CO at 294 K and 2 Torr of total pressure. The rate coefficient for exchange, k3 = (2.1 X 0.3) X lo-'* cm3 s-', is about an order of magnitude larger than the bimolecular rate for the addition reaction, k2 = (2.7 A 0.4) X High-pressure limiting bimolecular and low-pressure termolecular recombination rate cm3 s-I and 4.9 X lo-'' cm6 s-] are derived. The results are discussed in the context of previous coefficients of 1.1 X work on the title reaction and on the chemistry of singlet CH2.
Introduction The methylidyne radical is of considerable practical as well as fundamental interest. In hydrocarbon combustion, its reactions play a key role despite typically low CH concentrations. The reaction with N z at high temperatures to form atomic nitrogen and H C N is thought to be the primary formation mechanism for "prompt" NO.' The reaction between C H and O2leading to OH formation is sufficiently exoergic to produce the intense OH(A-X) chemiluminescence associated with hydrocarbon flam~3.2.~ The chemiionization reaction with atomic oxygen produces CH0+,4 which is probably important in the initiation stage of scot f~rmation.~ Practical interest in the C H + C O reaction is motivated by similarities with its sister process, C H + N2: C H + N2 + M HCN2 + M (1) +
CH
+ C O + M -HC2O + M
(2)
Not only are C O and N2 isoelectronic, but they share a electronic ground state. Each is held together by a strong, electron-rich triple bond, a perfect target for an electrophilic radical like CH. Reactions 1 and 2 are at least phenomenologically similar; consumption of C H by either reagent at moderate temperatures is dominated by recombination to a radical intermediate.6,7 Additional insight into the mechanism of the C H N, reaction could be obtained by studying C H CO, particularly if information on the dynamics of adduct formation and/or rearrangement could be exposed. The possibility of carbon atom exchange in the CH + CO system, as revealed by isotopic labeling
+
+
12CH
+ I3CO
-
I3CH + l 2 C 0
(3f)
I3CH
+ I2CO
-
IZCH
+ I3CO
(3r)
or
represents a unique opportunity to study the underlying reaction mechanism that has not previously been exploited. Fundamental interest in the title reaction is motivated by indirect but compelling evidence for efficient C exchange in the 'Present address: Questek, Inc., 44 Manning Road, Billerica, MA. *To whom correspondence should be addressed at School of Physics and Astronomy, University of Minnesota, Minneapolis, MN.
0022-3654/89/2093-1042$01.50/0
analogous singlet methylene ('CH2) system. Early radiotracer (14C) studies* showed that exchange, which may be accompanied by singlet-triplet conversion, was competitive with the overall removal of 'CH2 by CO, even at relatively high pressures where recombinative removal became evident. This implied a major role for a symmetric oxirene intermediate, a contention that has received considerable theoretical a t t e n t i ~ n . ~ - ' Unfortunately, ~ oxirene has never been observed in the laboratory. The chemical similarity of 'CH, and CH(X)I4 (due to the presence of at least one fully vacant orbital) suggests the tantalizing possibility that C exchange in C H C O might also be fast and begs the question of whether an oxiryl radical intermediate might be involved. The mere existence of C exchange near room temperature could set limits for the C-H bond strength in oxirene. From a theoretical point of view, exploration of the rich chemistry of the C H radical has only begun. Mechanistic studies using modern ab initio methods are now tractable, and this capacity has been exercised on one of the simplest C H reactions, C H H2.I5J6 The predicted mechanism agreed with that deduced from experimental measurements of the thermal rate
+
+
(1) Glarborg, P.; Miller, J. A.; Kee, R. J. Combust. Flame 1986,65, 177. (2) Gaydon, A. G. The Spectroscopy of Flumes, 2nd ed.; Chapman and Hall: London, 1974. (3) Lichten, D. A,; Berman, M. R.; Lin, M. C. Chem. Phys. Lett. 1984, 108, 18. (4) Vinckier, C. J . Phys. Chem. 1979, 83, 1235. (5) Calcote, H. F. Combusr. Flame 1981, 42, 215. (6) Berman, M. R.; Lin, M. C. J . Phys. Chem. 1983, 97, 3933. (7) Berman, M. R.; Fleming, J. W.; Harvey, A. B.; Lin, M. C. Nineteenth Symp. (In?.)Combust,;The Combustion Institute: Pittsburgh, 1982; pp 73-9. (8) Montague, D. C.; Rowland, F. S. J. Am. Chem. Soc. 1971.93, 5381. (9) Dewar, M. J. S.; Ramsden, C. A. J . Chem. Soc., Chem. Commun. 1973, 688. (10) Basch, H. In Chemical and Biochemical Reactiuiry; The Jerusalem Symposia on Quantum Chemistry and Biochemistry, VI; Israel Academy of Sciences and Humanities: Jerusalem, 1974; pp 183-8. (1 1) Strausz, 0. P.; Gosavi, R. K.; Denes, A. S.; Csizmadia, I. G. J . Am. Chem. Soc. 1976, 98,4784. (12) Dykstra, C. E. J . Chem. Phys. 1978, 68, 4244. (13) Tanaka, K.; Yoshimine, M. J . Am. Chem. Soc. 1980, 103, 7655. (14) James, F. C.; Choi, H. K. J.; Ruzsicska, B.; Strausz, 0. P. In Frontiers of Free Radical Chemistry; Academic: New York, 1980; pp 139-69. (15) Brooks, B. R.; Schaefer 111, H. F. J . Chem. Phys. 1977, 67, 5146. (16) Dunning, T. H.; Harding, L. B.; Bair, R. A.; Eades, R. A,; Shepard, R. L. J . Phys. Chem. 1986, 90, 344.
0 1989 American Chemical Society
