7746
J . Phys. Chem. 1991, 95, 7746-7751
sidered to be the first ion in the chain, is enhanced." However, ND3H+ is in fact the first, and most exothermic, step in the shuttling chain. It is slightly enhanced, while the other two values for ND2H2+and NH3D+ are simply intermediate between the expected values for pure ND, and pure ND2H, respectively. For a 75:25 ND3/ND2H mixture the experimental results show ND2H2+to be on the low side and NDH3+ to be somewhat on the high side but altogether the values are not far from the expected statistical distribution. Ion/Neutral Complexes. Convincing arguments have been put forth5 that ion/neutral complexes are important in unimolecular dissociations of acetamide radical cation, CH3CONH2'+. However, reinterpretation of kinetic energy release data argues against an ion/neutral complex as an intermediate in the formation of the M-CO distonic fragment ion.'* The (low pressure) FTICR (18) Morton, T. H. The 38th ASMS Conference on Mass Spectrometry and Allied Topics, Tucson, AZ, June 3-8, 1990; Org. Muss Specrrom., in press.
ketene ion/NH3 experiment did not detect an ion/neutral complex: This suggested the present (high pressure) SIFT experiment which is better suited for collisional stabilization of collision complexes. While NH4+.NH3was observed at a low yield, no CH2CO+/NH3complexes were observed under the present conditions, as noted earlier. Ab initio calculations5 indicate that the complex is rather weakly bound with respect to CH2NH,'+ + CO ( 1 6 kcal/mol), that there is a high activation barrier separating it from the acetamide radical cation H3CCONH;+, and that reaction l a is highly exothermic as noted above. These are the obvious reasons for the inability to stabilize collisionally any CH2CO'+/NH3 complex.
Acknowledgment. B.R. thanks the Royal Society and the Israel Academy of Science and Humanities for a Study Visit Award and South Bank Polytechnic for sabbatical leave. Registry No. NH,, 7664-41-7; ND,, 13550-49-7; HzO,7732-18-5; CH4, 74-82-8; H, 1333-74-0.
Rate Constant and Products of the Reaction between NOs and CIO over the Temperature Range 353-210 K Peter Biggs, Matthew H. Harwood, A. Douglas Parr, and Richard P. Wayne* Physical Chemistry Laboratory, South Parks Road, Oxford OX1 3QZ, UK (Received: March 7,1991; In Final Form: May 29, 1991) The rate constant for the reaction between NO3 and CIO has been examined at I .4 Torr over a range of temperature (T = 353-210 K) by using a newly commissioned discharge-flow apparatus. The rate constant for the reaction has been found cm3 molecule-I s-' (where to be temperature independent, with a mean value over all temperatures of kl = (5.0 f 1.4) X the errors represent 95%confidence limits, precision only). The approximate ratios for branching into the following channels have been investigated: NO3 + CIO ClOO + NO2 (la); NO3 + CIO OCIO + NO2 (lb). At T = 296 K, the ratio k l , / k l was determined to be 0.9 f 0.35. The ratio klb/kl was studied at several temperatures and was found to be 0.2 f 0.1 at T = 297 K, decreasing with decreasing temperature. No products other than OClO and ClOO (observed indirectly) were detected. Comparisons were made with other studies of the reaction, and some possible implications of OClO formation in polar stratospheres are discussed.
-
-
Introduction The C10 radical is an important stratospheric species associated with springtime ozone depletion over the Antarctic.'-' Elevated concentrations have been found in the Arctic stratosphere during January and February.* Significant stratospheric column abundances of NO3 have been observed in early December and February in the Arctic5 and in September in the Antarctice6 Chlorine dioxide, OCIO, is used as a probe of the perturbed chemisry of polar stratospheres and in particular of the chemistry of bromine.'** Cox et a1.9 reported the only previous determination of the rate constant between NO3 and CIO and suggested that the formation of OClO and NO2 (reaction 1b) could be a minor product channel of the reaction between NO, and CIO. Both NO3 and CIO exhibit large diurnal variation in concentration: NO, ( I ) Solomon, S. Nufure 1990.347, 347. (2) Anderson, J. G.;Brune, W. H.;Proffitt, M. H. J . Geophys. Res. 1989, 94, 11465. (3) DeZafra, R.L.;Jaramillo, M.; Parrish, A.; Soloman, P.; Connor, 9.; Barrett, J . Nufure 1987, 328, 408. (4) Brune, W. H.; Toohey, D. W.; Anderson, J. G.;Chan, K. R. Geophys. Res. Lert. 1990. 17. 505. ( 5 ) Fielder, M.; Gomer, T.; Platt, U. Presented at European Polar Stratospheric Ozone Workshop, Schliersee, Germany, 3-5 Oct 1990. (6) Sanders, R. W.; Solomon, S.; Mount, G. H.; Bates, M. W.; Schmeltekopf, A. L. J . Geophys. Res. 1987,92, 8339. (7) Solomon, S.;Sanders, R. W.; Miller, H. L., Jr. J . Geophys. Res. 1990, 95, 13807. (8) McElroy, M. 9.; Salawitch, R. J.; Wofsy, S.C. Geophys. Res. Leu. 1986. 13. 1296. (9) Cox, R. A.; Fowles, M.; Moulton, D.; Wayne, R. P. J . fhys. Chem. 1981. 91. 3361.
is rapidly photolyzedI0 during the day, and its concentration reaches a maximum during the night; CIO is, in general, photochemically generated, with the highest concentrations being obtained during the day. Peak concentrations of NO, are expected to be at an altitude of 40 km, while those of CIO have been found to be at an altitude of 20 km, so that there is no prima facie expectation of a reaction between the two species taking place in the atmosphere. However, measurements of [ClO] have established a secondary maximum, between altitudes of 35 and 40 km. Should significant concentrations of NO3 and CIO coexist in the same airmass, particularly during twilight (or, conceivably, as a consequence of unusual transport conditions), and reaction 1 b proceed at a significant rate, then formation of OClO via reaction l b might contribute to the abundance of this species. In addition to the potential atmospheric importance of reaction I , the process provides further information about radical-radical reactions from which a pattern of reactivity may emerge. Although there have been several previous direct determinations of the rate constants for the reactions of NO3 with H, 0, OH, H02, CI, and Br," showing these reactions to be generally fast, this work is the first to determine directly the rate constant for the reaction of NO3 with another transient radical over a range of temperature. CIO itself has been studied in reaction with a variety of other radical species (0, OH, H 0 2 , NO, BrO) over a wide temperature (IO) Smith, J . P.; Solomon, S. J . Geophys. Res. 1990, 95, 13819. !) Wayne, R. P.; Barnes, I.; Biggs, P.; Burrows, J . P.; Canosa-Mas, C. E.: Hiorth, J.; LeBras, G.; Moortgat, G.K.; Perner, D.;Poulet, G.;Restelli, G.;Sidebottom, H. Armos. Enoiron. 1991, 25A. I . (I
0022-3654/91/2095-7746%02.50/00 199 I American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 20, 1991 7747
Reaction between NO3 and CIO
t
TO FLOW PUMP SAMPLING REGKIN
ION REGION POLE CAN
milor 6 Ion lenses
cdu
DISCRIMINATOR
METER
TO DIFFUSION PUMPS
TO DIFFUSION PUMPS
TO DIFFUSION PUMPS
p RECORDER k q
Figure 1. Top: discharge-flow apparatus showing gas flows used in work determining kinetics. Bottom: quadrupole mass spectrometer.
range.12 Each of these reactions has been found to possess a rate coefficient that increases with decreasing temperature.
Experimental Section Apparatus. A schematic diagram of the apparatus is shown in Figure I , top. A flow of carrier gas was passed both through a sidearm of the flow tube and through the sliding injector into the flow tube. CIO was made in the sidearm by the reaction of C1 atoms with 0,: CI 0, CIO O2 (2) The chlorine atoms were produced either directly by a microwave discharge (2450 MHz, 50 W) passed through dilute Clz or by the reaction between fluorine atoms and hydrogen chloride: F HCI H F CI (3)
+
+
-
+
+
+
F atoms being produced in a microwave discharge through dilute F2 in helium. For the work on the determination of the rate constants of the reaction between NO3 and CIO, the NO, radicals were formed in the sliding injector (8-mm outer diameter) by the reaction of fluorine atoms with nitric acid: F + HNO3 NO3 + H F (4) and the CIO was in excess over NO3. For studies of the products of reaction, a second sidearm was added to the flow tube, and both CIO and NO, were produced, by the methods described above, in sidearms. The flow tube was made of Pyrex and was 1.5 m long with an internal bore of 36.8 mm. It had two fixed injectors, one near each end. For all experiments, it was internally coated with halocarbon wax. A glass screw-joint connected the downstream end of the tube to a Pyrex absorption cell of base path length 110 mm. The cell narrowed at the downstream end to a tube of internal bore 18 mm leading to the sampling region of the mass spectrometer. All gas-handling lines were constructed from Pyrex and PTFE greaseless taps were used. Flows of reactant from the storage bulbs were measured by using ball flowmeters (Jencon, RSI) and regulated with stainless steel needle valves (Nupro). +
(12) &More. W. B.; Sander, S. P.; Golden, D. M.; Molina, M. J.; Hampson, R. F.; Kurylo, M.J.; Howard, C. J.; Ravishankara, A. R. Chemical Kinetics and Photochemical Data for use in Stratospheric Modeling, J. P. L. Publication, 1990; 90-1.
For chlorine mixtures, a needle valve made of Monel metal was used. The main flow of helium was regulated by an electromechanical control valve coupled to a thermal conductivity flow sensor (Brooks 58 16-lA21). The NO, concentration in the flow tube was monitored by optical absorption at A = 662 nm. The absorption a t this wavelength was measured by using a dual-beam spectrophotometer, the wavelength-selecting element of which was an interference filter (transmission maximum a t X = 662 nm, fwhm bandwidth of PA = 3.5 nm). The sample beam made 12 passes through a White-cell optical system, giving a total absorption path length of 1.32 m. The detector was a red-sensitive photomultiplier tube (Hamamatsu R928), the output of which was amplified and passed to a dual-boxcar analyzer in which the sample and reference parts of the signal were demultiplexed, integrated, and subtracted. The effective absorption cross section for NO, in this system was found by titration with NO to be 1.6 X lo-'' cm2 molecule-' (this value includes a convolution factor for the bandwidth of the filter); typically it was possible to detect 2 X 10" molecules of NO3 at a signal-to-noise ratio of 1:l with an integration time of 5 s. The quadrupole mass spectrometer (Figure 1, bottom) was based on an Extranuclear Laboratories design. Radio-frequency and dc pole voltages were derived from a modified V.G. Micromass Q50 control and power supply. Sampling from the flow tube was via a 1-mm pinhole mounted on a reentrant section of the flow system. A second 1-mm pinhole at the apex of a coaxial steel cone ensured that a pseudomolecular beam entered the ionization region of the mass spectrometer. A cylindrical ionizer (Extranuclear Laboratories) was placed close to the second pinhole. The ion detector was an off-axis channel electron multiplier (Mullard B4 19BL). The charge pulse from the detector was amplified and passed to a discriminator and then to a rate meter. The sample region was pumped by a 6-in. oil diffusion pump backed by a rotary pump. The ionization region and pole region were each pumped separately by 2-in. oil diffusion pumps fitted with liquid nitrogen traps and backed by a rotary pump. At linear flow velocities of 10 m s-l and flow tube pressure of 1.4 Torr, the ionization region was at a pressure of about 2 X Torr. At the flow speeds used in these experiments, the pumping system was able to maintain an adequate vacuum in the mass spectrometer up to a flow-tube pressure of approximately 1.8 Torr. This relatively low maximum pressure prevented a study of the pressure dependence of reaction 1. Electron impact energies used for ionization were between 39 and 48 eV. Typically, a flow tube concentration of 10I2molecules of C10 could be measured at a signal-to-noise ratio of 1:l with an integration time of I5 s. During experiments, the flow tube was maintained at a pressure of around 1.4 Torr by a rotary pump (Edwards E2M80) with mechanical booster (Edwards EH250). Flow speeds were in the range 4.9-17.3 m s-I. The pressure drop in the flow tube along its length was found to be in agreement with that expected from the Poiseuille equation and was a maximum of 2% a t the flow speeds used in the experiments. The flow tube was held at the same temperature over a distance of 114 cm before the absorption cell by surrounding it with copper foil; the upstream 36 cm of the flow tube was left unregulated. The copper foil was wrapped with thermostatically regulated heating tape (Hotfoil type E). For experiments below room temperature, the flow tube was surrounded by an insulated box containing dry ice and the heater used to reach the desired temperature. Nine thermocouples were attached to the outside wall of the flow tube in order to check that an even temperature was maintained. The entire system was tested by the determination of the rate constant for the reaction between NO, and NOz. The measured rate constant at a pressure of 1.5 Torr of helium was (3.9 f 0.9) X cm3 molecule-' s-l, in agreement with the value calculated (3.7 X cm3 molecule-' s-)) for this pressure using the published falloff parameter^.'^ (13) Smith, C. A.; Ravishankara, A. R.;Wine, P. H. J . Phys. Chem. 198s. 89, 1423.
7748 The Journal of Physical Chemistry, Vol. 95, No. 20, 1991
Biggs et al.
TABLE I no.
2 5a 5b 5c 5d 6 7 8 9 la lb 11
12 13
rate constant, cm3 molecule-l s-I reaction (a) Mechanism and Rate Constants Used in Numerical Integrations to Calculate CIO Concentrations CI + 0, CIO + 0 2 2.9 X IO-" exp(-260/T) CIO t CIO OClO t CI 5.7 x 10-l2 exp(-2400/ r ) CIO CIO ClOO CI 3.0 X exp(-l940/T) CIO + CIO CI2 o2 3.4 x lo-12exp(-l940/T) CIO + CIO + M C1202 + M 5 X 10-33(T/300)-'.4[M] 5.9 x lo-" OClO + CI CIO + CIO 3 X 10-I3[M] ClOO + M CI + 0 2 + M CI wall see text CIO wall see text
-+ -+ -
+
+
-+
(b) Further Chemical Equations Used in Calculating Rate Constants ClOO NO2 see text NO3 CIO OClO + NO2 see text NO, + CI C10 + NO2 2.6 X IO-" CIO t NO2 + M CION02 + M 5 X 10-3'(T/300)-3~4[M] NO, t NO2 M NZO, + M 3.7 X 10-14(T/300)4.'[M]
+
12 27' 27' 27a 28 12 12
+
NO, t CIO
-+
ref
29 12 13
-
'The values given are quoted in ref 27 as being obtained by Hayman et aI.;)O however, the latter authors do not, in fact, provide information about the branching ratios in their Dam. and Burkholder et aL2' auwar to have used results provided by Dr. R. A. Cox at XVII Informal conference on photochemisGy, Boulder, CO, i986.
..
Materials. Helium (BOC, commercial grade) was passed through an oxygen-removing column (Oxisorb, Messer Griesheim) and then through two traps held at 77 K, one containing molecular sieve (BDH type 4A). HCI (BDH, 99.6%) and chlorine (BDH, 99.97%) were condensed at 77 K, and excess gas was pumped away before use. Fluorine (5% in He, BOC special gases) was used as supplied. Nitric acid (M&B Labs, 70%)was used as received, mixed with sulfuric acid (Fisons, 98%) in a 1:2 mixture held a t 260 K or below, and was carried into the flow tube by bubbling helium through the mixture. Nitric oxide (BOC Special Gases) was purified by fractional distillation before use. Chlorine dioxide was made by passing pure chlorine through NaCIOt (Aldrich). The impure OClO was trapped at 77 K and then held in a trap at 195 K, allowing any chlorine contaminant to evaporate. OClO (which has a vapour pressure of approximately 1 Torr at 195 K) was allowed to evaporate gradually in the dark and to recondense in a coldfinger held at 77 K. Any material that did not evaporate at 195 K was pumped away on warming to room temperature. The purity of OClO was checked by mass spectrometry. No trace of molecular chlorine could be seen in the mass spectrum. Mixtures of between 2% and 10%of OClO in helium were used. New mixtures of OClO were used on each day of experiments when OClO was required. Ozone was prepared as described previou~ly,'~ except that ozone at a concentration of approximately 50% was generated and it was then carefully diluted with helium and left to mix for 24 h or more. Concentrations of ozone in the mixtures were determined by optical absorption at X = 254 nm immediately before use; they were in the range of 5-1 5%. Results Kinetics. Determination of the CIO concentration was achieved by observing the loss of ozone (A[03]) on switching on the microwave discharge that created chlorine atoms (checks were made to ensure that no ozone was lost when no fluorine [CI2in some experiments] was flowing). The CIO concentration could be found by numerical simulation using FACSIMILE'' with the mechanism listed in Table la. The rate constants for wall loss of CIO were determined by titrating CIO with NO at different contact times and comparing [CIO] with that expected on the basis of numerical modeling; the signal-to-noise ratio for the detection of CIO in the mass spectrometer was too small to determine wall losses by injection of this radical from the sliding injector. Wall losses of CIO were found to be very small ( < I s-I) at room temperature ~
and at 210 K. Wall-loss rates for CI were determined by titrating with ozone and observing [CIO] for a variable injection position. The rate constant for wall loss of CI at room temperature was found to be 5 s-I and increased only to 9 s-* at 210 K. These low rate coefficients mean that the influence of wall losses on the determined [CIO] (and on the kinetics) is small. As a first approximation, [CIO] = A[03], but corrections of typically 10% (and of a maximum of 30%) were needed because of additional loss of ozone via reactions 5a and 5b followed by reaction 2. There remains uncertainty about the magnitude of the rate constants for reactions 5a-c. A recent publicationI6 has reported roomtemperature rate constants for these reactions which are, in total, 100%larger than the sum of the rate constants for reactions 5a-c used here. We note that use of the different rate constants would mean that the [ClO] is approximately 10% lower (on average) than we have calculated, with a consequent increase of ca. 10% in the calculated rate constant for reaction 1. A lower yield of CIO than expected when CI atoms reacted with ozone has been observed in several studies, as described by Burkholder et al.," and it has been suggested that reaction 2 could produce vibrationally excited CIO (CIO*). The excited species could react further in the process ClO* + c1- c12
+0
(10)
This reaction, if it occurred in our system, might interfere with the preparation of known quantities of the radical CIO from ozone. Addition of CF4 (as a vibrational quencher) to the Cl/03 system within 5 X lo4 s of the initiation of the reaction ([CF,] > I O l 5 molecules ~ m - [O,] ~, =5x molecules ~ m - [Cl] ~ , N 2 x lOI3 molecules ~ m - produced ~) no detectable change in the signals at m / e = 48 or m / e = 51. In addition, the titration of CIO with N O used in determining wall losses indicates that excited CIO does not interfere with our measurements, at least within experimental error. The choice of NO3as the minority component was determined by the chemistry and by the poorer signal-to-noise ratio obtained with the mass spectrometer compared with the optical absorption system. Reactions l a and 7 would be followed rapidly in the presence of excess NO3 by the reaction NO3 + CI C10 NO2 (1 1)
-
+
thus reducing the observed decay rate of CIO. While the reaction conducted in excess NO3 might be a reliable method for directly determining the rate of reaction 1b, the signal-to-noise available
~~
(14) Ralph, D. G.; Wayne, R. P.J. Chem. SOC.,Faraday Trans. 2 1982, 78, 1815. (IS) Chance, E.M.; Curtis, A. M.; Jones,I. P.;Kirby, C. R. FACSIMILE
Rep. AERE-R8775; Atomic Energy Research Establishment; Harwell: Oxford, 1979.
(16) Simon, F. G.;Schneider, W.;Moortgat, G. K.; Burrows, J. P. J . Photochem. Photobiol. 1990. 55, I . (17) Burkholder, J. B.;Hammer, P. D.; Howard, C. J.; Goldman, A. J . Geophys. Res. 1989, 94, 2225 and references therein.
The Journal of Physical Chemistry, Vol. 95, No. 20, 1991 1149
Reaction between NO, and CIO
401
+ +
301
t
25
15 -
-32
0
1
2
3
4
I
I
I
5
6
7
[Clo]/iO" molecule cm-' Figure 2. Plot of k'versus [CIO] for all kinetic runs; 0 , 353 K; 0, 298 K; B, 250 K; V, 210 K .
with our mass spectrometer did not allow reliable data to be obtained under these conditions. The initial stage of analysis was carried out assuming pseudo-first-order conditions and that there were no complicating kinetics. The relationship between the logarithm of the normalized concentration of NO3 and contact time, t , was linear and gave the observed pseudo-first-order decay rate constant, k', for a known CIO concentration. A weighted linear least-squares analysis of k'versus [CIO] then gave an initial estimate of the second-order rate constant at a particular temperature (Figure 2). Diffusional corrections to the pseudo-first-order rate constants were calculated from the formula of Keyser,'* using as diffusion coefficient the value for C 0 2in helium;I9 the corrections were always less than 1.5%. However, both the self-reaction of CIO in reactions 5a and 5b and reaction la followed by reaction 7 produce CI atoms that themselves consume NO, (via reaction 11). Thus, the true rate constant can be obtained only by a method involving numerical integration of the rate equations using an assumed mechanism (potentially all reactions in Table I). The value of kl was adjusted as part of the modeling process in order to minimize the residual sum of squares between the calculated [NO,] and observed [NO,]. The rate constants derived from the numerical integrations could be obtained (within a few percent) by using only reactions 1, 2, 5a, 5b and 7,and 1 1, and the other reactions listed in Table I make little difference to the results. Reactions of the CIO dimer, (ClO)*, make little contribution because of the low pressure at which these experiments were performed. The experiments were performed in the presence of excess ozone. The correction to k l resulting from the participation of reaction 1 1 is therefore small, since reaction 2 then dominates the scavenging of CI atoms. The overall rate constant at a particular temperature is derived from a weighted average of all the individual rate constants for each of the kinetic runs. The CIO, NO, and O3 concentrations in the reaction region, the observed decay rates, and the rate constants derived from numerical integration are shown in Table 11. The average rate constant at a particular temperature, together with that obtained by using the analytical method described earlier, are also shown in Table 11. The simulations apparently require knowledge of the branching ratio kla/klb;the simulations were performed after preliminary experiments had been done to identify the products (see below), thus allowing a better estimate of the ratio to be obtained. In any case, the rate constant derived from the integration for a particular run is only weakly dependent on the branching ratio. The differences between the rate constants obtained by the analytical method and those from the numerical integrations are small, demonstrating that, for the conditions under which the experiments were performed, the complications arising (18) Kcyset, L. F. J . Phys. Chem. 1984, 88, 4750. (19) Reid, R. C.; Rausnitz, J. M.; Shewood,T. K.The properties of gases McGraw-Hill: New York, 1977. and liquids, 3rd 4.;
4 I
I
I
I
I
I
2.5
3.0
3.5
4.0
4.5
5.0
IO'K/T Figure 3. Arrhenius plot for the reaction between NO3 and CIO. B, measured rate constants for the reaction 1 (error bars are 95%confidence limits, precision only). 0 , rate constants for reaction 1 b only (error bars
are 2a, precision only).
12 16 20 molecule cm-' Figure 4. Change in [OCIO] versus the change in [NO,] observed in work determining the products. V, 353 K; B, 298 K;0 , 210 K. 0
4
8
[NO,]/IOu
from the secondary chemistry are relatively small. The averaged rate constants derived from the numerical integrations show no clear dependence on temperature. A slight positive temperature dependence may be present (E,, = 0.5 f 0.8 kJ mol-', where errors are 2a), but we prefer to quote a temperature-independent value of the rate constant for reaction 1, obtained from a weighted mean of the kinetic runs at all temperatures, of k l = (5.0 f 1.4)X IO-', cm3 molecule-' s-l. The Arrhenius plot of In k l versus the reciprocal of the temperature is shown in Figure 3. Products. The experiments to determine the branching ratios into the product channels were performed in a different way from the kinetic experiments. The CIO and NO, radicals were formed in two sidearms at the upstream end of the flow tube and were in contact for fixed times between 0.23and 0.34s, while the initial concentrations of NO, were varied. The yield of OClO was observed with the mass spectrometer and compared with a calibration using previously prepared OCIO.The signal-to-noise ratio of the mass spectrometer did not allow the formation of OClO to be observed as a function of contact time. An initial estimate of the branching ratio can be derived from a linear least-squares analysis of OClO formed beyond that from the CIO self-reaction versus NO, lost (Figure 4), but, as for the kinetic work, the more accurate estimates can only be obtained from comparison of the experimental data with the results of numerical integrations of the rate equations. We found the rate constant for reaction between NO3and OClO to be approximately (3113, molecule-' s-I or less, and the reaction is too slow to have interfered with our measurements. Numerical integrations
7750 The Journal of Physical Chemistry, Vol. 95, No. 20, 1991 TABLE II: Kinetic Data Used in Deriving Rate Comtants 10-'*[N03], 10-'3[CIO],
molecules
molecules cm-]
1.95 2.05 2.63 2.83 3.22 3.50 3.71 4.56
1.64 1.61 1.76 1.35 1.08 1.72 1.57 1.28
1.19 1.23 I .55 1.73 1.81 2.51 2.90 2.97 3.29 3.30 3.36 4.34 6.54
1 .80 2.10 1 .80 1.65 2.70 1.70 2.30 2.30 1.62 1.22 1.60 1.10 1.80
2.05 2.2 2.22 2.37 2.12 4.54 5.20 5.25
1.38 2.28 1.58 2.27 1.87 1.74 1.58 1.44
1.18 1.62 1.70 2.22 2.82 3.04
1.65 1.43 1.63 1.46 1.56 1.61
Biggs et al. k ',
10-'3[0,],
molecules cm-) Temperature = 353 K
S-I
6.75 5.13 4.26 7.04 3.89 6.69 5.32 5.51
1013kl,cm3 molecule-' s-'
12.9 11.9 12.7 13.3 15.3 18.8 18.9 27.0
6.56 5.64 4.6 1 4.66 4.37 5.27 4.94 5.60 5.3 i 1.2a 5.7 i 0.36
5.9 6.9 10.5 10.0 7.3 14.2 16.7 11.4 17.1 17.0 14.5 22.9 34.4
4.69 5.27 6.61 5.49 3.89 5.31 5.33 3.79 4.90 4.91 4.07 5.17 5.00 5.1 f 1.1' 5.3 f 0.26
11.3 10.3 7.4 10.5 15.6 27.5 20.0 24.5
5.31 4.45 3.09 4.38 5.84 5.71 3.70 4.60 4.4 f 1.7" 4.4 f 0.46
6.2 6.7 10.7 12.6 19.1 14.5
5.05 3.88 5.86 5.53 6.52 4.37 4.8 i 2.3' 5.1 i 0.6b
Temperature = 299 K 2.33 3.01 5.06 3.13 3.41 2.93 2.56 6.67 2.83 3.23 3.20 5.72 3.16
Temperature = 250 K 5.9 1 5.05 2.58 8.63 10.0 4.04 4.48 7.36
Temperature = 210 K
a.
7.13 5.82 4.30 6.27 3.64 4.15
Weighted average. *Analytical method.
were carried out separately for each initial NO, concentration, with three parameters being varied as part of the fitting procedure: the initial concentration of CI atoms, k5,, and the branching ratio k l b / k l . The rate constant kS,was varied because it was thought originally that the amount of OClO derived from reaction 5a would be altered once the microwave producing NO, was switched on and that this change could not be observed independently of the OClO formed by reaction 1b. In fact, the branching ratio klb/,kl obtained from the numerical integrations is found to be insensitive to the adopted value of k5a. While the values of the rate constant derived for the reaction ( k 5 , = (4 f 2) X cm3 molecule-' s-I at room temperature, with an activation energy greater than ca. 10 kJ mol-') may not be very reliable, the values for k l b / k l are far more robust. Concentrations of OClO from reaction 5a were typically (0.3-1) X I O i 2 molecules cm-3 at the sampling pinhole of the mass spectrometer, although for some room-temperature data points the contribution was approximately 4 X IO'* molecules cm-,. The values of klb/kl derived from the numerical integrations (Yklb/kl integration") together with those obtained from the analytical method ( * k l b / k l analytical") are presented in Table 111. The logarithms of the rate constants corresponding to reaction 1b from the numerical integrations are plotted against
TABLE III: Branching Ratios for the Formation of OClO kiklki
temp, K
analytical
integration
353 297 216
0.10 i 0.03 0.16 f 0.05 0.036 i 0.014
0.14 i 0.13 0.20 i 0.10 0.035 i 0.05
the reciprocal of temperature in Figure 3. Further experiments were performed at room temperature in order to establish the additional loss of ozone caused by reaction 1 a followed by reactions 7 and 2. The procedure was similar to that used for the study of OClO except that the only parameters varied were the initial CI atom concentration and the branching ratio kla/kl. Comparison of experimental concentration data with those obtained from numerical integrations gave a value for k l , / k l of 0.9 f 0.35. Before starting the detailed examination of products, a search was made for a signal at m / e = 81 (empirical formula CIN02). No change in signal from the mass spectrometer was observed on mixing NO, and CIO, indicating that, as expected, neither ClNO2 nor CION0 is formed at detectable levels. The formation
The Journal of Physical Chemistry, Vol. 95, No. 20, 1991 7751
Reaction between NO, and CIO of chlorine nitrate and an oxygen atom is endothermic by over 150 kJ mol-I.
Discussion The only previous work on the reaction between NO, and Clog suggested a rate constant of 4.0 X lo-', cm3 molecule-' s-I at room temperature with an activation energy of 3.5 kJ mol-'. When the absorption cross section for NO, used by Cox et ala9in the calculation of the rate constant is adjusted in line with the recent evaluation," the revised rate constant is 4.9 X lo-', cm3 molecule-I s-I, in very good agreement with our value. The temperature dependence that Cox et aIs9observed is not entirely incompatible with our work, but a smaller dependence on temperature than theirs seems more likely on the basis of our results. Our findings concerning the branching ratio are similarly consistent with the previous work, with cox et quoting an upper limit of klb/kla C 0.4, compared to our value of 0.22. The reaction between NO, and CIO is slower than that of all bimolecular reactions between NO, and all other radical species (except for NO, itself) for which rate constants have been measured. Further, measured rate constants for the reaction of CIO with all other radicals (except for CIO itself) are also found to be greater than the rate constant for its reaction with NO3. There is no significant activation energy, implying that reaction l a at least proceeds along a monotonically attractive potential surface, and it is puzzling that the reaction is so slow (it proceeds at a rate approximately 800 times slower than the hard-sphere collision rate). The low rate constant can in part be explained by the spin and orbital multiplicity of the CIO and NO, species,20 but this cannot be the entire explanation. It may be that the low rate constant is a consequence of the reaction proceeding via a many-centered transition state or an adduct. However, the excellent agreement between our rate constant at room temperature and at a pressure of 1.4 Torr, and the value of Cox et ale9for experiments performed at atmospheric pressure, suggest that if an adduct is formed, it is not collisionally stabilized. We note that, should the C10 approach the NO, collinearly with one of the N - O bonds, then the transition state would be of point group Czuand both reactions l a and 1 b could be symmetry forbidden. However, the symmetry restriction does not apply to the reaction NO, + CIO CI + O2 NO2 (IC)
-
+
which could not be distinguished from reaction l a under the conditions of these experiments. If the transition state is of lower symmetry (C, or Cl), however, then the reaction would necessarily be symmetry allowed. The reverse of reaction 1 b has been investigated previously,2' and a value for the temperature-dependent second-order rate constant reported of 2.3 X exp(-5388/7') cm3 molecule-' s-I. Combining the rate constant derived from this expression at (20) Biggs, P.; Brown, A. C.; Canosa-Mas, C.; Carpenter, P.; Monks, P. S.; Wayne, R. P. In Physico-chemical behaviour of atmospheric pollutants; Restelli, G., Angeletti, G., Eds.; Kluwer Academic: Dordrecht, 1990. (21) Martin, H.; Gareis, R. 2.Electrochem. 1956, 60, 959.
T = 298 K with that for the effective rate constant for reaction 1b obtained from our previous work gives a value for the equilibrium constant of
Using the most recent thermodynamic data22*23 for So and for NO2 and CIO, we can establish the difference between the enthalpies of formation of NO, and OC10, two values not well established at present.J1-'2This difference is found to be 25.8 kJ mol-', a value entirely consistent with the NASA tabulation. However, the implication is that the lowest enthalpy of formation determined for NO, (64.4 kJ mol-')24 and employed in ref 11 is too small, because it is not consistent even with the lowest quoted value of the enthalpy of formation of OClO (96.1 kJ mol-').12 Rather, the higher values for the enthalpy of formation of NO, (72.4 and 73.2 kJ m01-')25*26come within the range of quoted enthalpies of formation for OC10. Reaction l b clearly becomes less important at low (stratospheric) temperatures. The possibility that reaction 1 contributes significantly to OClO formation, even supposing that the radicals were present in the same airmasses, is extremely small, and there is no evidence to suppose that the current assumption that all OClO is derived from the reaction between CIO and BrO is invalid. ArHO298 of
Acknowledgment. A.D.P. thanks the Department of the Environment for a maintenance grant administered by AEA Technology, Harwell, during the early part of the work and the CEC for a contract during the latter part. We thank Dr. C. E. Canosa-Mas for helpful discussions. Note Added in Proof. Since this paper was submitted, we have become aware of a further study by the group of Professor R. N. Schindler a t the University of Kiel, Germany, of the reaction between NO, and CIO conducted at room temperature. Although the overall rate coefficient that these workers obtain is in agreement with that reported in the present study, the new branching ratio, kIb/kl,is much larger than ours. The new work will be published in Ber. Bunsen-Ges. Phys. Chem. (E. Becker, U. Wille, M. M. Rahman. and R. N. Schindler). (22) Chase, M. W., Jr.; Davies, C. A.; Downey, J. R., Jr.; Frurip, D. J.; McDonald, R. A.; Syverud, A. N. JANAF thermodynamic tables, 3rd ed.; J. Phys. Chem. Re/. Dura 1985, 14, Suppl. No. 1. (23) Atkinson, R.; Baulch, D. L.; Cox, R. A.; Hampon, R. F., Jr.; Kerr, J. A.; Troe, J. J. Phys. Chem. Re/. Data 1989, 18, 881. (24) McDaniel, A. H.; Davidson, J. A.; Cantrell. C. A.; Shetter, R. E.; Calvert, J. G.J. Phys. Chem. 1988, 92, 4172. (25) Kircher, C. C.; Margitan, J. J.; Sander, S.P. J . Phys. Chem. 1984, 88. .-, 4310 - -. (26) Burrows, J. P.; Tyndall, G.S.;Moortgat, G. K. Chem. Phys. Len. 1985, 119. 193. (27) Burkholder, J. B.; Orlando, J. J.; Howard, C. J. J . Phys. Chem. 1990, 94, 687. (28) Trolier, M.; Mauldin, R. L., 111; Ravishankara, A. R. J. Phys. Chem. 1990, 94, 4896. 1291 Mellouki. A.: Le Bras. G.:Poulet. G. J. Phvs. Chem. 1987. 91. 5760. (30) Hayman, G.D.; Davies, J. M.; Cox, R. A.Geophys. Res. Lett. 1986, 13. 1347.
