J. Phys. Chem. 1082, 86, 785-790
785
Kinetics of the OH -t NOP -t M Reaction at High Total Pressures John S. Robertshaw and Ian W. M. Smlth' Depafiment of Physlcal Chemlstfy, Unlverstty Chemical Laboratories, CambrMge CB2 1EP. England (Received: May 5, 1981; In Final Form: August 3, 1981)
Using pulsed photolysis of HN03 to generate OH radicals and laser-inducedfluorescence (LIF) to observe their subsequent decay, we have measured the rate constants (k,)for OH + NOp + M (= Ar, CF4)-,HN03 + M at 295 f 4 K and at Ar pressures up to 4.0 atm and CF4pressures up to 8.6 atm. A t the highest total pressure, kl N 3.0 X lo-" cm3molecule-' s-l and may still not have reached the limiting high-pressure value (k,"). The results are compared with previous experimental data obtained at pressures below 1 atm, and with estimates of k,", and this new application of LIF is assessed.
Introduction Dissociation-association reactions which involve the breaking or making of a single bond can provide an especially searching test of theories of unimolecular reactions, since high-temperature studies of the dissociation can be combined, via detailed balancing, with low-temperature measurements on the reverse radical combination reaction. However, in order to make a thorough comparison with theory, it is important that rate constants are obtained throughout the "falloff" regime in which the reaction order changes. Typically, measurements are required over 4-5 orders-of-magnitudechange of totalpressure. This is most likely to be possible for reactions involving a total of about 5 to 8 atoms, since then the reaction will be in its falloff region throughout the most convenient range of total pressures. Even then experiments are needed at pressures greater than 1atm. The study of combination reactions at very high pressures has been pioneered by Troe and his coworkers.' They have determined the rates at which halogen atoms combine with themselves, and with NO, in some cases, at total pressures as high as 1000 atm. Their method involves pulsed photolysis with photoelectric monitoring of changes in continuum absorption. Here, we report the application of a new technique to the reaction OH + NO2 + M HN03 + M (1) -+
Because of its importance in stratospheric chemistry, this reaction has been studied extensively in recent years.2 Previous flash photolysis experiments, together with resonance f l u o r e s ~ e n c eor~resonance ~~ absorption5i6observation of OH concentrations, have supplied rate constants at pressures up to about 1atm. However, both techniques do become less sensitive as the total pressure is raised, resonance fluorescence because the A22+ state of OH is readily q u e n ~ h e d ,and ~ , ~resonance absorption because the (1)H. Hippler, K. Luther, and J. Troe, Chem. Phys. Lett., 16,174 (1972);Ber. Bunsenges. Phys. Chem., 77,1104(1973);H. Van den Bergh and J. Troe, Chem. Phys. Lett., 31, 351 (1975);H. Hippler and J. Troe, Int. J. Chem. Kinet., 8, 501 (1976);H.Van den Bergh and J. Troe, J. Chem. Phys., 64,736(1976);H. Hippler, S.H. Luu,H. Teitelbaum, and J. Troe, Znt. J. Chem. Kinet., 10, 155 (1978). (2) D. L. Baulch, R. A. Cox, R. F. Hampson, Jr., J. A. Kerr, J. Troe, and R. T. Watson, J. Phys. Chem. Ref. Data, 9,295 (1980). (3)R. Atkinson, R.A. Perry, and J. N. Pitta, Jr., J. Chem. Phys., 66, 306 (1976). (4)P. H.Wine, N. M. Kreutter, and A. R. Ravishankara, J. Phys. Chem., 83, 3191 (1979). (5)C. Morley and I. W. M. Smith, J. Chem. SOC.,Faraday Trans. 2, 68,1016 (1972). (6)C. Anastasi and I. W. M. Smith, J. Chem. SOC.,Faraday Trans. 2,72, 1459 (1976). 0022-3654/82/2086-0785$01.25/0
OH absorption lines pressure broaden. The present work demonstrates that, if the weak OH resonance lamp is replaced by a tunable laser, fluorescence can be used to observe OH to much higher total pressures. Furthermore, ow measurements show that the rate constant for reaction 1may still have not reached its limiting high-pressure value at about 10 atm and that this limiting value is at least twice that indicated by the extrapolation of results from lower pressures.6
Experimental Sectiong Our experimental method relied on creation of OH radicals by pulsed photolysis of HNO, and observation of their subsequent removal by laser-induced fluorescence (LIF). A flashlamp pumped dye laser (Chromatix, CMX-4) was tuned to a strong line in the A28+-X211(1,O) band of OH at about 2820 A and the amplitude of the (1,l) plus (0,O)fluorescence was measured for various delay times between the photolytic flash and the "probing" laser pulse. The apparatus is shown schematically in Figure 1. The reaction vessel was constructed from a (10 X 10 X 10 cm) block of stainless steel and had an internal volume of approximately 250 cm3. The laser beam entered and left the cell through 7-mm thick quartz windows positioned on opposite faces of the vessel, and fluorescence was observed perpendicular to the direction of the laser beam through a third quartz window, 10 mm thick. The side facing this third window had three tapped holes to which stainless steel tubing was connected via Gyrolock unions. The top tube admitted a low-pressure gas mixture from a conventional Pyrex vacuum line, the middle tube could take a thermocouple, and the bottom tube admitted diluent gas (i.e., M) at high pressure through a "rose" which protruded into the reaction vessel to facilitate mixing. This last inlet was connected to a manifold which could be filled directly from a cylinder containing the diluent gas. Pressures were measured on a mercury manometer or by metal Bourdon gauges. A flash lamp plus filter cell assembly was bolted to the top of the reaction vessel. The filter cell was machined from Monel metal and gave a path length of 18 mm. It was separated from the reaction vessel by a 14-mm thick Spectrosil window and from the flashlamp by a 9-cm focal length Spectrosil lens. The flashlamp was made from a 15 cm length of 5 cm diameter Pyrex tubing and was closed at its top end by (7)K. H. Becker and D. Haaks, 2.Naturforsch. A, 28, 249 (1973). (8)M. A. A. Clyne and S. Down, J. Chem. SOC.,Faraday Trans. 2,70, 253 (1974). (9)Further experimental details are available from J. S. Robertshaw, Ph.D. Thesis, Universityof Cambridge, 1981,or directly from the authors.
0 1982 American Chemical Society
The Journal of Physical Chemistty, Vol. 86,No. 5, 1982
786
Robertshaw and Smith
a 5 cm focal length mirror mounted with an O-ring seal. About 6 cm below the center of the mirror were two tungsten electrodes whose separation could be adjusted. The discharge between these electrodes dissipated 10 J and was fired at 5 Hz through 1 atm of slowly flowing N2. The main flash lasted about 20 ps but was followed by a longer weak tail. The effect of the mirror and lens combination was to collect, rather than strongly focus, the light from the discharge. Experiments in which fluorescent paper was inserted into the reaction cell indicated that the flash illuminated, fairly evenly, an area of about 3 cm2 in the center of the vessel. Flowing the N2 helped to keep the mirror and lens clean for longer periods, although the flashlamp could also be taken apart fairly easily for cleaning. The filter cell was generally filled with 500 torr of C12. This served to reduce scattered light at the wavelengths of the OH fluorescence, as well as reducing the (small) extent of NO2 photolysis. Attempts to use H 2 0 as a precursor for OH were unsuccessful. Presumably the thickness of the quartz lens and window absorbed the short wavelength radiation capable of dissociating H20. HN03 absorbs strongly2at longer wavelengths than H20 and has been used successfully in previous studies6,10of reactions of OH. (The yield of OH from HN03 (see below) and a comparison of the HNO, and NOz absorptions indicate that only about 1 in lo6 of the NOz would have been photodissociated.) The laser radiation used to excite OH was generated by frequency doubling the output from Fluorol7GA dye. A low finesse etalon was used in the laser cavity yielding an output bandwidth of 0.6 cm-'. The beam was expanded to about 1 cm diameter, passed through a filter (Fl) to remove the fundamental output, and collimated with irises before entering the reaction vessel. Before starting a series of experiments, the laser was tuned in two stages. First, a fraction of the radiation was reflected by a quartz plate into a 0.3-m focal length, calibrated monochromator (M2: Hilger-Watts, D330). In addition, the laser beam emerging from the reaction cell was passed through a small flame of hydrogen burning in air. LIF from the steady-state concentration of OH present in the flame could be detected through a small, low-resolution monochromator (M1: Spex, Micromate).
During the experiments proper, the laser was triggered, 30 ws to 1 ms after the flash lamp, by sending a signal from photomultiplier P1 through a delay unit. The fluorescent signals from OH generated by the flash could themselves be used for tuning once the experiments were under way. A quartz lens mounted just outside the observation window collected the fluorescence excited by the laser and directed it, through a Pyrex flat and an interference filter (F2: Corion; peak transmission at 3104 A, bandwidth (fwhm) of 136 A), onto photomultiplier P4 (E.M.I., 9783B). The distance between P4 and the lens could be adjusted to maximize the fluorescence signal. The filter combination passed fluorescence in the (1,l) and (0,O) bands but discriminated strongly against scattered laser light. The ability of laser-induced fluorescence to excite and detect OH at different wavelengths is an additional advantage over resonance fluorescence where both excitation and fluorescence occur in the (0,O) band. The output from P4 was observed directly on oscilloscope S2 and was also fed into a linear gate (Brookdeal, 415) which was opened for 10 p s by a square wave pulse from the delay unit. The fluorescence itself was extremely short lived ( 11.9 15.1
Quoted uncertainties correspond to a single standard deviation.
lo-"
i
5.9 8.24 10.7 12.3
3.65 5.84 6.30 9.36 10.1 9.4 10.4 13.2 13.6 14.8 15.0
[ N O , ] = 0.
1 . 2 x 10." 0.8 X 1 . 6 X lo-" 2.2 X 10'' - 2.6 X 2.2 x lo-"
98.7
2.4 5.43 6.67 9.37 13.9 10.9 13.7 10.6 16.7
9.4 10.9
7.1
0 0.51 1.02 1.53 2.04 2.55 3.06 3.57 4.08 4.59 5.11
k1,0/103
i
41.1
t
4K
based on "corrected" Lindemann plot o f data obtained with P H ~ G 300 torr RRKM calculations t o fit data of ref 5 semiempirical Kassel curves t o fit data obtained with PN,,PSF,4 500 torr modified Gorin model to fit data of ref 6 maximum free energy model; partition functions along reaction coordinate calculated semiempirically direct observation with PCF,d 8.6 atm
ciation of OH + NO2, because it is unlikely to be very efficient at quenching OH A2Z+,and because it will survive as a high-pressure gas in low-temperature experiments which might be carried out in the future. Although CF, has not been used previously as a third body for reaction 1, SF6, which usually has a similar third-body efficiency to CF4,16has. Figure 5 shows that our results with M = CF, are very similar to those with M = SF6 obtained by Anastasi and Smith6 and by Wine et al.4 below 1 atm. Comparing the two falloff curves in Figures 4 and 5, we calculate the ratio of third-body efficiencies" (PCF,/Pk) to be 3.3. Wine et al.4 found / ~ s F , J P=~ 4.0. There have been several previous attempts to estimate klm,the limiting high-pressure rate constant for reaction 1. These are summarized in Table III. The methods used (16)M.Quack and J. Troe in 'Gas Kinetics and Energy Transfer",P. G. Ashmore and R. J. Donovan, Ed., Specialist Periodical Reports, Chemical Society, London, 1977,Chapter 5. (17)Like Wine et al.,' we estimated this ratio by considering the horizontal displacement of the two falloff m e s in Figures 4 and 5. The quoted value for ( j 3 c ~/a&) does not, therefore, include any factor allowing for the different codision rates of CF, and Ar with (HN03)+. (18)W. Tsang, Znt. J. Chem. Kinet., 5,947 (1973). (19)G. P. Smith and D. M. Golden, Znt. J. Chem. Kinet., 10, 489 (1978).
ref 5 18 6 19 20 this work
are based on extrapolation of data obtained at total pressures below 1atm, on model calculations, or most often on a combination of the two. Our measurements at high [CF,] exceed all these estimates of klmand, although the data are scattered, it appears that the high-pressure limit has still not been reached. One possible explanation for the slow approach of the second-order rate constants to a higher asymptotic limit than has been previously suspected is that two isomers of HNO,, HONOz and HOONO, can form. Niki et have recently used Fourier transform infrared spectrometry to identify both ClN02 and ClONO as products of the C1+ NO2 + M association reaction, which is electronically very similar to reeaction 1. Niki's results have been discussed by Chang et a1.,22who calculated low-pressure rate constants for both channels according to a model developed by Troe.23 The branching ratio is proportional to the ratio (20)M. Quack and J. Troe, Ber. Bunsenges. Phys. Chem., 81, 329 11977). ~ - -_,_ .
(21)H.Niki, P. D. Maker, C. M. Savage, and L. P.Breitenbach, Chem. Phys. Lett., 59,78 (1978). (22)J. S. Chang, A. C. Baldwin, and D. M. Golden, J. Chem. Phys., 71,2021 (1979). (23)J. Troe, J. Chem. Phys., 66, 4758 (1977).
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of phase spaces associated with the energized adducts of the two isomers. The falloff behavior of two competing channels in a radical combination reaction will differ. In regard to high-pressure behavior, two cases can be distinguished. If the barrier between the potential minima associated with the two isomers is higher than the energy corresponding to separated radicals, formation of the isomers would proceed via different transition states, the rates of the two reactions would be additive rather than competitive, and the branching ratio could be a function of total pressure. On the other hand, if there is a low barrier so that the energized adducts of both isomeric forms can rapidly interconvert and are formed via the same transition state, the two channels can be said to be competitive and the branching ratio should be independent of total pressure. In the context of these arguments, it is worth noting that the rate constants which we measure at high [CF,] are within a factor of ten of the simple collision theory rate, despite the fact that the ground state of HNO, is only one of 8 electronic states that correlate with OH(211) + NO2 (X2Al). This suggests a second possible explanation for the observed high-pressure behavior. Reaction may be proceeding via one or more weakly bound excited states. The contribution of such pathways to the overall rate of reaction would only become appreciable at high total pressures when the collision rate transfering such electronically excited, internally energized, adducts into the ground electronic state became comparable to the rate at which these species redissociate. Unfortunately, such a suggestion has to remain speculative as no relevant spectroscopic information is available. The existence of such a route for association could reconcile our data with the results and conclusions of Jaffer and Smith.24 They found the rate constant for (24) D. H. Jaffer and I. W. M. Smith, Faraday Discuss.Chem. SOC., 67, 212 (1979).
Robertshaw and Smith
OH(U=l) + NO2
+
OH(u=O)
+ NO2
to be (1.6 f 0.4) X lo-'' cm3molecule-' s-'. They reasoned that this probably corresponded to the rate constant for formation of strongly bound complexes in which energy was rapidly randomized. This rapid energy randomization might not occur, however, in weakly bound, electronically excited, complexes. The present paper reports a novel application of the powerful LIF technique. As noted earlier, the signals decreased steadily as the pressure of the diluent gas was raised and this made it impossible to carry out satisfactory measurements at pressures above 8.6 atm. With the present apparatus, the LIF signal at the shortest delays became equal to the background ("flash-only" plus "laser-only") at about 3 atm CF4. With improved equipment, it should be possible to complete successful experiments at much higher pressures than those used in the present investigation. The most important modifications would be the use of pulsed laser photolysis and the reduction of background signals by careful baffling of both laser beams. The use of a laser for photolysis will not only make it easier to reduce the scattered light levels, it will also allow one to measure much faster first-order decays. It would be especially interesting to study the high-pressure limit of reaction 1 over a range of temperature since it remains uncertain whether the negative temperature dependence of kl" indicated by earlier measurement^^,^^ is genuine, or simply an artefact of the extrapolation procedures which were used. Acknowledgment. It is a pleasure to dedicate this paper to Professor Simon Bauer on his 70th birthday. We are grateful to S.R.C. for an equipment grant and for a research studentship (J.S.R.). (25) K. Glanzer and J. Troe, Ber. Bunsenges. Phys. Chem., 78, 71 (1974).
