J. Phys. Chem. 1984,88, 4909-4917
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FEATURE ARTICLE Kinetics of Elementary Radical Reactions in the Gas Phase F. Kaufman* Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 (Received: April 1 1 , 1984)
Elementary radical-molecule or radical-radical reactions have recently been investigated directly rather than by modeling of multistep mechanisms. Both flow reactor and (laser) flash photolysis techniques have been greatly improved through more sensitive detection and more versatile generation of radical species. These advances are briefly reviewed, and several classes of reactions are discussed and compared with theoretical predictions: (1) H-atom abstraction by OH from H2, paraffins, and substituted paraffins via simple, well-defined transition states; (2) OH reactions via adduct formation, e.g., with HN03 or olefins; (3) H 0 2 reactions with atoms (H, 0, N, C1, Br) and radicals (H02, OH); and (4) radical-molecule reactions that have multiple product channels including extensive rearrangements (0+ C2H4,NH2 + NO). The importance of strong interaction of experiment and theory is stressed. The outlook is bright and the field is as exciting as it is underappreciated.
Introduction Chemistry is the science that concerns itself with the structure and properties of pure substances and with the reactions that transform one set of substances into another. In the gas phase, these transformations usually occur in complicated sequences of elementary steps as the consequence of a quasi-least-action principle which states that multiple collisions and/or major bond rearrangements are too improbable and/or too costly in potential energy to provide an efficient reaction path. The elucidation of the sequence of steps, Le., of the reaction mechanism, has long been and continues to be the subject of much research. In order to characterize the single, elementary steps, it is necessary to measure their rate parameters as directly as possible and to understand their dynamics with the aid of reaction rate theory. That is the subject of this article. Clearly, these rate parameters are not primary molecular quantities such as those obtained in state-to-state (e.g., singlecollision, molecular beam scattering) measurements. They represent averages over distributions of initial and final states for rapidly equilibrated degrees of freedom (Le., translation, rotation, and some low-frequency vibrations) coupled by their specific interaction cross sections. Nevertheless, they carry much fundamental information. Furthermore, the experimental mapping of state-to-state scattering matrices becomes impossible for polyatomic reactants because these matrices are unmanageably large. Fortunately, present-day experimental methods for measuring elementary reaction rate parameters are sufficiently state selective that they are usually able to provide data on reactant removal or product formation in specific vibronic states. The resultant information is therefore much closer in substance to that of state-to-state experiments than it is to that of bulb experiments where only bulk properties or stable reactant and/or product species concentrations are measured. It has become increasingly possible to prepare the desired radicals in the absence of interfering species, to react them with other molecules or radicals, and to monitor reactant and product concentrations with great specificity and sensitivity, Le., to study a particular elementary reaction directly, in splendid isolation. What are the chief uses of such information? First and foremost, we gain insight into reaction events of increasing complexity. By measuring elementary radical reaction rate constants for the disappearance of selected vibronic states and for the formation of product species as functions of temperature, pressure, and diluent gas, we can usually establish the nature of the reactive 0022-3654/84/2088-4909$01.50/0
interaction in some detail. Experimental data may be compared with theoretical predictions that use a variety of semiempirical and ab initio models. For some radical-molecule reactions, the dependence of rate parameters on structure and substituent effects of the molecule is easily measured. This forms a growing data base that aids our understanding of chemical reactions. For radical-radical reactions and for those radical-molecule steps that proceed via bound adduct intermediates, the identification of product channels is as important as the measurement of the overall rate. Because of their complexity, these processes are less easily amenable to theoretical analysis. The broad application of elementary reaction rate data need hardly be emphasized. The individual reactions are the building blocks of nearly all gas-phase processes: industrial, atmospheric, pollution, combustion, and so on. The reactive radical species may be generated by photolysis, by thermal or nonthermal initiation, and the propagating steps differ from process to process. For a given process, the total number of elementary reactions will usually be very large, but relatively few of them are critically important. Sensitivity analysis' helps identify which ones are important when and where, but only after we know at least roughly how fast they are. Unavoidably, we are forced to guess, measure, or calculate the rate parameters if we want to control or modify the process. In this paper, elementary reaction rate measurements will first be described including the experimental methods used, the generation and detection of radicals, and the uncertainties of derived rate parameters. Examples of specific reactions will then be given, first of simple H-atom-abstraction reactions of OH with singlebonded molecules, then of more complicated OH reactions with *-bonded molecules, and lastly of radical-molecule and radical-radical reactions involving H 0 2 , NH,, and other species. Theoretical calculations will be mentioned and discussed where applicable. Triatomic A + BC reactions are excluded here even through they have been the workhorse of rate theory for half a century, because they represent only a tiny fraction of this rapidly expanding field. It is now both necessary and feasible to measure polyatomic reactions and to acquire a broader understanding of their rate parameters.
Experimental Methods In spite of the large variety of experimental approaches, viz., variants of flow reactor or flash photolysis techniques, the unifying (1) Rabitz, H.; Kramer, H.; Dacol, D. Annu. Rev. Phys. Chern. 1983,34, 419.
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description of “pump-and-probe” applies throughout. In conventional discharge-flow experiments, the “pump” phase may involve electrodeless discharge or laser photolytic generation of reactive species in certain vibronic states or it may involve a prereaction, e.g., F H202 HF HOz, in which a precursor atom/radical product (F) reacts rapidly and quantitatively to generate the desired species (HO,). Two reactant streams may be piepared carrying different, reactive radicals, each present in large excess of inert carrier gas (He, Ar), and then brought together to study radical-radical reactions. The time delay between “pump“ and “probe” is here given by the flow time between the mixing and detection regions and varies typically from a few to tens of milliseconds. In the flow reactor (discharge-flow, flowing afterglow) configuration, probe (Le., detection) methods are at their most versatile, including resonance and laser-induced fluorescence, resonance and laser (multipath) absorption, laser magnetic resonance, chemiluminescence, mass spectrometer, and others. The availability of stable, narrow band lasers covering ever-wider spectrql ranges has, of course, been a powerful driving force in this field, for both radical generation and detection. While it may be argued that, in spite of highly state-specific detection, such experiments lose their specificity both because of their initial-state distributions and because of the many-collision time delay, Le., that they are mere ”bulb” experiments, powerful counterarguments may be cited: (1) The high detection sensitivity and consequent low radical concentrations make radical-radical interactions among the minor (lower concentration) reactants negligibly slow within the available reaction time. ( 2 ) The most frequent collisions (normally by 3-6 orders of magnitude) are radical-He or -Ar collisions, efficient in thermalizing translation and rotation, but highly inefficient in changing vibronic-state distributions. (3) The experiments are normally carried out in difference mode such that carrier gas and other background effects are‘measured independently and subtracted. The pressure range of flow reactor experiments has so far been limited ( < l o torr), but extensions to higher pressures are now under way. In (laser) flash photolysis experiments, the pump-and-probe approach is more obvious and direct. Real-time measurements are carried out over intervals of (1 ms, such that transport effects are unimportant. The accessible pressure range is much wider. For these reasons, (laser) flash photolysis is the method of choice when applicable. Since it is more restrictive, however, both in the photolytic generation of reactant radicals and in the limitation on other reactants, flash photolysis is unable to provide rate data for many reactions, whereas the flow rector technique is nearly universally applicable. Flow Reactor Technique and Apparatus. Within the limitation of providing some examples of recent experimental work but without extensive review, this section first mentions progress in data interpretation in terms of the fundamental transport equation and then shows diagrams of some apparatuses used in several laboratories. Many earlier papers have dealt with the solution of the transport equation2-’ under various conditions of surface boundary conditions, average flow velocity, and rate parameters. This discussion deals principally with reactive atom/radical species whose surface removal can be sufficiently well inhibited (accommodation per gas kinetic surface collision) that very coefficient, y 5 large, average flow velocities (0 50-100 ms-l) and large radial concentration gradients can be avoided.
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( 2 ) Kaufman, F. Prog. React. Kine?. 1961, 1, 1. (3) Walker, R. E. Phys. Fluids 1961, 4 , 1211. (4) Huggins, R. W.; Cahn, J. H. J . Appl. Phys. 1967, 38, 180. (5) Ferguson, E. E.; Fehsenfeld, F. C.; Schmeltekopf, A. L. Adu. At. Mol. Phys. 1969, 5, 1. (6) Cher, M.; Hollingsworth, C. S . Ado. Chem. Ser. 1969, No. 80, 118. (7) Farragher, A. L. Trans. Faraday SOC.1970, 66, 1411. (8) Bolden, R. C.; Hemsworth, R. S.; Shaw, M. J.; Twiddy, N. D. J . Phys. B 1970, 3, 45. (9) Pokier, R. V.; Carr, R. W., Jr. J . Phys. Chem. 1971, 75, 1593. (10) Brown, R. L. J . Res. Natl. Bur. Stand. 1978, 83, 1. (11) Howard, C. J. J . Phys. Chem. 1979, 83, 3.
D I E LASER ~ C M X - ’ I - . - - - \ M I R R Q R
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Figure 1. Diagram of flow reactor apparatus with movable, concentric double injector (ref 16 and 68).
DISCHARGE L A
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Figure 2. Diagram of flow reactor apparatus with upstream infrared laser multiphoton dissociation radical source and detection by LIF, VUVRF, and modulated molecular beam MS. Reprinted with permission from ref 17. Copyright 1984 North-Holland Publishing Co.
In our own work (see Figures 1 and 2 ) , we use a movable injector or double injector to add the second reactant, usually in excess concentration in order to approach pseudo-first-order kinetics for the radical removal step. Exposed surfaces are covered with thin films of Teflon or halocarbon wax and effective surface removal rate constants, kIw, are measured in independent experiments, usually kIw < 10 s-l. The available temperature range is 3 atomic) reaction this interplay of theory and experiment marks an auspicious beginning. (ii) The agreement is fortuitously good, mainly because the ab initio barrier height is uncertain to about f 2 kcal/mol. (iii) More powerful and extensive quantum calculations of the potential energy surface are urgently needed (as they are for other reactions discussed below). (iv) The large tunneling correction, though somewhat atypical because of the thin barrier and sharp turning angle of the reaction coordinate, suggests more experimental work at low temperaures as well as more ab initio theoretical calculations. This reaction and its isotopic variants will continue to be a proving ground of rate theory and experiment. (31) Walch, S. P.; Dunning, T. H. J . Chem. Phys. 1980, 72, 1303. (32) Schatz, G.C.; Elgersma, H. Chem. Phys. Lett. 1980, 73, 21. (33) Isaacson, A. D.; Truhlar, D. G. J . Chem. Phys. 1982, 76, 1380.
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(34) Cohen, N. Int. J . Chem. Kinet. 1982, 14, 1339; 1983, 15, 503. (35) Benson, S. W. “Thermochemical Kinetics”, 2nd ed.; Wiley: New York, 1976. Golden, D. M. J . Phys. Chem. 1979,83, 108. (36) Zellner, R. J . Phys. Chem. 1979, 83, 18. (37) Tully, F. P.; Ravishankara, A. R. J . Phys. Chem. 1980, 84, 3126. (38) Jeong, K.-M.; Kaufman, F. J . Phys. Chem. 1982, 86, 1808. (39) Jeong, K.-M.; Hsu, K.-J.; Jeffries, J. B.; Kaufman, F. J . Phys. Chem. 1984,88, 1222. (40) Jeong, K.-M.; Kaufman, F. J . Phys. Chem. 1982, 86, 1816. (41) Zellner, R.; Steinert, W. Inr. J . Chem. Kinet. 1976, 8, 397. (42) Tully, R. P.; Ravishankara, A. R.; Carr, K. Int. J . Chem. Kinet. 1983, 15, 1111.
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Kaufman
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- XENONARCLAMP - FIRSTSURFACE MIRROR H - FLASH LAMP/ REACTOR HOUSING M 0 1 - 1/2METER MONOCHROMATOR A N D SPECTROGRAPH ,&lo2 - l I 4 M E T E R MONOCHROMATOR I - IRIS LE - LENS W - WHITE C E L L OPTICS T - 4' X 8' OPTICAL BENCH PMT - PHOTOMULTIPLIER TUBE B - B E A M SPLITTER LA - HELIUM.NEON LASER C
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Figure 10. Diagram of flash photolysis multiple path UV absorption apparatus.30 Reprinted with permission from ref 29.
and 3.09 f 1.01, A(300 K) = 16.3 X and 3.87 X cm3 and E(300 K) = 2.44 and 1.57 kcal/mol, respectively. Cohen's34 transition-state calculation gives n = 2.0, A(300 K) = 8.6 X and E(300 K) = 2.05 kcal/mol, midway between the experimental values. Further experimental work is required, yet our semiempirical calculation of A(300 K)39 is in moderate agreement (factor 1.7) with experiment. Since the energy barrier H2 and OH CH4, tunneling is lower than that for OH corrections should be smaller, but very little is known about them. Scattered information is available for the higher paraffins in fair agreement with Cohen's c a l c ~ l a t i o n sbut , ~ ~detailed discussion is not warranted here. In addition to CH4 and C2Hs,our own recent work has included e~~~ the measurement of nine C1- and F - s u b ~ t i t u t e d - m e t h a nand four substituted-ethane reactions39with OH by the flow reactor, R F technique over the -250-470 K temperature range, fitting to three-parameter expressions, and comparing the reduced A( 300 K) values with semiempirical transition-state calculations using the Benson/Golden model compound method.35 Our reason for this approach was that energy barriers cannot be estimated with sufficient accuracy to provide a meaningful test, but A factors, i.e., entropies of activation, can be calculated for assumed transition-state geometries. Furthermore, since both the experimental rate measurements and the transition-state calculations were done in a consistent, strictly comparable manner, there should be some cancellation of systematic errors. The results for all 15 compounds (including CHI and C2Hs) are encouraging in one respect and discouraging in another. For reasonable transition-state geometries, i.e., extensions of 0.3 A for the C-H and H-0 bonds and approach angles of 150-165', experient and theory agree well, on average: to within 20% if the T exponent, n, is assumed to be 2.0,"O somewhat worse if the n's are taken from the three-parameter fits.39 However, the scatter of individual ratios of theoretical to experimental A values is large and random, Le., bears no apparent relation to the physical nature of the halocarbon, e.g., size, dipole moment, van der Waals interaction, energy barrier, etc. Based on this scatter, the predictive power of the theory for A(300 K) is roughly a factor of 2 (up or down) at the single standard deviation level. This is perhaps not too surprising and serves as a reminder that we tend to take the predictions of thermochemical transition-state theory somewhat too seriously. OH Addition and Adduct Formation. Turning briefly to reactions of OH with a-bonded molecules, we recognize the possibility of adduct formation with subsequent redissociation, cleavage of a weaker bond, or collisional stabilization. This greatly complicates data analysis and interpretation, since there are now at least three sets of thermochemical parameters to contend with: S-I,
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(a) entrance-channel transition state; (b) exit-channel transition state; and (c) adduct configuration and well depth. The measured data are then functions of many parameters and unique interpretation becomes difficult. Yet, it is particularly important in these contentious cases to show that rational explanations exist that are in accordance with our understanding of rate processes. As a first example, the reaction OH HN03 H 2 0 NO3 may be cited, where there is no question regarding the identity of the p r o d u ~ t s . ~ There ~ - ~ ~is also no likelihood of a a-bonded adduct, and one may therefore expect, for a reaction that is 18 kcal/mol exothermic, either normal H-atom transfer or the formation of a hydrogen-bonded intermediate complex. Because the reaction is an important HO, removal step in the low stratosphere, it has recently received a great deal of attention. The experimental results are astonishing. Two early studies, one by flash photolysis,46 the other by flow reactor,'" were in good agreement and gave a strangely temperature-independent, yet small, rate constant of cm3 s-l. Since 1981, six flash photolysis and two (8-9) X flow reactor measurements have established that k(298 K) is about 1.3 X cm3 s-l, but that the reaction has a negative T dependence of about exp(+76O/T) over the 220-300 K range. Some experimental discrepancies remain, e.g., a small pressure dependence over the 20-100-torr He range48and a lower k(298 K) and T dependence by one flow reactor but the agreement is substantial, seven studies giving k(298 K) = (1.28 f 0.07) X cm3 s-I. Yet, how are we to interpret an Arrhenius expression exp(760/T) for a seemingly simple, one-channel, of 1 X elementary H-atom-transfer reaction? And why is there no direct H-atom transfer; Le., why is its activation energy >4 kcal/mol? Attempts have been made at rationalizing the result either via the formation of a tight, H-bonded intermediate complex48or via generalized transition-state theory,50 but with only marginal success. The reactions of O H with olefins, substituted olefins, aromatics, etc., mostly proceed by the adduct route with subsequent stabilization or product formation. This is a large and active field of elementary reaction rate research, but only a few examples can
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(43) Nelson, H. H.; Marinelli, W. J.; Johnston, H. S . Chem. Phys. Lett. 1981, 78, 495. (44) Jourdain, J. L.; Poulet, G.; LeBras, G. J. Chem. Phys. 1982,76,5827. (45) Ravishankara, A. R.; Eisele, F. L.; Wine, P. H. J . Phys. Chem. 1982, 86, 1854. (46) Smith, I. W. M.; Zellner, R. In?. J . Chem. Kinet. Symp. 1975, No. 1. 341. -, (47) Margitan, J. J.; Kaufman, F.; Anderson, J. G. In?. J . Chem. Kinet. Symp. 1915, No. 1 , 281. (48) Margitan, J. J..; Watson, R. T. J. Phys. Chem. 1982, 86, 3819. (49) Connell, P.; Howard, C. J. In?. J . Chem. Kinet., in press. (50) Marinelli, W. J.; Johnston, H. S . J . Chem. Phys. 1982, 77, 1225. ~
The Journal of Physical Chemistry, Vol. 88, No. 21, 1984 4915
Feature Article
be given here. The parent O H
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C2H4 reaction has no exothermic elimination channel and is therefore controlled by adduct formation. The reaction was studied by nine groups between 1970 and 1977, mainly by flash photolysis, and most recently by Tullysl by the laser photolysis/LIF technique. It is in the falloff region below about 400 torr of He at room temperature and its highpressure k(298 K) is about 8.5 X 10-l2cm3 s-l. Over the T range 291-438 K, the high-pressure k exhibits negative T dependence, exp(4601T) or which is not unexpected for such processes. The apparent, sharp drop of k at higher Tis due to the thermal instability of the adduct as shown by increasingly nonexponential OH decays as the reaction approaches equilibrium rather than going to completion. In that sense, the observed rate constant no longer represents the elementary step of adduct formation, but becomes a phenomenological mix of forward and reverse steps and should not be plotted on the same graph (Figure 4, ref 5 1) where it gives the erroneous impression of a sharp increase in the magnitude of the negative temperature coefficient. The same comment applies to similar graphs in several OH + aromatics paper^.^^^^^ In those cases, a simple H-abstraction channel ultimately becomes controlling at higher temperatures. For halogen-substituted olefins, two brief examples, OH C2HC13 and OH CZCl4, will suffice. These reactions were studied only at fairly low pressures, 1-6 torr in flow reactor ~ t u d i e s , ~but ~ Jsince ~ the lifetime of the OH adduct for C1 elimination is likely to be short compared to collisional stabilization or redissociation, the observed k is the high-pressure rate constant of adduct formation. For C2HC13,k(298 K) is 2.4 X and the T dependence is negative,55EIR = -445 f 41 K or T1.4,very reasonable values compared with CzH4 above, when reaction site available and steric hindrance are considered. For CzCl4, k(298 K) = 1.7 X cm3 s-' and the T dependence is positive,55E / T = 1200 f 55 K. There is a substantial barrier to O H addition and the corresponding Arrhenius A value is large, 9.4 X lo-'' cm3 s-l, because OH addition is unselective, albeit restricted by an energy barrier. In summary, the simple olefin and aromatic reactions are well characterized experimentally and qualitatively understood. Radical-Radical Reactions. The power of modern experimental methods is best exemplified by a brief discussion of elementary radical-radical rate measurement and their interpretation. The reactions of the H 0 2 radical, important in both combustion and atmospheric chemistry, with atom and other radical species, will serve as a focus of this discussion. For atomic reactants, we will consider H , 0, N, C1, and Br, and for molecular ones, OH and H 0 2 itself. When the flow reactor method is used, its versatility simplifies the problem: the two reactive species are prepared in different upstream sections of the apparatus and are mixed at the beginning of the reaction region, each in great dilution with inert carrier gas. In isolated cases, flash or laser photolysis is able to coproduce two different radical species whose reaction can then be monitored directly. The study of the 0 HOz reaction by Ravishankara et al.27is a fine example (see Figure 8). H202and O3 are both photolyzed a t 248.5 nm, [H20z] >> [O,];HOz is generated by OH H202and is in excess over 0 which is generated by 0, photolysis and subsequent quenching of O(lD) by added N2. The decay of [O] is measured by VUV resonance fluorescence. However, the H02 concentration cannot be measured directly but must be calculated on the basis of laser flux and H 2 0 2concentration, absorption coefficient, and quantum yield. In our study of this same reaction,s6 HOz is produced by F + HzO2in the movable double injector (Figure 1) and analyzed by LIF following
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(51) Tully, F. P. Chem. Phys. Lett. 1983, 96, 148. (52) Perry, R. A.; Atkinson, R.; Pitts, J. N., Jr. J . Phys. Chem. 1977, 81, 296, 1607. (53) Tully, F. P.; Ravishankara, A. R.; Thompson, R. L.;Nicovich, J. M.; Shah, R. C.; Kreutter, N . M.;Wine, P. H. J . Phys. Chem. 1981,85, 2262. (54) Howard, C. J. J . Chem. Phys. 1976, 65, 4771. (55) Chang, J.-S.; Kaufman, F. J . Chem. Phys. 1977, 66, 4989. (56) Sridharan, U. C.; Qiu, L. X.;Kaufman, F. J . Phys. Chem. 1982, 86, 4569.
quantitative conversion to OH with excess NO. 0 atoms are produced in a microwave discharge of H e plus a trace of Ozand monitored by VUVRF. If necessary, this concentration measurement is calibrated by producing known [O]from excess [N] via N NO N2 0 using small, measured NO additions. In the following brief summary of results, only direct measurements will be cited. For H HOz, this leaves only our own recent works6 analogous to the above description except that [HI was monitored by Lyman-a resonance fluorescence and calibrated by conversion to OH by using the H + NOz titration. The total rate constant, kT(298 K), was (7.4 f 1.2) X lo-" cm3 s-l, and its branches, a-c for the product channels O H + OH, H 2 0 + 0, and Hz 02,were found to be 0.87, 0.04, and 0.09 of kT, respectively; i.e., k, = 6.4, kb = 0.30, and k, = 0.67 X lo-'' cm3 s-' . For 0 HOZ,which has only the single product channel, OH Oz,there are now four independent, direct measurements, three by flow reactor and one by laser photolysis methods. The results are very encouraging: K e y ~ e r ' s6.1 ~ ~X lo-" cm3 s-l, ours6 5.4 X lo-" cm3 s-l, Brune et al.'s20 5.2 X lo-" cm3 s-l, and Ravishankara et a l . ' ~6.2~ X~ cm3 s-l are all well within the overlapping uncertainty limits of about f(0.8-1.1) X lo-" cm3 s-l claimed by these investigators. These values are about a factor of 2 larger than earlier, more indirect estimates. Only Keysers7 has studied the temperature dependence and reported exp[+200 over the 229-372 K range. f 28)/Tl corresponding to For N HOZ,only a single room-temperature measurement, by Brune et al.zo in a flow reactor system (see Figure 4), has recently become available. The reaction has four possible product channels, N H Oz, NO OH, H N O 0, and H NO2, of which only the first two seem plausible in terms of the required bond rearrangements. The decay of HOz was monitored by LMR in excess [N]. The measured k(298 K) = 2.2 X lo-" cm3 s-l includes all exothermic channels. The C1 H 0 2 reaction was studied directly by Lee and HowardSs from 250 to 414 K, who monitored HO,, OH, and C10 concentrations by LMR in a flow reactor system (see Figure 3). The reaction has two product channels, the highly exothermic (-53.9 kcal/mol) HCl O2(a), and the slightly endothermic (-2 kcal/mol) C10 + O H (b). These authors were able to measure both k, and kb over that T range and obtained k, = 1.8 X lo-" exp(l7O/T) and k b = 4.1 x lo-" exp(-450/T) cm3 s-l. At room temperature, k,(298 K) = 3.2 X lo-" and kb(298 K) = 0.91 X lo-" cm3 s-l. The value of kb is consistent with the rate constant of the reverse reaction measured recently by Ravishankara et aLS9 in a flow reactor system system and an earlier measurement of Leu and L h 6 0 It is interesting to note that the slightly endothermic kb channel is relatively fast. Finally, for Br H 0 2 , there is a recent measurement by Poulet et in a flow reactor apparatus using both LIF and mass spectrometry. Since there are several fast, simultaneous reactions occurring, the system had to be computer modeled, and the best cm3 s-l, much slower than the fit gave k(298 K) = 7.6 X Br H C O reaction for which a k(298 K) of 2.8 X 1O-Io cm3 s-l was reported. Although the latter reaction is much more exothermic (M029s = -70.8 kcal/mol) than the former (M",,, = -38 kcal/mol), it is unclear why Br + HOz should be so slow. In summary of this discussion of five atom HOz reactions, we should first note that these measurements represent substantial progress in gas-phase kinetics. It has become possible to isolate highly reactive, fast elementary processes, to determine their rate constants directly with good accuracy, and, in some cases, to ascertain the product channels and their individual rates. Turning next to possible generalizations and inferences regarding mechanism and dynamics, the picture is not clear. The formation of an intermediate adduct seems to be favored, certainly in the H
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(57) Keyser, L. F. J . Phys. Chem. 1982, 86, 3439. (58) Lee, Y.-P.; Howard, C. J. J . Chem. Phys. 1982, 77, 756. (59) Ravishankara, A. R.; Eisele, F. L.; Wine, P. H. J . Chem. Phys. 1983, 78, 1140. (60) Leu, M.-T.; Lin, C.-L. Geophys. Res. Lett. 1979, 6, 425. (61) Poulet, G.; Laverdet, G.; LeBras, J . Chem. Phys. 1984, 80, 1922.
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+ H0, case where highly excited H z 0 2 tis a logical intermediate and where the simple H abstraction to form H2 O2 is at least 1 order of magnitude slower. A similar mechanism may operate in the very fast 0 H 0 2 reaction, but this remains to be established by isotope substitution experiments. There is some O2 Hevidencez0 that the N O OH rather than the N H atom-transfer product channel is the major one in the N + HOz reaction. For C1 HOz, where the most extensive information is available, there is an apparent contradiction: the H-atom-transfer step has a negative T dependence, whereas the minor C10 OH channel which would logically originate via the ClOOH adduct shows a positive T dependence. Yet we may rationalize the entire process as follows: a highly excited ClOOH adduct is first formed which decomposes either by HCl elimination or by 0-0 bond cleavage. The latter process is slightly endothermic, therefore its positive T dependence. The former is controlled by adduct formation which usually exhibits slightly negative T dependence. The fact that four-center HC1 elimination occurs here but probably not in H HOz may be ascribed to the large size of C1 compared with H and to the HCl dipole moment. Since the adduct is presumably the lowest singlet state, HCl elimination should leave 0, in either the alAg or blZg+state, both energetically accessible by the 53.8 kcal/mol exothermicity. If this conjecture has merit, the much slower speed of the Br H 0 2reaction may be due to its smaller exothermicity (-38 kcal/mol) which would make the blZg+state of Oz inaccessible. More work is required to establish whether direct H abstraction does or does not occur in all of these interactions. Two important radical-radical reactions of H02, the self-reH2OZ 02,and the reaction with OH, action, H 0 2 HO, OH H 0 2 H 2 0 02,have also been reexamined recently. Five recent, low-pressure (
