THE J O U R N A L O F
PHYSICAL CHEMISTRY ?2 Copyright, 1979, by the American Chemical Society
Registered in I J S . Patent Office
VOLUME 83, NUMBER 1
JANUARY 11,1979
Symposium on Current Status of Kinetics of Elementary Gas Reactions: Predictive Power of Theory and Accuracy of Measurement National Bureau of Standards, Gaithersburg, Maryland June 19-21, 1978
Introductory Remarks Frederick Kaufman Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 Publication costs assisted by the Office of Chemistry and Chemical Technology, National Research Council
The symposium, most of whose papers are published in this issue of T h e Journal of Physical Chemistry, was held on June 19-21,1978 a t the National Bureau of Standards, Gaithersburg, Maryland. It was sponsored by the National Research Council-Clommittee on the Kinetics of Chemical Reactions aind by the U S . Department of Commerce, National Bureau of' Standards, National Measurement Laboratory. I t received funding support from the Petroleum Research Fund, administered by the American Chemical Society, from the U.S. Department of Energy, and from the National Science Foundation. All local arrangements were handled most excellently by an NBS committee headed by Dr. David Garvin. About 115 persons from 8 countries attended and took an active part in presenting and discussing 33 papers. The meeting ended with a brief general discussion on the afternoon of tlhe third day. It started on a note of great sadness, mourning the recent death of two giants of the world of chemical kinetics: 0022-3654/79/2083-0001$01 .OO/O
Professor R. G. W. Norrish of Cambridge and Professor 0. K. Rice of North Carolina. Professor Rudy Marcus briefly described some of his own experiences as a young postdoc in Professor Rice's group and paid tribute to Rice's unusual and highly stimulating methods of guiding his young associates by giving them great freedom to choose and develop ideas but by also exposing them to profound, critical analysis which helped them discover the crux of their problem. My reason for calling this meeting was the need to take stock of the present state of the field of elementary, thermal gas reaction kinetics; to assess the accuracy of measurement techniques; to discuss the compilation and evaluation of rate data; and, most importantly, to examine the predictive power of theory. These three concerns are dealt with, respectively, in the first eight, the next three, and the remaining seventeen papers of this journal issue. The stock-taking; was stimulated by recent advances in the direct experimental measurement of elementary atom or 8 1979 American Chemical Society
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The Journal of Physical Chemistry, Vol. 83, No. 1, 1979
radical reaction kinetics and by the preoccupation of theorists with problems of detailed state-to-state dynamics a t a time when the demand for rate constants, measured, calculated, or guessed, is growing rapidly in such diverse fields as atmospheric chemistry, combustion, and pollution. For these reasons, the subject matter of the symposium was sharply focused on thermal reaction rates of neutral, (electronic) ground-state species, not because state-to-state dynamics or excited state reactions or ion reactions are any less interesting, but because thermal reactions have recently been treated with much benign neglect. They did, after all, form the foundation of reaction rate theory in the 1920’s and ~ O ’ S , yet have only recently become open to direct experimental measurement of good accuracy. A large number of key questions ought once again to be asked and their answers examined in the light of laboratory results. They include the following: validity and limitations of transition state theory; potential energy surfaces, how to calculate them (ab initio vs. semiempirical) and what detail is required (in a cost-benefit analysis sense); classical trajectory calculations; quantum corrections based on one-, two-, and three-dimensional theory; nonequilibrium effects in two-body reactions; energy transfer in dissociation/recombination reactions and its dependence on excitation energy, molecular complexity, and temperature; prediction of rate parameters over large temperature ranges, for widely different molecular complexity or for series of reactants differing only in substituent effects; implication of energy disposal information for thermal rate constants; critical test cases, presently available or to be developed, for theory-experiment comparison. The reader of this journal issue must decide which of these and other issues have been brought closer to successful resolution. My own, brief appraisal would begin with the statement that the meeting seemed a useful and successful exercise, that it should probably be repeated in a few years, and possibly become a regularly scheduled event, albeit an infrequent one. In assessing the present state of affairs in the three topical areas, it is probably fair to say that the greatest progress has been achieved in the first area under discussion, that of experimental measurements. Here, the wide use of highly sensitive detection techniques (resonance fluorescence, laser induced fluorescence, laser magnetic resonance, molecular beam sampling mass spectrometry, etc.) and the wide range of atom or radical generation techniques (photolytic, discharge, thermal, chemical, etc.) has made it possible to make measurements on vastly more reaction systems than ever before, and to do so in a direct manner, Le., without recourse to classical methods of fitting analytical data to proposed mechanisms. The major experimental methods, flash photolysis and discharge-flow for the low temperature range and shock tube for high temperatures, continue to dominate the scene. Other methods, e.g., very low pressure pyrolysis (VLFP),are making major contributions, especially for bond fission reactions of larger molecules. Hybrid techniques, e.g., discharge flow shock tube, and extensions to high temperatures (high temperature fast flow reactor) are successfully bridging the gap between the widely separated temperature regimes of earlier studies. The realistic appraisal of experimental error still leaves much to be desired and we are fortunate in having the fine review paper by Cvetanovic, Singleton, and Paraskevopoulos to help us put our house in order. Experimental rate measurements of elementary reactions have certainly
Frederick Kaufman
“arrived” and their future looks very bright, indeed, both in regard to improved accuracy and to wide applicability to reaction systems. The second topical area, compilation and critical evaluation of rate data, suffers greatly from being underfunded. The papers devoted to this field and the ensuing discussion show the urgent need for increased support. This is due both to the proliferation of experimental studies and to increased “user” pressure, mainly for modeling calculations in atmospheric chemistry, combustion, or pollution studies. Rate data evaluation is a relatively small, inexpensive activity, but it is in great demand by many groups: by experimentalists to keep abreast with the field; by theorists to have reliable results to guide and check their calculations; and by modelers to provide them with input parameters for computer codes. It is clear, of course, that compilation and evaluation spans a wide spectrum and that different “customers” may have very different requirements. Yet the overall need for faster progress on all fronts, i.e., for greater funding support, seems well substantiated. The third area, predictive power of theory, makes up almost two-thirds of the symposium and of the published papers. It is also the most difficult to assess in a broad, overall sense. There has been clear progress on all fronts. Ab initio, three-dimensional, fully quantum calculations of the dynamics of some simple systems (H + Ha),routine three-dimensional classical trajectory calculations on many systems, ab initio and semiempirical potential energy surfaces, testing of various approximate theories against exact calculations in the easily accessible one-dimensional format (mainly for A + BC reactions), development of improved statistical theories of dissociation-recombination reactions, and continued application of transition state theory, particularly in its thermochemical variant (Benson, Golden), with excellent success to a host of complicated systems. To my nontheorist mind, many major questions remain unsettled: How extrapolatable are one-dimensional concepts and findings to the real world? What is the present and near-future accuracy of ab initio potential energy surface calculations and what impact can they be expected to have on elementary reaction rate calculations? How many and what kind of scaling parameters are needed in the characterization of semiempirical surfaces for thermal rate constant calculations? How are quantum (tunneling) effects best approximated in complex reaction systems? How serious are the necessary overestimates of equilibrium (transition state) theory rate constants due to “recrossing” effects. due to non-uniqueness of the transition state, due to specificity of energy disposal? What is a conservative estimate of the predictive power of thermochemical kinetics? As good as a factor of 2 or 3 in the Arrhenius A factor? The list of questions could be lengthened almost indefinitely, but enough. There is clearly much more work to be done. What impresses me, however, is the general usefulness and resilience of simple transition state theory which, after early triumphs went into a lengthy eclipse only to re-emerge as a surprisingly accurate (and sometimes as the only) tool of the gas phase kineticist. The characterization of dissociation and recombination reactions of polyatomics by a judicious mix of transition state, quantum statistical, and energy transfer theories, often at the semiempirical level, has also made very good progress. Because of the fairly limited range of collisional energy transfer probabilities for highly excited polyatomics, the predictive power of these theories sometimes exceeds that of the corresponding processes in diatomics. Yet even
Kinetic Measurements Using Flow Tubes
for diatomics substantial progress has been achieved, as evidenced by some of the present papers. Lastly, the symposium did achieve its major goal: to
The Journal of Physical Chemistry, Vol. 83, No. 1, 1979
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bring experimenitalists and theorists together and to show that the field of thermal elementary reaction kinetics is alive and well.
Kinetic Measurements Using Flow Tubes Carleton J. Howard Aeronomy Laboratory, NOAA Environmental Research Laboratory, Boulder, Colorado 80303 (Received October 6, 1978) Publication costs assisted by the NOAA Environmental Research Laboratory
This paper is a selective survey of the chemical kinetic literature involving flow tube measurements of elementary reaction rate constants. It describes the origins of the flow tube method, the experimental technique, the measurement of rate constants, and an analysis of the inherent errors. Emphasis is placed on the discussion of t h e strengths and limitations of the method as a source of kinetic data.
I. Introduction In recent years there has been an increasing demand for gas phase reaction rate data. Laser development,l atmospheric c h e m i ~ t r y ,and ~ , ~combustion4 are examples of fields of application of elementary reaction studies. Committees and organizations have been created to collect, evaluate, and disseminate kinetic information. In atmospheric chemistry, for example, there are serious economic and social implications derived from the application of kinetic data in computer models that assess the impact of anthropogenic chemicals on stratospheric ozone. Thui;, increased concern for the accuracy of rate constant measurements has developed concurrently with the demand for more data. The flow tube technique has been the most prolific source of kinetic data near 300 K. In evaluating the usefulness of this method for obtaining rate constant data it is instructive to make a comparison with the flashphotolysis technique. This comparison on the basis of seven different criteria is summarized in Table I. The emphasis of this discussion is not to demonstrate the superiority of one method in all categories but rather to show the complementary nature and strengths of both methods. In making such a comparison it is necessary to make some generalizations that are not accurate for every study. In this respect the discussion is influenced by the experiences we have had in the NOAA Aeronomy Laboratory using both techniques. A. Ternpcrature Range. The useful temperature range is nearly the same for both techniques. The upper temperature limit i s established by the onset of problems with the thermal stability of reactants and the selection of materials for fabricating the apparatus. At the low temperature extreme the flow tube method is somewhat more restricted than the flash-photolysis method because of heterogeneous reactions. It is often observed that the rate of destruction of radicals such as C1, OH, and H02 on the reactor surface increases significantly at temperatures below about 250 Ke5 Nevertheless, there have been several studies using flow itube techniques beyond these limits. For example, Trairior et aL6 have studied the recombination of atomic hydrogen down to 77 K, Westenberg and deHaas7s8have routinely studied reactions of 0 and OH up to 1000 K, and Fontijn et ale9have developed a flow reactor designed for operation up to 2000 K. The latter
TABLE I: Comparison of Flow Tube and Flash-Photolysis Kinetic Tecliniques flow tube
flash photolysis
temperature
200-600 K
100-600 K
range pressure
1-10 torr
5 torr-several
(1O - l o - l O - l ') cm3 molecule-' s - '
( 1 0 ~ ' o - 1 0 ~ 'cm3 8)
excellent
requires fast
range rate constant range detection versatility reactant versatility heterogeneous reactions expense
atmospheres molecule-'
excellent
detector limited
can be serious
none
low
moderate
s-l
work deals with reactions of metals and metal oxides in a special application of the flow tube technique and is described in detail elsewhere in this volume.1° B. Pressure Range. The flow tube technique is basically a low pressure technique as will be discussed later. Flash photolysis, on the other hand, can be used to very high pressures with the main limitation being the detection of reactants. If resonance fluorescence is used,for detection, some species such as OH are quenched by the buffer gas. In this case resonance absorption may be usedll to extend the pressure range. C. Rate Constant Range. Both flow tube and flashphotolysis techniques are used to measure fast reactions with rates up to gas kinetic collision rates. The greater pressure range of the photolysis method also allows larger reactant concentrations to be used and hence smaller rate constants to be measured. Thus photolysis systems have a significant advantage for studying slow reactions and termolecular reactions a t high pressures. D. Detector Versatility. One of the two major advantages of the flow tube technique is the immense variety of methods that can be used to detect the reactants and products. This advantage is derived from the steady-state nature of the flow system in which the progress of the reaction is frozen a t any fixed observation point along the tube. Since the concentrations of the reactants are constant a t that point, there are no constraints on the detector speed. A flash-photolysis experiment, on the other hand, is studied in real time and requires a detector
This article not subject to U S . Copyright. Published 1979 by the American Chemical Society
