Evaluation and Compilation of Reaction Rate Data
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Evaluation and Compilation of Reaction Rate Data Robert F. Hampson, Jr.,* and David Garvin National Bureau of Standards, Washington, D.C. 20234 (Received May 12, 1977) Publicatlon costs assisted by the National Bureau of Standards
Recently chemists have been attempting to analyze very complex chemically reacting systems in terms of the individual elementary chemical reactions. There is an attempt to reproduce the observed behavior of the complex system by mathematical simulations, that is, models. The validity of the analysis is limited by the reliability of the input data including rate constant values for the large number of elementary reactions. Recent activities in the area of compilation and critical evaluation of rate data have produced extensive tables of preferred values of rate parameters. This paper provides a guide to these sources of evaluated rate data. It discusses factors to be considered in the evaluation of rate data with emphasis on the kinds of information an author must provide in order for his results to be considered by the data evaluator. Current gaps in the kinetic data base for one particular system, namely, the chlorine-catalyzed destruction of stratospheric ozone, are indicated.
Introduction Recently chemists have been attempting to enlarge their area of understanding from deliberately simple, controlled chemical systems to systems of much greater complexity. These systems range from laboratory smog chamber studies to naturally occurring chemical systems such as the troposphere and the stratosphere both in their natural states and as perturbed by man-made activities. The basis for these analyses has been twofold-the development and refinement of high-speed reliable computational techniques and the accumulation of a reliable data base for the large number of elementary chemical reactions and photochemical processes involved. In these studies, there is an attempt to reproduce the observed behavior of the complex system by mathematical simulations that is, models. These models require large amounts of numerical input data including rate constants values for all the elementary reactions, which may well exceed 100 in number. The validity of the analysis is limited by the reliability of the input data. However, the modeler may not himself be an expert in the area of chemical kinetics. Where should he look to find reliable rate constant data? In this paper we will indicate sources of reliable, kinetic data. Also, factors to be considered in evaluating the reliability of published rate data will be discussed with emphasis on the kinds of information that must be furnished by an author in order for his results to be considered by the evaluator. Finally some current gaps in the kinetic data base for one particular system, namely, the chlorine-catalyzed destruction of stratospheric ozone, will be indicated. Sources of Rate Data A survey of reaction rate data compilation and evaluation activities was published in 1973.l It contained a description of the formal, continuing kinetics data programs on national and international levels. The appendix to ref 1 contained a listing of the then current tables, evaluations, bibliographies, and reviews of kinetics data. Since publication of ref 1 many papers containing rate data for atmospheric reactions have been published. There are only a small number of major programs which evaluate these published rate data and prepare tables of recommended values for rate parameters. The major evaluation activities concerned with homogeneous, gas phase, atmospheric reactions are the following: (a) High Temperature Reaction Rate Data Centre, Department of Physical Chemistry, University of Leeds, Leeds, England, D. L. Baulch, Director. This center evaluates rate data for gas phase, high temperature re-
actions. Three hard-cover volumes of evaluations have been issued (see entries in Table I under Baulch et al.). (b) Chemical Kinetics Information Center, Physical Chemistry Division, National Bureau of Standards, Washington, D.C., R. F. Hampson, Director. This center collects and indexes publications containing information on rates of chemical reactions. It provides bibliographic support for authors of critical reviews for the National Standard Reference Data System. The center prepares bibliographies on broad topics in kinetics and provides kinetics information services for the scientific public. The center issues rate data evaluations and tables of recommended rate and photochemical data (see entries in Table I under Hampson). (c) The Climatic Impact Assessment Program of the U.S. Department of Transportation organized an evaluation program covering chemical and photochemical reactions in the stratosphere. The results were published in CIAP Monograph 1, T h e Natural Stratosphere of 1974 (see Table I). (d) The NASA Upper Atmosphere Research Program organized a kinetics evaluation program for its assessment of the effects of chlorofluoromethanes on stratospheric ozone. This evaluation was made by the program’s Laboratory Measurements Committee. The results appear in NASA Reference Publication 1010, “Chlorofluoromethanes and the Stratosphere”, R. D. Hudson, Ed., Aug 1977. (e) The Committee on Data for Science and Technology (CODATA) of the International Council of Scientific Unions (ICSU) has very recently established a new Task Group on Chemical Kinetics which will prepare a table of recommended rate and photochemical parameters for atmospheric reactions. Table I provides a guide to recent sources of chemical kinetics data for atmospheric reactions and combustion. It emphasizes sources of evaluated rate data but also includes some large scale compilations. The time period covered is from 1973 to the present. A more comprehensive listing is given in NBS List of Publications No. 73, “Chemical Kinetics Tables, Data Evaluations and Bibliographies. A Guide to the Literature”, revised September 1976. This is available upon request from the Chemical Kinetics Information Center of the National Bureau of Standards.
Compilation and Evaluation of Rate Data We have already mentioned data evaluation. The concept of data evaluation implies that an expert has examined all the pertinent evidence and has made a recommendation. The Journal of Physlcal Chemistry, Vol. 81, No. 25, 1977
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R. F. Harnpson, Jr., and D. Garvin
TABLE I: Chemical Kinetics Tables, Evaluations, and Compilations
TABLE 11: Reactions for which Additional Rate Data Are Needed ~
Anderson, L., “Atmospheric Chemical Kinetics Data Survey”, Rev. Geophys. Space Phys., 1 4 , 1 5 1 (1976). Baulch, D. L., Drysdale, D. D., Horne, D. G., and Lloyd, A. C., “Evaluated Kinetic Data for High-Temperature Reactions”, Vol. 1, “Homogeneous Gas Phase Reactions of the H,-0, Systems”, Butterworths, London, 1972. Baulch, D. L., Drysdale, D. D., Horne, D. G., “Evaluated Kinetic Data for High-Temperature Reactions”, Vol. 2, “Homogeneous Gas Phase Reactions of the H,-N,-0, Systems”, Butterworths, London, 19 7 3. Baulch, D. L., Drysdale, D. D., Duxbury, J., and Grant, S., “Evaluated Kinetic Data for High-Temperature Reactions”, Vol. 3, “Homogeneous Gas Phase Reactions of the 0,-0, System, the CO-0,-H, System, and of Sulphur-Containing Species”, Butterworths, London, 1976. CIAP Monograph 1, “The Natural Stratosphere of 1974”, Department 3f Transportation DOT-TST-75-51, Sept. 1975 (Table 5.48 also published in Natl. Bur. Stand., Tech. Note, No. 866 (1975). See Hampson and Garvin, 1975). Demerjian, K. L., Kerr, J. A., and Calvert, J. G., “The Mechanism of Photochemical Smog Formation”. Adv. Environ. Sci. Technol., 4, 1 (1974): Engleman, V. S., “Survey and Evaluation of Kinetic Data o n Reactions in Methane/Air Combustion”, Report No. EPA-600/2-76-003(Jan, 1976) (EPA, Research Triangle Park, N.C.). Ferguson, E. E., “Rate Constants of Thermal Energy Binary Ion-Molecule Reactions of Aeronomic Interest”, At. Data Nucl. Data Tables, 1 2 , 159 (1973). Hampson, R. F., Ed., “Survey of Photochemical and Rate Data for Twenty-eight Reactions of Interest in Atmospheric chemistry”, J. Phys. Chem. R e f . Data, 2, 267 (1973). Hampson, R. F., and Garvin, D., Ed., “Chemical Kinetic and Photochemical Data for Modelling Atmospheric Chemistry”, Natl. Bur. Stand. Tech. Note, No. 866 (1975). Herron, J. T., and Huie, R. E., “Rate Constants for the Reactions of Atomic Oxygen ( 0 3 P ) with Organic Compounds in the Gas Phase”, J. Phys. Chem. Ref. Data, 2, 467 (1973). Kerr, J. A., and Parsonage, M. J., “Evaluated Kinetic Data on Gas Phase Addition Reactions: Reactions of Atoms and Radicals with Alkenes, Alkynes and Aromatic Compounds”, Butterworths, London, 1972. Kerr, J. A., and Parsonage, M. J., “Evaluated Kinetic Data on Gas Phase Hydrogen Transfer Reactions of Methyl Radicals”, Butterworths, London, 1976. Kondratiev, V. N., “Rate Constants of Gas Phase Reactions”, Reference Book translated by L. Holtslag and R. Fristrom, COM-72-10014 (1972). Lloyd, A. C., “Evaluated and Estimated Kinetic Data for the Gas Phase Reactions of the Hydroperoxyl Radical”, Int. J. Chem. Kinet., 6, 1 6 9 (1974). Niki, H., Daby, E. E., and Weinstock, B., “Mechanism of Smog Reactions”, Adv. Chem. Ser., No. 113, 1 6 (1972). Sieck, L. W., and Lias, S. G., “Rate Coefficients for IonMolecule Reactions. I. Ions Containing C and H”, J. Phys. Chem. R e f . Data, 5 , 1 1 2 3 (1976). Watson, R. T., “Rate Constants of C10, of Atmospheric Interest”, J. Phys. Chem. Ref Data, 6,8 7 1 (1977).
One may ask, “Why bother to reexamine published data?”. There are several reasons: By its very nature scientific research is a continuing, progressive process building upon previous work and extending areas of understanding. It is essential to be aware of what results have already been obtained. However the sheer bulk of the scientific literature constitutes an obstacle. Compilations of data have the effect of compressing the literature one must search, often by a large factor. Another reason is related to the increasing trend toward interdisciplinary research and multifaceted programs to The Journal of Physical Chemistry, Vol. 81, No. 25, 1977
(1) (2) (3) (4) (5) (6) (7)
(8) (9) (10) (11) (12) (13) (14)
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OH t NO, ( + M ) - + HNO, ( t M ) HO, t NO, ( + M ) - HO,NO, ( t M ) HO,NO, reactions OH + HO, H,O t 0, HO, t HO, -, H,O, + 0, HO, t N O - NO, t OH HO, t 0, OH t 2 0 , OH t 0, HO, t 0, 0 t HO, -* OH t 0, C10 + NO, ( + M ) ClONO, ( + M ) c10 + 0 - 0, t c1 C10 + N O + NO, + C1 C1 t CH, HC1 + CH, C1+ HO, -+ HCl + 0, -+
--f
-+
-f
-+
study large, complex problems. Often the result is that a scientist or an engineer finds himself dependent upon data whose reliability he cannot assess. He must depend upon the expertise of an evaluator. A kineticists will often require auxiliary rate data in the interpretation of his experiments. However disagreements in published values exist. Unless he is willing to compile and evaluate all the data he may choose a wrong value with resulting inaccuracy in his own reported result. The evaluator attempts to provide him with reliable base point data for the interpretation of his results. What are the factors an evaluator must consider in assessing the reliability of published rate data? A scholary exposition of principles governing the accuracy of kinetics techniques and the reliability of the data derived using them has been given by Cvetanovic et a1.* Much of what will be discussed here was considered by those authors. The evaluator must consider many factors. He must analyze the technique used. This must include an attempt to identify both the source and magnitude of potential systematic errors. This analysis must include those whose effects were considered and minimized and also those which could not be eliminated. The resolution possible and the sensitivity attainable with the measurement technique used must be considered. The evaluator must try to understand fully the chemical system and assure himself that the author considered all possible chemical interferences which could complicate the interpretation of his results. Here the evaluator may have the advantage of hindsight. Some previously unsuspected complications to the chemistry may have been recently discovered, permitting the evaluator to retrospectively examine earlier papers in an effort to assess the magnitude of this effect in the earlier work. It may turn out that there are not enough data given to permit this reanalysis in which case the results may have to be discarded. The evaluator must consider all the data published on a given system. Those data sets he decides to accept must be intercompared. To do this he must be able to find in the paper tabulated numerical data which still shows the experimental scatter. An Arrhenius expression unsupported by numerical data is almost worthless. There is no way for the data evaluator to assess realistically the reliability of a result determined by only one technique in one laboratory. Intercomparison of data sets obtained by different techniques and different workers is essential to the establishment of a recommended value with a high level of confidence. The evaluator should also consider the performance record of the technique when it has been applied in closely related systems. Finally, the pertinent material to be examined may be much more than the set of rate measurements on the
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Evaluation and Compilation of Reaction Rate Data
specific systems. Comparison with theory may be helpful in rejecting work which leads to unreasonable values for kinetic parameters. Comparison with rate data for similar reactions may indicate an anomaly. Molecular structure and thermochemical data may help establish or rule out a particular mechanism. Consideration of the information needs of the data evaluator has provided the basis for the specification of a set of guidelines for the presentation of experimentally determined numerical rate data. These guidelines have been formulated by the CODATA Task Group on Data for Chemical Kinetics and published as CODATA Bulletin No. 13.3 The guidelines are based on three general principles: (1) “The work should be described in sufficient detail to allow the equipment, conditions, procedure and, logically, the results to be reproduced, and to allow the quality of the work to be appraised. ( 2 ) The results should be presented in a form that will permit them to be reanalyzed and reintrepreted by others. (3) There should be an assessment of the accuracy and reproducibility of the numerical results”. The report contains specific guidelines and a checklist which should be considered by authors and referees. Careful observance of the report’s recommendations would do much to improve the quality of kinetics literature and to assure the long-term survival of the particular data reported.
Current Kinetic Data Needs in Atmospheric Chemistry Finally we will consider one complex problem currently being studied by modelers and point out existing gaps in the overall kinetic data base. The problem selected is the chlorine-catalyzed destruction of the stratospheric ozone layer initiated by fragments derived from the photodecomposition of halogenated organic compounds. The reactions for which data are needed are listed in Table 11. This listing of needed rate data is based on discussions of the Laboratory Measurements Committee of the NASA Upper Atmosphere Research Program’s Chlorofluoromethane Assessment Workshop in January 1977.4 The selection of these reactions as candidates for further study is based upon consideration of both the present status of the rate data and the results of a sensitivity study for this complex system. For purposes of these studies data are needed a t the temperature and pressure regimes characteristic of the stratosphere ( T 220-270 K and pressures of approximately 1-7 kPa (10-50 Torr)). Reaction 1, the formation of nitric acid, needs further study under typical stratospheric conditions. Only one systematic study of the rate below room temperature has been reported. I t needs verification. Pernitric acid, H02N02, has been the subject of several recent inve~tigations~f and suggested to be a potential sink for NO, in the atmosphere. There are very limited data on its formation by reaction 2 and none on its photochemistry or interactions with atoms and radicals. For the very important HOz reactions 4, 5 , and 6 there are inconsistencies among the reported absolute values and the reported rate ratios that cannot now be resolved. Reactions 7, 8, and 9 are important in the models. Uncertainties in their rate constants make significant contributions to the overall uncertainty in the results of the model calculations. Among the reactions of chlorine-containing species, the most important reaction is reaction 10, the formation of chlorine nitrate. Although there is good agreement among the reported result^,^ all have been obtained a t low
-
pressures (
