James B. Burkholder and Michael J. Kurylo
These tables present evaluated rate constants and equilibrium constants for bimolecular and termolecular reactions of a wide variety of species important in understanding and modeling the chemistry of the different components of the Earth's atmosphere. The data in these tables are taken from the latest data tables produced by the NASA Panel for Data Evaluation (Ref. 20). For a complete history of the Panel and its predecessors, please see the Introduction to this document in the Online Edition of the CRC Handbook available at <http://www.hbcponline.com/> or Refs. 1-20 below.
Bimolecular reaction rate constants
In Tables 1a through 1l, rate constants for bimolecular reactions are grouped into the following classes, with reaction constants for each class given in a separate table as follows.
The tables contain recommended bimolecular reaction rate constants k(T) at temperature T in Arrhenius form,
k(T) = A × exp^{(–E/RT)} (1)
The parameters for equation 1 given in Tables 1a to 1l are defined as follows.
The temperature ranges shown for each reaction indicate the range for the available experimental data. This is not necessarily the range of temperature over which the recommended Arrhenius parameters are applicable. See the corresponding note in Ref. 20 for each reaction for such information.
The parameters f_{298} and g can be used to calculate the estimated rate constant uncertainty over the recommended temperature range from the following expression:
f(T) = f_{298} × exp│g × {1/T – 1/298}│ (2)
where the exponent is an absolute value.
Note that f_{298} and g have been defined to correspond to approximately one standard deviation. Hence, f(T) yields a similar uncertainty interval. The more commonly used 95% confidence limits at a given temperature can be obtained by multiplying and dividing the recommended value of the rate constant at that temperature by the factor f^{2}(T). It should be emphasized that the parameter g has been defined for use with f_{298} in the above expression and should not be interpreted as the uncertainty in the Arrhenius activation temperature (E/R).
The uncertainty represented by f(T) is normally symmetric; i.e., the rate constant may be greater than or less than the recommended value, k(T) by the factor f(T). In a few cases, asymmetric uncertainties are given in the temperature coefficient. For these cases, the factors by which a rate constant is to be multiplied or divided to obtain, respectively, the upper and lower limits are not equal, except at 298 K where the factor is simply f_{298}.
Termolecular reaction rate constants
Table 2.1 gives rate constants for the termolecular reactions, which have a more complicated dependence on temperature and pressure. Hence, recommendations are made for the low-pressure limiting rate constant k_{0}(T) at temperature T and for the high-pressure limiting rate constant k_{∞}(T), also at temperature T, as well as the parameters required to obtain the effective second-order rate constant for a given temperature and pressure.
For the low-pressure limit, the following equation holds:
k_{0}(T) = k_{0}^{298} (T/298)^{-n} (3)
Similarly, for the high-pressure limit, the equation is as follows:
k_{∞}(T) = k_{∞}^{298} (T/298)^{-m} (4)
The parameters for equations 3 and 4 are given in Table 2.1 and are defined as follows.
More complete details on the equations in which these parameters are used are available in Ref. 20.
Rate constants for chemical activation reactions
Table 2.2 contains rate constants for chemical activation reactions.
k_{0}(T) = k_{0}^{298} (298/T)^{n} (5)
Similarly, for the high-pressure limit, the equation is as follows.
k_{∞}(T) = k_{∞}^{298} (298/T)^{m} (6)
Some association reactions produce not only recombination product(s) but also additional products that appear to originate from a simple bimolecular reaction. In these cases, the total rate constant k_{Total} is equal to the sum of the rate constant for the association reaction k_{f}(T,[M]) and the rate constant for the formation of products resulting from chemical activation k_{f}^{CA}(T,[M]). More details on this model are given in Ref. 20. k_{f}(T,[M]) is a function of k_{0}(T) and k_{∞}(T) (Ref. 20). k_{f}^{CA}(T,[M]) is dependent on the second order rate constant at [M] = 0 (i.e., the zero-pressure intercept), given by the Arrhenius expression as follows.
k_{int}(T) = Ae^{-BT} (7)
The parameters for equations 5, 6, and 7 are given in Table 2.2 and are defined as follows.
Equilibrium constants
Some three-body reactions form products that are thermally unstable at atmospheric temperatures. In such cases, the thermal decomposition reaction may compete with other loss processes, such as photodissociation or radical attack. Table 3 contains recommended equilibrium constants K_{eq}(T) for several reactions that may fall into this category. Each recommended equilibrium constant is given in Arrhenius form and contains the information shown below.
K_{eq}(T) = A exp^{(B/T)} for temperatures 200 K <T< 300 K (8)
The parameters for equation 8 are given in Table 3 and are defined as follows.
As for bimolecular reactions, f_{298} is the approximate one standard deviation uncertainty factor in the equilibrium constant at 298 K, and g is the parameter to be used to calculate the equilibrium constant uncertainty at temperatures other than 298 K.
The process of evaluating chemical kinetic data does not conform to a simple set of mathematical rules. There is no “one-size-fits-all” algorithm that can be applied and each reaction must be examined on a case-by-case basis. Consideration of uncertainties in the kinetic and photochemical parameters used in atmospheric models plays a key role in determining the reliability of and uncertainty in the model results. Quite often the cause(s) of differences in experimental results from various laboratories cannot be determined with confidence and making recommendations for the uncertainties of the rate constant is often more difficult than making recommendations for the Arrhenius parameters themselves. In many cases, investigators suggest possible qualitative reasons for disagreements among data sets. Thus, data evaluators necessarily must consider a variety of factors in assigning a recommendation, including such aspects as the chemical complexity of the system, sensitivities and shortcomings of the experimental techniques employed, similarities or trends in reactivity, and the level of agreement among studies using different techniques.
These data are the recommendations that appear in the most current NASA JPL document (JPL 19-5) (Ref. 20). Readers should consult the Online Edition of the CRC Handbook (see footnotes) for the descriptive notes and bibliography associated with each line entry. In particular, the notes contain important details about the latest revision date, changes from previous evaluations, data formats, units, and the actual use of the recommendations and their indicated uncertainties.
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