Rate constant databases

As section 14.3 notes, to date the majority of the estimation methods proposed have limitations due to the unavailability of specific properties of the molecule considered.Further-more, all of the methods use the measured rate constant database during their development. 

For example, the method Atkinson (1986) proposed requires a reliable rate constant database for the various classes of organic compounds in order to derive the various group rate constants and substituent factors. As may be expected, this estimation technique is reasonably reliable when used within its derivation database (Kwok and Atkinson, 1995). Thus, Kwok and Atkinson (1995) observed that the 298 K rate constants were predicted to within a factor of 2 of the experimental values for 90% of the 485 organic compounds that were considered and for which experimental data were available [and rate constants for alkyl nitrates are still predicted to within a factor of 2 after re-evaluation to take into account the recent kinetic and mechanistic data of Talukdar et al. (1997)]. 

The York University ion-molecule reaction rate constant database... 

By reducing an elementary reaction model taken fi om the database, a comprehensive gas-phase reaction model of propane pyrolysis was derived objectively. The reaction rate constants that were not accurate under the conditions of interest were found and refined by fitting with the experimental results. The obtained reaction model well represented the effects of the gas residence time and temperature on the product gas composition observed in experiments under pyrocarbon CVD conditions. 

Collections of reaction rate constants exist for explosions, atmospheres and astro-physical reactions. One such collection is the UMIST Astrochemistry Database and here the form of Equation 5.13 is generalised to ... 

In principle, it is now possible to construct a complete network of interconnecting chemical reactions for a planetary atmosphere, a hot molecular core or the tail of a comet. Once the important reactions have been identified the rate constants can be looked up on the database and a kinetic model of the atmosphere or ISM molecular cloud can be constructed. Or can it Most of the time the important reactions are hard to identify and if you are sure you have the right mechanisms then the rate constants will certainly not be known and sensible approximations will have to be made. However, estimates of ISM chemistry have been made with some success, as we shall see below. 

Using the UMIST Astrochemistry Database format for rate constants, calculate the rate constants at 20 and 100 K for the following reactions, giving the units for the rate constants in each case ... 

In spite of the paucity of data on the energy of R—H and In—H bonds, the rate constants of the reactions R02 + RH (2) and R02 + InH (7) have been measured for a great number of compounds (see Database [73]). This explains why these are the parameters that were taken as the kinetic characteristics of R02, In, RH, and InH (Table 14.5). The symbol denotes that these rate constants (k2 and k7) refer to a reaction temperature of 333 K. 

The experimental values of rate constants of R02 reactions with aromatic amines (AmH) are given in Database [52], The experimental measurement of the rate constant /c7 for aromatic amines from kinetics of oxidation faced with great difficulties. These difficulties arise due to the extremely high activity of aminyl radicals toward hydroperoxide [53-56], The reaction... 

At moderate temperatures (T < 350 K), these peroxides are stable but readily decompose at elevated temperatures to form radicals [58 60]. Peroxyl radicals can add to phenoxyl radicals at both para- and ort/zo-positions with the proportion dependent on the substituent. Thus, the peroxyl radical adds to 2,4,6-tris(l,l-dimethylethyl)phenoxyl in the para-position by eight times more rapidly than in the ortho-position, whereas it adds to 2-methyl-4,6-bis(l,l-dimethylethyl)phenoxyl preferentially in the ortho-position [59], The rate constants of the reactions of the peroxyl radical with 2,6-bis(l,l-dimethylethyl)-4-substituted phenoxyl radicals in benzene were measured in the presence of respective phenol and AIBN as a source of 1-cyano-l-methylethylperoxyl radicals [60,61]. The concentration of the formed phenoxyl radical in these experiments peaked at [ArO ]max (k7/kx)[ArOH]. Using this expression and taking the known kn values, the values of ks for various phenoxyl radicals 4-Y-2,6-(Me3C)2. Q,H20 were estimated at T 353 K (see Database [52]) ... 

The values of rate constants of the reaction of selected phenoxyls with ionol calculated by the IPM method are given in Table 18.4. Experimental data on reactions of phenoxyls with phenols, hydroperoxides, and hydrocarbons can be found in Database [41] and those calculated by the IPM method values in the Handbook of Antioxidants [4]. 

The values of activation energies and rate constants of the reactions of /t-benzoquinone with a few phenols and amines are presented in Table 18.10. For additional data on this reaction see handbook [4] and Database [41]. 

The rate constants (/c[and k]) and the stoichiometric coefficients (t and 1/ ) are all assumed to be known. Likewise, the reaction rate functions Rt for each reaction step, the equation of state for the density p, the specific enthalpies for the chemical species Hk, as well as the expression for the specific heat of the fluid cp must be provided. In most commercial CFD codes, user interfaces are available to simplify the input of these data. For example, for a combusting system with gas-phase chemistry, chemical databases such as Chemkin-II greatly simplify the process of supplying the detailed chemistry to a CFD code. 

These in vivo and in vitro human metabolism studies indicate that pyrethroids undergo rapid metabolism and elimination as observed in rats, and qualitative metabolic profiles (e.g., kinds of metabolites) of pyrethroids are assumed to be almost the same between humans and rats, suggesting that a large database of animal metabolism of pyrethroids could provide useful information for the evaluation of behavior of pyrethroids in humans. Nowadays, human pesticide dosing studies for regulatory propose are severely restricted in the US, and thus detailed comparison of in vitro metabolism (e.g., metabolic rate constants of pathways on a step-by-step basis) using human and animal tissues could be an appropriate method to confirm the similarity or differences in metabolism between humans and animals. 

CHEMRev The Comparison of Detailed Chemical Kinetic Mechanisms Forward Versus Reverse Rates with CHEMRev, Rolland, S. and Simmie, J. M. Int. J. Chem. Kinet. 37(3), 119-125 (2005). This program makes use of CHEMKIN input files and computes the reverse rate constant, kit), from the forward rate constant and the equilibrium constant at a specific temperature and the corresponding Arrhenius equation is statistically fitted, either over a user-supplied temperature range or, else over temperatures defined by the range of temperatures in the thermodynamic database for the relevant species. Refer to the website http //www.nuigalway.ie/chem/c3/software.htm for more information. 

Finally, the National Institute of Standards and Technology (NIST) in the United States has several chemical kinetics databases that are available for purchase from the Office of Standard Reference Data at NIST. The NIST Standard Reference Data Base 17 gives gas-phase rate constants through 1993 and Data Base 40 gives solution-phase data through 1992. In addition, aqueous-phase data are available through the Radiation Chemistry Data Center of the Notre Dame Radiation Laboratory (http //www.rcdc.nd.edu/). 

Each of these four second-order rate constants can be estimated from the structure of the compound of interest using group rate constants and substituent factors that have been derived from a large set of experimental data. This method has proven to be quite successful for prediction of k n0. for compounds that are well represented in this database (i.e., predictions within a factor of 2 to 3). Larger deviations are found,... 

In addition to the databases on thermochemistry and rate constants, a large number of kinetic models have been published. Most of these can be obtained electronically from the authors and a number of mechanisms are available directly on the Web. Some useful sites can be found in Refs. [3-6,8,366],... 

Find, using available chemical kinetics databases, previous determinations of the rate constant for this reaction. Select the most reliable value, and discuss the choice in terms of the way the rate constant was determined. 

Compare the collision theory rate with the database value and calculate the steric factor, that is, the ratio between the measured rate constant and the rate constant estimated from collision theory. 

Develop a reaction mechanism for iodine (I2-O2-H2 system) from the information in the NIST Chemical Kinetics Database [256], Start with the H2-O2 reaction subset hydrogen.mec. Using the database, identify the relevant reactions with I2. Add these reactions to the starting mechanism, including product channels and rate constants. List the additional I-containing species formed in reactions of I2. Extend the reaction mechanism with reactions of these species. Continue this procedure until reactions of all relevant iodine species in the I2-O2-H2 system is included in the mechanism. 

Select a starting mechanism for high-temperature methane chemistry (for example, GRIM30. mec). Based on the discussion of low-temperature methane chemistry in this chapter, add the necessary reactions with rate constants from an available database. 

Tang, W.Z. and Hendrix, T., An Internet Database of Kinetic Rate Constants and QSAR Models for Hydroxyl Radical Reactions and Ti02/U V, Dept, of Civil and Environmental Engineering, Florida International University, Miami, 1998. 

Methods to predict the hydrolysis rates of organic compounds for use in the environmental assessment of pollutants have not advanced significantly since the first edition of the Lyman Handbook (Lyman et al., 1982). Two approaches have been used extensively to obtain estimates of hydrolytic rate constants for use in environmental systems. The first and potentially more precise method is to apply quantitative structure/activity relationships (QSARs). To develop such predictive methods, one needs a set of rate constants for a series of compounds that have systematic variations in structure and a database of molecular descriptors related to the substituents on the reactant molecule. The second and more widely used method is to compare the target compound with an analogous compound or compounds containing similar functional groups and structure, to obtain a less quantitative estimate of the rate constant. 

Similarly, the most recent version of the Atmospheric Oxidation Program (version 1.8) gives agreement of the calculated and experimental room temperature rate constants to within a factor of 2 for 90% of 647 organic compounds, with a standard deviation of 0.242 log units (i.e., a factor of 1.75) (Meylan, 1997). However, extrapolation of this estimation method to organic compounds outside of the database used for its development and testing lacks reliability and is not recommended. 

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