Absolute Arrhenius parameters

The experimental errors associated with absolute Arrhenius parameters are rather large for radical addition reactions. It has been estimated that the minimum error in a determination of log (A/1 mol 1 s ) will be (0.6 to 1.2) at 350 K (Kerr and Parsonage, 1972a). The experimental results for addition of a number of alkyl and halogenoalkyl radicals to substituted ethylenes, which are shown in Table 19, are quite constant and vary by less than the possible experimental error over the whole range of radicals. 

Absolute Arrhenius parameters for gas phase radical addition reactions... 

In using the tables of the present chapter it should be borne in mind that while the results are presented in the form of absolute Arrhenius parameters, the bulk of the data have been derived from measurements of rate coefficient ratios and the absolute values are usually based on estimates of the rate coefficients of the reference reaction. This applies particularly to the radical reactions where the measurements are made relative to the radical combination reaction. Details of the assumptions involved with many of the reference reactions are discussed under the headings of the appropriate radicals. 

The absolute Arrhenius parameters for the H-abstraction reactions of perfluoroethyl radicals shown in Table 27 have been calculated by assuming A = 4 x 10 1 mole sec and E = 0 kcal mole for the... 

Absolute Arrhenius Parameters and Rate Constants at 164°C for Radical Additions to Ethylene... 

The past decade has been an extremely fruitful one in the field of quantitative free radical kinetics. Two papers can be identified as the starting point of much of this work. The first of these is the acetone photolysis study by Noyes and Dorfman1 which gave confidence to the use of acetone as a reproducible source of methyl free radicals in a fairly simple kinetic environment. The second is the study of Gomer and Kistiakow-sky2 of the absolute rate of recombination of CH3 radicals. The latter study made it possible to give absolute values for the Arrhenius parameters for the reactions of alkyl free radicals with stable molecules. It also opened the way for putting the reactions of methyl radicals with other alkyl radicals on an absolute basis. 

Fischer H (1986) Substituent Effects on Absolute Rate Constants and Arrhenius Parameters for the Addition of Tert-Butyl Radicals to Alkenes. In Viehe HG, Janousek Z, Merenyi R, (eds) Substituent Effects in Radical Chemistry. Reidel, Dordrect, p 123... 

Absolute rate constants and Arrhenius parameters have been determined for the thermal E,Z-isomerization of the stable disilene derivatives 92-96 in deuteriated aromatic solvents or THF-ds solution by XH or 29Si NMR spectroscopy133-136. With 1,2-dialkyl- and 1,2-diamino-l,2-dimesityldisilenes such as 92a-94, the (E)-isomers are considerably more stable than the (Z)-isomers, and so rate constants for E,Z-isomerization were determined after first generating mixtures enriched in the (Z)-isomer by UV-irradiation of the (El-isomer, and then monitoring the recovery of the solution to its equilibrium composition. On the other hand, little difference in thermodynamic stability is observed between the (Eland (Z)-isomers of tetraaryldisilenes such as 95a,b, and E,Z-isomerization kinetics were hence determined starting from solutions prepared from the individual, pure (or almost... 

TABLE 18. Absolute rate constants, deuterium kinetic isotope effects and Arrhenius parameters for addition of substituted phenols (112a-g) to tetramesityldisilene (110) in benzene solution at... 

The accurate determination of rate constants for the reactions of 19F atoms is often hampered by the presence of reactive F2 and by the occurrence of side reactions. The measurement of the absolute concentration of F atoms is sometimes a further problem. The use of thermal-ized 18F atoms is not subject to these handicaps, and reliable and accurate results for abstraction and addition reactions are obtained. The studies of the reactions of 18F atoms with organometallic compounds are unique, inasmuch as such experiments have not been performed with 19F atoms. In the case of addition reactions, the fate of the excited intermediate radical can be studied by pressure-dependent measurements. The non-RRKM behavior of tetraallyltin and -germanium compounds is very interesting inasmuch as not many other examples are known. The next phase in the 18F experiment should be the determination of Arrhenius parameters for selected reactions, i.e., those occurring in the earth s atmosphere, since it is expected that the results will be more precise than those obtained with 19F atoms. 

Since A° is constant for a given series satisfying equation (44), any structural change in the reactants must be reflected in the only parameter, E, determining the rate constant. If T = Tit the rate constants of all reactions in the given set will be identical. For that reason, Tt is called the isokinetic temperature . It is a mathematical consequence of equation (44) and has no physical meaning. (The only physically reasonable isokinetic temperature is the absolute zero.) Nevertheless, the value of as compared with a medium value of the temperature range of experiments (Texp) can help us to classify possible correlations of the Arrhenius parameters (Simonyi, 1967 Tiidos, 1969). 

Kinetic results are given in Table 9. The toluene carrier technique often yields low values of the activation energies and 4-factors. Results for the benzyl esters, however, appear to be reasonably good. Note that the estimated reaction enthalpies compare favorably with the observed activation energies and that -factors are reasonable (AS 5 cal.deg mole ). Since only absolute decomposition rate coefficients were reported for the allyl esters, we have estimated the Arrhenius parameters on the assumption that the -factors were all 10 sec The activation energies so obtained are reasonable, since they compare quite favorably to those estimated by group additivities and the accepted heats of formation of the product radicals (Column 3, Table 9). 

Absolute values of kj were obtained from the known parameters Ajp and Ejp (see Table 1.8). Table 1.7 summarizes the recommended Arrhenius parameters for the temperature range 600-800 K for a number of compounds RH used as additives in the C3H6 -I- O2 system. For alkenes, bearing in mind that only allylic C—H bonds undergo reaction (1) under the conditions used, Aip = Ai (per C—H bond) is assumed to obtain the activation energies. 

Table 1. Summary of the measured rate eonstants at 298 K obtained by absolute and relative methods and the Arrhenius parameters in the temperature range 230-372 K.

Table 9. The absolute values of k, and kt at 25 °C and their Arrhenius parameters for anionic polymerization of sodium polystyryl in various ethereal solvents...

The n-pentyl radical is the largest alkyl radical for which Arrhenius parameters have been determined for a gas-phase metathetical reaction. Problems with volatility of reactants and dimer products are considerable in studies involving radicals larger than C4. The few results available for n-pentyl are given in Table 21. but in fact for only the first of the four reactions listed was there an experimental determination the other three results were obtained from data for the reverse reactions and the equilibrium constants derived from thermodynamic data. The selection of the rate coefficient for the n-pentyl dimerization reaction, upon which to base the absolute data for the reaction... 

The kinetics of the reactions of alkoxy radicals have been reviewed and evaluated by Gray et al. [369]. Results for H-abstraction reactions of methoxy radicals are shown in Table 33. It is not possible with alkoxy radicals to monitor the radical concentrations by measuring the rates of formation of the dimers since these are peroxides which are difficult to analyse. The reactions of methoxy radicals with alkanes were studied indirectly by measuring the rates of consumption of the alkanes in competitive experiments involving pairs of reactants [366]. This yielded relative Arrhenius parameters which were put on an absolute basis by... 

The absolute rates for 1-butene are given in Table 28, and again these are recalculated from relative rates. They are, however, consistent, although this is not too surprising. After all, if one assumes that the rate data for ethylene are correct, then the inherently more accurate relative rate data will fall into line very nicely. No Arrhenius parameters are available. 

No attempt has been made to provide the intensive detail found in Howard s review of oxyradicals [10], the review of Hendry et al. [43] of H-atom transfer to several radicals or Anbar and Neta s review of HO-radical reactions [44]. Instead, we have attempted to extend the scope of those reviews in two ways (i) rate coefficients are provided for R02-radical addition to many olefins, for ring closures to form cyclic ethers, and for intramolecular abstraction (ii) for each reaction, we have estimated the best value Arrhenius parameters (A-factor and E) and, where such values have been measured they are also listed. We believe the value of absolute rate coefficients is improved substantially by the availability of reliable Arrhenius parameters, with which one can calculate the values of rate coefficients at other temperatures for use in experimental or modelling studies. 

Absolute rate coefficients and Arrhenius parameters have been obtained for the cycloaddition reaction of S( F2,1,0) atoms with a representative series of olefins and acetylenes. The activation energies are small, and they exhibit a trend with molecular structure which is expected for an electro-philic reagent The A-factors show a definite trend which can be attributed to steric repulsions and a generalized secondary a-isotope effect explained by activated complex theory. Secondary a-H/D kinetic isotope effects have been measured and their origin discussed. Hartree-Fock type MO calculations indicate that the primary product of the S( F) + olefin reaction is a ring-distorted, triplet state thi-irane, with a considerable energy barrier with respect to rotation around the C-C bond. 

Quantitative studies of S( F) atom reactions have been carried out with about two dozen olefins (5, 6). The rate coeflBcients and Arrhenius parameters are summarized in Table I. The absolute rate coeflBcients were determined in flash photolysis experiments using kinetic absorption spectrometry (6, 7). Mixtures of 0.1 ton COS and 200 torr CO2 were flash photolyzed in the presence of an olefin, and S( F) atoms concentrations were monitored by measuring the optical densities of the 1807 ( F2 ) and 1820A ( Fi) atomic transitions. 

Most of the researchers in AAS, starting with [26, 27], attributed these discontinuities in plots to changes in the vaporization mechanism and/or in the chemical form of the reactant, for instance, to a transition from oxide dissociation at low temperatures to sublimation of free metal in the high-temperature domain. Interpretation of the mechanism was attempted by identifying the only measured parameter E with thermal effects of a variety of conceivable processes (or of individual steps of the whole process). In 1981, a method was proposed [25] to determine absolute vaporization rates. It included measurement of both Arrhenius parameters (see Sect. 3.6). 

The rates are those for methane removal and have been measured at various reactant pressures, so that tiiey and the apparent Arrhenius parameters on which they are based have no absolute significance. 

Hydrogen Abstraction by Chlorine Atoms. Absolute rate constants and Arrhenius parameters for the chlorination of a series of dilorine bstituted methanes and their deuteriated analogues have been obtained by Oyne and Walker, using mass spectrometric analysis of molecular reactant consumption in excess dilorine atoms. Uncertainties in the kinetic parameters for Cl + H2, the reference reaction in competitive dilorinations, are discussed. 

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