Between the Arrhenius Parameters

One approach to estimating rate parameters for elementary steps rests on the relationship between the Arrhenius parameters defined by... 

Table 1 shows that there is no clear correlation between the Arrhenius parameters and exotheimicities of the oxygen atom transfer reactions which are important in the stratosphere, this divergence is made more remarkable by the recent work of Barnett, Marston and Wayne [7] who obtained a rate coefficient of 1 X 10-16 cm molecule" s" for the highly exothermic reaction... 

Kinetic studies at several temperatures followed by application of the Arrhenius equation as described constitutes the usual procedure for the measurement of activation parameters, but other methods have been described. Bunce et al. eliminate the rate constant between the Arrhenius equation and the integrated rate equation, obtaining an equation relating concentration to time and temperature. This is analyzed by nonlinear regression to extract the activation energy. Another approach is to program temperature as a function of time and to analyze the concentration-time data for the activation energy. This nonisothermal method is attractive because it is efficient, but its use is not widespread. ... 

The above form of the Arrhenius equation takes into account the high degree of correlation that exists between the kinetic parameters. This pivoting method solves a convergence problem that can occur during parameter fitting if all six parameters (Fm, Em, Fdl, Edl, Fd2, and Ed2) are allowed to vary. 

The slow combustion reactions of acetone, methyl ethyl ketone, and diethyl ketone possess most of the features of hydrocarbon oxidation, but their mechanisms are simpler since the confusing effects of olefin formation are unimportant. Specifically, the low temperature combustion of acetone is simpler than that of propane, and the intermediate responsible for degenerate chain branching is methyl hydroperoxide. The Arrhenius parameters for its unimolecular decomposition can be derived by the theory previously developed by Knox. Analytical studies of the slow combustion of methyl ethyl ketone and diethyl ketone show many similarities to that of acetone. The reactions of methyl radicals with oxygen are considered in relation to their thermochemistry. Competition between them provides a simple explanation of the negative temperature coefficient and of cool flames. 

The following example illustrates one particular quantitative application of compensation behavior for the comparison of levels of activity between different systems. The Arrhenius parameters for the steam reformation reaction over nickel alumina catalysts (290) are log A = 17.25 and E = 29.0. The position of this point on compensation diagrams would appear to be more realistically represented by the compensation relation found for oxidation and exchange processes on nickel oxide (Table V, G) than that for cracking on the metal (Table I, A). One possible mechanistic explanation for this distinction is that the active catalyst is an oxide phase [possibly including NiAl204 (290)1... 

Robinson, 1969a). It is probable that the hydrophobic nature of the phenyl groups of p-nitrophenyl diphenyl phosphate results in deep penetration of the neutral ester in the Stern layer, thus shielding the phosphoryl group from nucleophilic attack. Unlike other reactions between nucleophiles and neutral substrates catalyzed by cationic micelles (Bunton and Robinson, 1968, 1969a) and the hydrolysis of dinitrophenyl phosphate dianions in the presence of cationic micelles (Bunton et al., 1968), the catalysis of the hydrolysis of -nitrophenyl diphenyl phosphate by CTAB arises from an increase in the activation entropy rather than from a decrease in the enthalpy of activation. The Arrhenius parameters for the micelle-catalyzed and inhibited reactions are most probably manifestations of the extensive solubilization of this substrate. However, these parameters can be composites of those for the micellar and non-micellar reactions and the eifects of temperature on the micelles themselves are not known. Interpretation of the factors which affect these parameters must therefore be carried out with caution. In addition, the inhibition of the micelle-catalyzed reactions by added electrolytes has been observed (Bunton and Robinson, 1969a Bunton et al., 1969, 1970) and, as in the cases of other anion-molecule reactions and the heterolysis of dinitrophenyl phosphate dianions, can be reasonably attributed to the exclusion of the nucleophile by the anion of the added salt. 

Barson and Ensor [34] have studied the temperature dependence of the apparent transfer constant in telomerization of MM A with CBr4. They found dramatic differences in the Arrhenius parameters between the first, second, etc. monomer additions to the CBr radical (Table 2.) The authors explain these results by electron transfer from radical to monomer, yielding the combining ions... 

The magnitude of the solvent isotope effect and the absence of a carbon isotope effect confirm that proton transfer is rate-determining in the reactions referring to s. As far as the reactions referring to are concerned, the experimental values of these rate coefficients for the decarboxylation of 2- and 4-aminobenzoic acids, as well as the Arrhenius parameters, are comparable to those of the substituted salicylic acids if expected substituent effects are taken into account (Table 21) there is a correlation between log A and Ea. Therefore, it is reasonable to expect that the mechanism is the same. The observed general catalysis supplies additional evidence for rate-determining proton transfer from H30+ to S (sigma complex formation) in the decarboxylation of 4-aminobenzoic acid. 

Of the mechanistic issues dealt with in the full study (including kinetic trade-off s between different Co-+ spin states in the reaction product and the role of solvent and conformational fluctuations of the DBA system), we focus here on the activation parameters and related nuclear tunneling and entropy effects which are crucial for establishing meaningful contact with the Arrhenius parameters obtained from the experimental rate data [158]. The theoretical analysis also led to new insights... 

Relation (33) was confirmed in several cases for the photolysis between 100 and 250 °C and at pressures higher than 100 torr. The Arrhenius parameters of reaction (32), determined by means of eqn. (33), are summarized in Table 17. When evaluating 33 from the ratio 32/ 26. the value 26 = 3.7x10 ... 

In a system where acetone is photolyzed in the presence of a hydrogen containing compound (RH), a competition exists between the acetone and the RH molecules for the methyl radicals. The Arrhenius parameters of several hydrogen transfer reactions of the methyl radical have been determined in such systems ... 

Early kinetic experiments on the thermal decomposition of nitro compounds established that for the simplest derivative, nitromethane, the process was first order, but that the reaction was chemically complex owing to further reactions between the products and nitromethane. Cottrell et re-examined the nitromethane pyrolysis and reported values of = 53.2 kcal.mole" and log A = 13 for the Arrhenius parameters of the homogeneous decomposition a radical mechanism was proposed, initiated by C-N cleavage... 

Baldwin, Walker and Brewery [68] showed that both the experimental and the derived activation energies are reasonably consistent with the thermochemistry and the Arrhenius parameters of the reverse reactions. In particular, they showed that for the homolysis of trimethylbutyl radicals there is a very good correlation between log/c at 753 K and AU, the internal energy change (Fig. 1.9). Although the data are limited, for the other radicals the rate constant and Arrhenius parameters for homolysis fit a common pattern when allowance is made for the strain energy in all the species involved. Hence, for the homolysis. 

Schwab has pointed out that the following relationship between the two parameters of the Arrhenius equation is frequently encountered. A decrease in the activation energy of a given reaction, for a series of catalysts, often does not increase the reaction rate to the extent calculated, because of a simultaneous decrease of the frequency factor. Cremer (106) confirmed this for the decomposition of ethyl chloride on various chloride catalysts. These findings will be discussed here with due regard to the relation between adsorption and elementary reaction rates dealt with in the preceding section. 

At relatively high temperatures typical for CsHg oxidation, oxygen diffusion is rapid compared to other steps (for the Arrhenius parameters for this process, see [21]). In this case, the relation between and 6q is given by the grand canonical distribution, i.e., one should have... 

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