Rate constant prediction

When this is done, the dependence of k(Ee) upon Ee is even greater than predicted by the dipole-alignment model, and the thermal rate constant predicted from this variation, extrapolated to thermal energies, is more than twice the thermal rate constant found experimentally by using the pulsing technique. 

EPR studies on electron transfer systems where neighboring centers are coupled by spin-spin interactions can yield useful data for analyzing the electron transfer kinetics. In the framework of the Condon approximation, the electron transfer rate constant predicted by electron transfer theories can be expressed as the product of an electronic factor Tab by a nuclear factor that depends explicitly on temperature (258). On the one hand, since iron-sulfur clusters are spatially extended redox centers, the electronic factor strongly depends on how the various sites of the cluster are affected by the variation in the electronic structure between the oxidized and reduced forms. Theoret-... 

It can be difficult to estimate theoretically the bond lengths and vibrational frequencies for the activated complex and the energy barrier for its formation. It is of interest to assess how the uncertainty in these parameters affect the rate constant predicted from transition state theory (TST). For the exchange reaction... 

Except for mevinphos, agreement between experimental volatilization rate constants and rate constants predicted using k ... 

An alternate form of the rate constant predicted by transition state theory using a statistical mechanical approach for the equilibrium constant K is Eq. (R) ... 

In summary, the measured rate constants (based on bulk concentrations) increase as the pressure is decreased near the critical point. This cannot be explained solely on the basis of the pressure effect on the rate constant predicted from transition state theory or cage effects. As a result, we believe that local composition increases near the critical point play an important role in the rate increase. 

The equation for the high pressure rate constant predicts strict Arrhenius behaviour, and also a non-linear plot for l/kobs versus 1/[A], both in agreement with experiment. 

The rate constant predicted by conventional transition-state theory can turn out to be too small, compared to experimental data, when quantum tunneling plays a role. We would like to correct for this deviation, in a simple fashion. That is, to keep the basic theoretical framework of conventional transition-state theory, and only modify the assumption concerning the motion in the reaction coordinate. A key assumption in conventional transition-state theory is that motion in the reaction coordinate can be described by classical mechanics, and that a point of no return exists along the reaction path. 

Studies of proton transfers involving small ions with localized charge have shown that these reactions may proceed indeed with rate constants close to or even slightly larger than the collision rate constants predicted by the ADO theory (Mackay et al., 1976). However, rate-constant measurements of proton-transfer reactions between delocalized anions (Farneth and Brau-man, 1976) and sterically hindered pyridine bases (Jasinski and Brauman, 1980) and of SN2 displacement reactions (Olmstead and Brauman, 1977 Pellerite and Brauman, 1980 Pellerite and Brauman, 1983 Caldwell et al., 1984 for a review see Riveros et al., 1985) have shown that the rate constants can span the range from almost collision controlled values down to ones too slow to be observed. For these reactions the wide variation in rate constants has been explained on the basis of a double potential-well model which for a hypothetical SN2 substitution is schematically shown in Fig. 4. 

MRRKM theory, from RIT, from direct trajectory simulations, and, for reference purposes, from the RRKM theory. In particular, a test of the effect of the RRKM choice of transition state on the predicted rate of isomerization is made by neglecting the contribution of intramolecular energy transfer (Model No. 1). It is seen that the RRKM choice of transition state leads to considerable error the isomerization rate constant predicted is greater than those from the MRRKM theory and RIT by as much as a factor of 4. With intramolecular bottlenecks taken into account, both RIT and the MRRKM theory agree well with trajectory calculations. 

Collette, T.W. (1990) Ester hydrolysis rate constant prediction from infrared interferograms. Environ. Sci. Technol. 24, 1671-1676. 

It is interesting to note that while feed carbon number is dominating for rate constant prediction, yield selectivity is influenced more by hydrogen content and iso- and normal-paraffin contents. 

Figure 12a indicates how the measured rate constant kf for the endergonic electron transfer oxidation of methylbenzene by (phen)3-iron(III) complexes varies with the driving force. It is seen from these data that, provided that AG° < 10 kcal/mol, a very good fit is obtained with the rate constant predicted from Eqs. (78) through (80) on the basis of an... 

Ion-molecule rate constants, predicted by these theories, show a satisfactory agreement with the experimental ones, as illustrated in Table XII for a set of proton-transfer processes. Furthermore, the data in Table XII confirm the view that rate coefficients for reactions between a given ionic species and homologous substrates are expected to fall within the same order of magnitude. Similar rate-constant coefficients for reactions involving a given ionic species and molecules with comparable polarizability and dipole momentum can therefore be expected. 

The rate constant predicted by simple collision dynamics is thus... 

The aim of this section is to illustrate how the mechanistic principles as well as the rate constant predictions outlined in the previous section can be used to simulate the overall rate of a reaction. It also provides an opportunity to discuss more extensively the meaning of relation (13). This is done using experimental data on the hydroisomerization of hexane. 

TABLE 3. Comparison between Observed Rate Constants for Complex Dissociation" and Rate Constants Predicted from Measured Formation Rate Constants and Equilibrium Constants for... 

Figure 5.2 Left potential energy curves for an electron transfer reaction in an encounter complex of an acceptor A and a donor D, (A--D) — (A --D + ). Right rate constants predicted by combining Equations 5.6 and 5.7. The diagrams are explained in the text...

Table I. Unimolecular Rate Constants Predicted by RRKM Theory for C—and C—C Reaction Channels in Rotationally Cold (300 K) and Rotationally Hot (6000 K) Model Ethane Molecules"...

Deviations have been found in the predictions of the LSW theory. For instance, a practical system could have broader and more symmetrical steady-state distribution in difiusion-controUed coarsening. Also, the rate constant predicted by the LSW theory is often different from the experimental observations. In addition, volume fraction of the precipitates sometimes should be included, which is assumed to have no effect in the LSW theory [14-16]. It is found that as the volume fraction increases, the rate constant is increased while the size distribution function is broadened with increasing volume fraction of the precipitates [17]. Further modifications have made the LSW theory to be more agreement with experimental results [15, 18]. 

General Remarks. Johnston discussed rate constant prediction for reactions in SCF, to be used in process design, where knowledge of the rate constant is required as a function of density and temperature. The outstanding issues are knowledge of the Gibbs free energy of reaction as function of density and temperature and dynamic effects which are... 

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