Reaction rate prediction variational transition state theory

In the present work, information about the potential energy surfaces for these systems is obtained by the BAC-MP4 method [28-33]. This method has been very successful for predicting the thermochemistry of molecules and radical species, and has been extended to calculating the potential information along reaction paths needed for the variational transition state theory calculations. In the latter case, the method has been shown to be capable of quantitative predictions for a gas phase chemical reaction [33]. In the present study our interests are in estimates of the order of magnitude of reaction rates, and in studies of qualitative trends such as the effect of cluster size on the magnitude of quantum tunneling. The methods employed here are more than adequate for these types of studies. 

The case of m = Q corresponds to classical Arrhenius theory m = 1/2 is derived from the collision theory of bimolecular gas-phase reactions and m = corresponds to activated complex or transition state theory. None of these theories is sufficiently well developed to predict reaction rates from first principles, and it is practically impossible to choose between them based on experimental measurements. The relatively small variation in rate constant due to the pre-exponential temperature dependence T is overwhelmed by the exponential dependence exp(—Tarf/T). For many reactions, a plot of In(fe) versus will be approximately linear, and the slope of this line can be used to calculate E. Plots of rt(k/T" ) versus 7 for the same reactions will also be approximately linear as well, which shows the futility of determining m by this approach. 

The final limitation of the pure electrostatic theory is its inability to predict solvent effects for reactions involving isopolar transition states. Since no creation, destruction, or distribution of charge occurs on passing from the reactants to the activated complex of these reactions, their rates are expected to be solvent-independent. However, the observed rate constants usually vary with solvent, although the variations rarely exceed one order of magnitude [cf. Section 5.3.3). These solvent effects may be explained in terms of cohesive forces of a solvent acting on a solute, usually measured by the cohesive pressure of the solvent [cf. Section 5.4.2). 

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