No Barrier Theory

Alternatively one can make use of No Barrier Theory (NBT), which allows calculation of the free energy of activation for such reactions with no need for an empirical intrinsic barrier. This approach treats a real chemical reaction as a result of several simple processes for each of which the energy would be a quadratic function of a suitable reaction coordinate. This allows interpolation of the reaction hypersurface a search for the lowest saddle point gives the free energy of activation. This method has been applied to enolate formation, ketene hydration, carbonyl hydration, decarboxylation, and the addition of water to carbocations. ... 

J. P. Guthrie, No Barrier Theory Calculating Rates of Chemical Reactions from Equilibrium Constants and Distortion Energies, ChemPhysChem 2003,4, 809. 

No Barrier Theory—A New Approach to Calculating Rate Constants in Solution... 

No Barrier Theory (NBT) [1,2] is a new approach to calculating rate constants in solution that uses an experimental equilibrium constant and an assumed mechanism as the only empirical information needed in order to calculate a rate constant. What is directly calculated is the free energy of activation but conversion of this to a rate constant is trivial. The saving thing about these calculations is that relatively low-level quantum chemistry computational methods suffice in many cases semiempirical methods are sufficient. NBT also provides a way to think qualitatively about whether a reaction is likely to be slow or fast thus, it can be used both qualitatively to think about mechanisms and quantitatively to predict rates. 

NO BARRIER THEORY—A NEW APPROACH TO CALCULATING RATE CONSTANTS... 

Even more intriguing is the possibility that No Barrier Theory could provide a way to predict yields in synthetic reactions by predicting the rates of the competing reactions— provided that one thought of all the alternatives Where the competing reactions differed in sensitivity to reactant concentrations or catalysis, it might well be possible to devise conditions to improve the yield. 

All of these possible apphcations of No Barrier Theory depend on its having been shown to be rehable for a wide range of reactions. This testing is underway and thus far the method has proven rehable but clearly more testing is needed. 

Because No Barrier Theory is a new approach, it has been necessary to develop the language to describe it. A glossary of the terms we have found necessary is given in Table 5.1. 

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