Beyond the classical formulation

Further improvement of TST requires a critical analysis of its fundamental assumptions. The development of this theory was based on the following approximations (i) the distribution of energy in the reactants and transition state follows the Boltzmann distribution law, (ii) every system that crosses the transition state becomes a product and (iii) the movement along the reaction coordinate is adiabatic and can be described by classical mechanics. Next, we analyse each one of these approximations to identify the conditions in which they fail and see how the theory can be improved in terms of both generality and accuracy. 

The quasi-equilibrium assumption, that is the basis for the use of the Boltzmann distribution law, may lose its validity for rapid reactions. In such reactions, the most energetic reactant molecules may disappear very rapidly and the concentration of species at the transition state may be lesser than that for a trae equilibrium. In practice, even when EJRTk 3, as in the Cl+HBr— HCH-Br hydrogen atom abstraction, internal-state nonequilibrium effects are very small [6]. 

The last assumption of the classical TST concerns the adiabaticity of the movement along the reaction coordinate. The term adiabatic has been used in many different 

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