The activated behavior of the reaction rate can be understood from a simple model of two colliding atoms. In this chapter, we discuss the connection to transition state theory, which takes into account the internal degrees of freedom of larger molecules and explains not only the activation energy, but also the prefactor of the Arrhenius law. We formulate transition state theory in a thermodynamic context and discuss kinetic isotope effects. Finally, we present quite general rate expressions based on the flux over a saddle point.
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