Chemical kinetics is the study of reaction rates and rate constants for chemical processes. Reaction rates are measured under varying conditions from which rate laws and reaction mechanisms are derived. Typically, the depletion of a reactant or the formation of a product involves multiple sequential and competing reaction steps. Whereas it is difficult to analyze individual reaction steps experimentally, computational methods can focus in on single chemical events. Rate constants for elementary reactions are determined by entropic and energetic changes during reaction progress. Commonly, an energetic barrier has to be crossed before reaction can occur. The point of highest potential or Gibbs free energy along the minimum energy path is termed transition state and is the object of interest in computational chemical kinetics. Within transition state theory, the rate constant depends exponentially on the energy difference between transition state and reactants. This implies that the accurate computation of energies is imperative to successful kinetic predictions. However, as computational power and thereby accessible system size increases, entropic contributions gain importance. In particular, we are interested in transition states of organic molecules containing about 50 atoms that are characterized by a number of low-frequency modes with significant deviation from harmonic behavior, the latter being typically assumed. Since low-frequency motion contributes the most to the vibrational entropy, we address this issue in our computational work.