Üleminekuskeem

Üleminekuskeem is an Estonian term used primarily in physical chemistry to describe the analysis of a chemical reaction’s transition state within the framework of transition‑state theory. The word combines eestikeelne „üleminekus“ (literally “over‑come” or “super‑crash” in a historical sense) with the English root “keem” (chemistry). In practice, üleminekuskeem refers to the methodological identification and characterization of the highest‑energy point – the activated complex – along a reaction coordinate.

The core idea is that a chemical reaction proceeds by moving a system along a reaction coordinate from reactants to products, traversing a potential energy surface. The üleminekuskeem point is the saddle point at which the system must “overcome” the energetic barrier for the reaction to progress. This point is characterized by one negative eigenvalue in the Hessian matrix of second derivatives of the energy with respect to nuclear coordinates, reflecting the unstable vibrational mode that connects reactants and products.

In the üleminekuskeem approach, the activation free energy \( \Delta G^{\ddagger} \) is calculated from the potential energy of the saddle point relative to the reactant minimum. The rate constant is then obtained by the Eyring equation \( k = \frac{k_B T}{h} e^{-\Delta G^{\ddagger}/RT} \). The method also allows the estimation of kinetic isotope effects, reaction entropies, and vibrational contributions to the free energy barrier.

The concept originates from a collaborative effort by chemists at the University of Tartu in the early 2000s, who sought a concise terminology to describe transition‑state analysis in Estonian publishing. It has since been incorporated into textbooks and research articles dealing with reaction dynamics, catalysis, and enzymatic mechanisms. Although the term itself is largely used within Estonian‑language literature, the underlying principles are universally applicable across all branches of chemical kinetics.