What does a negative entropy of activation tell you about a reaction?

It indicates that the activated complex is more ordered or constrained than the separate reactants, typical of associative steps where two molecules come together into a single, more rigid transition structure.

Why is transition state theory called an absolute rate theory?

Unlike the empirical Arrhenius equation, it derives the rate constant from the molecular partition functions and energy of the activated complex, so in principle it predicts the rate from structure alone without fitting experimental rate data.

Transition State Theory

Transition state theory calculates reaction rates from the properties of an activated complex poised at the top of the energy barrier, giving absolute rate constants from molecular structure and statistical mechanics.

Definition

Transition state theory is a framework for computing reaction rate constants by treating the activated complex at the maximum of the potential energy barrier as a species in quasi-equilibrium with the reactants, decaying to products at a universal frequency.

Scope

This topic covers the theory of reaction rates based on the activated complex: the potential energy surface and the reaction coordinate, the saddle point and activated complex, and the assumption of quasi-equilibrium between reactants and the transition state. It develops the Eyring equation and the activation entropy and enthalpy, compares the theory with simple collision theory, and notes extensions such as the role of the transmission coefficient and Marcus theory for electron transfer. The empirical Arrhenius parameters and reaction mechanisms are treated in sibling topics.

Core questions

What is the activated complex, and where does it sit on the potential energy surface?
How does the quasi-equilibrium assumption lead to the Eyring equation?
How are the activation enthalpy and entropy interpreted molecularly?
How does transition state theory improve on simple collision theory?

Key concepts

Potential energy surface and reaction coordinate
Activated complex and saddle point
Quasi-equilibrium assumption
Eyring equation and the transmission coefficient
Activation enthalpy and entropy

Key theories

Eyring activated complex theory: The rate constant is the product of a universal frequency factor and the quasi-equilibrium constant for forming the activated complex, expressing the rate in terms of the activation Gibbs energy and hence the activation enthalpy and entropy.
Marcus theory of electron transfer: For electron-transfer reactions the activation barrier is built from the reaction free energy and a reorganization energy, predicting a characteristic inverted region where rates fall as reactions become more exergonic.

Clinical relevance

Transition state theory frames the design of catalysts and enzymes through the concept of transition-state stabilization, the interpretation of kinetic isotope effects, the rates of electron-transfer steps in respiration and photosynthesis, and the quantitative modelling of reaction rates across chemistry.

History

Transition state theory was formulated independently in 1935 by Eyring and by Evans and Polanyi, building on London's potential energy surfaces; Marcus extended the activated-complex picture to electron transfer in the 1950s, a development recognized with the 1992 Nobel Prize in Chemistry.

Key figures

Henry Eyring
Michael Polanyi
Rudolph A. Marcus

Seminal works

eyring1935
marcus1956
laidler1987

Frequently asked questions

What does a negative entropy of activation tell you about a reaction?: It indicates that the activated complex is more ordered or constrained than the separate reactants, typical of associative steps where two molecules come together into a single, more rigid transition structure.
Why is transition state theory called an absolute rate theory?: Unlike the empirical Arrhenius equation, it derives the rate constant from the molecular partition functions and energy of the activated complex, so in principle it predicts the rate from structure alone without fitting experimental rate data.