Reaction coordinate

Lore & Background

In chemistry, a reaction coordinate is chosen to represent progress along a reaction pathway, often as a geometric parameter like bond length or bond angle. For example, in the homolytic dissociation of molecular hydrogen, the bond length serves as an apt coordinate. Non-geometric parameters such as bond order are also used, though direct representation can be difficult for complex reactions. In computer simulations, collective variables are employed for target-oriented sampling, reducing many individual variables into a lower-dimensional set that describes crucial system characteristics. These collective variables span the reaction coordinate with a continuous function, and applying a bias to them enables enhanced simulations that can 'steer' toward desired destinations. Special collective variables that distinguish reactants from products are known as order parameters, a term originating in phase transition work. Reaction coordinates are special order parameters that describe the entire pathway from reactants through transition states to products. They may be defined using chemically intuitive variables like bond lengths, splitting probabilities (committors), or eigenfunctions corresponding to reactant-to-product transitions.

Reader's Guide

The reaction coordinate is a foundational concept in chemical kinetics and computational chemistry, as it parameterizes reaction progress at the molecular level. It differs from extent of reaction, which measures progress in terms of system composition. In transition-state theory, the reaction coordinate for each step is one of a set of curvilinear coordinates that leads smoothly from reactants to products via the transition state, typically following the path defined by the potential energy gradient—shallowest ascent and steepest descent. Free energy is often plotted against reaction coordinates to show potential energy profiles. The concept enables the study of rare events in simulations by using collective variables and enhanced sampling techniques, making it indispensable for understanding and predicting chemical reactivity.

Did You Know?

• A reaction coordinate is an abstract one-dimensional coordinate chosen to represent progress along a reaction pathway.
• In the homolytic dissociation of molecular hydrogen, an apt reaction coordinate is the bond length.
• Reaction coordinates are special order parameters that describe the entire pathway from reactants through transition states to products.
• In transition-state theory, the reaction coordinate is typically chosen to follow the path defined by the potential energy gradient – shallowest ascent/steepest descent – from reactants to products.