reactioncoordinate

A reaction coordinate is a parameter that represents progress along a chemical reaction from reactants to products. In a molecular system, it is typically a one-dimensional coordinate that captures the essential structural changes during the transformation, such as changes in bond lengths, bond angles, or combinations of several internal coordinates.

The intrinsic reaction coordinate (IRC) is a common formalization of the reaction coordinate. It is defined as the minimum energy path on the potential energy surface that connects reactants to products through the transition state. The energy profile along the reaction coordinate shows a maximum at the transition state and lower energies at the reactant and product valleys. The activation energy is the energy difference between the reactants and the transition state along this path.

In theoretical and computational chemistry, the reaction coordinate is used to study kinetics and mechanism. It can be constructed by constrained optimization that probes successive points along the coordinate, or by representing the reaction as a vector in multidimensional configurational space that traces the reaction path. The reaction coordinate is not unique; different coordinates can describe the same reaction, especially in complex rearrangements.

For multidimensional reactions, a set of coordinates or an entire reaction path may be employed, with related concepts such as the minimum energy path, transition state theory, and quasi-harmonic approximations guiding interpretation. Experimentally, reaction coordinates are inferred from kinetic data, isotope effects, and spectroscopic observations, often in conjunction with computational models to relate observables to progress along the reaction. A clear reaction coordinate helps identify which bonds are formed or broken and the sequence of events during the transformation.