Transition state

A transition state is the highest-energy configuration along a particular chemical reaction pathway, where bonds are partly broken and formed. It is fleeting and cannot usually be isolated.

Reaction coordinate: A variable that tracks progress along a chemical reaction pathway. It locates the transition state along the pathway from reactants to products.
Chemical bond: An interaction that holds atoms together in a molecule or solid. Partial bond formation and breaking define the structure of many transition states.
Arrhenius equation: An equation relating a reaction rate constant to temperature and activation energy. Its activation energy summarizes the temperature-sensitive barrier leading to the transition state.
Reactant: A starting substance consumed during a chemical reaction. Reactants occupy an energy basin before the pathway climbs toward its transition state.
Activation energy: The energy barrier reactants must overcome for a reaction to proceed. The barrier height is measured from the reactants to the transition state.
Chemical reaction: A process in which substances are transformed through changes in atomic connectivity. A transition state belongs to a particular pathway connecting reactants and products.
Eyring equation: An equation relating a reaction rate constant to temperature and activation parameters. It connects rate measurements to the free energy of activation for forming the transition state.
Product (chemistry): A substance formed by a chemical reaction. Products lie beyond the transition state along the reaction pathway.
Potential energy surface: A map of a system’s potential energy as a function of its atomic positions. Its energy landscape contains the transition state and the pathways around it.
Molecular geometry: The three-dimensional arrangement of atoms in a molecule. Transition-state structure describes atomic positions at the reaction’s energy barrier.

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