Activated complex

An activated complex is a short-lived, high-energy arrangement of atoms near the top of a reaction’s energy barrier, as reactants progress toward products. It is a transition-state configuration, not an isolable intermediate.

Transition state: A configuration at a potential-energy surface’s saddle point, where motion along a reaction coordinate changes from reactants toward products. The activated complex is the molecular configuration associated with this point on the reaction pathway.
Chemical reaction: A process in which atoms are rearranged to form substances with different chemical compositions or structures. An activated complex occurs during the conversion of reactants into products.
Catalysis: The acceleration of a chemical reaction by a substance that is regenerated rather than consumed overall. Catalysts change reaction pathways and lower the barrier to the activated complex.
Collision theory: A model that explains reaction rates through collisions with sufficient energy and suitable molecular orientation. It emphasizes reactive collisions, while activated-complex theory describes a barrier-top configuration.
Reaction coordinate: A parameter describing progress along a chemical reaction pathway, often tracking changes in molecular geometry. It represents the progress by which reactants approach and pass the activated complex.
Chemical kinetics: The study of the rates of chemical reactions and the factors that affect them. The lifetime and formation rate of activated complexes help explain reaction speed.
Enzyme catalysis: The acceleration of chemical reactions by enzymes through substrate binding and stabilization of reaction pathways. Enzymes can favor configurations resembling the activated complex, reducing the effective barrier.
Molecular dynamics: A computational method that simulates atomic motion by numerically integrating equations of motion. It follows trajectories over time rather than assuming a statistical activated-complex population.
Activation energy: The minimum energy barrier that reacting particles must overcome for a reaction to proceed. The activated complex lies at the energy barrier whose height determines this threshold.
Reaction intermediate: A species formed in one reaction step and consumed in a later step, with a finite lifetime on the reaction pathway. Unlike an intermediate, an activated complex is not a local energy minimum or isolable species.

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