unimolekuláris

Unimolekuláris, or unimolecular, refers to processes in which a single molecule undergoes a chemical transformation without requiring a collision with another distinct reactant molecule in the rate-determining step. In simple cases the rate law is first order: rate = k [A]. This contrasts with bimolecular reactions, where the rate depends on the product of the concentrations of two reacting species, and with termolecular reactions that involve three bodies.

In gas-phase kinetics unimolecular reactions are often described by mechanisms in which energy transfer by collisions is needed to reach an activated state. The classic Lindemann–Hinshelwood mechanism treats A + M ⇌ A* + M followed by A* → products. Depending on pressure, the observed rate can be first order in A (high-pressure limit) or fall off to a more complex dependence with respect to A and M (low-pressure limit). More modern treatments use RRKM theory to relate the rate constant to the density of vibrational states and the energy of the molecule.

Unimolecular processes include various decompositions and isomerizations of molecules, photodissociation, and other energy-driven transformations. They are central to gas-phase, atmospheric, and combustion chemistry, where first-order kinetics often dominate the fate of reactive intermediates following an initial activation step.

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