unimolekularen

Unimolekularen, or unimolecular processes, describe chemical transformations in which the rate-determining step involves only a single molecule. In this view, a molecule rearranges, fragments, or isomerizes without requiring a direct collision with another molecule to initiate the reaction. Unimolecular steps are common in gas-phase chemistry, combustion, atmospheric chemistry, and certain solid-state or solution-phase transformations where energy redistribution within a molecule governs reactivity.

In the gas phase, unimolecular reactions typically require the molecule to acquire sufficient internal energy to reach an activated state. A classic framework is the Lindemann mechanism, which posits a two-step process: A + M ⇌ A* + M, followed by A* → products. The first step involves collisional excitation to a reactive intermediate A*, and the second step converts A* into products. The observed rate can depend on the concentration of a third body (M) at low pressures and becomes essentially independent of M at high pressures, producing a characteristic fall-off behavior.

The modern, more general description uses RRKM theory (Rice–Ramsperger–Kassel–Marcus), which connects the unimolecular rate to the density of available quantum states of the energized molecule and the probability of crossing a transition state. This framework accounts for how energy distribution and molecular structure control the likelihood of dissociation or isomerization, especially at high internal energies.

Unimolecular reactions also occur in condensed phases where solvent interactions influence energy flow, or via direct thermal processes in solids. They play a key role in modeling atmospheric degradation, combustion chemistry, mass spectrometry, and polymer reactions, providing a foundational concept for understanding how single-molecule transformations proceed.