unimoleculare

Unimoleculare, in chemistry often referred to as unimolecular, describes reactions or processes in which a single molecule undergoes a chemical transformation. In many cases the reactive event proceeds without a direct collision with another molecule, but the surrounding environment can influence the process by providing or removing energy through collisions with a third body.

Kinetic behavior of unimolecular reactions is commonly first order with respect to the reacting species, especially in the gas phase at moderate to high pressures. The classical picture is the Lindemann–Hinshelwood mechanism, in which a molecule A is first energized through collision with a third body M to form A*, which can then either deactivate back to A or proceed to products. At low pressures, the rate can depend on both A and M, giving a more complex, pressure-dependent form; at high pressures, the rate becomes effectively first order in A, determined by the rate of conversion of the energized intermediate to products.

Theoretical treatments of unimolecular processes include transition-state theory and RRKM (Rice–Ramsperger–Kassel–Marcus) theory. RRKM provides a microcanonical rate constant by accounting for the density of states of the energized molecule and the energy distribution, explaining how internal energy and vibrational modes influence dissociation or isomerization rates.

Applications span gas‑phase chemistry, atmospheric and combustion chemistry, and mass spectrometry. Classic examples include the thermal decomposition of tert-butyl chloride to isobutene and HCl, and the fragmentation of molecular ions produced in mass spectrometry. Unimolecular mechanisms also underpin photodissociation and various rearrangements that occur without a bimolecular collision as the rate-determining step.