Energy Exchange and Transition-State Theory

Associative reactions can only proceed if energy is removed from the product molecule. To this end, three-particle reactions are required. At low pressures, energy transfer to reactants can become rate limiting. 

Under such conditions monomolecular reactions such as dissociation or isomerization behave as bimolecular reactions. 

Here we will discuss the phenomena that result in chemical conversion reactions when transfer of energy and momentum become important. As demonstrated, the removal of translational energy (or momentum) is crucial in association reactions. The reverse process, collisional excitation of reactants is also important, for two types of unimolecular reactions, namely dissociation 

Examples are the dissociation of molecular bromine, Br2, the decomposition of sulfuryl, SO2CI2, into SO2 and CI2, in the isomerization of cyclopropane to propylene. All these reactions are elementary, and thus truly unimolecular. 

In the early twenties, several scientists puzzled over how a reacting molecule in an unimolecular reaction could acquire sufficient internal energy to react. Jean Perrin proposed in 1919 that energy was provided by radiation from the walls of the reaction vessel. Frederick Lindemann, the later Lord Cherwell, strongly objected against this Radiation Theory of Chemical Action and presented an alternative view in 1921, which is still regarded as essentially correct. It explains a remarkable feature of unimolecular reactions The rate of a unimolecular reaction is first order in the reactant at normal pressure, but may become second order at low pressure. 

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