Unimolecular electronic seen

Since the early transition state does not involve covalent interaction with the nucleophile, the rate-determining intermediate also does not involve covalent interaction with the nucleophile, and is best represented as an ion pair. A structural representation of this ion pair cannot be specified precisely, but certainly there has been massive electron reorganization with little covalent interaction between the nuclei in the bond being broken. Mutual polarization of the carbonium ion and counter-ion can be assumed, as well as coulombic attraction. The intermediate formed in the rate-determining step is then rapidly converted to an intermediate which does involve covalent interaction with solvent or other nucleophiles present. Thus, the rate-determining step is unimolecular, and the observed kinetics are first-order. As will be seen shortly, the stereochemical course of the reaction can also be accommodated. 

The general dominance of the lowest energy pathway in thermally activated systems is also seen in unimolecular reactions. Studies of the dissociation of N2O and of related isoelectronic molecules illustrate this, even though another factor restricts the lowest energy channel. In spite of the spin-forbidden nature of the reaction, the thermal decomposition of N2O yields products in their electronic ground states ... 

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