Nucleophilic displacement reactions (the S(N)2 reaction) of ions in th
e gas phase are a prototypical reaction system that allows a study of
dynamics, mechanisms, and structure-energy relations. This article rev
iews aspects of the kinetics (especially the applicability of statisti
cal reaction rate theory), the relation of structure and reactivity, a
nd the effects of small numbers of solvent molecules on the reaction a
nd compares the behavior of the ionic reaction in the gas phase with t
hat in solution.