GAS-PHASE ION CHEMISTRY VERSUS SOLUTION CHEMISTRY

The basic requirements for a correct comparison between kinetic and mechanistic data concerning ionic processes in the gaseous phase and in solution are presented. They entail extension of the classical kinetic, structural, and stereochemical methodologies of investigation typical of solution chemistry to the gas phase as well, which must refer to ionic processes involving thermally equilibrated ionic species, including the encounter complexes between an ion and a neutral molecule, with a Maxwell-Boltzmann energy distribution in all their degrees of freedom. Literature kinetic data of several representative ion/molecule reactions indicate that this condition is readily attainable even in a relatively large temperature range, provided that the ion/molecule reaction takes place at sufficiently high pressures (e.g. P > 760 Torr). A critical survey of the most common techniques normally employed in the kinetic and mechanistic analysis of gas-phase ionic processes indicates that only a few of them, such as gamma-radiolysis, nuclear decay, and ion mobility mass spectrometry, guarantee this condition. Other techniques, operating normally at much lower pressures (< 10 Torr), such as ion cyclotron resonance, chemical ionization mass spectrometry, flowing afterglow, selected-ion flow tube, pulsed electron high pressure mass spectrometry, and electron bombardment flow radiolysis, while providing invaluable information upon the nature and stability of the ionic intermediates involved in ionic reactions, cannot nevertheless ensure their thermal equilibration prior to conversion to products, providing therefore phenomenological kinetic and mechanistic data which refer to excited species and thus can hardly be correlated with those relative to the same processes in solution. Direct evidence about this point is provided by a recent comparison of kinetic and mechanistic data concerning classical ionic reactions in solution, such as unimolecular isomerization of elusive ionic species, electrophilic aromatic substitutions, neighboring group participation reactions, and base-induced elimination processes, with those relative to the same processes carried out in the gas phase under both low and high pressure conditions.