HOST-GUEST CHEMISTRY IN THE MASS-SPECTROMETER

The study of host-guest chemistry in the gas phase has been pursued sy stematically only in the last 5 years, after sporadic interest in the 1980s, despite extensive investigation of this chemistry in the conden sed phase and the success encountered in several fields of application . Most gas-phase studies have been performed under the controlled cond itions that can be achieved inside a mass spectrometer, where solvent effects are not present. At this point, the first evaluation of the ac hievements and knowledge emerging from these mass spectrometric invest igations can be attempted. At the same time, it is possible to evaluat e the most promising subjects for future research. This review underta kes these tasks, by presenting (i) applications where mass spectrometr y was used to investigate condensed phase equilibria and, more extensi vely, (ii) experiments where host-guest complexes were formed directly in the gas phase. The latter processes are discussed in detail in rel ation to the structural and electronic effects, the energetic requirem ents and the dependence on size, rigidity, spatial geometry and functi onal groups of both hosts and guests. All these effects combined are l ikely to contribute to the strength and multiplicity of the non-covale nt bonds that allow the complex to be formed. The macro(poly)cyclic ho sts considered include crown ethers, cryptands, cyclodextrins, calixar enes, cryptophanes, cavitands, carcerands and macrolides. The guests a re either metal cations or organic molecules and ions.