Host–Guest Chemistry in the Mass Spectrometer

The study of host–guest chemistry in the gas phase has been pursued systematically only in the last 5 years, after sporadic interest in the 1980s, despite extensive investigation of this chemistry in the condensed phase and the success encountered in several Ðelds of application. Most gas-phase studies have been performed under the controlled conditions that can be achieved inside a mass spectrometer, where solvent e†ects are not present. At this point, the Ðrst evaluation of the achievements and knowledge emerging from these mass spectrometric investigations can be attempted. At the same time, it is possible to evaluate the most promising subjects for future research. This review undertakes these tasks, by presenting (i) applications where mass spectrometry was used to investigate condensed phase equilibria and, more extensively, (ii) experiments where host–guest complexes were formed directly in the gas phase. The latter processes are discussed in detail in relation to the structural and electronic e†ects, the energetic requirements and the dependence on size, rigidity, spatial geometry and functional groups of both hosts and guests. All these e†ects combined are likely to contribute to the strength and multiplicity of the non-covalent bonds that allow the complex to be formed. The macro(poly)cyclic hosts considered include crown ethers, cryptands, cyclodextrins, calixarenes, cryptophanes, cavitands, carcerands and macrolides. The guests are either metal cations or organic molecules and ions.