Host–guest complexation in the gas phase. Investigation of the mechanism of interaction between cavitands and neutral guest molecules

Abstract

Host–guest complexes are formed in the gas phase by vaporizing cavitand 1 within a mass spectrometer ionsource filled with a mixture of a buffer gas (methane) and one or more candidate guests in known relative concentration. The main purpose of this study was to investigate the inherent mechanism for the process. From experimental data, it was deduced that the complexation is strongly influenced by the geometric and electronic properties of the host as well as on the guest structure and functional groups. The occurrence of this high selectivity, together with the evaluation of reactive species, led us to conclude that the observed supramolecular interactions involve undoubtedly neutral guests, whereas the host might either be a charged or a neutral reactant. To distinguish between these two cases, various experimental observations have been made: (i) the data obtained in the positive- and the negative-ion modes are generally very similar; (ii) the extent to which complexation occurs depends on the residence time of ionic species in the mass spectrometer reaction chamber; (iii) the mass-selected charged host is capable of forming complexes with neutral guests in the octapole collision cell of a tandem mass spectrometer, and their relative abundances are similar to those observed in the experiments conducted in the ion source. It is concluded that the mechanism involving the preliminary ionization of the host and its subsequent interaction with a neutral guest is predominant, but a simultaneous interaction mechanism between neutral species might also occur to some extent. Another conclusion of this study is that CH-π interactions are the dominant interactions between cavitand 1 and aromatic guests.