Trapping Alkali Halide Cluster Ions Inside the Cucurbit[7]uril Cavity

In this study, the distinctive behavior of cucurbit[n]uril (CB[n]), which captures a variety
of alkali
halide clusters inside the cavity during the droplet evaporation,
has been investigated by using ion mobility spectrometry–mass
spectrometry. Complexes of CB[7] with various alkali chloride cluster
cations or anions generated during the electrospray ionization were
studied, and their collision cross-section (CCS) values were obtained
to determine whether these clusters were trapped inside the cavity
or not. It was found that the clusters smaller than a specific critical
size were trapped inside the CB[7] cavity in the gas phase, although
trapping of alkali halide clusters at the given concentration is supposed
to be unfavorable in solution. We suggest that the rapid solvent evaporation
rapidly increases ion concentrations and subsequently forms alkali-chloride
contact ion pairs; therefore, it may provide a specific environment
to enable the formation of the inclusion complexes.