Supramolecular chemists frequently use mass spectrometry as a tool to determine the stoichiometry of complexes. However, modern tandem mass spectrometric techniques can yield much additional useful information, and can be designed to probe the gas phase behavior of the complexes for comparison with behavior in condensed media. We employ Fourier transform ion cyclotron resonance (FTICR) mass spectrometry with electrospray ionization to study the gas phase behavior of complexes based on cucurbit[n]uril hosts. Patterns in the mass spectra suggest that the alkali cation caps bound to cucurbit[5]uril (CB[5]) can strongly influence the binding of neutral guests within the CB[5] cavity. For instance, the use of capping potassium ions strongly enhances binding of neutral formic acid in the CB[5] cavity, whereas the use of sodium ions as caps disfavors the binding of formic acid. Further, analysis of the pressure dependence of FTICR mass spectrometric linewidths yields ion-neutral collision cross sections that are in reasonable agreement with the results of more commonly-practiced ion mobility measurements. These results, in conjunction with computational modeling, give conformational information about cucurbituril complexes that can rule out some proposed structures and provide evidence for others. For instance, cross section measurements show that chloride ions bind inside the CB[5]cavity, whereas bromide can bind either inside or outside, depending on which alkali cation is used to cap the complex.

Gas phase cucurbituril chemistry

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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