Hexameric capsules studied by magic angle spinning solid-state NMR spectroscopy: Identifying solvent molecules in pyrogallol[4]arene capsules

Powders of pyrogallol[4]arene hexamers were produced by evaporation from organic solvents and were studied, for the first time, by magic angle spinning solid-state NMR (MAS ssNMR). Evaporation selectively removed non-encapsulated solvent molecules leaving stable hexameric capsules encapsulating solvent molecules. After exposure of the powder to solvent vapors, ¹H/¹³C heteronuclear correlation MAS ssNMR experiments were used to assign the signals of the external and encapsulated solvent molecules. The formed capsules were stable for months and the process of solvent encapsulation was reversible. According to the ssNMR experiments, the encapsulated solvent molecules occupy different sites and those sites differ in their mobility. The presented approach paves the way for studying guest exchange, guest affinity, and gas storage in hexamers of this type in the solid state. Peeking inside: Powders of pyrogallol[4]arene hexamers obtained by slow evaporation of the corresponding solutions were studied by various magic angle spinning solid-state NMR techniques. As a result, signals of the encapsulated and non-encapsulated solvent molecules were identified, the encapsulation process was shown to be reversible, and it was demonstrated that the encapsulated solvent molecules occupy different sites having different mobilities.