To probe the metastable nature of cage C, we first isolated guest-free C by extracting phenolphthalein 9 from inclusion complex 9ÌC, as described above. Then, we recorded a series of 1 H NMR spectra in D2O at 60 ºC (see Figure ); integrating the spectra allowed us to plot the spontaneous conversion C→T. By fitting C's decay to first-order kinetics (Figure ), we obtained k = 0.63±0.02 h -1 (at 60 ºC). ) Two different views of the X-ray crystal structure of the host-guest inclusion complex 10ÌC (host and guest protons, water molecules, and counterions omitted for clarity). The molecular orientation in (d) highlights the Td symmetry of cage C (the molecular surface of 10 was calculated using PyMOL (v. 2.0, Schrödinger, LLC.) using a probe radius of 1 Å). The cartoon on the bottom-right shows the orientation of the imidazole groups within the TImB panels (pink panel = clockwise rotation of the imidazole groups; gray panels = counter-clockwise rotation of the imidazole groups). (e) Monitoring the decay of cage C at 60 ºC (for the NMR spectra, see Figure ). Markers: experimental data points; line: fit to a first-order rate equation.