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Endofullerenes are supramolecular complexes where one small (endohedral) atom/molecule is completely confined within a bigger, fullerene, molecule which acts as an enclosing cage.1 Endofullerenes offer an ideal “particle in a box” nano-laboratory to observe quantum mechanical phenomena.
The noble gas endofullerenes He@C₆₀ and Ne@C₆₀ have been investigated using INS (Inelastic Neutron Scattering) and THz (far-Infra Red) spectroscopy.2 Surprisingly, He@C₆₀ is observed to absorb THz light and scatter neutrons, which translationally excite the Helium atom. The atomic translational quantization arising due to confinement is probed using these techniques. Simulating the experimental results delivers the confining interaction potential between the noble gas atom and the confining C₆₀ fullerene cage; providing valuable non-covalent interaction parameters, which serve as benchmark for quantum chemistry calculations.
An unexpected NMR (Nuclear Magnetic Resonance) interaction, J-coupling, between ³He and ¹³C in the C₆₀ cage was observed due to confinement, in the ³He@C₆₀ endofullerene.3 This is attributed to a scalar ⁰J$_{HeC}$-coupling, where the “zero” represents the number of chemical bonds between Helium and Carbon. Indication of similar ⁰J-couplings between ¹H and ¹³C are also observed in endofullerenes containing endohedral hydrogen.
When the C₆₀ molecule is filled with an endohedral species, the C₆₀ ¹³C NMR resonance is shifted downfield, relative to C₆₀, depending on the species (endohedral shift). The magnitude of the shift scales proportionally with the size of the endohedral species. The endohedral shift appears to reflect the “pressure” a single atom/molecule exerts on its container, in this case the C₆₀ molecule. Solution and solid state NMR measurements together with an interpretation of the endohedral shifts will be presented.
1-Y. Murata et al., Science, 2005, 307, 238
2-G. R. Bacanu et al., JCP, 2021, 155, 144302
3-G. R. Bacanu et al., JACS, 2020, 142, 16926
