Spectroscopy of non-metallic C60 endofullerenes

C60 fullerene molecules are closed carbon structures with hollow inside. Placing an atom or molecule inside the cavity leads to endofullerenes. The encapsulated, endohedral, species display quantization of their translational degrees of freedom. Such translational eigenstates define the non-bonded (non-covalent) interaction between the host (fullerene) and the guest (endohedral). Encapsulated molecules behave as quantum rotors, displaying essentially free rotation even at cryogenic temperatures. For symmetric molecules, spin isomerism can be observed in the condensed phase. The translational and rotational motion can interact strongly, leading to translational–rotational coupling, which can couple to the vibrational degrees of freedom. Endofullerenes act as nano-laboratories that isolate the encapsulated atoms/molecules from the environment, ideal for spectroscopic investigations into the quantum behavior of the endohedral species. This article is a comprehensive review of all experimental physico-chemical investigations, mostly spectroscopic, of non-metallic C60 endofullerenes. In metallic endofullerenes, the endohedral species interacts strongly with the fullerene cage, usually leading to chemical bonding and/or charge transfer. Non-metallic endofullerenes display weaker guest–host interactions, mostly of non-covalent character, that isolate the endohedral species from the environment. All spectroscopic studies are reviewed, together with experimental techniques related to structure, voltammetry, electron transport, heat capacity, etc. The spectroscopic methods include nuclear magnetic resonance, electron paramagnetic resonance, inelastic neutron scattering, Raman, THz and infrared, electronic structure, and ionization techniques. The article is organized in order of ascending energy probed by or required by the experimental techniques. Theoretical background is provided for the quantum behavior and the spectroscopies used to probe them.