Construction of a Metal-Free Electron Spin System by Encapsulation of an NO Molecule Inside an Open-Cage Fullerene...

A reactive radical species, nitric oxide (NO), was encapsulated in a unimolecular form inside an open-cage fullerene derivative under high-pressure conditions in the solid state. Surprisingly, the molecular complex showed sharp ¹ H NMR signals despite the existence of the paramagnetic species inside the carbon cage. Owing to the paramagnetic shifts, the escape rate of the NO was determined experimentally. After constructing a stopper on the rim of the opening, the NO was found to stay inside the cage even at 50 °C. The ESR measurements of the powdery sample showed paramagnetic properties at low temperature. The single-crystal X-ray structure analysis clearly demonstrated the existence of the encapsulated NO molecule, suggesting rapid rotation inside the cage. The ¹ H NMR chemical shifts displayed a large temperature dependence owing to the paramagnetic effects.