Isolation and Spectral Properties of Kr@C₆₀, a Stable van der Waals Molecule

The stable van der Waals molecule krypton inside C 60 was isolated by HPLC from a mixture containing 99.9% empty C 60 . Four passes through a semipreparative PYE [2-(1-pyrenyl)ethylsilyl] column provided 140 μg of 90% pure Kr@C 60, as confirmed by mass spectrometric analysis. The 13 C NMR spectrum of Kr@C 60 contains a single line shifted 0.39 ppm downfield from the C 60 resonance. UV−visible spectroscopy shows that krypton incorporation causes a red shift of ca.. 45 cm - 1 in the lowest singlet transition of C 60 . Three of the four allowed vibrational fundamentals are observed by infrared spectroscopy; each shows a small frequency increase in Kr@C 60 . A 12% acceleration in triplet state decay is measured at 77 K and attributed to an “endohedral heavy atom effect” induced by the krypton atom. It is concluded that coupling between the krypton atom and the π-electron system of the fullerene is small but significant.