Isolation and spectral properties of Kr@C-60, 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 semiprepar ative PYE [2-(1-pyrenyl)ethylsilyl] column provided 140 mu g of 90% pure Kr @C-60, as confirmed by mass spectrometric analysis. The C-13 NMR spectrum o f Kr@C-60 contains a single line shifted 0.39 ppm downfield from the C-60 r esonance. 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 spec troscopy; each shows a small frequency increase in Kr@C-60 A 12% accelerati on in triplet state decay is measured at 77 K and attributed to an "endohed ral heavy atom effect" induced by the krypton atom. It is concluded that co upling between the krypton atom and the pi-electron system of the fullerene is small but significant.