Synthesis and Kr-83 NMR spectroscopy of Kr@C-60

Synthesis of Kr@C-60 is achieved by quantitative high-pressure encapsulation of the noble gas into an open-fullerene, and subsequent cage closure. Krypton is the largest noble gas entrapped in C-60 using 'molecular surgery' and Kr@C-60 is prepared with >99.4% incorporation of the endohedral atom, in ca. 4% yield from C-60. Encapsulation in C-60 causes a shift of the Kr-83 resonance by -39.5 ppm with respect to free Kr-83 in solution. The Kr-83 spin-lattice relaxation time T-1 is approximately 36 times longer for Kr encapsulated in C-60 than for free Kr in solution. This is the first characterisation of a stable Kr compound by Kr-83 NMR.