![]() (NiOEP)♲C 6H 6, with the occupation level of krypton being only 9% ( 21).It should be mentioned that the position of the helium is similar to that of krypton reported by the X-ray diffraction data of 60/C 60) 1, the encapsulated helium atom was unambiguously observed at the centre of the C 60 cage with 100% occupation level, probably owing to the enrichment during crystallization. (NiOEP) 2 without any disorders of the C 60 cage ( Supplementary Tables S1–S3).The least-square refinements gave the structure of 60 The X-ray diffraction data were collected using synchrotron radiation at 173 K at SPring-8. To prevent such a rotational or orientational disorder in the single crystal, co-crystals of the 60 and nickel(II) octaethylporphyrin (NiOEP) were prepared from the solution in o-xylene 15, 21. In addition, we also report the close contact of the helium atom with a nitrogen atom by placing a nitrogen atom inside 60 and 70 by the use of nitrogen radio-frequency (RF) plasma method 17, 18, 19.Īlthough C 60 is a crystalline material, a C 60 cage is known to rotate even in the solid state 20. Here we report the single crystal X-ray observation of a helium atom inside C 60. The encapsulation level of 60 thus obtained was enhanced to 95% by purification with recycling high-performance liquid chromatography (HPLC) 14. This method involves insertion of a helium atom into open-cage fullerene derivatives followed by restoration of the openings under mild conditions 14. 30% encapsulation levels in macroscopic scales. On the other hand, we have developed rational synthetic methodology 7, 8, 9, 10, 11, 12, 13, 14, 15, so-called ‘molecular surgery’ 16, and recently succeeded in the chemical syntheses of 60 and 70 with ca. However, the encapsulation level of the helium atom was quite low such as 0.15–0.4% (refs 5, 6) because of the inefficiency of this physical method, although the inner 3He atom is useful for the clarification of the internal magnetic field of fullerenes 5. 3 in 1994 in which 3He atom was forced into pristine C 60 and C 70 under drastic conditions, such as 650 ☌ under 3000, atm of helium gas 3, 4. Endohedral C 60 and C 70 encapsulating a helium atom, that is, 60 and 70 were first reported by Saunders et al. Fullerene C 60 is a good candidate to entrap a helium atom. In contrast, if a proper single crystal is available, a helium atom should be observable by the single X-ray analysis because a helium atom possesses two electrons in its 1s electronic shell which is isoelectronic with the lithium cation widely observed in the literature 2. Thus, it is crucial to embed a helium atom selectively into some crystalline material with a discrete structure if one wishes to observe it by X-ray analysis.īecause the strength of the X-ray diffraction of single crystal X-ray analyses depends on the total number of electrons of all atoms existing in the unit cell, it is usually difficult to observe atoms with poor electron density, such as hydrogen, after structural refinements. This is because neither helium nor a helium-incorporating material becomes crystalline solid under atmospheric pressure even in the vicinity of 0 K (ref. Helium is well known as the smallest noble gas atom and it has refused to be directly observed by single crystal X-ray analysis, a powerful method to determine molecular structure. The importance of helium is widely seen in its common use in helium balloons or respirator gas, as well as in its scientific use as a refrigerant for superconductive magnets and physical research at low temperature. Electron spin resonance analysis reveals that the encapsulated helium atom has a small but detectable influence on the electronic properties of the highly reactive nitrogen atom coexisting inside the fullerene, suggesting the potential usage of helium for controlling electronic properties of reactive species. In addition, the close packing of a helium atom and a nitrogen atom inside fullerenes is realized using two stepwise insertion techniques, that is, molecular surgery to synthesize the fullerenes encapsulating a helium atom, followed by nitrogen radio-frequency plasma methods to generate the fullerenes encapsulating both helium and nitrogen atoms. Here we report the X-ray diffraction study of 60 and the clear observation of a single helium atom inside C 60. However, crystallographic data containing helium has not been reported, owing to the difficulty in embedding helium into crystalline materials. Single crystal X-ray analysis has been used as a powerful method to determine the structure of molecules. ![]()
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