Fullerene helium atom

The possibility that an even larger impact caused the P T extinction received support when Becker and Poreda found that helium and argon atoms were present in the inner cores of some of the fullerenes from the P T boundary sediments (The cover of this book shows a helium atom inside a mol ecule of Ceo) What is special about the fullerene trapped atoms is that the mixtures of both helium and argon isotopes resemble extraterrestrial isotopic mixtures more than earthly ones The He/ He ratio in the P T boundary fullerenes for example is 50 times larger than the natural abundance ratio... 

NMR chemical shifts of the appropriate endohedral fullerenes (fullerenes that contain helium atoms)... 

Because He has a spin of 1/2 and a high gyromagnetic ratio, He-NMR spectroscopy of fullerenes and chemically modified fullerenes containing helium atoms produces useful information in regard to the electronic and... 

For pristine Geo, insertion of a helium atom ( He) required drastic conditions such as 650 °C and 3000 atm to give He C6o in an incorporation rate of 0.1% [58, 84). The incorporation was improved to 1% when Cso vvas pre-treated with KCN the reason for this is as yet unknown [85]. In contrast, theoretical calculations showed that insertion of a helium atom or a hydrogen molecule into open-cage fullerene 65 would take place under milder conditions such as at 124 or 397 °C for helium or hydrogen, respectively (Scheme 9.35) [59]. Actually, such insertion into 65, which was obtained in a similar maimer as 41, was achieved for the first time in 2001 by Rubin and coworkers [59]. The treatment of 65 as a crystalline powder with He (288-305 °G, ca. 475 atm, 7.5 h) gave the He-encapsulating molecule, He 65, in 1.5% yield. The NMR chemical shift of the He incorporated in 65 appeared at AS = —10.10 ppm relative to the free dissolved He gas. The He atom slowly escaped when He 65 was heated above 80 °C in a solution of... 

The aromaticity of fullerenes has been investigated theoretically and substantiated experimentally by NMR [154]. The best experimental method for studying the aromaticity of fullerenes was devised by Saunders and Cross, who encapsulated helium atoms inside fullerenes and measured their NMR spectra [155]. He NMR measurements show that C o exhibits the least aromatic character whereas C70 has the highest, and that the aromaticity of higher fullerenes lies between these two extremes. This is in contradiction to the initial suggestion in 1985 that C o is covered by a sea of n electrons that make it aromatic [14a, 156], and to the assumption of high aromaticity due to the large number of possible Kekule structures [157]. The reduction of fullerenes is not expected to modify their shape or symme-... 

He C6o and He C7o The He NMR spectra of endohedral helium atoms were used to monitor the aromaticity of the fullerenes as a function of electron addition upon reduction. The reduction of He C6o and He C7o to their hexaan-ions was followed by a study of their He shifts [17]. The reduction was carried out with Li metal (in the presence of 2), and the NMR spectra of the anions were... 

Two helium atoms inside fullerenes Another signal that was measured in the He NMR spectra is that of two helium atoms inside a fullerene, e.g. He2 C . ... 

What a storyi Fullerenes formed during the ex plosion of a star travel through interstellar space as passengers on a comet or asteroid that eventually smashes into Earth Some of the fullerenes carry pas sengers themselves—atoms of helium and argon from the dying star The fullerenes and the noble gas atoms silently wait for 251 million years to tell us where they came from and what happened when they got here... 

One of the aspects that has been of interest is the incorporation of an external atom in the spheroidal cavity. A variety of metal atoms can, in principle, be trapped in this cavity. Some of the studies have claimed that it is possible to push atoms such as lanthanum, iron and helium inside the spheroidal cavity of CgQ and other fullerenes. Substitution of the carbon in CgQ by boron and nitrogen has been attempted. Interestingly, nitrogen not only substitutes for carbon in the cage but also adds on to Cgo and C-iq. 

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