Recognition of hydrogen isotopomers by an open-cage fullerene.

We present our study on the recognition of hydrogen isotopes by an open-cage fullerene through determination of binding affinity of isotopes H2/HD/D2 with the open-cage fullerene and comparison of their relative molecular sizes through kinetic-isotope-release experiments. We took advantage of isotope H2/D2 exchange that generated an equilibrium mixture of H2/HD/D2 in a stainless steel autoclave to conduct high-pressure hydrogen insertion into an open-cage fullerene. The equilibrium constants of three isotopes with the open-cage fullerene were determined at various pressures and temperatures. Our results show a higher equilibrium constant for HD into open-cage fullerene than the other two isotopomers, which is consistent with its dipolar nature. D2 molecule generally binds stronger than H2 because of its heavier mass; however, the affinity for H2 becomes larger than D2 at lower temperature, when size effect becomes dominant. We further investigated the kinetics of H2/HD/D2 release from open-cage fullerene, proving their relative escaping rates. D2 was found to be the smallest and H2 the largest molecule. This notion has not only supported the observed inversion of relative binding affinities between H2 and D2, but also demonstrated that comparison of size difference of single molecules through non-convalent kinetic-isotope effect was applicable.

X-ray observation of a helium atom and placing a nitrogen atom inside He@C60 and He@C70.

Single crystal X-ray analysis has been used as a powerful method to determine the structure of molecules. However, crystallographic data containing helium has not been reported, owing to the difficulty in embedding helium into crystalline materials. Here we report the X-ray diffraction study of He@C60 and the clear observation of a single helium atom inside C60. 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. 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.

Rational synthesis, enrichment, and (13)C NMR spectra of endohedral C(60) and C(70) encapsulating a helium atom.

Endohedral fullerenes encapsulating a helium atom, i.e., He@C(60) and He@C(70), at occupation levels of 30% were prepared by rational chemical synthesis. The existence of weak interactions between the inner helium and the outer fullerene cages was demonstrated by experimental and computational investigations.

Can H2 inside C60 communicate with the outside world?

The quenching rate constants of singlet oxygen by C60, H2@C60, D2@C6o, H2, and D2 in solution were measured. The presence of a hydrogen (H2@C60) or deuterium (D2@C60) molecule inside the fullerene did not produce any observable effect based on triplet lifetime or EPR measurements. However, a remarkable effect was found for the 1O2 quenching by C60, H2@C60, D2@C6o, H2, and D2. Singlet oxygen was generated by photosensitization or by thermal decomposition of naphthalene endoperoxide derivatives. Comparison of the rate constants for quenching of 1O2 by H2@C60 and D2@C60 demonstrates a significant vibrational interaction between oxygen and H2 inside the fullerene. The quenching rate constant for H2 is 1 order of magnitude higher than that of D2, in agreement with the results observed for the quenching of 1O2 with H2@C60 or D2@C60.

Fine tuning of the orifice size of an open-cage fullerene by placing selenium in the rim: insertion/release of molecular hydrogen.

A newly synthesized open-cage fullerene containing selenium in the rim of the 13-membered-ring orifice allows milder conditions for hydrogen insertion, and the rate for hydrogen release is ca. three times faster than its sulfur analogue.