Cryogenic NMR spectroscopy of endohedral hydrogen-fullerene complexes

Abstract

We have observed 1H NMR spectra of hydrogen molecules trapped inside modified fullerene cages under cryogenic conditions. Experiments on static samples were performed at sample temperatures down to 4.3 K, while magic-angle-spinning (MAS) experiments were performed at temperatures down to 20 K at spinning frequencies of 15 kHz. Both types of NMR spectra show a large increase in the intramolecular 1H-1H dipolar coupling at temperatures below 50 K, revealing thermal selection of a small number of spatial rotational states. The static and MAS spectra were compared to estimate the degree of sample heating in high-speed cryogenic MAS-NMR experiments. The cryogenic MAS-NMR data show that the site resolution of magic-angle-spinning NMR may be combined with the high signal strength of cryogenic operation and that cryogenic phenomena may be studied with chemical site selectivity.

Quantum Translator-Rotator: Inelastic Neutron Scattering of Dihydrogen Molecules Trapped inside Anisotropic Fullerene Cages

“…Applying Refs. [11,12] it may be shown that, the fullerene cage has been estimated previously from the molecular structure [7] and to a first approximation the H 2 molecule occupies a sphere with root mean square radius (rms), Therefore, although in this provisional analysis the cage has been approximated with spherical shape, a very consistent picture has emerged, confirming the assignments to the rotational and translation manifolds. In fact the translational band has structure and is best fitted as a doublet with energies 17.10 and 17.90meV.…”

“…In spherical symmetry the first excited translational state is three-fold degenerate but in the presence of an anisotropic cage this degeneracy is fully lifted. In this case the translational states are described by three quantum numbers One axis of the ATOCF cage is significantly narrower than the other two with an effective radius estimated to be 0.60Å compared with a rms radius of 0.78Å for the cage as a whole [7]. Scaling the observed translations at ≈17.5meV by the …”

“…[4,5,6 ] The dynamics of several endohedral H 2 -fullerenes have been studied by NMR [7,8] and the data have been interpreted alongside models that incorporate quantised rotational and translational states of H 2 . However, the energy level separations are very much larger than the characteristic energy scale associated with the magnetic interactions so that NMR is only able to probe the translation-rotation manifold indirectly.…”

HD in C 60 : theoretical prediction of the inelastic neutron scattering spectrum and its temperature dependence

“…Subsequent IR spectroscopic studies of H 2 @C 60 [4] as well as of endohedral HD and D 2 in C 60 [7] have provided a great deal of valuable information about the T-R energy levels of the guest molecules and their interaction potentials with C 60 . The physical properties of hydrogen endofullerenes have also been probed by nuclear magnetic resonance [1,8,9]. What has enabled the experimental investigations is the synthetic organic breakthrough, known as 'molecular surgery', by Komatsu and co-workers [10,11] for producing macroscopic amounts of H 2 @C 60 and HD@C 60 .…”

Molecular surgical synthesis of H 2 @C 60 : recollections

“…But, much more detailed and sophisticated studies on the NMR of this molecule have been conducted by the group of Levitt and co-workers [7]. The molecular hydrogen inside the fullerene cage gives us a good opportunity to study its spin chemistry and physics, as well as NMR, which have been carried out actively by the group of Turro and co-workers [8].…”