Nuclear Magnetic Resonance of Hydrogen Molecules Trapped inside C₇₀ Fullerene Cages

Abstract

We present a solid-state NMR study of H2 molecules confined inside the cavity of C70 fullerene cages over a wide range of temperatures (300 K to 4 K). The proton NMR spectra are consistent with a model in which the dipole–dipole coupling between the ortho-H2 protons is averaged over the rotational/translational states of the confined quantum rotor, with an additional chemical shift anisotropy δHCSA=10.1 ppm induced by the carbon cage. The magnitude of the chemical shift anisotropy is consistent with DFT estimates of the chemical shielding tensor field within the cage. The experimental NMR data indicate that the ground state of endohedral ortho-H2 in C70 is doubly degenerate and polarized transverse to the principal axis of the cage. The NMR spectra indicate significant magnetic alignment of the C70 long axes along the magnetic field, at temperatures below ∼10 K.

Symmetry-breaking in the endofullerene H2O@C60 revealed in the quantum dynamics of ortho and para-water: a neutron scattering investigation

“…The first such physical investigation of H 2 O@C 60 was undertaken in 2012 at low temperature using three forms of spectroscopy; far-infrared, inelastic neutron scattering (INS) and solid state nuclear magnetic resonance (NMR). 4 Analogous to earlier investigations on other small molecule endofullerenes such as H 2 @C 60 , [5][6][7][8][9][10][11][12][13][14][15][16] these first experiments revealed the quantum rotation of the water molecule and conclusively identified its nuclear spinisomers, ortho-H 2 O and para-H 2 O, in the various spectra. The existence of these two species is founded in the requirement for the total wavefunction of the water molecule to be antisymmetric upon exchange of two fermions, in this case the two 1 H nuclei.…”

NMR of molecular endofullerenes dissolved in a nematic liquid crystal

“…This leads to a contribution to the RCSA which is independent of the orientational ordering of the molecules with respect to the cage. Anisotropic shielding fields of this kind have been studied in detail for the endofullerene H 2 @C 70 , in which the the enclosing carbon cage has a permanent elongation 41 . The 19 F spectrum of HF@C 60 in the solid state also displays a signature of cage-induced CSA 3 .…”

EPR Study of NO radicals encased in modified open C60 Fullerenes

“…In contrast to encapsulated atoms, we have also to consider for the NO case a librational mode of the radical with respect to the cage axis. In a study of H 2 encapsulated in C60 or C70, the eigenstates of H 2 were determined numerically by invoking the appropriate 5-dimensional potential surface, describing translational and rotational degrees of freedom (Xu et al, 2009;Mamone et al, 2013). Lacking numerical values for the potential surface in our more complicated case, it is only possible to estimate typical values for the librational mode by approximating the interconversion between up/down (its z axis) of the radical axis in a potential well of 80 meV (645 cm −1 ).…”