MAS spectra of (inset) as a function of temperature. All experiments were recorded at a spinning frequency and a field of , using a spin echo sequence of duration to suppress the exohedral signal.
Solid lines: static spectra of as a function of temperature, recorded with a solid echo sequence of duration at a field of . Dotted lines: best fit simulations using a temperature-dependent biaxial dipole-dipole coupling (see Fig. 8).
Signal intensity vs temperature , as determined from static spectra at a field of . The behavior is consistent with the Curie law, indicating no evidence of ortho-para interconversion.
(a) Magic-angle-spinning spectra of as function of temperature, using single-pulse excitation at a field of . The broad feature and the narrow spinning sidebands are from the exohedral and endohedral protons, respectively. (b) Estimates of the endohedral spinning sideband amplitudes. (c) Best fits of the spinning sideband amplitudes to simulations using a temperature-dependent biaxial dipole-dipole coupling (see Fig. 8) and a chemical shift anisotropy interaction. The chemical shift anisotropy interaction was fixed at and , with the same principal axis system as the DD interaction.
spin-lattice relaxation time constants for as a function of temperature . Black squares: of a static sample, in a magnetic field of Grey diamonds: of a static sample, in a magnetic field of . Circles: of a magic-angle-spinning sample (spinning ), in a magnetic field of .
spin-lattice relaxation time constants for as a function of inverse temperature . Black squares: of a static sample, in a magnetic field of . Grey diamonds: of a static sample, in a magnetic field of . Solid line: best fit of the data to Eq. (27), using . Dashed line: best fit of the data to Eq. (27), using .
Hypothetical rototranslational energy level diagram for molecular hydrogen inside ATOCF cavity. (a) Rotational energy levels and rotational degeneracies of a free rotor. (b) Splitting of the rotational state due to translational quantization in a spherical box. Only the two lowest levels are shown. The states are labeled , where is the rotational sublevel quantum number and are translational quantum numbers. The total degeneracies (both translational and rotational) are displayed. (c) Splitting of the translational excited state due to cavity elongation. The translational excited state levels are labelled , where to indicate quenching of orbital angular momentum and translational polarization along the cavity axis system. (d) Splitting of each translational state due to a nonisotropic rotational potential. The energy levels are labelled with to indicate quenching of rotational angular momentum and rotational polarization along the principal axis system of the rotational potential.
(a) Dipole-dipole coupling constant and confidence limits as a function of temperature for , determined from the static spectra (squares) and the MAS spectra (circles). (b) Dipole-dipole biaxiality and confidence limits as a function of temperature for , determined from the static spectra (squares). The dashed lines are the best theoretical fits, using , , and while imposing .
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