Synthesis and properties of open fullerenes encapsulating ammonia and methane

Abstract

We describe the synthesis and characterisation of open fullerene (1) and its reduced form (2) in which CH₄ and NH₃ are encapsulated, respectively. The ¹H NMR resonance of endohedral NH₃ is broadened by scalar coupling to the quadrupolar ¹⁴N nucleus, which relaxes rapidly. This broadening is absent for small satellite peaks, which are attributed to natural abundance ¹⁵N. The influence of the scalar relaxation mechanism on the linewidth of the 1H ammonia resonance is probed by variable temperature NMR. A rotational correlation time of τ_c = 1.5 ps. is determined for endohedral NH₃, and of τ_c = 57 ± 5 ps. for the open fullerene, indicating free rotation of the encapsulated molecule. IR spectroscopy of NH₃@2 at 5 K identifies three vibrations of NH₃ (ν₁, ν₃  and ν₄ ) redshifted in comparison with free NH₃, and temperature dependence of the IR peak intensity indicates the presence of a large number of excited translational / rotational states. Variable temperature ¹H NMR spectra indicate that endohedral CH₄ is also able to rotate freely at 223 K, on the NMR timescale. Inelastic neutron scattering (INS) spectra of CH₄@1 show both rotational and translational modes of CH₄. Energy of the first excited rotational state (J = 1) of CH₄@1 is significantly lower than that of free CH₄.

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Identifiers

ORCID for Sally Bloodworth: orcid.org/0000-0003-2219-3635

ORCID for Karel Kouril: orcid.org/0000-0003-4884-1637

ORCID for Neil J. Wells: orcid.org/0000-0002-4607-5791

ORCID for Malcolm H. Levitt: orcid.org/0000-0001-9878-1180

ORCID for Richard J. Whitby: orcid.org/0000-0002-9891-5502

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