Theory and spectroscopy of dihydrogen endofullerenes

Abstract

Recent advances in synthetic chemistry made available a new class of fascinating
compounds in which a molecule of hydrogen is encased inside the hollow cavity of
the highly symmetric fullerene C60. There is a great interest in studying endohedral
fullerene complexes owing to the rigidity, symmetry, homogeneity and isolation provided
by the carbon cages. On one side these properties result in very detailed and wellresolved
spectra even in the condensed phase. On the other side such supermolecules
are a unique real-world example of a quantum rotor confined in an almost spherical trap,
whose dynamics can be directly and accurately treated from first principles. The study
of the quantum dynamics and the analysis of the spectroscopic observations of H2@C60
provide a stepping stone for the characterization of the carbon-hydrogen interaction in
curved nanocarbons.

In this work symmetry is employed to simplify the analytical treatment of the dynamics
of the endohedral hydrogen, by expanding the non-bonding confining potential
in terms of spherical multipoles and by using a spherical basis to represent the effective
rotational-translational Hamiltonian. The thesis features the first study of an endohedral
hydrogen fullerenes, H2@C60, by infrared spectroscopy . The quantization of the
translational motion, the high rotational freedom and rotational-translational couplings
clearly show up in the infrared spectra. The successful assignment of the IR peaks to
transitions between the quantum states of the confined hydrogen by means of the presented
theoretical model has lead to the first accurate determination of the molecular
vibro-rotational parameters and effective hydrogen-cage potentials in H2@C60.

The nuclear spin dynamics is very sensitive to the local environment around the
hydrogen molecules. Nuclear magnetic resonance spectroscopy at cryogenic temperatures
shows that the symmetry at the center of the cages is reduced by solid state effects
and/or occluded impurities. The observations of small but not yet completely understood
discrepancies in the lineshapes and relaxations of two samples of H2@C60 stress
the influence of the preparation, storage and manipulation on the nuclear spin dynamics
of endohedral hydrogen fullerenes.

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Identifiers

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

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