Infrared spectroscopy of small-molecule endofullerenes

Abstract

Hydrogen is one of the few molecules that has been incarcerated in the molecular cage of C 60 to form the endohedral supramolecular complex H 2 @C 60 . In this confinement, hydrogen acquires new properties. Its translation motion, within the C 60 cavity, becomes quantized, is correlated with its rotation and breaks inversion symmetry that induces infrared (IR) activity of H 2 . We apply IR spectroscopy to study the dynamics of hydrogen isotopologues H 2 , D 2 and HD incarcerated in C 60 . The translation and rotation modes appear as side bands to the hydrogen vibration mode in the mid-IR part of the absorption spectrum. Because of the large mass difference of hydrogen and C 60 and the high symmetry of C 60 the problem is almost identical to a vibrating rotor moving in a three-dimensional spherical potential. We derive potential, rotation, vibration and dipole moment parameters from the analysis of the IR absorption spectra. Our results were used to derive the parameters of a pairwise additive five-dimensional potential energy surface for H 2 @C 60 . The same parameters were used to predict H 2 energies inside C 70 . We compare the predicted energies and the low-temperature IR absorption spectra of H 2 @C 70 .