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 .

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.…”

5D quantum dynamics of the H2@SWNT system: Quantitative study of the rotational-translational coupling

“…Four of the SPFs sets used in the present work (ρ, φ, x and y) are represented in a Fourier method grid. However, a particular issue appears when representing the orientation of the H 2 , θ: due to the 1 sin 2 θ term that appears in Eq. (3), a singularity arises in the Hamiltonian for θ = 0, i.e., if the H 2 molecule is oriented parallel to the nanotube's axis.…”

“…5 The first five-dimensional study of hydrogen confined in carbon nanotubes, considering hydrogen's vibration, was later carried out by some of us. 13 This research field has been greatly benefited lately by a) Electronic mail: fermin.huarte@ub.edu the valuable contributions of Bačić and coworkers, who have made rigorous calculations of the hydrogen molecule in metalorganic frameworks (MOFs), 7 of hydrogen endofullerenes, 2 and of the HD molecule confined in a clathrate hydrate structure.…”

“…[1][2][3] This interest arises due to the unique phenomena that take place when a molecule is embedded in cavities within the nanometric scale, which may be seen as quasizero or quasi-one-dimensional spaces. Among these effects, we find distortions of the electronic structure and geometry of the species, 4 as well as changes in their dynamic behavior due to a strong translation-rotation coupling.…”

Nanolaboratories: physics and chemistry of small-molecule endofullerenes

“…Murata and co-workers report on kinetic and thermodynamic differences between H 2 , HD and D 2 when incorporated into an open-cage fullerene [12]. Rõõm and co-workers [13] provide a detailed exposition of the infrared spectroscopy of H 2 , HD and D 2 encapsulated in C 60 . The theoretical treatment of this problem requires solution of the quantum dynamics for a vibrating quantum rotor moving in a three-dimensional spherical potential.…”