Symmetry-breaking in the endofullerene H₂O@C₆₀ revealed in the quantum dynamics of ortho and para-water: a neutron scattering investigation

Abstract

a Inelastic neutron scattering (INS) has been employed to investigate the quantum dynamics of water molecules permanently entrapped inside the cages of C 60 fullerene molecules. This study of the supramolecular complex, H 2 O@C 60 , provides the unique opportunity to study isolated water molecules in a highly symmetric environment. Free from strong interactions, the water molecule has a high degree of rotational freedom enabling its nuclear spin isomers, ortho-H 2 O and para-H 2 O to be separately identified and studied. The INS technique mediates transitions between the ortho and para spin isomers and using three INS spectrometers, the rotational levels of H 2 O have been investigated, correlating well with the known levels in gaseous water. The slow process of nuclear spin conversion between ortho-H 2 O and para-H 2 O is revealed in the time dependence of the INS peak intensities over periods of many hours. Of particular interest to this study is the observed splitting of the ground state of ortho-H 2 O, raising the three-fold degeneracy into two states with degeneracy 2 and 1 respectively. This is attributed to a symmetry-breaking interaction of the water environment.

Quantum Tunneling of Water in Beryl: A New State of the Water Molecule

“…INS spectra of water in beryl do not show transitions between the para-and ortho-spin isomer forms of water, which were observed in H 2 O@C 60 endofullerene [12,13], where water also demonstrates absence of HB and rotates almost freely, but did not exhibit evidence of tunneling. The absence of the transitions between the para-and ortho-states in the INS spectra of beryl is probably linked to the fact that water is not as free as in the case of fullerenes (it has a dipole moment perpendicular to the c-axis and it occupies 6-fold equivalent positions across the channel).…”

“…Confined water can be almost free [11], without hydrogen bonds (HB), and display significant quantum behavior. Inelastic neutron scattering (INS) studies have demonstrated that in fullerenes such water exhibits quantum transitions between the para-and ortho-states [12,13], and it has been suggested that quantum tunneling of water explains the fine structure of the optical spectra of beryl [14][15]. INS provides a unique method to study the vibrational and quantum tunneling dynamics of hydrogencontaining materials because of the anomalously large incoherent neutron scattering crosssection of protium (H-1).…”

Synthesis and Properties of Open Fullerenes Encapsulating Ammonia and Methane

“…INS spectrometers and IN1‐Lagrange is a triple‐axis spectrometer equipped with a large area graphite analyser. IN4c and IN1‐Lagrange have been described in earlier papers, IN6 is designed for quasi‐elastic and inelastic neutron scattering. It operates on a cold neutron source and provides good resolution at low energy transfer, accessing both neutron energy (NE) gain and NE loss components of the spectrum.…”

Probing Thermal Conductivity of Fullerene C60 Hosting a Single Water Molecule

“…More importantly, there is no physical bonding reaction between the encapsulated water molecule and carbon atoms and only nonbonding interaction between them are expected. [8][9][10][11][12][13] With the encapsulation of a single water molecule inside, fundamental properties of C 60 dramatically change, and even show completely novel and unique properties. [14][15][16][17] For example, Insulated by the outer spherical carbon shell, the encapsulated single water molecule shows an enhanced self-diffusion coefficient in comparison with that in bulk water.…”