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.

Articles you may be interested in 194306 (2014) Nuclear spin conversion of water inside fullerene cages detected by low-temperature nuclear magnetic resonance The water-endofullerene H 2 O@C 60 provides a unique chemical system in which freely rotating water molecules are confined inside homogeneous and symmetrical carbon cages. The spin conversion between the ortho and para species of the endohedral H 2 O was studied in the solid phase by lowtemperature nuclear magnetic resonance. The experimental data are consistent with a second-order kinetics, indicating a bimolecular spin conversion process. Numerical simulations suggest the simultaneous presence of a spin diffusion process allowing neighbouring ortho and para molecules to exchange their angular momenta. Cross-polarization experiments found no evidence that the spin conversion of the endohedral H 2 O molecules is catalysed by 13 C nuclei present in the cages.

The quantum dynamics of a hydrogen molecule encapsulated inside the cage of a C 60 fullerene molecule is investigated using inelastic neutron scattering (INS). The emphasis is on the temperature dependence of the INS spectra which were recorded using time-of-flight spectrometers. The hydrogen endofullerene system is highly quantum mechanical, exhibiting both translational and rotational quantization. The profound influence of the Pauli exclusion principle is revealed through nuclear spin isomerism. INS is shown to be exceptionally able to drive transitions between ortho -hydrogen and para -hydrogen which are spin-forbidden to photon spectroscopies. Spectra in the temperature range 1.6≤ T ≤280 K are presented, and examples are given which demonstrate how the temperature dependence of the INS peak amplitudes can provide an effective tool for assigning the transitions. It is also shown in a preliminary investigation how the temperature dependence may conceivably be used to probe crystal field effects and inter-fullerene interactions.

Nuclear magnetic resonance (NMR) techniques are extensively used in many areas of basic and clinical research, as well as in diagnostic medicine. However, NMR signals are intrinsically weak, and this imposes substantial constraints on the amounts and concentrations of materials that can be detected. The signals are weak because of the low energies characteristic of NMR and the resulting very low (typically 0.0001-0.01%) polarization of the nuclear spins. Here, we show that exposure to very low temperatures and high magnetic fields, in conjunction with nanoparticle-mediated relaxation enhancement, can be used to generate extremely high nuclear polarization levels on a realistic timescale; with copper nanoparticles at 15 mK and 14 T, (13)C polarization grew towards its equilibrium level of 23% with an estimated half-time of about 60 hours. This contrasts with a (13)C half-time of at least one year in the presence of aluminium nanoparticles. Cupric oxide nanoparticles were also effective relaxation agents. Our findings lead us to suspect that the relaxation may be mediated, at least in part, by the remarkable magnetic properties that some nanoparticle preparations can display. This methodology offers prospects for achieving polarization levels of 10-50% or more for many nuclear species, with a wide range of potential applications in structural biology and medicine.