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H2O@C60 provides the almost unique opportunity to study water molecules in isolation without hydrogen bonding. However, unlike H2, H2O possesses an electric dipole moment, so the properties of the H2O@C60 complex are characterised by interactions between the electric dipoles in different cages. H2O is also a quantum rotor, which exhibits nuclear spin isomerism, possessing para (total nuclear spin I=0) and ortho (I=1) species.

The dynamics of the endohedral H2O is different than that of H2 trapped in C60 due to the lower symmetry of the H2O molecule. The endohedral H2O behaves like an asymmetric top and it requires quantum numbers J, Ka, Kc to fully describe the rotational angular momentum of the quantum rotor, where J is the total angular momentum quantum number, while Ka and Kc are the angular momentum quantum numbers in the prolate and oblate symmetric tops respectively. [1]

The energy levels of H2O@C60 (Figure 1) closely resemble that of the H2O in free space at low temperatures . This is because the trapped H2O are free to rotate and translate within their cages while being isolated from one another. The main difference between the energy levels of H2O@C60 and H2O in free space at low temperatures is the splitting of the ground state of ortho-H2O (101 state). [1] Similar sized splittings are also detected in other higher energy para-H2O and ortho-H2O states. This splitting is postulated to be due to the electric dipole of H2O causing Jahn-Teller distortion on the C60 cage.

The H2O@C60 exhibits nuclear spin conversion from ortho-H2O to para-H2O at low temperatures. This nuclear spin conversion is measured to have a half-life of ~14 hours at 4.2K. The mechanism behind this nuclear spin conversion at low temperature is still not fully understood. It is believed to be due to inter-fullerene and intra-fullerene interactions.

Figure 1. The energy level diagram based on recent INS results.


Figure 2. NMR experiment showing ortho to para conversion of H2O in the H2O@C60 sample at 4.2K and 0.86T over the course of 48 hours. The NMR signal decays over time because para-H2O is NMR silent unlike ortho-H2O. After 24 hours, 95% of the ortho-H2O are converted to para-H2O, and by the 36th hour of the experiment, all the ortho-H2O are fully converted to para-H2O.