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, K_a, K_c to fully describe the rotational angular momentum of the quantum rotor, where J is the total angular momentum quantum number, while K_a and K_c 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.