Spin-isomer conversion of water at room temperature, and quantum-rotor-induced nuclear polarization, in the water-endofullerene H$_2$O@C$_{60}$

Water exists in two forms, para and ortho, that have nuclear spin states with different symmetries. Here we report the conversion of fullerene-encapsulated para-water to ortho-water. The enrichment of para-water at low temperatures is monitored via changes in the electrical polarizability of the material. Upon rapid dissolution of the material in toluene the excess para-water converts to ortho- water. In H$_2{}^{16}$O@C$_{60}$ the conversion leads to a slow increase in the NMR signal. In H$_2{}^{16}$O@C$_{60}$ the conversion gives rise to weak signal enhancements attributed to quantum-rotor-induced nuclear spin polarization. The time constants for the spin-isomer conversion of fullerene-encapsulated water in ambient temperature solution are estimated as 30$pm$4 s for the $^{16}$O-isotopologue of water, and 16$pm$3 s for the $^{17}$O isotopologue.