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

TL;DR

This study demonstrates the conversion between water spin isomers within fullerene cages at room temperature, revealing quantum-rotor effects and enabling nuclear polarization enhancements observed via NMR.

Contribution

It reports the first observation of water spin-isomer conversion inside fullerenes at ambient conditions, highlighting quantum-rotor-induced nuclear polarization effects.

Findings

Spin-isomer conversion time: ~30 seconds for H2O@C60.

Detected nuclear polarization enhancements due to quantum rotor effects.

Conversion rates differ between isotopologues, indicating isotope dependence.

Abstract

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.