Despite being forbidden by quantum mechanics, the isomerization reaction of the nuclear spin isomers (NSI) of the water molecule (ortho- H2O and para-H2O) is known to proceed readily under the confinement of inert matrices and endofullerenes1,2. The mechanism is thought to proceed through intra- and inter-molecular hyperfine interactions, notably the nuclear spin-rotation and nuclear spin-spin couplings. Differences between interconversion kinetics reported for the various confinement media, as well as comparisons between calculated rates for the free H2O molecule in the gas-phase and under confinement can help understand the isotope and temperature dependence observed experimentally3.
A simple confined rotor model is used to describe confinement effects on the ro-translational eigenstates while the quantum relaxation model is used to describe the interconversion rates between the NSI of water. Advances towards a quantitative description of confinement effects on the mechanism and rates for interconversion between the NSI of H2O will be examined in detail.

(1) Suzuki, H.; Nakano, M.; Hashikawa, Y.; Murata, Y. Rotational Motion and Nuclear Spin Interconversion of H2 O Encapsulated in C60 Appearing in the Low-Temperature Heat Capacity. J. Phys. Chem. Lett. 2019, 10 (6), 1306–1311. https://doi.org/10.1021/acs.jpclett.9b00311.
(2) Meier, B.; Kouřil, K.; Bengs, C.; Kouřilová, H.; Barker, T. C.; Elliott, S. J.; Alom, S.; Whitby, R. J.; Levitt, M. H. Spin-Isomer Conversion of Water at Room Temperature and Quantum-Rotor-Induced Nuclear Polarization in the Water-Endofullerene H 2 O @ C 60. Phys. Rev. Lett. 2018, 120 (26), 266001. https://doi.org/10.1103/PhysRevLett.120.266001.
(3) Chapovsky, P. L.; Mamrashev, A. A. Nuclear Spin Conversion in H 2 O Revisited. Phys. Rev. A 2021, 104 (5), 052816. https://doi.org/10.1103/PhysRevA.104.052816.