Giant splitting of the hydrogen rotational eigenenergies in the C${}_2$ filled ice

Hydrogen hydrates present a rich phase diagram influenced by both pressure and temperature, with the so-called C${}_2$ phase emerging prominently above 2.5 GPa. In this phase, hydrogen molecules are densely packed within a cubic ice-like lattice and the interaction with the surrounding water molecules profoundly affects their quantum rotational dynamics. Herein, we delve into this intricate interplay by directly solving the Schrödinger’s equation for a quantum H${}_2$ rotor in the C${}_2$ crystal field at finite temperature, generated through Density Functional Theory. Our calculations reveal a giant energy splitting relative to the magnetic quantum number of $\pm$3.2 meV for $l=1$. Employing inelastic neutron scattering, we experimentally measure the energy levels of H${}_2$ within the C${}_2$ phase at 6.0 and 3.4 GPa and low temperatures, finding remarkable agreement with our theoretical predictions. These findings underscore the pivotal role of hydrogen–water interactions in dictating the rotational behavior of the hydrogen molecules within the C${}_2$ phase and indicate heightened induced-dipole interactions compared to other hydrogen hydrates.