Giant splitting of the hydrogen rotational eigenenergies in the C₂ filled ice

Hydrogen hydrates present a rich phase diagram influenced by both pressure and temperature, with the so-called C₂ 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 Schr\"odinger's equation for a quantum H₂ rotor in the C₂ crystal field at finite temperature, generated through Density Functional Theory. Our calculations reveal a giant energy splitting relative to the magnetic quantum number of 3. 2 meV for l=1. Employing inelastic neutron scattering, we experimentally measure the energy levels of H₂ within the C₂ 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 C₂ phase and indicate heightened induced-dipole interactions compared to other hydrogen hydrates.