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 ±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.