Giant Splitting of the Hydrogen Rotational Eigenenergies in the C_{2} Filled Ice.

Hydrogen hydrates exhibit 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 icelike 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 good 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 van der Waals interactions compared to other hydrogen hydrates.