Pressure-Induced Amorphization of Methane Hydrate: An Ab Initio Molecular Dynamics Study of Guest Molecule Solvation

Pressure-induced amorphization (PIA) of clathrate hydrates provides valuable insights into the fundamental properties of water and aqueous systems. We performed ab initio molecular dynamics (AIMD) simulations to elucidate the molecular-level mechanisms underlying the experimentally observed vibrational spectroscopic changes during PIA of methane hydrate (MH). The simulations successfully reproduced key experimental observations, including the blue-shift of C-H stretching vibrations with compression, the convergence of signals from small (S) and large (L) cages upon amorphization, and spectral broadening at high densities. By analyzing the correlation between vibrational spectra and methane hydration structures, we demonstrate that amorphization effectively averages the system density with respect to C-H stretching vibrations. Spectral decomposition conditioned on solvation number reveals that the broadening of C-H stretching peaks originates from the increased diversity of local solvation environments caused by cage collapse. At densities of 1.4-1.8 g cm^-3, the peak position exhibits clear positive correlation with the number of proximate water molecules, whereas this correlation is negligible when the cage structure is maintained at lower densities. Analysis of metastable amorphous states at low densities demonstrates that competing effects govern the C-H stretching vibrational frequency: the approach of first-nearest-neighbor water molecules (blue-shifting) and the weakening of hydrogen bond rigidity between surrounding water molecules (red-shifting). This solvation-state-based analysis represents a key advantage of MD simulations and reveals that focusing on the number of particularly close water molecules is essential for understanding the molecular picture of PIA.