Raman Measurements of Pure Hydrogen Clathrate Formation from a Supercooled Hydrogen–Water Solution

The nucleation and growth of a solid clathrate hydrate from the liquid phase is a process that is even less understood and more difficult to study than the nucleation of a solid phase from a pure liquid. We have employed in situ Raman spectroscopy to study the hydrogen–water supercooled solution undergoing clathrate formation at a pressure of about 2 kbar and temperature of 263 K. Raman light scattering detects unambiguously the H₂ molecules inside of clathrate crystallites, which change stoichiometry during growth. The spectral intensity of the hydrogen vibrational band shows the time evolution of the population of the large and small cages, demonstrating that, in the initial stages of clathrate formation, the occupation of the large cages is quite lower than its equilibrium value. From the measurement of the growth rate of the crystallites, we demonstrate that the growth of the clathrate in the liquid is a diffusion-limited process

Vibrational Modes of Hydrogen Hydrates: A First-Principles Molecular Dynamics and Raman Spectra Study

We have employed classically propagated molecular dynamics (MD), within the framework of density functional theory (DFT), to calculate vibrational spectral band of molecular hydrogen trapped in clathrate hydrate, with large-cage occupancy from 1 to 4, at ∼260 K and ∼2 kbar. The predicted vibrations, obtained by applying a state-of-the-art generalized gradient approximation (GGA) functional with nonlocal correlation (VdW-DF), reproduce satisfactorily our own accurate Raman spectra (at the same temperature and pressure conditions). We decomposed the MD-sampled vibrational band to individual peaks and assigned them to the vibration of H₂ molecules enclosed in small and large cages of SII hydrate. By summing the resulting spectral bands, we have demonstrated that the measured spectral response is a complex composition of signals originating from H₂ molecules experiencing different local, intracage environments