3 research outputs found
Raman Measurements of Pure Hydrogen Clathrate Formation from a Supercooled HydrogenâWater 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 hydrogenâwater
supercooled solution undergoing clathrate formation at a pressure
of about 2 kbar and temperature of 263 K. Raman light scattering detects
unambiguously the H<sub>2</sub> 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 âŒ260 K and
âŒ2 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 H<sub>2</sub> 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 H<sub>2</sub> molecules experiencing
different local, intracage environments
Impact of the Condensed-Phase Environment on the TranslationâRotation Eigenstates and Spectra of a Hydrogen Molecule in Clathrate Hydrates
We
systematically investigate the manifestations of the condensed-phase
environment of the structure II clathrate hydrate in the translation-rotation
(TR) dynamics and the inelastic neutron scattering (INS) spectra of
an H<sub>2</sub> molecule confined in the small dodecahedral cage
of the hydrate. The aim is to elucidate the extent to which these
properties are affected by the clathrate water molecules beyond the
confining cage and the proton disorder of the water framework. For
this purpose, quantum calculations of the TR eigenstates and INS spectra
are performed for H<sub>2</sub> inside spherical clathrate domains
of gradually increasing radius and the number of water molecules ranging
from 20 for the isolated small cage to more than 1800. For each domain
size, several hundred distinct hydrogen-bonding topologies are constructed
in order to simulate the effects of the proton disorder. Our study
reveals that the clathrate-induced splittings of the <i>j</i> = 1 rotational level and the translational fundamental of the guest
H<sub>2</sub> are influenced by the condensed-phase environment to
a dramatically different degree, the former very strongly and the
latter only weakly