S. R. Gough

Laboratory analysis of a naturally occurring gas hydrate from sediment of the Gulf of Mexico

Intact natural gas hydrate of structure II made up more than 80% of the water present in nearbottom core material recovered from the Gulf of Mexico. Solid-state carbon-13 nuclear magnetic resonance with magic-angle spinning gave resolved lines from ethane, propane and isobutane and apparently from methane in the two sizes of cage in the hydrate lattice. Low-temperature dielectric loss peaks were assigned to reorientation of encaged propane, isobutane, H2S and CO2 molecules.

Ordering of guest-molecule dipoles in the structure I clathrate hydrate of trimethylene oxides

Abstract The rotational mobility of encaged trimethylene oxide (TMO) molecules was studied down to 1.8°K by sub-MHz dielectric measurements of the structure I H2O clathrate and by proton magnetic resonance measurements of the corresponding D2O clathrate. The results indicate that below a transitional temperature range about 105°K most TMO dipoles assume parallel alignment along the 4 axes of the cages. Below the transition the proton second moment suggests the presence of hindered rotation of TMO about its polar axis until the rigid-lattice condition is reached below 5°K. Some residual very broad dielectric absorption (activation energy 2.1 kcal/mole) persists to very low temperatures. Guest-guest and guest-host interaction energies are calculated for simple models

A wide‐line NMR study of reorientation of some spherical‐top molecules enclathrated in water

NMR spectra at temperatures down to 1.8 K are given for the 19F resonance of SF6 and CF4 in H2O and D2O clathrates and for the 1H resonance in CH4–D2O and CH4–tetrahydrofuran‐d8–D2O clathrates. The second moments correspond effectively to isotropic reorientation of encaged SF6, CF4, and CH4 molecules at temperatures above 13, 22, and 4 K, respectively. The CH4 spectra are only slightly broadened at 1.8 K. For SF6 and CF4 a low‐temperature transition in second moment is characterized by the superposition on the rigid lattice band of a narrow component whose intensity increases progressively with rise of temperature. This ’’apparent phase‐change effect,’’ after Resing, is attributed to a very broad distribution of reorientational correlation times, here associated with orientational disorder of the water molecules of the lattice. The behavior during the transition agrees with a model which assumes a Gaussian distribution of activation energies about a mean value of 207 cal/mol for SF6 and 360 cal/mol for CF4.

Some structural and thermodynamic studies of clathrate hydrates

X-ray and neutron diffraction studies show argon and krypton to preferentially form clathrate hydrates of structure II, rather than structure I as previously assumed; methane and hydrogen sulphide do form structure I. Re-examination of solid-solution thermodynamic theory shows that structure II is basically the more stable; structure I is generally formed only when the guest molecule is in the size range that favours occupancy of the 14-hedral over the 12-hedral cages. For molecules too large to enter the 12-hedra the relative stability of structure II is greatest at 0°C, in agreement with the observed sequence of change of stability of cyclopropane hydrate: I to II at -16° and II to I at 1.5°. Carbon dioxide hydrate is observed to decompose on prolonged standing at 105K in accord with the low-temperature instability predicted by Miller.

Some physical and thermophysical properties of clathrate hydrates

Oxygen, nitrogen and air, like argon and krypton (1), are found to preferentially form gas hydrates of structure II, rather than structure I as previously expected for gas hydrates of small guest molecules. Lattice parameters from X-ray diffraction are given in the Table.

Dielectric and 13C NMR Studies of Sulfur Dioxide-Hydroquinone Clathrates*

Dielectric measurements of SO2 quinol clathrates show that the reorientation of encaged SO2 molecules is very rapid and depends greatly on the degree of cage occupancy. For aβ-quinol sample of cage occupancyθ = 0.57, the reorientation rate was ∼ 1 MHz at 6 K, with a reorientational activation energy of 673 J/mol. For a sample identified by13C NMR asα-quinol, and for aβ-quinol sample with most cages filled with Xe, SO2 reorientation rates are even greater, with activation energies of only some tens of J/mol. The low temperature dielectric studies show that some ethanol may be enclathrated inβ-quinol recrystallized from this solvent. The13C NMR spectra confirm the X-ray results that the lattice becomes distorted with increased SO2 content.