S. K. Garg

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.

NMR behavior of the clathrate hydrate of tetrahydrofuran. I. Proton measurements

Abstract The motion of guest THF and host water molecules has been studied to 2°K by proton CW measurements and by proton T1 measurements of the THF-H2O, THF-d8-H2O, and THF-1320 systems. Reorientation rates of THF and water molecules and the corresponding activation energies (0.92 and 7.2 kcal/ mole, respectively) agree well with dielectric results, as does the NMR evidence of a very broad distribution of orientational correlation times of THF molecules. The latter is attributed to the variability of electrostatic fields in different cages arising from disordered orientations of the water molecules. Second moments show that THF molecules effectively undergo isotropic reorientation near 250°K and become rigid only below 10°K. The exponential magnetization recovery curves observed at all temperatures show guest and host protons to have common spin temperatures. T1ϱ measurements of THF-d8·17H2O suggest that both reorientation and diffusion of water molecules contribute to relaxation, the former being dominant at lower temperatures in contrast to the case in ice Ih.

NMR behavior of the clathrate hydrate of tetrahydrofuran. II. Deuterium measurements

Deuterium NMR lineshapes, spin-echo signal shapes, and spin-lattice and spin-spin relaxation times of the hydrate of perdeuterated tetrahydrofuran (THF-d8·17H2O) were measured between 6 and 277 K. Below about 20 K the D quadrupole coupling constant for encaged THF-d8 is 173 kHz. At higher temperatures guest-molecule reorientation occurs with an effective activation energy of 0.92 kcal/mole, in agreement with previous proton NMR and dielectric results. The D measurements clearly show a degree of inequivalence among the preferred orientations of the encaged molecules up to about 200 K. This anisotropy disappears at higher temperatures, where reorientation of water molecules leads to an averaging of the 16-hedral cage coufigurations to the tetrahedral symmetry shown by X-ray studies. The D quadrupole coupling constant and asymmetry parameter of THF·17D2O are 215 kHz and 0.11, respectively, at 37 K. An activation energy of 7.2 kcal/mole for reorientation of D2O molecules is derived from T1 measurements.

Nuclear magnetic resonance and relaxation properties of solid SF6

Line shape and relaxation studies of the low temperature solid phase of SF6 stable below 94 K show the presence of two inequivalent lattice sites, one‐quarter of the SF6 molecules being characterized by a barrier to isotropic reorientation of 1.9 and the remainder by a barrier of 2.8 kcal/mole. The chemical shift anisotropy (σ∥−σ⊥) of the S–F bond in SF6 is found to be 300±50 ppm. The sensitivity of line shape to rf field level in the cubic phase between 94 and 135 K is adequately described by the Goldman–Provotorov theory of ’’saturation narrowing’’. The activation energy for translational diffusion in this phase is 9.8 kcal/mole.

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.

Reorientation and diffusion of water molecules in xenon hydrate and other structure.

In contrast to ice Ih, in which the translational diffusion of water molecules is faster than their reorientation, the good correlation between proton magnetic resonance line narrowing and dielectric relaxation rates of the water molecules which form the structure I clathrate hydrate lattice suggests that reorientation is the faster process. This is certainly so for the hydrate of ethylene oxide-d4, for which two separate line-narrowing regions are observed. Both processes depend on the nature of the guest molecule.

Methyl group tunneling effects on 1H NMR line shapes of solids at low temperatures

Methyl group tunneling side bands have been observed in the proton NMR line shapes of solid dimethyl ether, propane, propane‐2, 2d2, and dimethyl sulfide at low temperatures. The line shape theory after Apaydin and Clough was used to derive tunneling frequencies. Assuming a threefold, sinusoidal potential barrier for methyl group rotation, application of the Hecht and Dennison relation yields barrier heights of 3.85, 3.58, 3.60, and 3.51 kcal/mole, respectively, for dimethyl ether, propane, propane‐2, 2d2, and dimethyl sulfide.

Analysis of nmr lineshapes of rigid-lattice multispin systems. III. Bond length and 19F chemical shielding in SF6, SeF6, and TeF6☆

Abstract Low-temperature 19 F NMR lineshapes were recorded for the clathrate deuteriohydrates of SF6, SeF 6 and TeF 6 , the formation of a TeF6 clathrate being reported for the first time. Analysis of the rigid-lattice lineshapes of these six-spin systems gave the following bond distances (±0.01 A) and 19 F chemical shielding anisotropies: 1.58 A, 310±30 ppm for SF 6 ; 1.68 A, 370±20 ppm for SeF 6 ; 1.84 A 215±25 ppm for TeF 6 . The corresponding spin-rotation tensors were calculated by the method of Flygare.