Crystallographic Data for Some New Type II Clathrate Hydrates 1

The absorptivity of tetrahydrofuran clathrate hydrate in the range 70-7 cm-' has been measured a t several temperatures in the range 17--80 K. There are two broad bands with maxima a t 25 and 38 cm-I which are due to the rotational oscillations of tetrahydrofuran molecules in their cages. The integrated absorptivity yields an effective dipole moment for the oscillation of 1.63 D, which isclose to the gas-phase value. The negativesecond moment of the absorptivity yields thecontribution0.105 i -0.007 to thelowfrequency refractive index, in good agreement with a less accurate value from dielectric measurements. The orientalional disorder of the water molecules causes a distribution of potentials hindering the rotational oscillations of the guest molecules, and a detailed analysis of the shapes of the bands yields directly the distribution of force constants for the oscillations. friquence, ce q i~i est en accord avec la valeur moins precise obtenue par mesures dielectriques. Le desordre d'orientation des niolicules d'eau provoque ilne distribution de potentiels perturbant les oscillations de rotation des molCcules-hBtes. Une analyse dttaillee de la forme des bandes a permis d'obtenir directenlent la distribution des constantes de force pour les oscillations.

Section: ( I ) Integrated Absorptivitymentioning

confidence: 95%

The Rotational Oscillations of Tetrahydrofuran in Tetrahydrofuran Clathrate Hydrate

Klug¹,

Whalley²

1973

“…Structure I1 melting point of 4.38 "C at an approximate comhydrates are formed by dimethyl ether (I), by position of THF.16H20. Several nugatory irtrimethylene oxide, tetrahydrofuran, 2,5-dihydrofuran, propylene oxide, and 1,3-dioxolane (4, 5) and by acetone (6, 7) and cyclobutanone (5,8). With the exception of the study by Glew and Rath (9) of the variable composition of ethylene oxide hydrate, there appears to be no accurate information about the degree of occupancy of the cages or the heat of formation of these hydrates.…”

Section: Canadian Journal O F Chemistry 49 2691 (1971)mentioning

confidence: 99%

Composition of Tetrahydrofuran Hydrate and the Effect of Pressure on the Decomposition

Gough¹,

Davidson²

1971

150

103

The decomposition temperature of the structure I1 clathrate hydrate of tetrahydrofuran has been followed to pressures above 3 kbars, where the hydrate is found to decompose incongruently in the regions of stability of ices I11 and 11, with no evidence of formation of a denser hydrate. From measurements of liquid solution densities and of volume changes at hydrate decomposition the density of the hydrate was obtained. Comparison with the X-ray lattice dimension indicates that at least 98% of the large cages are occupied, the principal uncertainty being in the lattice parameter. A similar treatment is made of Tammann and Krige's data (23) for chloroform hydrate. Analysis of the results of 18 composition determinations of structure I1 hydrates shows no statistical evidence of departure from the ideal composition of 17 mol of water per mol of hydrate former.La temperature de dCcomposition du clathrate hydrate de structure I1 du tktrahydrofuranne a Ct C suivie jusqu'a des pressions superieures a 3 kbar; il a kt6 trouve que dans ces regions de stabilitk des glaces I11 et I1 l'hydrate se decompose d'une f a~o n aberrante sans preuve de formation d'un hydrate plus dense. A partir des mesures de densite des solutions liquides et des changements de volume a la dCcomposition de I'hydrate, il a Ct C possible d'obtenir la densite de l'hydrate. La comparaison avec la dimension du reseau obtenu par rayons X indique qu'au moins 98% des grandes cages sont occupees, ]'incertitude principale &ant dans le parametre retuculaire. A partir des donnCes de Tamman et Krige une analyse similaire est effectuee pour le chloroforme hydrate. Les analyses des resultats se rapportant a 18 determinations de composition d'hydrates ayant la structure I1 ne montrent aucune preuve statistique d'ecart par rapport la composition idkale de 17 mol d'eau par mole de gCnCrateur d'hydrate. Canadian Journal o f Chemistry, 49, 2691 (1971)Among the diverse molecules now known to therefore most easily distinguished from ice and form "gas hydrates" of von Stackelberg's has been chosen for further study. structures I and I1 (1) are a number of ethers and Tetrahydrofuran hydrate was first reported ketones which are distinguished from the classical by Palmer (10) who found from melting point us. gas-hydrate formers by their solubility in water concentration curves a composition of between and by the possibility of hydrogen bonding with 13 and 16 molecules of water per molecule of the host water molecules bf the clathrate struc-tetrahydrofuran (THF), with a probable formula tures. Ethylene oxide (2, 3) and trimethylene of THF.14H20. Erva (1 1) obtained a congruent oxide (4) form hydrates of type I. Structure I1 melting point of 4.38 "C at an approximate comhydrates are formed by dimethyl ether (I), by position of THF.16H20. Several nugatory irtrimethylene oxide, tetrahydrofuran, 2,5-dihydrofuran, propylene oxide, and 1,3-dioxolane (4, 5) and by acetone (6, 7) and cyclobutanone (5,8). With the exception of the study by Glew and Rath (9) of the variable composition o...

“…The mean activation energies for orientation of ethylene oxide and tetrahydrofuran are less than 1 kcal/mol. For these and such other polar guest molecules (27) as trimethylene oxide (12), propylene oxide (12), dihydrofuran (12), acetone (28), and dioxolane (29) in structure I1 hydrates and methyl chloride (30) in structure I, the contribution of the guest molecule to the permittivity shows no significant departure at temperatures down to 77 to 100 O K from that expected for isotropic rotation, or the equivalent contribution to the permittivity from reorientation between two or more symmetrically disposed sites of the same energy. The electrostatic fields of the water molecules therefore appear to introduce no great anisotropy at high temperatures.…”

Section: Electrostatic Fields Of the Water Moleculesmentioning

confidence: 99%

The Motion of Guest Molecules in Clathrate Hydrates

Davidson¹

1971

The reorientation of molecules encaged in hydrates of structures I and I1 is controlled mainly by the cage geometry and the electrostatic fields of the water molecules. Departure from spherical symmetry of the short-range interactions between the guest molecule and the water molecules which form the cages leads to preferred orientations which become increasingly occupied at low temperatures. A modified Lemard-Jones potential energy calculation shows the preferred positions of C1, to lie offcenter and close to the equatorial plane of the tetrakaidecahedral cage with the C1 atoms near the axial planes of symmetry. In an attempt to account for the remarkable reorientational freedom of polar guest molecules, the electrostatic fields of the water molecules are treated in detail. It is shown that the geometry of the cages causes the sum of the fields of the cage water dipoles to almost vanish at points near the cage center, but that this is not true of the resultant quadrupolar fields. It is suggested that the quadrupolar fields are also relatively small because the hydrogen bonding to four neighbors considerably reduces the quadrupole moment of the water molecule. The orientational disorder of the water molecules results in a wide distribution of resultant electrostatic fields in different cages. This appears to be the origin of the great width of the dielectric absorption observed at low temperatures in tetrahydrofuran hydrate and of the wide temperature range of variation of the proton magnetic resonance (p.m.r.) line width in tetrahydrofuran deuterate.La rdorientation des mol6cules encagkes dans des hydrates de structures I et I1 est principalement contrBlde par la gComCtrie de la cage et les champs electrostatiques des molCcules d'eau. L'ecart a la symktrie sphkrique des interactions a courte distance entre les molbules d'eau qui forment les cages et la molbule qui y est emprisonnee conduit a des orientations prefkrentielles qui deviennent plus frdquentes a basse temperature. Un calcul d'knergie potentiel de Lennard-Jones modifid montre que les positions preferentielles de CI, sont decentrkes et proches du plan equatorial de la cage tetrakaidecahedrale avec les atomes de C1 proches des plans axiaux de symetrie. Dans une tentative de rendre compte de la liberte remarquable de reorientation des molbules polaires emprisomCes, les champs dlectrostatiques des mol6cules d'eau sont trait& en detail. On montre que la gkomktrie des cages entrainent la somme des champs dus aux dipBles aqueux de la cage a presque s'annuler en des points proches du centre de la cage, mais ceci n'est pas vrai des champs quadrupolaires resultants. On suggere que les champs quadrupolaires sont aussi relativement petits parce que la liaison hydrogene avec quatre voisins reduit considerablement le moment quadrupolaire de la molCcule d'eau. Le desordre d'orientatlon des mol6cules d'eau entraine une distribution Btendue des champs Blectrostatiques resultant dans les differentes cages. Ceci apparait etre a I'origine de la grande largeur d'absorption...