New Clathrate Hydrate Structure: High‐Pressure Tetrahydrofuran Hydrate with One Type of Cavity

Abstract

Since the last decade interest in clathrate hydrates has increased mainly because of the finding of immense deposits of hydrocarbon gas hydrates (clathrates) in the Earth's crust. [1,2] For scientists, interest is augmented by prospective investigations into the types of water arrangement in supramolecular structures (water being the most abundant chemical compound on earth). Herein we report on a new type of polyhedral clathrate framework, which was detected in a high-pressure [D 8 ]tetrahydrofuran hydrate. Its peculiarity consists in there being only one type of framework-building polyhedral cavity.Clathrate hydrates are inclusion compounds where the host is a polyhedral framework built of hydrogen-bonded water molecules in which guest molecules are trapped. Hostguest interactions are usually van der Waals in nature. Structures of clathrate hydrates are classified by the type of water framework, which is considered to be a three-dimensional packing of polyhedral cages with tetra-, penta-, and hexagonal faces, oxygen atoms as vertices, and hydrogen bonds as edges. The cages can act as hosts to a variety of guest molecules, from inert gases to large organic molecules (including those with hydrophilic groups). The shape and size of the guest molecules define the structural type of hydrate formed. Among the clathrate hydrates which arise under ambient pressure, three structural types are prevalent: cubic structures I and II, and hexagonal structure III (structure H), and these have been considered in detail in the literature. [3][4][5] Reports have appeared on bromine hydrate, which was shown to have a structure previously found only in semiclathrate hydrates, [6] and the hydrate of dimethyl ether, which was found to have a quite new structure.[7] For a long time the question of the effect of pressure on the structure of the clathrate hydrate formed remained open. It was known that under a pressure of hundreds of MPa, high-pressure phases existed in almost all the water-guest systems studied, though information on their structures was lacking. [8][9][10][11][12] Systematic studies of clathrate hydrate structures carried out in situ under high pressures were developed only during the last few years. In 2002, two new structural types of clathrate hydrates were identified in water-methane and water-argon systems under high pressure. [13,14] Hydrates of the structural type first found in methane hydrate were then also found in waternitrogen and water-argon systems. [15,16] The hydrate of the structural type first found in argon hydrate was then also found in the water-nitrogen system.[15] The characteristic feature of the structures under ambient pressure was the presence of several types of polyhedral cages packed in the water framework without any noticeable distortion of the lengths and angles of the hydrogen bonds. In contrast, the new high-pressure structures are built from one type of spacefilling polyhedral cage (in methane hydrate they can be considered as a system of one-dimensional channels). Such polyhedron...

Phase formation and structure of high-pressure gas hydrates and modeling of tetrahedral frameworks with uniform polyhedral cavities

“…Though our results on the topology of hypothetical space-fillings were incomplete (vide infra), it proved to be useful for crystal chemical applications and prognosis. This was confirmed by our discovery of OS III type of high-pressure gas hydrate exemplified by THF hexahydrate [38], the starting model for structure solution being chosen from our set of the space-fillings (packing No. 16) [98].…”

“…The low-pressure hydrate has the CS II type structure [14]; it transforms into the CS I hydrate at 2.85 kbar [61]. At $6 kbar, the formation of the hydrate is observed, which has a new structure designated above as OS III (figure 7) [38].…”

“…For a long time the only exceptions known were the sodalite type hydrates [20,29] containing uniform water cages (not taking into account ices Ih, Ic and II). However, recent structural studies of high-pressure hydrates of methane [36], argon [37], and THF [38] revealed new examples of clathrate hydrates (table 1) with the water frameworks constructed from the uniform cages. Note that the cavities in 2Ar Á 6H 2 O [37] and THF Á 6H 2 O [38] correspond to space-filling simple polyhedra, similarly to the situation observed for the truncated octahedra in the sodalite structure.…”

“…However, recent structural studies of high-pressure hydrates of methane [36], argon [37], and THF [38] revealed new examples of clathrate hydrates (table 1) with the water frameworks constructed from the uniform cages. Note that the cavities in 2Ar Á 6H 2 O [37] and THF Á 6H 2 O [38] correspond to space-filling simple polyhedra, similarly to the situation observed for the truncated octahedra in the sodalite structure. Following the above notation and numeration of the hydrate frameworks [1], we shall call the new structures orthorhombic structure II (OS II, figure 5), tetragonal structure IV (TS IV, figure 6) and orthorhombic structure III (OS III, figure 7), respectively.…”

“…We collected diffraction data from a powder sample of the acetone clathrate hydrate (CH 3 ) 2 CO Á 6H 2 O quenched from P ¼ 800 MPa at T ¼ 78 K. The X-ray diffraction pattern showed a considerable structural similarity between it and the clathrate hydrate THF Á 6H 2 O (the OS III type) [38]. However, acetone hexahydrate has a lower symmetry (space group P2 1 2 1 2 1 as compared to Pnma for THF hexahydrate) due to guest molecule ordering within the cavity ( figure 13).…”