Clathrate hydrates crystal structures

G.S. Nolas, C.A. Kendziora, Raman scattering study of Ge and Sn compounds with type-I clathrate hydrate crystal structure. Phys. Rev. B 62, 7157 (2000)... 

Potential Energy and Free Energy of Various Clathrate Hydrate Crystal Structures... 

Tabushi et al. (1981) suggested that the 15-hedron (51263) is absent from Figure 2.5 and in all clathrates except bromine due to an unfavorable strain relative to the other cavities in si and sll. In their review of simple and combined cavities, Dyadin et al. (1991) suggested that in addition to the cavities found in si, sll, and sH, there are 4258 and 51263 cavities. In Jeffrey s (1984) list of a series of seven hydrate crystal structures (Table 2.3), additional cavities to those found in si, sll, and sH are 51263, 4454,43596273, 4668. 

Structure identification and relative cage occupancies. The hydration number and relative cage occupation for pure components and guests were measured by Sum et al. (1997), Uchida et al. (1999), and Wilson et al. (2002). Raman guest spectra of clathrate hydrates have been measured for the three known hydrate crystal structures si, sll, and sH. Long (1994) previously measured the kinetic phenomena for THF hydrate. Thermodynamic sl/sll structural transitions have been studied for binary hydrate systems (Subramanian et al., 2000 Schicks et al., 2006). 

Equilibrium properties of the C02/sea-water system have been well researched from an experimental standpoint. In particular, the clathrate hydrate forming conditions T < 285K andP>4MPa) are well established. Several experiments have been performed under conditions mimicking the direct injection process and have attempted to study the dissolution rate of CO2 in seawater. Under direct injection conditions, the injected CO2 is in the form of a liquid droplet and a thin spherical shell of CO2 clathrate hydrate of structure I is observed to form around the CO2 drop, separating it from the sea water. The process of hydrate formation has many similarities with that of crystallization, i.e., it can be divided into a nucleation phase and a growth phase. For CO2 clathrates, the nucleation phase involves the formation of a... 

According to these authors all gas hydrates crystallize in either of two cubic structures (I and II) in which the hydrated molecules are situated in cavities formed by a framework of water molecules linked together by hydrogen bonds. The numbers and sizes of the cavities differ for the two structures, but in both the water molecules are tetrahedrally coordinated as in ordinary ice. Apparently gas hydrates are clathrate compounds. 

In the next section we shall give a brief account of the crystal structure of the hydroquinone clathrates and of the gas hydrates, as far as is needed for a proper understanding of the subsequent parts. The reader who is interested in the phenomenology of other clathrate compounds should consult one of the many review articles7,8 39 on inclusion compounds. 

Let us consider a clathrate crystal consisting of a cage-forming substance Q and a number of encaged compounds ( solutes ) A, B,. . ., M. The substance Q has two forms a stable modification, which under given conditions may be either crystalline (a) or liquid (L), and a metastable modification (ft) enclosing cavities of different types 1,. . ., n which acts as host lattice ( solvent ) in the clathrate. The number of cavities of type i per molecule of Q is denoted by vt. For hydroquinone v — for gas hydrates of Structure I 1/23 and v2 = 3/23, for those of Structure II vx = 2/17 and v2 = 1/17. 

Nevertheless, as in many previous observations, the clathrate formation by dipolar host compounds could not have been predicted in advance. In fact, there are no channels in the crystal structures of hydrated moxnidazole hydrochloride (closely related species to furaltadone hydrochloride) and of hydrated furaltadone base (Fig. 13)37). Rather, the latter two structures are best described as solvates with the H20 molecules contained in local voids between adjacent moieties of the host. 

A piperidene-based intermediate was found to crystallize as either an anhydrate or a hydrate, but the impurity profile of the crystallized solids differed substantially [26], Considerations of molecular packing led to the deduction that there was more void volume in the anhydrate crystal structure than in that of the hydrate form, thereby facilitating more clathration in the anhydrate than in the hydrate phase. This phenomenon was led to a decision to crystallize the hydrate form, since lower levels of the undesired impurity could be occluded and greater compound purity could be achieved in the crystallization step. 

In an oligonucleotide-drug hydrate complex, the appearance of a clathrate hydrate-like water structure prompt a molecular dynamics simulation (40). Again the results were only partially successful, prompting the statement, "The predictive value of simulation for use in analysis and interpretation of crystal hydrates remains to be established." However, recent molecular dynamics calculations have been more successful in simulating the water structure in Ae host lattice of a-cyclodextrin and P-cyclodextrin in the crystal structures of these hydrates (41.42). 

Readers of different backgrounds will wish to follow different paths through the chapters. Both the engineer and the researcher may wish to read Chapter 1 that provides a historical overview of clathrate hydrates. One cannot deal with hydrates without some knowledge of the all-important crystal structures provided in Chapter 2. Chapter 3 on hydrate kinetics gives the current picture ofhydrate time-dependence to supplement the time-independent phase equilibria in Chapter 4, the last chapter that should be of common interest to both the engineer and the... 

The initial limitations of the book are still largely present in the third edition. First the book applies primarily to clathrate hydrates of components in natural gases. Although other hydrate formers (such as olefins, hydrogen, and components larger than 9 A) are largely excluded, the principles of crystal structure, thermodynamics, and kinetics in Chapters 2 through 5 will still apply. 

Many gases, such as Ar, Kr, Xe, N2, O2, CI2, CH4 and CO, can be crystallized with water to form ice-like clathrate hydrates. The basic structural components of these hydrates are the (H2O)20 pentagonal dodecahedron and other larger polyhedra bounded by five- and six-membered hydrogen-bonded rings, which can accommodate the small neutral molecules. The inclusion properties of water imply that such polyhedra are likely to be present in liquid water as its structural components. 

An interesting example where infrared O-H frequencies were used to correlate structures is for choline chloride dihydrate, which is postulated to have a semi-clathrate hydrate structure by analogy with the known crystal structure of tetraethyl ammonium fluoride pentahydrate [162]. 

Hydrogen-bonding patterns in crystal structures of the cydodextrins and the simpler carbohydrates differ. The infinite, homodromic chains are common both in the low molecular-weight carbohydrates and in the cydodextrins. The principal difference lies in the frequency of occurrence of the homodromic and antidromic cycles, which are common in the cyclodextrin crystal structures and rare in the mono-, di-, and trisaccharides. The cyclic patterns are the rule in the clathrate hydrates and in the ices. From this point of view, the hydrogen-bonding patterns of the hydrated cydodextrins lie between those of the simpler hydrated carbohydrates and those of the hydrate inclusion compounds, discussed in Part IV, Chapter 21. 

The original X-ray studies of the gas clathrate hydrate structures were based on powder diffraction data, since the gas hydrates are notoriously difficult to obtain as single crystals. Clathrate hydrates formed by compounds which are liquids above 0°C, such as those of ethylene oxide and tetrahydrofuran, can be readily grown as large single crystals, and X-ray single crystal studies of both these hydrates have been carried out [787, 788]. 

Here the layers contain only water molecules which form antidromic pentagons, quadrilaterals and homodromic hexagons (Fig. 21.10). Clathrate or semi-clathrate structures have been postulated for choline chloride hydrate, (H3Q3N+CH2CH2OH 2H20 CP, on the basis of similarities in the solid-state infrared spectra [162], but this has not been confirmed by crystal structure analysis. 

Fig. 23.10 a, b. Comparison of fused water pentagons a in crambin and b in the clathrates of ethylene oxide deutero hydrate and of n-propylamine hydrate. Pentagon arrangements which are comparable in both crystal structures are indicated by thick lines [844]... 

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