Hydrated Crystals

Balke [55] obtained crystal hydrate of lithium hexafluorotantalate, LiTaF6 H20, by evaporating a solution prepared by the dissolution of Ta205 and Li2C03 in HF. Crystal hydrate of sodium heptafluorotantalate, Na2TaF7-H20, was prepared in the same way [56], while re-crystallization of Na2TaF7-H20 from water yielded sodium octafluorotantalate, Na3TaF8 [29]. 

Table 28 presents structural characteristics of compounds with X Me ratios between 6 and 5 (5.67, 5.5, 5.33, 5.25). According to data provided by Kaidalova et al. [197], MsNbsC Fu type compounds contain one molecule of water to form M5Nb303Fi4-H20, where M = K, Rb, Cs, NH4. Cell parameters for both anhydrous compounds [115] and crystal-hydrates [197] were, nevertheless, found to be identical. Table 28 includes only anhydrous compound compositions because IR absorption spectra of the above compounds display no bands that refer to vibrations of the water molecule... 

It is, therefore, required that all initial compounds be dried properly prior to performing the reduction. This procedure is not at all trivial and refers, first of all, to the diluent salts, and especially to potassium fluoride, KF, which is characterized by a strong hygroscopic property and a tendency to form stable crystal hydrates. The problem of contamination due to hydrolytic processes can usually be resolved in two manners. The first is to apply another tantalum-containing complex fluoride compound that does not undergo hydrolysis. The second involves the adjustment of the reduction process parameters and use of some additives that will "collect" the oxygen present, in the form of water, hydroxyl groups or other compounds. 

Crystal field splitting parameter, 2, 309 Crystal field theory, 1, 215-221 angular overlap model, 1, 228 calculations, 1, 220 generality, 1,219 low symmetry, 1,220 /-orbital, 1, 231 Crystal hydrates, 2, 305,306 bond distances, 2, 307 Crystals... 

Between 1865 and 1887, Dmitri 1. Mendeleev developed the chemical theory of solutions. According to this theory, the dissolution process is the chemical interaction between the solutes and the solvent. Upon dissolution of salts, dissolved hydrates are formed in the aqueous solution which are analogous to the solid crystal hydrates. In 1889, Mendeleev criticized Arrhenius s theory of electrolytic dissociation. Arrhenius, in turn, did not accept the idea that hydrates exist in solutions. 

Ions not solvated are unstable in solutions between them and the polar solvent molecules, electrostatic ion-dipole forces, sometimes chemical forces of interaction also arise which produce solvation. That it occurs can be felt from a number of effects the evolution of heat upon dilution of concentrated solutions of certain electrolytes (e.g., sulfuric acid), the precipitation of crystal hydrates upon evaporation of solutions of many salts, the transfer of water during the electrolysis of aqueous solutions), and others. Solvation gives rise to larger effective radii of the ions and thus influences their mobilities. 

Solids that form specific crystal hydrates sorb small amounts of water to their external surface below a characteristic relative humidity, when initially dried to an anhydrous state. Below this characteristic relative humidity, these materials behave similarly to nonhydrates. Once the characteristic relative humidity is attained, addition of more water to the system will not result in a further increase in relative humidity. Rather, this water will be sorbed so that the anhydrate crystal will be converted to the hydrate. The strength of the water-solid interaction depends on the level of hydrogen bonding possible within the lattice [21,38]. In some hydrates (e.g., caffeine and theophylline) where hydrogen bonding is relatively weak, water molecules can aid in hydrate stabilization primarily due to their space-filling role [21,38]. 

A thorough understanding of the hydration profile for a solid forming a crystal hydrate is important for several reasons. First, since an anhydrate and hydrate(s) are distinct thermodynamic species, they will have different physical-chemical properties (e.g., solubility) that may affect bioavailability. Second, a desired hydrate species can be formed and used (and retained) simply by controlling the desired, established environmental conditions. Third, since significant quantities of water can be sorbed/liberated as a hydrate becomes hydrated/dehydrated, the physical-chemical properties of the immediate system (including other nearby solids) can be markedly affected. 

A simplified series of reactions between a hafnium salt and sulfuric acid is given in Fig. 4.3. The reactions showcase important facets of thin-film synthesis (but do not address the precise identities of intermediates or complexities of aqueous hafnium chemistry.) In the first step, a hafnium oxide chloride crystal hydrate is dissolved in water to disperse small hafnium-hydroxo molecular clusters. Sulfato ligands are subsequently added in the form of sulfuric acid. Since sulfato binds more strongly than chloro, hafnium-hydroxo-sulfato aqueous species are created. Under mild heating, these species readily poly-... 

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). 

Similarly, Tuckerman (excerpt 12K) cites works that emphasize widespread interest in the research area, highlighting, for example, that crystal hydrates have attracted the attention of crystallographers and spectroscopists over several decades (46—28, 123-125). Specific benefits of crystal hydrates are touted, including their possible use as proton conductors (J26) and as important media for the study of proton motion. The latter is currently of interest in the field of low temperature spectroscopy (127-129). 

Ionic radii in the figure are measured by X-ray diffraction of ions in crystals. Hydrated radii are estimated from diffusion coefficients of ions in solution and from the mobilities of aqueous ions in an electric field.3-4 Smaller, more highly charged ions bind more water molecules and behave as larger species in solution. The activity of aqueous ions, which we study in this chapter, is related to the size of the hydrated species. 

Bridging water molecules occur in several crystal hydrates,53 and in a very few ternary complexes (Table 2).53 55... 

Many crystal hydrates contain discrete [M(OH2)JC]"+ units, which are simply binary aquo complexes. The most common coordination number is, of course, six, with octahedral stereochemistry,... 

Binary [M(OH2)J"+ entities are not known in crystal hydrates for x > 9 (indeed no ML, (L monodentate) has yet been characterized178), but ternary aquo complexes are known in which the coordination number of the metal ion is 10, 11 or 12 (Table 5).178 183 The [M(OH2)7]"+ unit does not appear to have been characterized crystallographically, but again ternary aquo complexes of this coordination number exist, e.g. [U02(0H2)5]2+ in the salt U02(C104)2 7H20.11... 

Table 5 Examples of Crystal Hydrates Containing Ternary Complexes of High Coordination Number Which Include Coordinated Water...

Cation X-Ray diffraction EXAFS Neutron diffraction Crystal hydrates Ionic radii ... 

Order arising through nucleation occurs both in equilibrium and nonequilibrium systems. In such a process the order that appears is not always the most stable one there are often competing processes that will lead to different structures, and the structure that appears is the one that nucleates first. For instance, in the analysis of the different possible structures in diffusion-reaction systems17-20 one can show, by analyzing the bifurcation equations, that there are several possible structures and some of them require a finite amplitude to become stable if this finite amplitude is realized through fluctuation, this structure will appear. In the formation of crystals (hydrates) the situation is similar the structure that is formed depends, according to the Ostwald rule, on the kinetics of nucleation and not on the relative stability. 

Neutron diffraction—single crystal Hydrate phase No Several hours Typically 1 atm., 20-5 K Definitive structure determination... 

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