Solvate crystals

Solid compounds can have four morphic states polymorphic, pseudo-polymorphic (solvates), amorphous, and desolvated solvates. Crystals usually exhibit narrow melting point ranges and defract light under an optical microscope. When a change in the arrangement of... 

An understanding of crystallization is important for the systematic development of crystal engineering, but it is not a simple phenomenon and many would agree that it is still far too difficult to study in a rigorous way, either experimentally or theoretically. However, indirect approaches to the study of crystallization are evolving. Three possible types of crystals that may be pertinent to this endeavor are (1) polymorphs - these represent cases of alternative crystallization, (2) pseudosymmetric structures with multiple molecules in the asymmetric unit - these could represent cases of incomplete crystallization, and (3) solvated crystals or pseudopolymorphs -these may represent cases of interrupted crystallization. These three scenarios are now sketched very briefly and the treatment given is necessarily selective. 

All of these interactions involve a host and a guest as well as their surroundings like solvation, crystal lattice, and gas phase. Electrostatic interactions are the driving force behind the ion pairing (ion-ion, ion-dipole, dipole-dipole, etc.) interactions, which are undeniably important in natural and supramolecular systems. The electrostatic interaction energy E is given by... 

Polymorphs and solvated crystals is generally observed in pharmacentical indnstry [1], The bioavailability, stability, solnbility, and morphology of the pharmacentical products are very influenced by polymorphs [2-7], therefore the control of the polymorphic crystallization is very important. The crystallization process of polymorphs and solvated crystals is composed of competitive nucleation, growth, and transformation from a meta-stable form to a stable form [4], Furthermore, the crystallization behavior is influenced by various controlling factors such as temperature, supersaturation, additives and solvents [8], In order to perform the selective crystallization of the polymorphs, the mechanism of each elementary step in the crystallization process and the key controlling factor needs to be elucidated [8], On the other hand, we reported for L-Glutamic acid and L-Histidine system previously [4] that the nucleation and transformation behaviors of polymorphs depend on the molecular stractures. If the relationship between molecular stmcture and polymorphic crystallization behavior is known, the prediction of the polymorphism may become to be possible for the related compound. However, detail in such relationship is not clearly understood. 

Themal treatment of the dimeric species 279 brings about a rearrangement to tetramer 281 (equation 56). The structure of 281 can be characterized by H, C H, P H and Li NMR spectroscopies. Recrystallization of 281 from toluene yields solvated crystals of formula 281-3PhMe, which show a core 12-membered macrocycle of C, P and Li atoms according to XRD crystallography ... 

In a solid-gas reaction involving a molecular crystal, the reactants are respectively the molecules in the crystalUne solid and the molecules in the gas phase and the product is the product crystal, which can be crystalUne or amorphous. Vapour uptake to generate a solvate crystal (e.g. hydration) is a related process. In fact the difference between a crystal solvation process and a solid-gas reaction leading to new molecular/ionic species is mainly in the energetic scale of the processes and in the fact that in solvation processes, molecules retain their chemical identity. On this premise there is a relevant analogy between the uptake of smaU molecules by a nanoporous material [16] and the reaction between a molecular crystal and molecules to yield a co-crystal or a salt (e.g. acid-base... 

The authors made no attempt at quantitative emission-intensity measurements. They did make the qualitative observation, however, that the rare-earth-ion emissions from the D20 solvated crystals were much more intense than the emissions from the H20 solvated ones. 

P-Benzilmoitoxime. The a-oxime is converted into the /J-form by treatment with a solution of hydrogen chloride in benzene (CAUTION) (or ether) at room temperature. From benzene, solvated crystals which melt on rapid heating at about 65 °C are obtained. Removal of benzene of crystallisation in an oven at 50 °C and recrystallisation from carbon disulphide (CAUTION) yields pure /J-benzilmonoxime, m.p. 112°C. The product gives no colour change with aqueous-alcoholic copper acetate solution if it is contaminated with the a-form a greenish colour is produced. (Conversion of the a-form into the / -form may also be effected by boiling in benzene solution in the presence of animal charcoal, which presumably contains adsorbed acidic catalysts.)... 

Aakeroy and coworkers used the seven new cocrystals of 1,4-diiodotetrafluorobenzene they discovered to critically discuss the progress made in the field, and to comment on challenges that remain unsolved [22]. The goal of the work was to match the desired supramo-lecular outcome with a strategy for synthesis of the intended system, and the investigators were able to obtain the anticipated intermolecular interactions in several cases. The crystallization of one intended system was complicated by the production of a solvated crystal form, and it was concluded that the serendipitous inclusion of solvent molecules in a crystal lattice was not always predictable, and the outcome not always favorable. 

Single crystal X-ray diffraction measurements have shown the importance of these attractive second order interactions in stabilisation of a relatively constant structure of TA in diastereoisomeric salts [14], DBTA in complexes [15, 16] and of DBTA derivatives in sole or solvated crystals, too. [17] In the most cases, strong H-bridges stick together the TA or DBTA molecules into long chains and chanels having chiral surface for discrimination between enantiomers. 

The material tenaciously holds hydrocarbons, such as pentane, hexane, and petroleum ether, which cannot be removed even under high vacuum. The solvated crystals show hydrocarbon protons in the NM and exhibit a broad melting point. However, we have found that cyclohexane is not retained in the crystals. 

The solvated crystals effloresce upon drying, leaving a white solid which is easily powdered. 

Crystals suitable for x-ray structure determination were grown slowly ( 1 hr) from dichloromethane by the gradual addition of methanol until incipient crystallization. Under these conditions the crystals obtained contained one molecule of dichloromethane per molecule of 1. The dichloromethane of crystallization was lost upon standing in air or by drying in a vacuum (vide supra). The solvated crystals of 1 were... 

Analogously, the sandwich cation [(r 6-C6H6)2Ru][BF4]2 can be crystallized from nitromethane as the solvated form [(r 6-C6H6)2Ru][BF4]2 MeN02. The solvate crystals, if exposed to air, rapidly convert to a different crystalline material, as ascertained by powder diffraction [73]. 

On this conceptual premise, uptake and release of solvent can be paralleled to a solid-gas reaction, whereby the reactants are the molecules in the crystalline solid and the molecules in the gas phase and the product is the solvated crystal. Clearly, the same reasoning applies to the reverse process, i.e. generation of a new crystalline form by means of gas release. In gas-solid reactions, gases are reacted directly with crystals or amorphous phases to give complete conversion and usually quantitative yields. What would then be the difference between a solvation reaction and a reaction leading to new molecular/ionic species if not the energetic scale of the processes and the fact that in solvation processes molecules retain their chemical identity ... 

Silver perchlorate forms deliquescent crystals, which decompose when heated to 486 Celsius. It is freely soluble in water saturated solution contains 85% by weight silver perchlorate making it one of the most water soluble compounds known lithium perchlorate being number 1. It is also soluble in aniline, pyridine, benzene, nitromethane, glycerol, and chlorobenzene. It can form solvated crystals with aniline, benzene, and toluene all explode on percussion. Silver perchlorate forms a hydrate, which melts at 43 Celsius. It can be made by reacting sodium hypochlorite (bleach) with silver bromide. 

What is implied by the observations in the preceding section is that the crystal lattices of substances of the IL type, in common with salt hydrate (and salt solvate crystals in general), become thermodynamically unstable with respect to their liquid phases (Guq < Gcrys) at temperatures that are very low, relative to their cohesive energies. 

The influence of methanol proportions in solvents, and temperature, on the solubility and the transformation behavior of 2-(3-cyano-4-isobutyloxyphenyl) -methylthiazole-5-carboxylic acid (BPT) was investigated. The transformation behavior was explained by the chemical potential difference between the stable and metastable forms. It was shown that a specific solute-solvent interaction contributes to the preferential nucleation and growth of the stable or metastable forms and influences the transformation behaviors, and the solubility of the solvated crystals is much more influenced by the solvent compositions than the true polymorphs. The solubility ratio of the solvated crystals depends on the solvent composition, whereas the solubility ratio of the true polymorphs is considered to be independent of the solvents. The crystallization behavior was also investigated. The transformation rate after crystallization appeared to depend on the initial concentration of BPT and the addition rate of the antisolvent. The cause of this phenomenon was presumed to be a slight inclusion of the stable form in the metastable form <2005PAC581>. 

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