Crystalline instabilities

Metallic Palladium films pass H2 readily, especially above 300°C. a for this separation is extremely high, and H2 produced by purification through certain Pd alloy membranes is uniquely pure. Pd alloys are used to overcome the crystalline instability of pure Pd during heating-cooling cycles. Economics limit this membrane to high-purity applications. 

Chemical conversion of compounds to intermediates of known absolute configuration is a method routinely used to determine absolute configuration (86). This is necessary because x-ray analysis is not always possible suitable crystals are required and deterrnination of the absolute configuration of many crystalline molecules caimot be done because of poor resolution. Such poor resolution is usually a function of either molecular instability or the complex nature of the molecule. For example, the relative configuration of the macroHde immunosuppressant FK-506 (105) (Fig. 8), which contains 14 stereocenters, was determined by x-ray crystallographic studies. However, the absolute configuration could only be elucidated by chemical degradation and isolation of L-pipecoUc acid (110) (80). 

It may occasion surprise that an amorphous material has well-defined energy bands when it has no lattice planes, but as Street s book points out, the silicon atoms have the same tetrahedral local order as crystalline silicon, with a bond angle variation of (only) about 10% and a much smaller bond length disorder . Recent research indicates that if enough hydrogen is incorporated in a-silicon, it transforms from amorphous to microcrystalline, and that the best properties are achieved just as the material teeters on the edge of this transition. It quite often happens in MSE that materials are at their best when they are close to a state of instability. 

Diels-Alder reaction of the furan derivative 148 with homochiral bicyclic enone 149 is the key step [56] in the total synthesis of the diterpenes jatropho-lone A and B, 151 and 152, respectively, isolated from Jatropha gossypiifolia L [57], Initial efforts to carry out the cycloaddition between 148 and 149 under thermal or Lewis-acid conditions failed due to diene instability. Application of 5kbar of pressure to a neat 1 1 mixture of diene and dienophile afforded crystalline 150 with the desired regiochemistry (Scheme 5.23). Subsequent aromatization, introduction of the methylene group, oxidation and methylation afforded (-l-)-jatropholones 151 and 152. 

A difiiculty with this mechanism is the small nucleation rate predicted (1). Surfaces of a crystal with low vapor pressure have very few clusters and two-dimensional nucleation is almost impossible. Indeed, dislocation-free crystals can often remain in a metastable equilibrium with a supersaturated vapor for long periods of time. Nucleation can be induced by resorting to a vapor with a very large supersaturation, but this often has undesirable side effects. Instabilities in the interface shape result in a degradation of the quality and uniformity of crystalline material. 

Phase transitions, whether first-order or second-order, are potent sources of instability of solid drugs and can usually be detected and studied by thermal methods of analysis (e.g., DSC, TGA, TMA, ODSC, DMA, DEA). In crystalline solids, typical first-order transitions are polymorphic or desolvation transitions. In amorphous solids, second-order transitions, such as glass transitions, are common. 

Molybdenum (IV) bromide is a black crystalline solid which is very sensitive to oxidation and hydrolysis it should be handled only under a dry inert atmosphere. Its solubility in bromine at 51° is ca. 3.0 g./lOOO g. of bromine, and bromine solutions were found to be nonconductors. For example, at 25° both the solvent bromine and a 0.96 X 10 3 M solution in molybdenum (IV) bromide exhibited a specific conductance of 1.3 X 10-10 ohm-1 cm.-1 At 110 to 130° in vacuo, solid molybdenum (IV) bromide decomposes quantitatively into molyb-denum(III) bromide and bromine, and because of this thermal instability it cannot be sublimed except under a bromine atmosphere. 

The instability of these chiral monolayers may be a reflection of the relative stabilities of their bulk crystalline forms. When deposited on a clean water surface at 25°C, neither the racemic nor enantiomeric crystals of the tryptophan, tyrosine, or alanine methyl ester surfactants generate a detectable surface pressure, indicating that the most energetically favorable situation for the interfacial/crystal system is one in which the internal energy of the bulk crystal is lower than that of the film at the air-water interface. Only the racemic form of JV-stearoylserine methyl ester has a detectable equilibrium spreading pressure (2.6 0.3dyncm 1). Conversely, neither of its enantiomeric forms will spread spontaneously from the crystal at this temperature. 

When considering reduction of particle size, one should be aware of potential instability enhancement. It is well known that grinding can reduce crystallinity [65] the resulting noncrystalline material may be more hygroscopic... 

Na-cefazolin is instable in its amorphous state. Takeda [1.32J described a method to ensure complete crystallization in which micro crystalline Na-cefazolin were added to at 0 °C supersaturated Na-cefazolin solution, frozen and freeze dried. The product did not contain amorphous or quasi-crystalline components. 

The temperature dependences of the isothermal elastic moduli of aluminium are given in Figure 5.2 [10]. Here the dashed lines represent extrapolations for T> 7fus. Tallon and Wolfenden found that the shear modulus of A1 would vanish at T = 1.677fus and interpreted this as the upper limit for the onset of instability of metastable superheated aluminium [10]. Experimental observations of the extent of superheating typically give 1.1 Tfus as the maximum temperature where a crystalline metallic element can be retained as a metastable state [11], This is considerably lower than the instability limits predicted from the thermodynamic arguments above. 

AIH3 has several crystalline and amorphous forms it appears as a non-volatile solid instable above 150°C. In a-A H3, aluminium is octahedrally surrounded by 6 hydrogen atoms. 

Self-sustaining thermal decomposition of NPK fertilisers is apparently possible if crystalline potassium nitrate is present from a exchange reaction of ammonium nitrate [1], Almost pure fertiliser grades of ammonium nitrate are legally restricted in some countries because of its instability [2],... 

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