Amorphous compounds

The modeling of amorphous solids is a more dilhcult problem. This is because there is no rigorous way to determine the structure of an amorphous compound or even dehne when it has been found. There are algorithms for building up a structure that has various hybridizations and size rings according to some statistical distribution. Such calculations cannot be made more efficient by the use of symmetry. 

Measurements [113,368] of interfacial (contact) potentials or calculated values of the relative work functions of reactant and of solid decomposition product under conditions expected to apply during pyrolysis have been correlated with rates of reaction by Zakharov et al. [369]. There are reservations about this approach, however, since the magnitudes of work functions of substances have been shown to vary with structure and particle size especially high values have been reported for amorphous compounds [370,371]. Kabanov [351] estimates that the electrical field in the interfacial zone of contact between reactant and decomposition product may be of the order of 104 106 V cm 1. This is sufficient to bring about decomposition. 

Visual inspechon frequently cannot differenhate between an amorphous or crystalline material, e.g. at Pfizer medicinal chemists were required to submit only crystalline and not amorphous compounds to an automated thermodynamic solubility assay. In prachce half the white powders that they produced for the assay and that they thought were crystalline were actually amorphous. Prior to 2000 the vast majority of these medicinal chemistry labs had no melting point equipment and it was only in 2000 that the pharmaceuhcal sciences department started a workshop to teach medicinal chemists the importance of solid state properhes, how to crystallize compounds and the importance of salt forms. 

Precipitation of compound from a DMSO stock is an issue of compound solubility in DMSO [14]. However, operationally, most organizations dissolving compounds in DMSO do not encounter problems in the solubilization process. This apparent paradox is explained by the issue of amorphous compounds. The vast majority of compounds solubilized into DMSO stocks are amorphous and inherently more soluble in DMSO (as previously discussed). The vast majority of the same compounds precipitating from DMSO are crystalline and much less soluble in DMSO (as previously discussed). The problem of compounds precipitating from DMSO occurs because so many compounds entering into screening libraries are amorphous. The precipitation of compounds from DMSO would be far less of a problem if screening compounds were entirely crystalline as they were 20 years ago. Quite simply DMSO insoluble crystalline compounds would never have been successfully dissolved in DMSO in the first place. 

Solids exist as either amorphous compounds or crystalline compounds [22]. In the latter, the molecules are positioned in lattice sites. Usually, amorphous substances decompose by first-order kinetics, and as such distinguish themselves from crystalline compounds [23]. 

Fig. 7 Dependence of vapor pressure and solubility for an enantiotropic pair (I/II), a monotropic pair (I/III), and an amorphous compound (a).

A shock- and heat-sensitive amorphous compound of uncertain structure. A sample exploded violently when scraped from a glass sinter, under impact from a hammer, on sudden heating to 260°C or when added to cone, sulfuric acid. 

SiBr4 Under pressure Colourless amorphous compound 300)... 

Nicropore blocking by binding agents, amorphous compounds, reaction products, successive phase transition and chemical interaction between the zeolite and the binder [7,25,87,88]. 

The metallorganic compounds (I, II) employed in presence of a heterogeneous phase containing an amorphous compound of a low-valency, strongly electropositive transition metal, generally polymerize a-olefins to amorphous polymers. In a similar fashion, the soluble reaction products of such metallorganic compounds with compounds of transition metals, chemisorbed on amorphous substrates, polymerize a-olefins to amorphous polymers 6, 9). 

Hydrochloric, nitric, and sulphuric acids transform the powder into black amorphous compounds containing acidic residues in place of tire hydroxo-groups.3... 

The union of mercury and sulfur is effected in the most simple way of all by merely rubbing the two substances together. The product is a black, amorphous compound, quite unlike the bright scarlet crystalline mineral cinnabar. To convert it into the crystalline form, advantage is taken of the following principle An amorphous form is more unstable than a crystalline form and tends to go over into it. The unstable amorphous form is also more soluble in solvents than the stable crystalline form. If, therefore, a solvent can be found in which both are slightly soluble, the amorphous form dissolves to saturation, but this produces a solution oversaturated with respect to the stable form, and some of this crystallizes out. More of the unstable... 

The action of liquid ammonia on selenium monochloridc is exceedingly vigorous and results in the deposition of red selenium. The monochloride in ether solution, however, yields with liquid ammonia a pale brown amorphous compound of formula Se2NCl.5... 

For amorphous compounds stored isothermally, the time for crystallization generally decreases with an increase in temperature, due to an increase in viscosity (Saleki-Gerhardt and ZograL, 1994) A mixture of two amorphous compounds will exhibit a single glass transition temperature intermediate between the two original components (Hancock and ZograL, 1994). If the densities of the... 

Among the various classes of materials, some have not yet kept their early promise. This is the case of amorphous compounds, whose use is also hampered by the severe conditions often employed in electrolysis cells. In the case of sulphides it is not yet clear how much of their activity is due to the chemical composition of the surface and how much to the structure resulting from the modification of the surface under cathodic load. In the case of composite materials, it is necessary to take into account that the surface area is normally higher for multicomponent phases, depending in particular on the method of preparation. 

Utilizing an amorphous form of an API is not, however, universally desirable. Amorphous compounds are often metastable. As a result, there is a real risk that they will transform to crystalline materials in the final dosage form. Novobiocin again provides a case in point. The amorphous form, in aqueous suspension, will transform on standing into the inactive crystalline form.10 Similarly, the highly soluble metastable crystals of riboflavin revert to less soluble forms if they are washed with water above 10°C.9... 

The second amorphous compound isolated had a molecular weight of 421. From the spectral data, these workers concluded that this alkaloid contained a lycoc-tonine skeleton, an N-ethyl group, and three methoxy-groups. 

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