Solid modifications

When the free enthalpy of reaction AG for the transformation of the structure of a compound to any other structure is positive, then this structure is thermodynamically stable. Since AG depends on the transition enthalpy AH and the transition entropy AS, and AH and AS in turn depend on pressure and temperature, a structure can be stable only within a certain range of pressures and temperatures. By variation of the pressure and/or the temperature, AG will eventually become negative relative to some other structure and a phase transition will occur. This may be a phase transition from a solid to another solid modification, or it may be a transition to another aggregate state. 

The aP4 white form is the most volatile, reactive (and highly toxic) and thermodynamically the least stable solid modification. The slow oxidation of the vapour over the solid results in the phosphorescence characteristic of this element. By heating above 180°C, white phosphorus is transformed into red phosphorus which... 

A substance A exists in two solid modification A j andA2 as well as liquid and vapour under one atmospheric pressure. 

Equation (3) is applicable to various equilibria such as solid-liquid equilibria, liquid-vapour equilibria and equilibria between two solid modifications. The Clapeyron s equation for these various equilibria can be easily obtained as follows ... 

Sulfur trioxide exists in at least three different solid modifications, as well as a liquid and a gas (Figure 18). 

The recrystallization procedure yields one of three solid modifications of this compound which have melting points of 65 to 66°, 70 to 71°, and 85 to 86°, respectively. The three forms have identical infrared spectra and the same composition. They differ in their superficial appearance, as may be seen by examination under a microscope. The form which melts at 65 to 66°, when melted and recrystallized by cooling, yields the modification which melts at 70... 

A single substance which can exist in two or more solid modifications has therefore several triple points, the maximum number of which can be computed by the law of combinations. Thus for sulphur, which can exist in two crystalline forms (rhombic and monoclinic), we have the following triple points ... 

Water is an example of a substance which can exist in five different phases. It has been obtained in three solid modifications. The j), T diagram of water has been very accurately determined recently by Tammann. 

Before going further, however, two remarks should be made. First the importance of certain structural modifications of the surface of the irradiated catalysts should be stressed. Among these modifications we cite the sometimes considerable transformations of the physical texture of the solid (modifications of the pore spectrum, etc.) ( 4, 7 S) due to sintering, as well as the modifications of the chemical nature of the surface (decomposition of oxides, of silanol groups,. . . ) (74). These structural modifications may have a particularly important influence in the case of large surface solids such as the silica and alumina used in our experiments. 

Properties Exists in three solid modifications a mp 62C, (i mp 32.5C, y mp 16.8C. The a form appears to be the stable form but the solid transitions are commonly slow a given sample may be amixture of the various forms and its melting point not constant. The solid sublimes easily. 

Sulindac (Shen et al. 1971) is another NS AID and exists in two nonsolvated solid modifications. The chemical consistency of the two solid phases was demonstrated by various methods, including solution NMR. The two polymorphs were designated Forms I and n of sulindac. Differences in the solid state were detected by XRPD and DTA (Fig. 3-3). 

By deuterium NMR, we have ascertained that the limit of solubility of 8a in the crystal-B phase is in fact somewhat less than 1.0 mol%. Under the conditions of our flash photolysis experiments, 8a is distributed between both a ketone-rich nematic phase and a ketone-depleted smectic phase at temperatures between 53-35°C. At 35°, the ketone-rich phase is transformed into a "p-phase" in which the ketone exhibits isotropic H and C NMR behavior, and which eventually crystallizes (though not under these conditions) to form a stable binary smectic or solid modification consisting of 8-10 mol% ketone and CCH-4. Thermal microscopy experiments with mixtures containing 1-3... 

The explanation of such a behaviour is, that if the determination of the melting-point is carried out rapidly, the point 0, the melting-point of the metastable solid form, may be realised. At this temperature, however, the liquid is metastable with respect to the stable solid form, and if the temperature is not allowed to rise above the melting-point of the latter, the liquid may solidify. The stable solid modification thus obtained will melt only at a higher temperature. 

Uranium disulphide is a particularly simple case, because it is solid throughout the experiment, and exists in only one crystalline form. Many compounds, however, undergo solid modifications or phase changes. These changes are easily catered for, because they occur at fixed temperatures. 

Reddish-brown vapor. Two solid modifications exist. One is reddish-brown to black-red, unstable (especially when exposed to visible, ultraviolet or infrared light) and can be stored only by chilling the freshly condensed product in liquid nitrogen with exclusion of light. Rhombic or monoclinic crystals. Vapor pressure (0°C) 24 mm. 

Formula weight 162.38. Reddish-brown liquid at ordinary temperatures exists in two solid modifications a-ICl, ruby-red needles (m.p. 27.19°C) )S-IC1, brownish-red plates (m.p. 13.9°C), labile form. 

Modifications of phosphorous pentoxide. Phosphorus pentoxide forms three solid modifications, of which the metastable M form is the ordinary commercial P30g. This modification crystallizes as rhombohedra with a molecular lattice (P40 (,) and sublimes readily at 250°C and 10 mm. (Glixelll and Boratynskl). Above 260°C and even more quickly above 500°C, form M changes Into form R. The latter crystallizes in a three-dimensional atomic lattice of PO tetrahedra and is less volatile. A form S, which... 

Solid-state NMR is extremely useful in investigating solid modifications of both the drug substance and the solid drug product. Crystalline material typically affords sharp and narrow C resonance peaks similar to solution spectra. In addition, because the molecules in one polymorphic form of a drag are arrayed differently than those in another polymorphic form, differences in the solid-state spectra of the polymorphs are quite apparent. For example. Fig. 7.13 clearly shows that there are a number of differences in the solid-state C CP/ MAS spectra of the two prednisolone polymorphs [56]. 

An Br n.q.r. examination of InBrg shows that there are at least two solid modifications of this compound. ... 

First, ice has a large number of solid modifications hexagonal or ordinary ice, 1 cubic ice, 1 ices II-IX vitreous ice. Most of them exist at elevated pressure or need special formation conditions. Under ordinary conditions, only hexagonal ice is formed, which has therefore been investigated more frequently than other modifications. In this chapter we shall deal only with ordinary ice. 

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