Modification polymorphic

Lead oxide (PbO) exists in two modifications (polymorphs) (1) red tetragonal lead oxide (tet-PbO) (also known as a-PbO or litharge) and (2) yellow orthorhombic lead oxide (orthorhomb-PbO) (also known as P-PbO or massicot). Tet-PbO is stable at low temperatures and low pressures. The transition temperature of tet-PbO to orthorhomb-PbO is 486—489 °C and the thermal effect of the transition is 1.35 kJ mol When orthorhomb-PbO is cooled rapidly, it may remain unchanged and continue to exist at low temperatures. Eventually, it is slowly transformed into tet-PbO under external physical action. Lead oxide exists also in amorphous form. The latter s amount depends on the method of manufacture of PbO. 

Polymorphism. Many crystalline polyolefins, particularly polymers of a-olefins with linear alkyl groups, can exist in several polymorphic modifications. The type of polymorph depends on crystallisa tion conditions. Isotactic PB can exist in five crystal forms form I (twinned hexagonal), form II (tetragonal), form III (orthorhombic), form P (untwinned hexagonal), and form IP (37—39). The crystal stmctures and thermal parameters of the first three forms are given in Table 3. Form II is formed when a PB resin crystallises from the melt. Over time, it is spontaneously transformed into the thermodynamically stable form I at room temperature, the transition takes about one week to complete. Forms P, IP, and III of PB are rare they can be formed when the polymer crystallises from solution at low temperature or under pressure (38). Syndiotactic PB exists in two crystalline forms, I and II (35). Form I comes into shape during crystallisation from the melt (very slow process) and form II is produced by stretching form-1 crystalline specimens (35). 

Calcium Pyrophosphates. As is typical of the pyrophosphate salts of multiple-charged or heavy-metal ions, the calcium pyrophosphates are extremely insoluble ia water. Calcium pyrophosphate exists ia three polymorphic modifications, each of which is metastable at room temperature. These are formed progressively upon thermal dehydration of calcium hydrogen phosphate dihydrate as shown below. Conversion temperatures indicated are those obtained from thermal analyses (22,23). The presence of impurities and actual processing conditions can change these values considerably, as is tme of commercial manufacture. 

Another important pigment in this class is Pigment Red 170 [6985-95-17, which provides medium shades of red, and when particle-grown produces an opaque modification which shows improved migration resistance and lightfastness. Like some other Naphthol AS pigments it shows the phenomenon of polymorphism. 

Crystalline Silica. Sihca exists in a variety of polymorphic crystalline forms (23,41—43), in amorphous modifications, and as a Hquid. The Hterature on crystalline modifications is to some degree controversial. According to the conventional view of the polymorphism of siHca, there are three main forms at atmospheric pressure quart2, stable below about 870°C tridymite, stable from about 870—1470°C and cristobaHte, stable from about 1470°C to the melting point at about 1723°C. In all of these forms, the stmctures are based on SiO tetrahedra linked in such a way that every oxygen atom is shared between two siHcon atoms. The stmctures, however, are quite different in detail. In addition, there are other forms of siHca that are not stable at atmospheric pressure, including that of stishovite, in which the coordination number of siHcon is six rather than four. 

The thermodynamically most stable polymorph of boron is the /3-rhombohedral modification which has a much more complex structure with 105 B atoms in the unit cell (no 1014.5 pm, a 65.28°). The basic unit can be thought of as a central Bn icosahedron surrounded by an icosahedron of icosahedra this can be visualized as 12 of the B7 units in Fig. 6.1b arranged so that the apex atoms form the central Bn surrounded by 12 radially disposed pentagonal dishes to give the Bg4 unit shown in Fig. 6.3a. The 12 half-icosahedra are then completed by means of 2 complicated Bjo subunits per unit cell,... 

The determination of precise physical properties for elemental boron is bedevilled by the twin difficulties of complex polymorphism and contamination by irremovable impurities. Boron is an extremely hard refractory solid of high mp, low density and very low electrical conductivity. Crystalline forms are dark red in transmitted light and powdered forms are black. The most stable ()3-rhombohedral) modification has mp 2092°C (exceeded only by C among the non-metals), bp 4000°C, d 2.35 gcm (a-rhombohedral form 2.45gcm ), A77sublimation 570kJ per mol of B, electrical conductivity at room temperature 1.5 x 10 ohm cm- . 

Phosphorus (like C and S) exists in many allotropic modifications which reflect the variety of ways of achieving catenation. At least five crystalline polymorphs are known and there are also several amorphous or vitreous forms (see Fig. 12.3). All forms, however, melt to give the same liquid which consists of symmetrical P4 tetrahedral molecules, P-P 225 pm. The same molecular form exists in the gas phase (P-P 221pm), but at high temperatures (above 800°C) and low pressures P4 is in equilibrium with the diatomic form P=P (189.5 pm). At atmospheric pressure, dissociation of P4 into 2P2 reaches 50% at 1800°C and dissociation of P2 into 2P reaches 50% at 2800°. 

In the vapour phase As is known to exist as tetrahedral Asa molecules with (As-As 243.5 pm) and when the element is sublimed, a yellow, cubic modification is obtained which probably also contains Asa units though the structure has not yet been determined because the crystals decompose in the X-ray beam. The mineral arsenolamprite is another polymorph, e-As it is possibly isostructural with metallic orthorhombic P. 

Unsubstituted phthalocyanines and also halogenated phthalocyanines show very poor solubility in organic solvents. In the solid state, unsubstituted phthalocyanines exhibit, in most cases, inclined one-dimensional stacking of the planar molecules. Besides other polymorphous modifications PcCu shows an x- and / -arrangement.74-75... 

In the case of PcCu and PcH2, the most stable modification, which is formed during synthesis, is the -modification. For a more detailed discussion on the polymorphism in phthalocyanines, see refs 36 and 76. 

The Ca-Cu system has been reexamined using thermal analysis and x-ray diffraction methods an independent study of the CaCuj-Cu section has also been completed. The resultant phase diagram, although similar to that in ref. 3 at the Cu-rich end, differs markedly for Ca-rich alloys. Supporting evidence for the modifications has been obtained from the Ca-Mg-Cu ternaiy system. Three intermediate compounds are formed in the system CaCuj (950 C) melts congruently, whereas CajCu (488 C) and CaCu (567°C) are formed in peritectic reactions. Single-crystal x-ray diffraction studies verify the stoichiometry of CajCu and examine the polymorphism of CaCu. ... 

In 1933, Bernal and Crowfoot [1] reported on the solid state polymorphism of p-azoxyanisole. They found two crystalline modifications of this compound, a stable yellow form and an unstable white polymorph. Krigbaum et al. [31 reexamined the crystal structure of the stable yellow form. The compound shows an imbricated structure which is the basic packing required for nematic behaviour according to Gray [132]. 

Ice I is one of at least nine polymorphic forms of ice. Ices II to VII are crystalline modifications of various types, formed at high pressures ice VIII is a low-temperature modification of ice VII. Many of these polymorphs exist metastably at liquid nitrogen temperature and atmospheric pressure, and hence it has been possible to study their structures without undue difficulty. In addition to these crystalline polymorphs, so-called vitreous ice has been found within the low-temperature field of ice I. It is not a polymorph, however, since it is a glass, i.e. a highly supercooled liquid. It is formed when water vapour condenses on surfaces cooled to below — 160°C. 

It is not appropriate in this chapter to give a detailed review of the solid-state behaviour of water in its various crystalline modifications. However, there are some general structures which are relevant and worth highlighting. Firstly, water molecules in these various solids have dimensions and bond angles which do not differ much from those of an isolated water molecule. Secondly, the number of nearest neighbours to which each individual molecule is hydrogen-bonded remains four, regardless of the ice polymorph. 

Budowle B and Baechtel FS (1990) Modifications to improve the effectiveness of restriction fragment length polymorphism typing appMed theories. Electrophoresis 1 181-187. 

Stereochemistry. - There has been a far infra-red spectroscopic study of ethyl phosphine,130 and in combination with X-ray fluorescence, further work completed on the conformational analysis of dialkylphenyl phosphines.131 The influence of water on the conformational equilibria of trimethyl phosphate has received attention.132 There has been a low temperature solid state and matrix isolation study of methyl phosphorodichi oridate,133 and a conformational study of polymorphic modifications of diphenyl-phosphinyl acetic acid hydrazide. 34... 

Molecules having the same composition but different structures are called isomers. The corresponding phenomenon for crystalline solids is called polymorphism. The different structures are the modifications or polymorphic forms. Modifications differ not only in the spatial arrangement of their atoms, but also in their physical and chemical properties. The structural differences may comprise anything from minor variations in the orientation of molecules up to a completely different atomic arrangement. 

Different modifications of a compound are frequently designated by lower case Greek letters a, j3,..., e.g. a-sulfur, j3-sulfur, or by roman numerals, e.g. tin-I, tin-II etc. Polymorphic forms of minerals have in many cases been given trivial names, like a-quartz, P-quartz, tridymite, cristobalite, coesite, keatite, and stishovite for Si02 forms. 

Knowledge of polymorphic forms is of importance in preformulation because suspension systems should never be made with a metastable form (i.e., a form other than the stable crystal form). Conversely, a metastable form is more soluble than a stable modification, and this can be of advantage in dissolution [Eq. (9)]. There are two types of polymorphism, a fact illustrated in the following discussion. 

The classic disilene 1 is unusual in that it exists in at least three crystalline modifications orange and yellow unsolvated forms and a yellow toluene solvate (Fig. 2). The orange polymorph has a helical conformation in which all of the mesityl substituents are twisted in the same direction thus molecules of 1 in this form are chiral.51 The toluene solvate has an unusual conformation in which two mesityl rings cis to each other are nearly coplanar with the Si=Si bond, while the other two cis mesityl groups are nearly orthogonal.41 The structure of the yellow unsolvated form is not yet known. Because of the flat potential surface for the Si=Si... 

It seems also meaningful to recall that, for both PVL and iPP, the metastable chiral modification is not obtained from solution. This fact is hard to rationalize if polymorphic discrimination occurs on the basis of the secondary nucleation site which should exist also in the presence of the solution it rather points to diffusion and to transport problems in the melt, or thermodynamic control in solution. 

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