THE CRYSTALLINE STATE

If the chemical contents of a polymorph are different than other forms, it is designated as a pseudopolymorph.Most often this occurs due to the presence of differing amounts of solvent, and may alter physical properties of the crystals such as melting points and solubilities. Polymorphism and pseudopolymorphism may be observed when different experimental conditions are used for synthesis. For example, if crystals are grown by sublimation, changing the temperature will often yield different crystal structures, possibly even metastable phases that are kinetically favored. 

A subclass of polymorphism known as polytypism is found for one-dimensional close-packed and layered structures such as SiC, Cdl2, GaSe, micas and clay 

When different compounds yield almost identical crystals, the forms are referred to as isomorphs. The word almost is indicated here, as isomorphs are not exactly the same although the arrangement of atoms/ions in the lattices are identical, one or more of the atoms in the lattice have been replaced with another component. For example, alums of the general formula (M)2(S04) (M)2 (804)3 24H2O may crystallize as isomorphs where one of the monovalent or trivalent metals is substituted with another. 

Single crystals comprise an infinite array of ions, atoms, or molecules, known as a crystal lattice. The strength of the interactions between the species comprising the crystal is known as the lattice energy, and is dependent on the nature and degree of 

The rate of a polymorphic phase transition depends on nucleation and growth processes, which are related to the mobility of atoms/molecules in the solid state. The Avrami equation (Eq. 7) may be applied to describe the degree of transformation, X, as a function of time, f  

External pressure may also be used to convert one form into another. When this medium is used, a polymorph with higher density will typically result due to the local confinement of lattice species through the externally applied pressure. It should be noted that multiple forms of an amorphous material i.e., lacking long-range structural order - see Section 2.4) are denoted as polyamorphs, as illustrated by the polyamorphism exhibited by silica at elevated pressures.  

---

The boron so obtained is an amorphous powder. It can be obtained in the crystalline state by reducing the vapour of boron tribromide with hydrogen, either in an electric arc or in contact with an electrically-heated tungsten filament ... 

From the standpoint of thermodynamics, the dissolving process is the estabHsh-ment of an equilibrium between the phase of the solute and its saturated aqueous solution. Aqueous solubility is almost exclusively dependent on the intermolecular forces that exist between the solute molecules and the water molecules. The solute-solute, solute-water, and water-water adhesive interactions determine the amount of compound dissolving in water. Additional solute-solute interactions are associated with the lattice energy in the crystalline state. 

Urease is one of the enzymes which have been obtained in the crystalline state. This has been done by stirring jack bean meal with 30°o aqueous acetone, filtering and allowing the filtrate to remain at o for several hours. The urease which crystallises out is separated by centrifuging and is then recrystallised. Like crystalline pepsin and trypsin, it is a protein. 

A left-handed double-heUcal stmcture has been proposed for geUan in the crystalline state, based on x-ray diffraction studies (227). The presence of acetyl groups presumably dismpts interchain aggregation, since these groups are postulated to be on the outside of the heUces. The role played by acetyl and glyceryl ester groups and their influence on the double-heUcal stmcture has been studied using computet models (232). 

Physical Properties. Table 3 Hsts physical properties of stereoregular polymers of several higher a-olefins. Crystal ceU parameters of these polymers ate available (34—36). AU. stereoregular polyolefins have helix conformations ia the crystalline state. Their densities usually range from 0.90 to 0.95 g/cm. Crystalline PMP, however, represents an exception its density is only 0.812—0.815 g/cm, lower even than that of amorphous PMP (0.835—0.840 g/cm ), thus making it one of the lowest densities among plastics. 

The pentahalides of phosphorus, PX, in the gas phase exhibit varying tendencies to dissociate into trihaUde and halogen. InstabiUty increases with increasing ionic radius of the halogen. The pentafluoride appears to be thermally stable. Dissociation of the pentachloride, a few percent at 100°C and 101.3 kPa (1 atm), is essentially completed at 300°C (36). The pentabromide is partially dissociated in the Hquid state and totally dissociated above ca 35°C (39). Pentaiodide does not exist. The molecules of PF and PCl in the vapor phase are trigonal bipyramids. In the crystalline state, both pentachloride and pentabromide have ionic stmctures, ie, [PClJ IPClg] and [PBr4]" PBrJ , respectively. The PX" 4 cations are tetrahedral and the PX anion is octahedral (36,37). 

Charge-transfer compounds can be isolated ia the crystalline state, although low temperatures are often required. The bromine—dioxane compound, for example, has a chain stmeture (42). 

Photochromism Based on Tautomerism. Several substituted anils of saHcylaldehydes are photochromic but only in the crystalline state. The photochromic mechanism involves a proton transfer and geometric isomerization (21). An example of a photochromic anil is /V-sa1icylidene-2-ch1oToani1ine [3172-42-7] C H qCINO. 

Photochromism Based on Redox Reactions. Although the exact mechanism of the reversible electron transfer is often not defined, several viologen salts (pyridinium ions) exhibit a photochromic response to uv radiation in the crystalline state or in a polar polymeric matrix, for example,... 

In the foregoing discussion polymers have been used as a medium for smaU molecules in comparison with the crystalline state. It has also been observed that there are changes in polymeric geometry and various rotational motions introduced by pressure (10—14). These are at times reflected in the absorption spectmm (usuaUy in the ultraviolet) or in the emission spectmm and are a form of piezochromism. 

From studies reported in the references in Table 5 (Section 4.04.1.3.1) the dihedral angle between a phenyl and a pyrazole ring in the crystalline state, falls between 4° and 22° when the phenyl group is in the 3- or 4-position. The planar conformation of C-formylpyrazoles (57) and the resonance interaction between them (87) has already been discussed in connection with H NMR (Section 4.04.1.3.3(i)) and IR studies (Section 4.04.1.3.7(iii)). 

Vibrational spectra including Raman data of 3,3-dimethyldiaziridine and its hexadeutero compound were recorded in the gas phase and in the crystalline state. Assuming C2 symmetry and employing isotopic shifts and comparison with azetidine, a classification of bands which regarded 33 normal modes could be given (75SA(A)1509). 

This effect is also observed with some polymers. The trans form of a hydrocarbon chain requires an energy about 0.8 kcal/mole less than the gauche. The trans form leads to an extended molecule and in hydrocarbons this becomes more favoured as the temperature is lowered. Linear polyethylenes take up this conformation in the crystalline state. 

The commercial poly-(4-methypent-1-ene) (P4MP1) is an essentially isotactic material which shows 65% crystallinity when annealed but under more normal conditions about 40%. For reasons given later the material is believed to be a copolymer. In the crystalline state P4MP1 molecules take up a helical disposition and in order to accommodate the side chains require seven monomer units per two turns of the helix (c.f. three monomers per turn with polypropylene and polybut-I-ene). Because of the space required for this arrangement the density of the crystalline zone is slightly less than that of the amorphous zone at room temperature. 

Study of the structure of cellulose (Figure 22.2) leads one to expect that the molecules would be essentially extended and linear and capable of existing in the crystalline state. This is confirmed by X-ray data which indicate that the cell repeating unit (10.25 A) corresponds to the cellobiose repeating unit of the molecule. 

Bragg, W.H. and Bragg, W.L. (1939) The Crystalline State A General Survey (Bell and Sons, London). 

Many high molecular weight synthetic polymers, such as polyethylene and polypropylene, have a large percentage of their molecules in the crystalline state. Prior to dissolution, these polymers must usually be heated almost to their melting points to break up the crystalline forces. Orthodichlorobenzene (ODCB) is a typical mobile phase for these polymers at 150°C. The accuracy and stability of the Zorbax PSM columns under such harsh conditions make them ideal for these analyses (Fig. 3.8). 

The triphenyl analogues are also monomeric in solution but tend to associate into chain structures in the crystalline state as a result of weak interraolecular M---C interactions GaPhr mp... 

A synthetic strategy which ensures retention of the monomeric form of SnR2 even in the crystalline state is to use functionalized R groups which contain a chelating substituent, e.g. by replacing the H atom in -CH(SiMe3)2 with a 2-pyridyl group. 

Orthophosphoric acid is a remarkable substance it can only be obtained pure in the crystalline state (mp 42.35°C) and when fused it slowly undergoes partial self-dehydration to diphosphoric acid ... 

JCS (P2) 1651]. For a compound existing in an enol-keto tautomeric equilibrium, the name used in this review is that of the tautomer found in the crystalline state. 

Studies of presumed azido-tetrazole tautomerism for 3-azido-l,2,4-benzotria-zine 1-oxides 3 show that compounds 3 exist only in the form of azides, both in the crystalline state and in solution. Similar results were obtained in the studies of 3-azido-1,2,4-triazine 1-oxides 4, which have never been detected in the tetrazole form (69CB3818, 77JHC1221, 82JOC3886). 

---