Crystalline forms

Aluminium oxide occurs naturally as emery (an impure form) and as corundum. Corundum is a crystalline form which may be coloured by traces of impurity, for example as ruby (red) and sapphire (blue). Small synthetic rubies and sapphires have been made by heating alumina with the colouring oxide in an oxy-hydrogen flame. 

Silicon burns when heated in air to red heat giving silieon dioxide, Si02- Several crystalline forms of Si02 are known. 

The solid is essentially ionic, made up of Pb and Cl ions. The vapour contains bent molecules of PbCh (cf. SnCh). Lead chloride is precipitated when hydrochloric acid (or a solution of a chloride) is added to a cold solution of a lead(ll) salt. It dissolves in hot water but on cooling, is slowly precipitated in crystalline form. It dissolves in excess of concentrated hydrochloric acid to give the acid H2[Pb"Cl4]. 

It is a white, deliquescent solid, very powdery, which exhibits polymorphism on heating, several different crystalline forms appear over definite ranges of temperature -ultimately, the P4O10 unit in the crystal disappears and a polymerised glass is obtained, which melts to a clear liquid. 

This occurs naturally as a white solid in various crystalline forms, in all of which six oxygen atoms surround each titanium atom. Titanium dioxide is important as a white pigment, because it is nontoxic. chemically inert and highly opaque, and can be finely ground for paint purposes it is often prepared pure by dissolving the natural form in sulphuric acid, hydrolysing to the hydrated dioxide and heating the latter to make the anhydrous form. 

Many molecules are obtained and used in a crystalline form, the nature of which can have e significant impact on their properties and behaviour. Moreover, it is sometimes possible foi a given material to exist in more than one crystalline form, depending upon the conditions under which it was prepared. This is the phenomenon of polymorphism. This can be important because the various polymorphs may themselves have different properties. It is Iberefore of interest to be able to predict the three-dimensional atomic structure(s) that a gi en molecule may adopt, for those cases where it is difficult to obtain experimental data and also where one might wish to prioritise molecules not yet synthesised. 

Many different approaches have been suggested as possible approaches to this problem, from the 1960s onwards [Verwer and Leusen 1998]. What is obvious from all of these ellorts is that this is an extremely difficult problem. Both thermodynamics and kinetics can be important in determining which crystalline form is obtained under a certain se1 of experimental conditions. Kinetic effects are particularly difficult to take into accouni and so are usually ignored. A proper treatment of the thermodynamic factors would lequire one to deal with the relative free energies of the different possible polymorphs... 

Both maltose and lactose, being reducing sugars, give osazones which differ from one another and from glucosazone in crystalline form. Sucrose (G-r-r-F), having no potential aldehyde or ketone grouping, does not form an osazone. 

If it is desired to observe the crystalline form of the osazone, draw up in a glass tube a few drops of the cold filtrate containing the fine crystals, and transfer to a microscope slide. Cover the drops with a slip and examine under the microscope unless the filtrate has been cooled very slowly and thus given moderately-sized crystals, the high power of the microscope will probably be required. Note the fine yellow needles aggregated in the form of sheaves. Compare with Fig. 63(A). 

Conduct the preparation in the fume cupboard. Dissolve 250 g. of redistilled chloroacetic acid (Section 111,125) in 350 ml. of water contained in a 2 -5 litre round-bottomed flask. Warm the solution to about 50°, neutralise it by the cautious addition of 145 g. of anhydrous sodium carbonate in small portions cool the resulting solution to the laboratory temperature. Dissolve 150 g. of sodium cyanide powder (97-98 per cent. NaCN) in 375 ml. of water at 50-55°, cool to room temperature and add it to the sodium chloroacetate solution mix the solutions rapidly and cool in running water to prevent an appreciable rise in temperature. When all the sodium cyanide solution has been introduced, allow the temperature to rise when it reaches 95°, add 100 ml. of ice water and repeat the addition, if necessary, until the temperature no longer rises (1). Heat the solution on a water bath for an hour in order to complete the reaction. Cool the solution again to room temperature and slowly dis solve 120 g. of solid sodium hydroxide in it. Heat the solution on a water bath for 4 hours. Evolution of ammonia commences at 60-70° and becomes more vigorous as the temperature rises (2). Slowly add a solution of 300 g. of anhydrous calcium chloride in 900 ml. of water at 40° to the hot sodium malonate solution mix the solutions well after each addition. Allow the mixture to stand for 24 hours in order to convert the initial cheese-Uke precipitate of calcium malonate into a coarsely crystalline form. Decant the supernatant solution and wash the solid by decantation four times with 250 ml. portions of cold water. Filter at the pump. 

Dissolve 34 g. of o-nitroaniline in a warm mixture of 63 ml. of concentrated hydrochloric acid and 63 ml. of water contained in a 600 ml. beaker. Place the beaker in an ice - salt bath, and cool to 0-5° whilst stirring mechanically the o-nitroaniline hydrochloride will separate in a finely-divided crystalline form. Add a cold solution of 18 g. of sodium nitrite in 40 ml. of water slowly and with stirring to an end point with potassium iodide - starch paper do not allow the temperature to rise above 5-7 . Introduce, whilst stirring vigorously, a solution of 40 g. of sodium borofluoride in 80 ml. of water. Stir for a further 10 minutes, and filter the solid diazonium fluoborate with suction on a sintered glass funnel. Wash it immediately once with 25 ml. of cold 5 per cent, sodium borofluoride solution, then twice with 15 ml. portions of rectified (or methylated) spirit and several times with ether in each washing stir... 

Amorphous or "plastic" sulfur is obtained by fast cooling of the crystalline form. X-ray studies indicate that amorphous sulfur may have a helical structure with eight atoms per spiral. Crystalline sulfur seems to be made of rings, each containing eight sulfur atoms, which fit together to give a normal X-ray pattern. 

Once the reaction mix has cooled after reflux, 500mL of room temperature dHsO can be added and the whole solution extracted with DCM. The DCM layer is separated and the solvent removed by distillation to give the li-nitropropene as an oil of all things. This oil can then be recrystallized in hot methanol just like the crystalline form was [38]. 

The irradiation of tetra-/-butylcyclopentadienone with 254 nm light at 77 K produced a tricyclopentanone which, upon extended irradiation, lost carbon monoxide. Tetra-f-butyltetrahedrane was formed. This derivative of the second fundamental hydrocarbon of molecular formula (CH), namely tetrahedrane, is stable at room temperature and could be isolated after chromatography on silica gel in crystalline form (G. Maier, 1978). 

In Parylene C, the single crystalline form observed is very similar to the d form of Parylene N. Its detailed crystal stmcture has been deterrnined (a = 596 pm, b = 1269 pm, c (chain axis) = 666 pm, /5 = 135.2°) (47). X-ray studies on the crystal stmcture of Parylene D have not been reported. 

Absorption and Fluorescence Spectra. The absorption spectra of actinide and lanthanide ions in aqueous solution and in crystalline form contain narrow bands in the visible, near-ultraviolet, and near-infrared regions of the spectmm (13,14,17,24). Much evidence indicates that these bands arise from electronic transitions within the and bf shells in which the Af and hf configurations are preserved in the upper and lower states for a particular ion. 

In milk fat, cholesterol is associated with Hpoproteins in the milk fat globule. It is also a component of animal membranes and controls rigidity and permeabihty of the membranes. Cholesterol has interesting surface properties and can occur in Hquid crystalline forms. Plants contain sterols such as P-sitosterol [83-46-5] (4b) or stigmasterol [83-48-7] (4c). Their functions in plant metaboHsm are not yet well understood. Analysis of sterols has proven useful for detection of adulteration of edible fats (9). 

At very low concentrations of water, or in foods held below the free2ing point of water, physical conditions may be such that the available water may not be free to react. Under these conditions, the water may be physically immobi1i2ed as a glassy or plastic material or it may be bound to proteins (qv) and carbohydrates (qv). The water may diffuse with difficulty and thus may inhibit the diffusion of solutes. Changes in the stmcture of carbohydrates and proteins from amorphous to crystalline forms, or the reverse, that result from water migration or diffusion, may take place only very slowly. 

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