Ammonium halides, structure

These considerations also explain the occurrence of cases of dimorphism involving the sodium chloride and cesium chloride structures. It would be expected that increase in thermal agitation of the ions would smooth out the repulsive forces, that is, would decrease the value of the exponent n. Hence the cesium chloride structure would be expected to be stable in the low temperature region, and the sodium chloride structure in the high-temperature region. This result may be tested by comparison with the data for the ammonium halides, if we assume the ammonium ion to approximate closely to spherical symmetry. The low-temperature form of all three salts, ammonium chloride, bromide and iodide, has the cesium chloride structure, and the high-temperature form the sodium chloride structure. Cesium chloride and bromide are also dimorphous, changing into another form (presumably with the sociium chloride structure) at temperatures of about 500°. 

A very small hump in the heat capacity curve at —30° was also found for ammonium fluoride the interpretation of this is uncertain (the structure of this crystal is not the same as that of the other ammonium halides). 

They find that heating cyclotri- and cyclotetrasilazanes with ammonium halide catalysts at temperatures of 160°C for 6-8 h results in a condensation polymerization process that evolves ammonia and leads to the formation of waxy polysilazanes. Analytical results suggest that these polysilazanes consist of rings linked by silyl bridges as illustrated by the following structure ... 

Among the alkali halides, the cesium chloride structure is found only in CsCl CsBr, and Csl at ordinary pressures, but all of the alkali halides except the salts of lithium can be forced into the CsCl structure at higher pressures. It is also adopted by the ammonium halides (except NH4F), TIC1, TlBr, T1CN, CsCN, CsSH, CsSeH, and CsNH2. 

Following the findings of Butler et al. [5], that diallyl quaternary ammonium halides form water soluble polymers, the structures of polymers produced via a ring-closing mechanism have been the subject of intensive research. A cyclic structure was elucidated and the mechanism was defined as an alternating intra-intermolecular chain propagation, later termed cyclopolymerization [3]. Based on the general scheme presented in Fig. 6 the chemical structure of PDADMAC is determined by... 

A number of other receptors structurally relating to 36 were described that possess overall similar binding properties although some characteristic differences were noted. Compound 37, for example, containing a cyclotriveratrylene-derived cap instead of a TREN moiety, also binds imidazolidin-2-one or ammonium halides, but since this compound lacks the tertiary amino group it does not allow binding properties to be controlled by protonation [84],... 

As the coordination number of an ion is thus increased from 6 to 8, it appears that the ionic radius also suffers a slight increase, presumably because the repulsion forces, exerted by the electron clouds of neighboring ions, increase. For the rubidium halides and the ammonium halides which can assume either structure (depending upon pressure and temperature), the interionic distances in the structures having coordination number 8 are about 3 percent greater than the distances in structures with coordination number 6. This increase is close to the value that would be... 

Figure 4. Structures of ammonium halides (a) CsCl-type of structure shown by NH4CI, NFUBr, and NH4I (b) wurtzite (ZnS) structure, shown by NH4F, and induced by the formation of N-H--F hydrogen bonds.

The crystal structure of the ammonium halides, which, with the exception of NH4F, have modifications with the CsCl-and the NaCl lattice, cannot be understood along these lines the structure of the NH4+ ion plays an important part. 

When a solution of a quaternary ammonium halide is treated with silver oxide, silver halide precipitates. When the mixture is filtered and the filtrate is evaporated to dryness, there is obtained a solid which is free of halogen. An aqueous solution of this substance is strongly alkaline, and is comparable to a solution of sodium hydroxide or potassium hydroxide. A compound of this sort is called a quaternary ammonium hydroxide. It has the structure R4N OH. Its aqueous solution is basic for the same reason that solutions of sodium or potassium hydroxide are basic the solution contains hydroxide ions. 

The ammonium ion is about the same size ( ,. = 151 pm) as the potassium ion (r = 152 pm) and this is a useful fact to remember when explaining the resemblance in properties between these two ions. For example, the solubilities of ammonium salts are similar to those of potassium salts. Explain the relation between ionic radius and solubility. On the other hand, all of the potassium halides crystallize in the NaCI structure with C.N. = 6 (see Chapter 4). but none of the ammonium halides does so. The coordination numbers of the ammonium halides are either four or eight. Suggest an explanation. 

Some crystalline compounds formed by AS2O3 with alkali and ammonium halides, for example, NH4CI.AS2O3. H2O, KCl. 2 AS2O3, etc., are closely related structurally to arsenious oxide. In the former, there are hexagonal AS2O3 sheets, with all O atoms directed to one side of the sheet, arranged in pairs with the... 

This article describes the solid state polymerization of 1,i-disubstituted butadiene derivatives in perovskite-type layer structures, in layered structures of organic ammonium halide salts, and in lipid layer structures. Recent investigations by spectroscopic methods and x-ray structure analyses are described. The studies clearly indicate that the photolysis in the crystalline state leads to the formation of 1,i-trans-polymers exclusively. Crystal structure analyses of monomeric and polymeric layer perovskites demonstrate that upon y-irradiation a stereoregular polymer is obtained in a lattice controlled polymerization. 

When the pseudo-spherical ammonium ion is mostly replaced by a truly spherical ion the complex sequence of phase changes found in the pure ammonium halides is suppressed. The mixed potassium ammonium halide salts retain their NaCl cubic structure down to the lowest temperatures. The alkali metal ions support the structure leaving the ammonium ions as free to rotate at 1 K as at 300K [13]. The INS spectrum of this system is quite different from the pure salt and there are no sharp features in any region of the spectrum. We shall analyse the impact that this freedom has on the internal modes about 1400 cm. ... 

The structures of a number of ammonium salts have already been described. In many of them the NH4+ ion is in free rotation and can be treated as a spherical ion of radius 1 48 A so that the structures observed are determined by the usual geometrical considerations. Thus most of the ammonium halides have either the sodium chloride or the caesium chloride structure. In ammonium fluoride, however, the cation is not in free rotation and the wurtzite structure results. 

We have already seen ( 8.06) that all the ammonium halides, except the fluoride, have either the caesium chloride or the sodium chloride structure and that the NH4+ ion is in free rotation and behaves as a sphere of effective radius 1-48 A. (They are, in fact, all dimorphous with a transformation from the former to the latter structure as the temperature is raised.)... 

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