Spherical halide ions

Imakubo T, Shirahata T, Herve K, Ouahab L (2006) Supramolecular organic conductors based on diiodo-TTFs and spherical halide ion X—(X=C1, Br). J Mater Chem 16 162-173... 

Due to the favorable conformation for multiple H-bonds, tren-based tripodal tris (urea) receptors, including 38a-j, have proven to be a promising scaffold for anion binding. Such receptors not only complementarily encapsulate oxoanions (sulfate, carbonate, and phosphate) but also display excellent binding ability to spherical halide ions. Several groups studied tripodal trisurea receptors with different substituents. 

A majority of the host systems mentioned so far have been shown to encapsulate halide guests as a consequence of the symmetrical nature of their binding pockets, which readily adapt to the spherical symmetry of the halide ion. A study of the bis-tren macrobicyclic ligand 19-6H, however, has revealed that, in addition to accommodating halide anions, it is also able to encapsulate azide, N, within its cylindrical cavity. Solution stability constant measurements indicate that the host... 

Spherical recognition of halide ions is displayed by protonated macropolycyclic polyamines. Thus, macrobicyclic diamines yield katapinates [3.9]. Anion cryptates are formed by the protonated macrobicyclic 16-6H+ [2.52] and macrotricyclic 21-4H+ [2.97] polyamines, with preferential binding of F and Cl- respectively in an octahedral and in a tetrahedral array of hydrogen bonds. 

The non-complementarity between the ellipsoidal 33-6H+ and the spherical halides results in much weaker binding and appreciable distortions of the ligand, as seen in the crystal structures of the cryptates 35 where the bound ion is F , Cl-, or Br-. In these complexes, F- is bound by a tetrahedral array of hydrogen bonds whereas Cl- and Br- display octahedral coordination (Fig. 4). Thus, 33-6H+ is a molecular receptor for the recognition of linear triatomic species of a size compatible with the size of the molecular cavity [3.11]. 

Mass transport of developer to, and oxidized developer and halide ions from, the silver speck is calculated by the method of spherical diffusion. The surface concentrations of the active species are then used in the Nemst equation to calculate the surface potential of the developing nucleus. The rate equation (79) obtained is ... 

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. ... 

In a completely ionic situation, an S state halide ion has spherical symmetry and the coupling constants are small, arising from lattice, shielding and overlap effects. In a partially covalent situation charge transfer will significantly affect /q if there is an excess of bonding over antibonding electrons. Thus, for a cen-... 

The alkali and halide ions have played an important role in studies of ionic solutions. The reasons for this are obvious. The alkali halides form simple salts which are soluble in water. The ions have spherical symmetry and vary sufficiently in size, thus permitting studies of the dependence of solvation on ionic diameter. 

In the present work, Eq. (17) was used to calculate the ion-dipole interaction energies for the alkali halide water dipole system. Here, water was treated as a simple spherical molecule of radius 0.14nm with a point dipole of moment 1.85D. Also, based on steric and hydration effects, three cases—A, B, and C—were used to calculate the ion-dipole interaction energies. (See Table 6.) Case A considers the alkali halides KF, RbF, CsF, CsCl, CsBr, and Csl, which have an alkali ion radius close to that of the water molecule and a ratio of hydration numbers of alkali ion to halide ion of less than 2. Case B considers the alkali halides LiF, NaF, LiCl, LiBr, NaBr, Lil, Nal, and Kl, which have an alkali ion radius less than that of the water molecule and a ratio of hydration numbers of alkali ion to halide ion of more than 2. Finally, case C considers the alkali halides KCl, RbCl, KBr, RbBr, and Rbl, which have an alkali ion radius close to that of the water molecule and a ratio of... 

The simplest recognition process is that of spherical substrates which may be cationic (alkali and alkaline-earth ions) or anionic (halide ions). 

Anion cryptates are formed by macrotricycles like (5) in their tetraprotonated state with the spherical halide anions [8]. (5)-4H binds the chloride ion very strongly and very selectively, giving the [Cl" c (5)-4H J cryptate (7), but does not complex other types of anions. These properties are unique at present with respect to both synthetic and natural halide binding sites, very little being known about the latter. Non-complementarity between an ellipsoidal cryptand and the spherical halides results in appreciable ligand distortions in the cryptates formed and in lower binding constants [9, 10] (see also below). 

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°. 

Figure 14-2 Atomic-orbital representation of the transition state for Sn2 displacement of 3-chloropropene (allyl chloride) with iodide ion. The halide orbitals are represented here as spherical for simplicity. The purpose of this figure is to show the -n bonding, which can stabilize the transition state.

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