Halide structures

Figure 2.2 Variation of Schottky formation energy of the sodium chloride structure halides MX M = Li, Na, K X = F, Cl, Br, I.

Type of crystal structure Halide Vapour species... 

The ionic conductivity of fluorite-structured halides such as SrF2, CaF, BaF, (3- Pb F 2, and SrCh, shows a rapid but continuous increase on heating, reaching values very dose to those shown by the liquid state [67-69]. The transition to the superionic state is also associated with peak in the specific heat Cp (the maximum value of which is generally taken to define the superionic transition temperature, f), anomalous behavior ofthe elastic constants and changes in the lattice expansivity (for details and references, see Refs. [38, 70]). 

Of all the fluorite-structured halides, P-PbF2 has attracted the most attention, since it has the lowest superionic transition temperature (Tc = 711 K). The ionic conductivity and structural properties of a number of compounds based on PbF2 have been studied, with the ternary compound PbSnF4 possessing one of the highest values of ionic conductivity of any F ion conductor at ambient temperature (<7, = 10 S cm- [76]). 

The induction energy is inlierently non-additive. In fact, the non-additivity is displayed elegantly in a distributed polarizability approach [28]. Non-additive induction energies have been found to stabilize what appear to be highly improbable crystal structures of the alkalme earth halides [57]. 

Miller T M, Leopold D G, Murray K K and Lineberger W C 1986 Electron affinities of the alkali halides and the structure of their negative ions J. Chem. Phys. 85 2368-75... 

Salaita G N, Lu F, Laguren-Davidson L and Flubbard A T 1987 Structure and composition of the Ag(111) surface as a function of electrode potential in aqueous halide solutions J. Electroanal. Chem. 229 1-17... 

Matsumoto H, Inukai J and Ito M 1994 Structures of copper and halides on Pt(111), Pt(IOO) and Au(111) electrode surfaces studied by in situ scanning tunneling microscopy J. Eiectroanai. Chem. 379 223-31... 

The melting and boiling points of the aluminium halides, in contrast to the boron compounds, are irregular. It might reasonably be expected that aluminium, being a more metallic element than boron, would form an ionic fluoride and indeed the fact that it remains solid until 1564 K. when it sublimes, would tend to confirm this, although it should not be concluded that the fluoride is, therefore, wholly ionic. The crystal structure is such that each aluminium has a coordination number of six, being surrounded by six fluoride ions. 

The solid anhydrous halides of some of the transition metals are often intermediate in character between ionic and covalent their structures are complicated by (a) the tendency of the central metal ion to coordinate the halide ions around it, to form an essentially covalent complex, (b) the tendency of halide ions to bridge, or link, two metal ions, again tending to covalency (cf. aluminium chloride, p. 153 and iron(III) chloride, p. 394). 

In the gold(lll) halides (except the fluoride) there is evidence for the formation of double molecules, AujXg (cf. chlorides of iron(III) and aluminium) so that the coordination is brought up to four, but with a planar structure ... 

The summation is over the different types of ion in the unit cell. The summation ca written as an analytical expression, depending upon the lattice structure (the orij Mott-Littleton paper considered the alkali halides, which form simple cubic lattices) evaluated in a manner similar to the Ewald summation this typically involves a summc over the complete lattice from which the explicit sum for the inner region is subtractec... 

CHEOPS is based on the method of atomic constants, which uses atom contributions and an anharmonic oscillator model. Unlike other similar programs, this allows the prediction of polymer network and copolymer properties. A list of 39 properties could be computed. These include permeability, solubility, thermodynamic, microscopic, physical and optical properties. It also predicts the temperature dependence of some of the properties. The program supports common organic functionality as well as halides. As, B, P, Pb, S, Si, and Sn. Files can be saved with individual structures or a database of structures. 

As well as the cr-complexes discussed above, aromatic molecules combine with such compounds as quinones, polynitro-aromatics and tetra-cyanoethylene to give more loosely bound structures called charge-transfer complexes. Closely related to these, but usually known as Tt-complexes, are the associations formed by aromatic compounds and halogens, hydrogen halides, silver ions and other electrophiles. 

Tertiary acetylenic halides give unsatisfactory results owing to further isomerization of the allenic halide into a conjugated diene system under the influence of the copper salt. Bromo- and iodoallenes with the structures R R2C=C=CH-X can also be synthesized by an aqueous procedure, consisting of reaction between... 

The terminal amino group of 2-hydrazino-4-phenylthiazole is also the reactive center in reactions with activated aryl halides such as 288. A solution of the product (289) obtained from this reaction when shaken with PbOj gives a deeply colored radical, whose structure has been studied by ESR (Scheme 173) (532. 533). 

Mercuric halides, silver nitrate, and copper nitrates form stable complexes with bis-2.2 -thiazolylazo compounds (1591). for which the X-ray structure is not yet known. 

While developing the connections between structure reaction and mechanism we will also extend the fundamentals of lUPAC nomenclature to functional group families beginning with alcohols and alkyl halides... 

Wnte structural formulas for each of the following alcohols and alkyl halides... 

The 8n2 mechanism is believed to describe most substitutions m which simple pri mary and secondary alkyl halides react with anionic nucleophiles All the examples cited in Table 8 1 proceed by the 8 2 mechanism (or a mechanism very much like 8 2— remember mechanisms can never be established with certainty but represent only our best present explanations of experimental observations) We 11 examine the 8 2 mecha nism particularly the structure of the transition state in more detail in 8ection 8 5 after hrst looking at some stereochemical studies carried out by Hughes and Ingold... 

In general 8 2 reactions of alkyl halides show the following dependence of rate on structure... 

The Stvfl mechanism is an ionization mechanism The nucleophile does not participate until after the rate determining step has taken place Thus the effects of nucleophile and alkyl halide structure are expected to be different from those observed for reactions pro ceedmg by the 8 2 pathway How the structure of the alkyl halide affects the rate of Stvfl reactions is the topic of the next section... 

Substitution can take place by the S l or the 8 2 mechanism elimination by El or E2 How can we predict whether substitution or elimination will be the principal reac tion observed with a particular combination of reactants The two most important fac tors are the structure of the alkyl halide and the basicity of the anion It is useful to approach the question from the premise that the characteristic reaction of alkyl halides with Lewis bases is elimination and that substitution predominates only under certain special circumstances In a typical reaction a typical secondary alkyl halide such as iso propyl bromide reacts with a typical Lewis base such as sodium ethoxide mainly by elimination... 

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