Covalently extended

Figure 7.4 (a) Schematic illustration of the tethering with covalent extenders technique. Reprinted from Reference 43, with permission from Macmillan Publishers Ltd, Copyright (2003). (b) Schematic illustration of the site-specific DCC technique. Reprinted from Reference 27, with permission from Elsevier, Copyright (2008). 

We have shown some of the structures exhibited by inorganic compounds. Others will be described as we examine the chemistry of particular elements and their compounds. Also, a great many covalent extended structures (e.g., diamond, graphite, quartz, boron nitride, etc.) will be discussed in later chapters as we deal with the chemistry of those materials. The discussion presented here is sufficient to serve as a basis for describing the behavior and properties of many inorganic substances. This is an important objective for understanding descriptive chemistry. 

Covalent (extended anion) as for ionic solids Mineral dehydrations (diffusive escape of product HjO) 7,8 5... 

Building Units Constructed from Covalently Extended Pillararenes... 

Organosilanes, such as trichlorosilanes or trimethylsilanes, can establish SA monolayers on hydroxylated surfaces. Apart from their (covalent) binding to the surface these molecules can also establish a covalent intennolecular network, resulting in an enlranced mechanical stability of the films (figure C2.4.11). In 1980, work was published on the fonnation of SAMs of octadecyltrichlorosilane (OTS) 11171. Subsequently, the use of this material was extended to the fonnation of multilayers 11341. 

If a covalent bond is broken, as in the simple case of dissociation of the hydrogen molecule into atoms, then theRHFwave function without the Configuration Interaction option (see Extending the Wave Function Calculation on page 37) is inappropriate. This is because the doubly occupied RHFmolecular orbital includes spurious terms that place both electrons on the same hydrogen atom, even when they are separated by an infinite distance. 

Here N is the number of bonds or molecules of a given type in the crystal, and is a geometric tensor associated with a particular microscopic polarizabiHty P this tensor is related to the crystallographic orientation of the bond. In extended systems such as covalent soHds it becomes difficult to define a species to which one can assign a unique value of P, and thus the value of P for a given group can only be an approximate representation. In... 

The NMR spectra of both the parent [2,3-f ] and [3,4-f ] pyridopyrazine systems have been analyzed (66JCS(C)999). Shift values are given in Table 3. These studies were extended to the phenomenon of covalent hydration in both systems (66JCS(C)999,79JHC301) (see Section 2.15.13.2), as well as the addition of other nucleophiles such as amide ion (79JHC301, 79JHC305). 

The P-—N and P-N bonds arc equivalent in these compounds and they could perhaps better be written as [X3P" N= PX3], etc. Like the parent phosphorus pcntahalidcs (p. 498). these diphosphazenes can often exist in ionic and covalent forms and they are part of a more extended group of compt unds which can be classified into several general series Cl(Cl2PN) PCl4. [Cl(Cl2PN) PCl3] Cl, ... 

The cation of 4,4 -biquinazolinyl and its 2,2 -dimethyl derivative readily add water across the 3,4- and 3, 4 -double bonds, but the cation of 2,2 -biquinazolinyl is not hydrated. Hydration in the 4,4 -isomers has been attributed to restricted rotation about the 4,4 -bond, a steric effect which is relieved by hydration. The ultraviolet spectrum of 2,2 -biquinazolinyl (neutral species and cation) shows that there is considerable conjugation between the quinazoline groups. Covalent hydration is absent from the latter compound because it would otherwise destroy the extended conjugation present. 

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