Halogen-centred cations

These short Au-Au contacts may be compared with distances of 2.88 A in metallic gold and 2.60 A in gaseous Au2. The term aurophilicity has been coined by H. Schmidbaur to describe the phenomenon [189,194], The interactions can occur as pairs, squares, linear chains or two-dimensional arrays of gold centres. Examples include the association between dimer units in the dithiocarbamates Au(dtc)2 (Figure 4.16) and the ionic tetrahydrothiophen complexes Au(tht)2 AuXJ (X = halogen), where cations and anions stack with Au—Au 2.97-2.98 A (X = I). The interaction is such that Au(S203)2 pair up, despite their charge, with Au-Au 3.24 A in the sodium salt. Likewise in Aupy AuCl, cations pair up at 3.42 A apart [10, 195]. 

Impurity and Aperiodicity Effects in Polymers.—The presence of various impurity centres (cations and water in DNA, halogens in polyacetylenes, etc.) contributes basically to the physics of polymeric materials. Many polymers (like proteins or DNA) are, however, by their very nature aperiodic. The inclusion of these effects considerably complicates the electronic structure investigations both from the conceptual and computational points of view. We briefly mentioned earlier the theoretical possibilities of accounting for such effects. Apart from the simplest ones, periodic cluster calculations, virtual crystal approximation, and Dean s method in its simplest form, the application of these theoretical methods [the coherent potential approximation (CPA),103 Dean s method in its SCF form,51 the Hartree-Fock Green s matrix (resolvent) method, etc.] is a tedious work, usually necessitating more computational effort than the periodic calculations... 

U.19 SALTS OF CARBON-, BORON-, HALOGEN- AND PNICTOGEN-CENTRED CATIONS AND OF SULFUR-AND ARSENIC-CONTAINING HETEROCYCLES... 

The presence of a heteroatom adjacent to the cationic centre and bearing an unshared pair of electrons, like nitrogen, oxygen or halogen increases stability... 

The analysis of close contacts at divalent sulphur and selenium centres reveals a picture very similar to that observed for halogen atoms. Close contacts again fall into two classes (Ramasubbu et al., 1986 Sakurai et al., 1963 Rosenfield et al., 1977), depending on whether the second centre involved is an electrophile E (a metal cation or an electrophilic hydrogen, or carbon halogen atom), or a nucleophilic anion Nu. Similar preferences are observed for S-S contacts (Ramasubbu et al., 1986). 

The stability of the carbocation is also increased due to the presence of heteroatom having an unshared pair of electrons, e.g. oxygen, nitrogen or halogen, adjacent to the cationic centre. Such carbocations are stabilized by resonance. The methoxymethyl cation is obtained as a stable solid, MeOCH2 SbF4. ... 

Various heteroatoms have been used to facilitate carbenium ion formation at less substituted centres, including halogens,9"" nitrogen,12 selenium,13 and sulfur.14 These reactions generally proceed via activation of the alkene, through formation of a cyclic cationic intermediate 25.15... 

The two types of stabilization are not eguivalent the cation and the bromonium ion are different molecules with different shapes, while the two representations of the oxonium ion are just that—they aren t different molecules. This stabilization of an adjacent cationic centre by a heteroatom with at least one lone pair to form a three-membered ring intermediate is not restricted to bromine or the other halogens, but is also an important aspect of the chemistry of compounds containing oxygen, sulfur, or selenium, as you will see in Chapter 27. 

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