Centres cationic

The idea that a metal-centred cation, arising from a BIE, can cationate an alkene and thus be an initiator could have been propounded by this author when he discovered the BIE of titanium tetrachloride [30] ... 

Bimolecular ion/molecule reactions of dienes and polyenes have been extensively studied for several reasons. Some of them have been mentioned implicitly in the previous sections, that is, in order to structurally characterize the gaseous cations derived from these compounds. In this section, bimolecular reactivity of cationic dienes, in particular, with various neutral partners will be discussed, and some anion/molecule reactions will be mentioned also (cf Section IV). In addition, the reactions of neutral dienes with several ionic partners will also be discussed. Of this latter category, however, the vast chemistry of reactions of neutral dienes with metal cations and metal-centred cations will not be treated here. Several reviews on this topic have been published in the last decade178. 

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... 

A A, Aj, A2 AC Ac AIBN As A °p acac [al] acceptor dilatometric constant, conversion factor active centre cation, cationic part of an acid 2,2 -azo-bis-isobutyronitrile solvated anion radical excited acceptor surface area of a particle of volume V 2,4-pentanedione-3yl (acetylacetonate) concentration of a monomeric organoaluminium compound at the surface of a catalyst... 

In cationic polymerizations, initiation occurs by attachment of a proton or some other Lewis-acidic cation X" to the H2C=CR2 double bond of a vinyl monomer to form a new carbon-centred cation of the type XH2C-CR2, which then grows into a polymer chain by subsequent H2C=CR2 additions (Figure 2, bottom). This type of polymerization works well - and is used in practice - only for olefins such as isobutene, where 1,1-disubstitution stabilizes the formation of a cationic centre. Since side reactions, such as release of a proton from the cationic chain end, occur rather easily, cationic polymerization usually gives shorter chains than anionic polymerization. 

Calculations by Hiickel s method predict a greater stability for the three-centre cation 549 this is in accord with the data obtained in the acetolysis of a deuterated sample. While ion 549 is homoaromatic, ion 550 is antihomoaromatic (4 delocalized electrons). 

SATTS OF CARBON-. BORON-. HAT.OGF.N- AND PNICTQGEN-CENTRED CATIONS AND OF SULFUR- AND ARSKNTC-CONTAINING HETEROCYCLES... 

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