Delocalization of bonding electrons

Delocalization of bonding electrons in the transition complex is an important event. In the absence of this far-reaching delocalization, even the most acid alkenes, ethylene and propene only undergo both radical and ionic polymerization reluctantly. 

At present, then, aromaticity is best defined in terms of stability derived from the delocalization of bonding electrons. An aromatic molecule is characterized by appreciable stabilization relative to a noncyclic polyene. An antiaromatic molecule is one that is destabilized relative to a polyene model, and the term nonaromatic can be applied to molecules for which the calculated energy and energy of the polyene model are comparable. Cyclobutadiene, with an estimated destabilization energy of 15-20 kcal/mol, is a good example of an antiaromatic species. 

Hydrophilic (Section 19 5) Literally water loving a term applied to substances that are soluble in water usually be cause of their ability to form hydrogen bonds with water Hydrophobic (Section 19 5) Literally water hating a term applied to substances that are not soluble in water but are soluble in nonpolar hydrocarbon like media Hydroxylation (Section 15 5) Reaction or sequence of reac tions in which an alkene is converted to a vicinal diol Hyperconjugation (Section 4 10) Delocalization of a electrons... 

FIGURE 4.16 Hyperconjugation in ethyl cation. Ethyl cation is stabilized by delocalization of the electrons in the C—H bonds of the methyl group into the vacant 2p orbital of the positively charged carbon. 

This reanangement is shown in orbital terms in Figure 5.8. The relevant orbitals of the secondary car bocation are shown in structure (a), those of the transition state for reanangement in (b), and those of the tertiary carbocation in (c). Delocalization of the electrons of the C—CH3 a bond into the vacant p orbital of the positively charged car bon by hyperconjugation is present in both (a) and (c), requires no activation energy, and... 

In the ring molecules containing a n bond, delocalization of tt electrons occurs through the interaction with o orbitals [19]. 

It is interesting to note in the latter connection that nucleophilic substitution transition states in which there apparently is not strong delocalization of pi electron density into the substituent tend to fall into the <7 type (cf. reactions 18 and 19 of Table VIII). In set 18, there are two ortho nitro groups which apparently take up much of the pi charge (thus it is unavailable to X), whereas in set 19, the positive piperidinium center may cause (perhaps with assistance from the NH hydrogen bonding permitted by the aprotic solvent) the... 

The history and development of polysilane chemistry is described. The polysilanes (polysilylenes) are linear polymers based on chains of silicon atoms, which show unique properties resulting from easy delocalization of sigma electrons in the silicon-silicon bonds. Polysilanes may be useful as precursors to silicon carbide ceramics, as photoresists in microelectronics, as photoinitiators for radical reactions, and as photoconductors. 

The polysilanes are compounds containing chains, rings, or three-dimensional structures of silicon atoms joined by covalent bonds. Recently, polysilane high polymers have become the subject of intense research in numerous laboratories. These polymers show many unusual properties, reflecting the easy delocalization of sigma electrons in the silicon-silicon bonds. In fact, the polysilanes exhibit behavior unlike that for any other known class of materials. 

It is important that these forms differ in the strength of the chemisorption bonding, i.e., in the heat of adsorption. The charged form is always stronger than the neutral form. Indeed, in the first case, unlike the second, desorption must be accompanied by the delocalization of an electron or hole this is always an endothermic process. 

As our quantum-chemical calculations show, similar transformation and delocalization of bonds takes place in the conductive forms of some other types of CPs (polyaniline, polypyrolle, etc.). Delocalization of chemical bonds after activation leads to appearance of an electronic conductivity in such types of conducting polymers and creates prerequisites for their application as electrode materials of electrochemical power sources. Such activation can be stimulated by intercalation of ions, applying the potential, and by use of some other low energetic factors. 

The orbital bonding nature within carbon nanotubes creates unique electrical properties within a non-metallic molecule, which is a result of the delocalization of the -electron donated by each atom. Electrical conductivity can take place along the entire nanotube due to the freedom of -electron flow, making possible the design of circuits of extremely low nanometer diameter. 

The bonding pattern of the last molecule in Table 4 is rather different from that of 1,3-butadiene, a fact which is probably connected to the ethynyl substituent that allows a further delocalization of the --electrons in this molecule. 

Substituted ethylenes, with hydrogen atoms replaced by various alkyl groups, have a common feature in their electronic spectra, i.e. an absorption band at ca. 164 — 180 nm. This band is interpreted to show that the delocalization of the electron pair is largely confined to the vicinity of the unsaturated centre, commonly referred to as the C=C double bond. If the de-localization is assumed not to exceed a linear distance of one bond length on either side of the double bond, the electron pair remains in a linear potential box of width 3d, with allowed energy levels of... 

As seen, in confirmation of the charge delocalization of the electron transfers, most distances inside the dmit ligand remain essentially unaltered. Nevertheless, the progressive shortening of the Ni-S bond suggests a non negligible contribution of the metal d orbitals to the frontier orbitals. 

The cyclic delocalization of jt-electrons in diazadiboretidines (XBNRjj, or borazines, (XBNRjj, etc., will be indicated by dotted lines over bonds, again favored over mesomeric structures. 

In 1949 Cox and co-workers ° from X-ray structural analysis and quantum-mechanical calculations suggested that thieno[3,2-b]thiophene (2) possesses a ground state intermediate form between the extremes with completely localized double bonds and that with complete delocalization of TT-electrons. The discrepancy between the theoretical and experimental (Fig. 1) values (0.05 A) for the central C,—Cg bond was noted, and the thienothiophene (2) molecule was shown to be planar and to have a center of symmetry. 

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