Delocalization of -bonding

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 treatment of the nitro group from the molecular orbital point of view is based on the formulae IV. Delocalization of bonds increases its... 

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. 

Osmabenzene 1 is a dark red-brown crystalline solid which is air stable both as a solid and in solution. X-ray structural analysis of 1 shows an essentially planar metallacycle and delocalization of bonding within the carbon portion of the ring the... 

Aromaticity is associated with delocalization of bonds and like the concept of a chemical bond itself, it is ill-defined. Various properties of molecules can be used to construct measures of aromaticity [51-53], Rigorous approaches estimating energy of the resonance stabilization are computationally expensive. A magnetic-property-based criterion of aromaticity called nuclear-independent chemical shift index (NICS) is a widely accepted measure of local aromatic character of a molecule... 

Boranes are typical species with electron-deficient bonds, where a chemical bond has more centers than electrons. The smallest molecule showing this property is diborane. Each of the two B-H-B bonds (shown in Figure 2-60a) contains only two electrons, while the molecular orbital extends over three atoms. A correct representation has to represent the delocalization of the two electrons over three atom centers as shown in Figure 2-60b. Figure 2-60c shows another type of electron-deficient bond. In boron cage compounds, boron-boron bonds share their electron pair with the unoccupied atom orbital of a third boron atom [86]. These types of bonds cannot be accommodated in a single VB model of two-electron/ two-centered bonds. 

FIGURE 4 16 Hyper conjugation in ethyl cation Ethyl cation is stabilized by delocalization of the elec trons in the C—H bonds of the methyl group into the vacant 2p orbital of the posi tively charged carbon... 

The first step protonation of the double bond of the enol is analogous to the pro tonation of the double bond of an alkene It takes place more readily however because the carbocation formed m this step is stabilized by resonance involving delocalization of a lone pair of oxygen... 

The degree to which allylic radicals are stabilized by delocalization of the unpaired electron causes reactions that generate them to proceed more readily than those that give simple alkyl radicals Compare for example the bond dissociation energies of the pri mary C—H bonds of propane and propene... 

C 1 IS more reactive because the intermediate formed by electrophilic attack there IS a relatively stable carbocation A benzene type pattern of bonds is retained m one nng and the positive charge is delocalized by allylic resonance... 

The orbital and resonance models for bonding in arylamines are simply alternative ways of describing the same phenomenon Delocalization of the nitrogen lone pair decreases the electron density at nitrogen while increasing it m the rr system of the aro matic ring We ve already seen one chemical consequence of this m the high level of reactivity of aniline m electrophilic aromatic substitution reactions (Section 12 12) Other ways m which electron delocalization affects the properties of arylamines are described m later sections of this chapter... 

The carbocation is stabilized by delocalization of the tt electrons of the double bond and the positive charge is shared by the two CH2 groups Substituted analogs of allyl cation are called allylic carbocations Allyl group (Sections 5 1 10 1) The group... 

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... 

A conjugated sp - -sp - single bond (for example, the bond joining the two phenyl rings of biphenyl, the central bond of butadiene, with delocalized aromatic bonds, or phenyl amine, where N-C bond is labeled aromatic and nitrogen is sp - hybridized) is described by a two-fold barrier, V2=10 kcal/mol. 

The results obtained for 1-phenylpyrazole (32) and its conjugate acid (34) are consistent with those of Minkin. The bond order between thd two rings decreases by protonation (from 0.341 to 0.241) and this is in agreement with the expected effect of the N-2 positive charge on the delocalization of the lone pair on N-1 over the phenyl ring. 

One of the structural implications of the delocalization of the negative charge, the identity of the two C—O bond lengths, intermediate between those of single and double bonds, has been verified by many crystal structure determinations. 

The meaning of the word aromaticity has evolved as understanding of the special properties of benzene and other aromatic molecules has deepened. Originally, aromaticity was associated with a special chemical reactivity. The aromatic hydrocarbons were considered to be those unsaturated systems that underwent substitution reactions in preference to addition. Later, the idea of special stability became more important. Benzene can be shown to be much lower in enthalpy than predicted by summation of the normal bond energies for the C=C, C—C, and C—H bonds in the Kekule representation of benzene. Aromaticity is now generally associated with this property of special stability of certain completely conjugated cyclic molecules. A major contribution to the stability of aromatic systems results from the delocalization of electrons in these molecules. 

The EPR spectrum of the ethyl radical presented in Fig. 12.2b is readily interpreted, and the results are relevant to the distribution of unpaired electron density in the molecule. The 12-line spectrum is a triplet of quartets resulting from unequal coupling of the electron spin to the a and P protons. The two coupling constants are = 22.38 G and Op — 26.87 G and imply extensive delocalization of spin density through the a bonds Note that EPR spectra, unlike NMR and IR spectra, are displayed as the derivative of absorption rather than as absorption. 

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... 

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