Delocalisation of electrons

Fig. 12.14 Delocalisation of electrons in aniline, naphthylamine and diphenylamine molecules...

Interpretable Laser Raman and Laser mass spectra have been obtained from certain other microstructures ( Ramsaysphaera ) (Fig. 32). The Laser mass spectra are characterized by CN and CNO ions (Fig. 33). Raman lines (Fig. 32) appear at 1360, 1600, 2720, 2960 cm-1 within the organic range of the spectrum. The strong line at 1360 cm-1 may be atrributed to a symmetric N—O vibration of the N02 group, the weaker line at 1600 cm-1 is characteristic of aromatic double bonds C = C. The first overtone of the 1360 cm-1 line is observed at 2720 cm-1. The spectrum has the features of a resonant Raman spectrum. It is very often obtained with this type of product in which a large delocalisation of electrons is possible. 

In contrast, 2- and 4-pyrones are considered to have relatively little aromatic character. Whereas in an analogous nitrogen series 4-pyridone 5.23 has significant aromatic character (mesomeric representation 5.23a making a considerable contribution to the overall electronic distribution), aromatic mesomeric representation 9.3a makes less of a contribution to the overall electronic structure of 4-pyrone. As with furan, the higher electronegativity of oxygen leads to heterocycles of little aromaticity in cases where delocalisation of electron density from the heteroatom is a prerequisite for that aromaticity. 

When both A and B are electropositive, they form a metallic bond. Such a bond involves delocalisation of electrons and the electrons are shared between atoms but in less specific way and without directional character. 

Amino acids are combined (linked together) through peptide bonds (-C-N-) (Figure 8.1) the peptide bond formed is planar (flat), due to the delocalisation of electrons that form the partial double bond, restricting rotation about the bond. The rigid peptide dihedral angle, co (the bond between C and N), is always close to 180°. The dihedral angles phi (the bond between N and Ca) and psi (the bond between Ca and C) can only have a number of possible values, and so effectively control the protein s three-dimensional structure. 

A molecular grouping which is specifically responsible for absorption is described as a chromophore, usually a conjugated system with extensive delocalisation of electron density. Any... 

In the structure of ylides, the ability of sulfur to stabilise an adjacent negative charge is an interesting phenomenon which probably involves the delocalisation of electron density into the low-lying 3d-orbitals of sulfur. Essentially, the question revolves around the relative importance of the resonance forms (2a) and (2b) (Scheme 1) in the ground state structure of sulfur ylides. The evidence tends to indicate that sulfur ylides do not contain appreciable double-bond character, so that the structure (2a) is probably the major resonance contributor, which would be in agreement with the preferred geometry of carbon and sulfur.1... 

Curly arrows can be used to show the delocalisation of electrons onto the carbon atom bearing the nitrogen. 

Benzene can be written with a circle within the ring to show the delocalisation of electrons (see Section 7.1). However, this does not show the six Jl-electrons, which makes drawing reaction mechanisms impossible. A single resonance form, showing the three C=C bonds, is therefore most often used. 

The delocalisation of electrons in aromatic compounds gives rise to characteristic chemical shift values in the 111 NMR spectra (see Section 10.5.1). 

The delocalisation of electrons means that benzene is unusually stable. It has a heat of hydrogenation which is approximately I50k.lmol 1 less than that which would be expected for a cyclic conjugated triene. This is known as the resonance energy. 

FP calculations are found to accurately account for iono-covalent bonding [45] and can thus be used to complement STM experiments by indicating which bands are localised on which atoms. Some caution is however appropriate, as current DFT functionals suffer from systematic over-delocalisation of electrons. 

Photophysical effects will here be taken to include light-induced changes in the extent of adsorption as well as photoelectronic effects involving the localisation or delocalisation of electrons at the illuminated interface. They will be differentiated from photochemical effects by the phenomenological criterion that photochemical effects involve rupture and/or rearrangement of bonds other than those between adsorbate and adsorbent. 

The H and C NMR analysis of poly(3-octyl thiophene) give a good evaluation of the ratio between head-to-head and head-to-tail sequences [158], This estimation is important because the presence of a head-to-head sequence is going to induce a sterical hindrance, with a slight rotation of two neighbouring cycles as a consequence of this constraint. The effect of this rotation is a weaker overlap of n orbitals and therefore a weaker delocalisation of electrons along the chain, which leads to an uneasier conduction. 

Scheme 32. Delocalisation of electron density from Ti-basic heteroatom alkylidyne substituents...

In this product delocalisation of electrons can only involve the two nitrogen atoms and the carbon atoms in the seven-membered ring. 

MINDO/3 calculations predicted that the valence isomer of butalene, singlet 1,4-benzyne (XXI), should be more stable than butalene itself [211], although MNDO calculations predict the opposite [212], Both indicate appreciable delocalisation of electrons in the periphery. Irrespective of whether (XX) or (XXI) represents the energetically preferred structure, valence tautomorism between them may take place readily and there is chemical evidence which implicates the possible presence of a biradical structure in reactions which may generate butalene [213]. 

The tribenzo derivative (XXIV) has been prepared [222,223]. Its H-n.m.r. spectrum suggests that there i.s no delocalisation of electrons over the 10 ir-electron moiety and it presumably exists preferentially as a bridged tribenzenoid structure as inferred in formula [XXIV). With potassium it forms a dibenzocyclooctatotraenide dianion [223,224]. 

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