7t-electron delocalisation

The aromatic character of ferrocene originates in the anionic cydopen-tadienide system (18), which has six 7t-electrons delocalised over a symmetrical cyclic five-carbon system. Coordination of two such ring systems with a metal ion (iron in the case of ferrocene) in a sandwich-like structure (20) gives rise to a remarkably stable compound. 

The energy difference between the two-structure calculation and a one-structure calculation at the same geometry with optimal orbitals but lacking resonance/7t-electron delocalisation gives the vertical resonance energy (VRE) [70]. 

Several qualitative models, e.g. Platt s ring perimeter model [88], Clar s model [89] and Randic s conjugated circuits model [90-92] have either been or are frequently used for the rationalisation of their properties. All these qualitative models rationalise the properties of aromatic and anti-aromatic hydrocarbons in terms of the Hiickel [4n+2] and [4n] rules. The extra stability of a PAH, due to 7t-electron delocalisation, can also be determined, computationally or experimentally, by either considering homodesmotic relationships [36] or by the reaction enthalpy of the reaction of the PAH towards suitable chosen reference compounds [93],... 

In this early application of the TURTLE program, the ability to restrict the wavefunction is used to ascertain the relative importance of 7t-electron delocalisation and induction to the enhanced acidity of carboxylic acids and enols compared to alcohols. These generic classes of molecules are represented by formic acid, vinyl alcohol and ethanol respectively. 

From a consideration of the optimised geometries, it could be concluded that both the acids and the deprotonated anions are subject to some 7t-electron delocalisation. In accord with chemical intuition, the effect of delocalisation is more important in the carboxylate and enolate anions than in the other species. However, the geometry changes that the acids undergo under deprotonation are only partly explained by 7t delocalisation. 

Indole is a ten-7t electron aromatic system. As with pyrrole, delocalisation of the lone pair of electrons from the nitrogen atom is necessary for aromaticity. The single overall electronic structure of indole is not completely described by structure 7.1, because this implies localisation of the lone pair on the nitrogen atom. Mesomeric representation 7.1a makes a contribution to the electronic structure of indole, as to a lesser extent do mesomeric representations where the negative charge occurs on the benzenoid ring. 

The first question that needed answering was whether the unsaturated NHC are indeed aromatic. The Hiickel rule states that aromatic systems are monocyclic, homonuclear, planar and possess a delocalised 4n-n2 jr-electron system [96]. Unsaturated NHC fulfil all the criteria except homonuclearity, which would make them heteroaromatic compounds. However, there was doubt as to the delocalisation of the 6 7t-electron system. Althongh there are 6 n-electrons, their distribution is extremely unequal, the backbone carbon atoms C and C contribute one rr-electron each, the nitrogen atoms provide a lone pair each, but the carbene carbon atom C gives none. Can there be a true delocalisation in such circumstances ... 

The sodium condensation reaction of a,co-bis(chlorosilyl)-substituted compounds and the coupling reaction of dilithio derivatives of compounds bearing 7t-electron systems with dichlorosilanes offer a convenient route to various silicon containing polymers. However, the polymers prepared by these methods always contain a small proportion of siloxy units in the polymer backbone, which would interrapt the electron delocalisation. Therefore, new synthetic routes to organosilicon polymers have been developed in which no alkali metal halide condensations are involved [6, 7]. We report syntheses of organosilicon... 

X-ray crystal structures of the 7-methoxycarbonyl derivative of (XXXIXB) [183] and of ontz-bis(difluoromethano)[14]annulene [184] show the presence of alternate single and double bonds in their peripheries. The 14-membered rings are non-planar. There are misalignments as high a.s 70° between adjacent p-orbitals in the ring, which clearly rules against cyclic delocalisation of the 7T-electrons [183],... 

The fundamentals of electrolytic expansion in polyaniline films have been discussed. Ion insertion and exclusion by electrolytic oxidation and reduction are the primary mechanisms. However, it is also evident that the changes in molecular conformations, arising due to the delocalisation of 7t-electrons and the electrostatic repulsion between the polycations, are other mechanisms operating in a conducting polymer microactuator. By investigating the molecular structure and the higher order structure to optimise the electrolytic expansion, it should be possible to improve the expansion ratio and the force for practical usage. 

The exact reverse of the above is seen with aniline (13), which is a very weak base (pK = 4-62) compared with ammonia (pIC = 9-25) or cyclohexylamine (pK = 10-68). In aniline the nitrogen atom is again bonded to an sp hybridised carbon atom but, more significantly, the unshared electron pair on nitrogen can interact with the delocalised 7T orbitals of the nucleus ... 

Electrons are transferred singly to any species in solution and not in pairs. Organic electrochemical reactions therefore involve radical intermediates. Electron transfer between the electrode and a n-system, leads to the formation of a radical-ion. Arenes, for example are oxidised to a radical-cation and reduced to a radical-anion and in both of these intermediates the free electron is delocalised along the 7t system. Under some conditions, where the intermediate has sufficient lifetime, these electron transfer steps are reversible and a standard electrode potential for the process can be measured. The final products from an electrochemical reaction result from a cascade of chemical and electron transfer steps. 

Electrochemical oxidation of alkenes results in the removal on one electron from the alkene function to give a 7t-radical-cation where the electron deficiency is delocalised over tire conjugated system. The majority of alkene radical-cations cannot be characterised because they readily lose an allylic proton in aprotic sol-... 

Reduction of benzenoid hydrocarbons with solvated electrons generated by the solution of an alkali metal in liquid ammonia, the Birch reaction [34], involves homogeneous electron addition to the lowest unoccupied 7t-molecular orbital. Protonation of the radical-anion leads to a radical intermediate, which accepts a further electron. Protonation of the delocalised carbanion then occurs at the point of highest charge density and a non-conjugated cyclohexadiene 6 is formed by reduction of the benzene ring. An alcohol is usually added to the reaction mixture and acts as a proton source. The non-conjugated cyclohexadiene is stable in the presence of... 

Although cyclic structures are unhkely for the sulfur-rich anions, S4N and SsN, because of ring strain, it should also be noted that the tc and 7t levels would be fully occupied in planar rings. In fact, the anions adopt open chain structures in which each terminal sulfur atom accommodates two lone pairs of electrons and the anti-bonding levels of the delocalised 7t-system are only partially occupied. 

The molecule is linear and without dipole moment, the intermolecular distances being C—N, 1.16 A and C—C, 1.37 A. Since the lengths are 1.15 A for the C=N bond and 1.54 A for the C—C single bond, it suggests that the 7t bonds of the C N groups are sufficiently delocalised to reduce the electron density between carbon and nitrogen and increase it between carbon and carbon. 

Mesomerism. Except in the case of side-chain anions, the anion orbital is orthogonal to the 7t-system and so it is not mesomerically delocalised. However, electron density, and therefore C-hydrogen acidity at ring carbons, is affected by resonance effects. 

Table 3.4 shows that, in the final complex (Figure 3.If), the Nd-C bond does not exist any longer, the new C4-C bond is formed, and the C3-C4 bond is ordinary, the population is 0.85. All these facts suggest that the insertion took place. The second double bond is delocalised, the electron populations of C1-C2 and C2-C3 bonds are close to 1.01 and the three carbon atoms Cj, C2, and C3 form a 7t-allylic bond with the Nd atom. 

These are a (sigma) bonds covalent bonds with the pair of electrons found mainly between the nuclei of the atoms bonded to each other. Each carbon atom forms three a bonds. That leaves one electron spare on each of the six carbon atoms. Each carbon atom contributes this one electron to a n (pi) bond (see page 57). However, the n bonds formed are not localised between pairs of carbon atoms as in an alkene C=C bond (see page 193). Instead, the Tt bonds in benzene spread over all six carbon atoms in the hexagonal ring. The six electrons in the 7t bonds are said to be delocalised. 

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