7r-electron

Neighboring group participation (a term introduced by Winstein) with the vacant p-orbital of a carbenium ion center contributes to its stabilization via delocalization, which can involve atoms with unshared electron pairs (w-donors), 7r-electron systems (direct conjugate or allylic stabilization), bent rr-bonds (as in cyclopropylcarbinyl cations), and C-H and C-C [Pg.150]

In TT-complexes formed from aromatic compounds and halogens, the halogen is not bound to any single carbon atom but to the 7r-electron structure of the aromatic, though the precise geometry of the complexes is uncertain. The complexes with silver ions also do not have the silver associated with a particular carbon atom of the aromatic ring, as is shown by the structure of the complex from benzene and silver perchlorate. ... 

In the Hiickel theory, the 7r-electron energy of a conjugated molecule can be expressed by the following equation ... 

The problem of electrophilic substitution into the anilinium ion has been examined by the methods of m.o. theory. Attempts to simulate the --inductive effect in Hiickel M.o. theory by varying the Coulomb integral of C(j) (the carbon atom to which the NH3+ group is attached) remove 7r-electrons from the o- and -positions and add them to the... 

M.o. theory has had limited success in dealing with electrophilic substitution in the azoles. The performances of 7r-electron densities as indices of reactivity depends very markedly on the assumptions made in calculating them. - Localisation energies have been calculated for pyrazole and pyrazolium, and also an attempt has been made to take into account the electrostatic energy involved in bringing the electrophile up to the point of attack the model predicts correctly the orientation of nitration in pyrazolium. ... 

Beyer synthesis, 2, 474 electrolytic oxidation, 2, 325 7r-electron density calculations, 2, 316 1-electron reduction, 2, 282, 283 electrophilic halogenation, 2, 49 electrophilic substitution, 2, 49 Emmert reaction, 2, 276 food preservative, 1,411 free radical acylation, 2, 298 free radical alkylation, 2, 45, 295 free radical amidation, 2, 299 free radical arylation, 2, 295 Friedel-Crafts reactions, 2, 208 Friedlander synthesis, 2, 70, 443 fluorination, 2, 199 halogenation, 2, 40 hydrogenation, 2, 45, 284-285, 327 hydrogen-deuterium exchange, 2, 196, 286 hydroxylation, 2, 325 iodination, 2, 202, 320 ionization constants, 2, 172 IR spectra, 2, 18 lithiation, 2, 267... 

Another structure with a 10-7r-electron conjugated system is bicyclo[6.2.0]deca-1,3,5,7,9-pentaene. The crystal structure of the 9,10-diphenyl derivative (Fig. 9.6) shows the conjugated system to be nearly planar. " ... 

The seven-membered CN3S3 ring, another ten 7r-electron system, was first obtained as the ester 12.19 (R = COaMe), which is a minor product of the reaction of S4N4 with dimethylacetylene dicarboxylate. It has a planar structure with bond lengths that indicate delocalization. The parent 1,3,5,2,4,6-trithiatriazepine 12.19 (R = H) is obtained as a colourless solid by carefully heating the ester with aqueous HCl followed by decarboxylation. ... 

The off-diagonal elements of P corresponding to directly bonded atoms are referred to as (7r-electron) bond orders. Hexatriene shows bond-length alternation there are distinct single and double bonds. Notice that the bond orders for the bonded atom pairs show a related alternation. 

Aj[ the beginning of this chapter, I introduced the notion that the 16 electrons iU ethene could be divided conceptually into two sets, the 14 a and the 2 n electrons. Let me refer to the space and spin variables as xi, Xj, > xi6, and for the minute I will formally label electrons 1 and 2 as the 7r-electrons, with 3 through 16 the cr-electrons. Methods such as Huckel rr-electron theory aim to treat the TT-electrons in an effective field due to the nuclei and the remaining a electrons. To see how this might be done, let s look at the electronic Hamiltonian end see if it can be sensibly partitioned into a rr-electron part (electrons 1 and 2) and a cr part (electrons 3 through 16). We have... 

We often speak of the PPP (Pariser-Parr-Pople) model in honour of these three authors. Thinking of the pyridine example above, we still divide the 42 electrons into two groups, the 6 7r-electrons and the 36 o--electrons. The spirit of the PPP model is that the cr-electrons and the atomic nuclei provide a potential for the TT-electrons. Attention focuses on the 7r-electrons. 

In the PPP model, each first-row atom such as carbon and nitrogen contributes a single basis functiqn to the n system. Just as in Huckel theory, the orbitals x, m e not rigorously defined but we can visualize them as 2p j atomic orbitals. Each first-row atom contributes a certain number of ar-electrons—in the pyridine case, one electron per atom just as in Huckel 7r-electron theory. 

For bonded atoms, the off-diagonal terms (where i j) are taken to depend on tjje type and length of the bond joining the atoms on which the basis functions y- and Xj 0 centred. The entire integral is written as a constant, 0ij, which is not the same as the fixY in Hiickel 7r-electron theory. The are taken to be parameters, fixed by calibration against experiment. It is usual to set Pij to zero when the pair of atoms are not formally bonded. 

It is widely accepted that the basis functions used in ZDO 7r-electron tho... 

Ideally one should remove all the unwanted spin components before minimizing the energy. In semi-empirical 7r-electron theory, Amos and Snyder (1965)... 

The integrals are again parameterized as in eq. (3.79). The approximations for the one-electron integrals in CNDO are the same as for INDO. The Pariser-Pople-Parr (PPP) method can be considered as a CNDO approximation where only 7r-electrons are treated. 

The foregoing resonance structures describe the influence of the substituents on the 7r-electron distribution in the thiophene ring. Besides this effect the inductive effect of the substituents on the a-electi on system must be considered when discussing physical and chemical properties of thiophenes. 

The electronic spectrum of a compound arises from its 7r-electron system which, to a first approximation, is unaffected by substitution of an alkyl group for a hydrogen atom. Thus, comparison of the ultraviolet spectrum of a potentially tautomeric compound with the spectra of both alkylated forms often indicates which tautomer predominates. For example, Fig. 1 shows that 4-mercaptopyridine exists predominantly as pyrid-4-thione. In favorable cases, i.e., when the spectra of the two alkylated forms are very different and/or there are appreciable amounts of both forms present at equilibrium, the tautomeric constant can be evaluated. By using this method, it was shown, for example, that 6-hydroxyquinoline exists essentially as such in ethanol but that it is in equilibrium with about 1% of the zwitterion form in aqueous solution (Fig. 2). 

The pyridopyrimidines possess the same 7r-electron structure as naphthalene. The electronic transitions between the 77-orbitals would therefore be expected to give rise to similar ultraviolet spectra. As in the case of the quinazolines and the pteridines, this has proved to be so. 

The term aromatic will be used in a strict non-historical sense to mean possessing a cyclic 7r-electron system (6 and 10 electrons for the mono- and bi-cyclic rings discussed in this review). Heteroaromatic compounds, like carboaromatics, have widely different degrees and types of electronic dissymmetry and polarizabihty. Consequently, their reactivity varies tremendously with any one reagent and their relative reactivity changes drastically with the type of reagent. In this sense, aromatic compounds show differences in reactivity but not in aromaticity. The virtues of this qiuilitative concept of aromaticity and the pitfalls of trying to use it as a quantitative concept in modern context have been ably presented by Peters and by Balaban and Simon. ... 

The exclusion of nitrogen-bridged bicyclic azines is purely arbitrary since the work on such azinoazines with a 10 7r-electron system (and the necessarily accompanying quaternary nitrogen-bridgehead atom) shows the activation expected of other aromatic azinium... 

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