N-Delocalization

The predictions of relative stability obtained by the various approaches diverge more widely when nonbenzenoid systems are considered. The simple Hiickel method using total n delocalization energies relative to an isolated double-bond reference energy (a + fi) fails. This approach predicts stabilization of the same order of magnitude for such unstable systems as pentalene and fulvalene as it does for much more stable aromatics. The HMO, RE, and SCF-MO methods, which use polyene reference energies, do much better. All show drastically reduced stabilization for such systems and, in fact, indicate destabilization of systems such as butalene and pentalene (Scheme 9.2). 

The allyl radical would be expected to be planar in order to maximize n delocalization. Molecular structure parameters have been obtained from EPR, IR, and electron diffraction measurements and confirm that the radical is planar. ... 

Based on experimental results and complementary calculations, an out-of-plane n-delocalization is suggested for thiirene dioxides39. As far as the thiirene oxide is concerned, theoretical calculations predict possible spiroconjugative-type53 interaction between the n c—c orbital of the ring and the jr-orbitals of the SO (which leads to aromatic stabilization and a n charge transfer backward from the SO to the C=C). There exists, however, a rather strong destabilization effect, due to the jr so(d )-orbital. 

Decarbonylation, photochemical 885 Decarboxylation 605 Dehydrohalogenation 417 n-Delocalization 392 Demeton sulphones, mass spectra of 155... 

The observed planarity and bond length equalization in 1,3,2-diazaphospholenium cations likewise suggest that these compounds have substantial n-electron delocalization and possess possibly aromatic character. Several studies were undertaken to quantify the degree of n-delocalization by computational calculations using the interpretation of population analyses, ELF calculations, evaluation of magnetic criteria [nucleus independent chemical shift (NICS) values], and the... 

As shown in Table 3.21, the leading n— n versus 7t—>- n delocalizations of these species are rather similar 76 versus 79 kcal mol-1. The pi NBOs of 20 are strongly polarized toward the ionic nonbonding center,... 

One case of n—5 —n delocalization was demonstrated by Stevenson et al. (2006). The potassinm anion-radical salt of l-(9-methyl-9H-fluoren-9-yl)-4-methyl benzyl is characterized by the delocalization of an nnpaired electron within the fluorenyl moiety only. Its ESR spectrnm completely coincides with the spectrnm of the potassium anion-radical salt of the 9,9-dimethyl fluorene anion-radical in THE However, the cesium anion-radical salt of the fluorenyl methylbenzyl derivative produces the ESR spectrum corresponding to the placement of this cation between the fluorenyl and methylbenzyl moiety. The conditions of n—s—n delocalization appear An unpaired electron spends its time within both fluorenyl and methylbenzyl fragments. The situation is explained in Scheme 3.54. 

Electron distributions are ascertained by means of a Mulliken population analysis of the ir-electron atomic populations (in the case of complete n-delocalization, each atom in the 67r-electron five-membered ring would have 1.20 7r-electrons associated with it) by determining the spatial extent of the localized orbitals of both the out-of-plane lone pair of the heteroatom and the C=C double bonds as well as through comparing the total electron density plots in planes parallel to the molecular plane. 

The chemistry of unsaturated azepines is dominated by their polyene character. The absence of n-delocalization confers instability on the ring system as witnessed by the many and various ring transformations undergone in acid and base solution, and under thermal and photolytic conditions. Most of the major reactions of azepines involve the neutral molecule e.g. cycloadditions (Section 5.16.3.8.1), metal carbonyl complexation (Section 5.16.3.8.2), dimerizations (Section 5.16.3.2.3) and photo- and thermo-induced valence isomerizations (Section 5.16.3.2.1). 

In contrast to 87b, the geometric isomer in 88 was shown to resist the development of n delocalization irrespective of the nature of X, the substances listed under 88 having been found to be uniformly unstable and strictly atropic by XH NMR. 

By way of general conclusion it is perhaps worth stressing that all information now available on the heteroannulenes serves unequivocally to establish that the notion of a lone pair replacing a double bond in the development of n delocalization is a realistically sound one having received experimental verification in a variety of structural environments widely differing in size and shape. 

Finally, if the electron is considered to be delocalized on a large part of the molecule as the result of n delocalization, the problem is further complicated. Pseudocontact shift is also provided by spin density in a p orbital which has an orbital contribution to the magnetic moment different from zero. This is very relevant for heteronuclei like 15N or 13C which may bear unpaired spin density on pz orbitals. The picture is of the type of Fig. 2.1 where, besides spin polarization, dipolar interactions occur. For example, fractions of unpaired electrons delocalized onto nitrogen p orbitals in Fe(CN) - are predicted to affect... 

Data is also available for addition reactions of electrophilic radicals, CH(CN)2, and so-called ambiphilic radicals, CH(C02Et)2 and CH2C02-/-Bu, which derive their electrophilic character from n-delocalization of the carbon-centered radical onto electron-attracting substituents [108-111], and for which the enthalpy of the addition process rather than the polar nature of the radicals may be the primary rate-determining factor [112,113]. 

From EXAFS studies it has been concluded that there are at least two sulfur atoms at an average Mo—S distance of 2.41 A. Recent x-ray results of sulfite oxidase show that molybdenum is coordinated by two sulfur atoms of one MPT ligand and one sulfur from a cysteine thiolate. As deduced from the energy of the Raman active C=C and C—S stretching modes, there is considerable n delocalization in the 1,2-enedithiolate [139],... 

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