Sigma* delocalization

Much of the interest in the polysilanes, polygermanes, and polystannanes involves their sigma delocalization and their sigma-pi delocalization when coupled with arenes or acetylenes. This is not unexpected since silicon exists as a covalent network similar to diamond. In exhibiting electrical conductivity, germanium and tin show more typical metallic bonding. Some polystannanes have been referred to as molecular metals. ... 

In summary, it appears that poly silanes and polygermanes have large third order nonlinear susceptibilities, particularly for materials containing only a saturated sigma bonded polymer backbone. This is consistent with their large polarizabilities and the extensive sigma delocalization along the backbone. In some instances, the... 

The aromatic rings present the highest and the antiaromatic systems the lowest bifurcation values of ELF. The bifurcation of the ELF occurs close to 0.75, except in system where sigma delocalization exists. In this way, the aromaticity of polycyclic aromatic hydrocarbons was well predicted, and also the aromaticity of new molecules was corroborated. In the all-metal aromatic compound Al - important contributions to stability from the two tt aromatic electrons and the cr system in the plane of the molecule were observed. The isosurfaces of the total ELF, ELFOT and ELF functions are showed in the Figure 6. 

This phenomenon is not possible in p-nitrobenzoic acid hence, p-nitrophenol is a stronger acid with respect to p-nitrobenzoic acid than is expected on the basis of a comparison of substituents in which this resonance delocalization is not an important factor. It was, therefore, recommended that Op = 1.27 be used for p-nitro derivatives of phenols and anilines, rather than the Op = 0.78 given in Table 7-10. These enhanced sigma constants, symbolized a, apply primarily to electron-withdrawing groups in reactions aided by low electron density at the reaction site. 

Valence bond theory (Chapter 7) explains the fact that the three N—O bonds are identical by invoking the idea of resonance, with three contributing structures. MO theory, on the other hand, considers that the skeleton of the nitrate ion is established by the three sigma bonds while the electron pair in the pi orbital is delocalized, shared by all of the atoms in the molecule. According to MO theory, a similar interpretation applies with all of the resonance hybrids described in Chapter 7, including SO S03, and C032-. 

Another species in which delocalized pi orbitals play an important role is benzene, QHg. There are 30 valence electrons in the molecule, 24 of which are required to form the sigma H H bond framework ... 

A good correlation with ordinary Hammett a values was based on 16 well-behaved substituents, and p-SOMe conformed well to this. Various other substituents showed deviations which were attributed to enhanced + R effects. These included p-SPh and this was explained in terms of 7t(pd) bonding, which was thus taken to play no part in the effect of p-SOMe on the methyl hyperfine splitting. More recently several 4-substituted benzyl radicals of the type RSO C6H4CH2 (n — 0,1 or 2 R = Me, Ph, Tol, COMe or OMe) have been examined by ESR spectroscopy249. The ability to delocalize spin density onto the substituent decreases in general as n increases and the effect of R depends on the oxidation state of sulfur. These authors have devised a new scale of substituent effects (sigma dot... 

The history and development of polysilane chemistry is described. The polysilanes (polysilylenes) are linear polymers based on chains of silicon atoms, which show unique properties resulting from easy delocalization of sigma electrons in the silicon-silicon bonds. Polysilanes may be useful as precursors to silicon carbide ceramics, as photoresists in microelectronics, as photoinitiators for radical reactions, and as photoconductors. 

The polysilanes are compounds containing chains, rings, or three-dimensional structures of silicon atoms joined by covalent bonds. Recently, polysilane high polymers have become the subject of intense research in numerous laboratories. These polymers show many unusual properties, reflecting the easy delocalization of sigma electrons in the silicon-silicon bonds. In fact, the polysilanes exhibit behavior unlike that for any other known class of materials. 

In any event, between 1951 and 1975, no papers appeared on polysilane high polymers. However, linear permethylpolysilanes of the type MelSiMezhiMe were prepared and studied, especially by Kumada and his students,(5) and cyclic polysilanes were being investigated in several laboratories.(6,7) Studies of the permethyl-cyclosilanes, (Me2Si)n where n = 4 to 7, showed that these compounds exhibit remarkable delocalization of the ring sigma electrons, and so have electronic properties somewhat like those of aromatic hydrocarbons.(6)... 

NBOs 3-5 are the three fluorine lone pairs ( LP ). As shown by the occupancies and hybrid composition, these lone pairs are inequivalent. LP(1) is the s-rich sigma-type sp0 26 lone pair (nF(cr) 79% s character), directed along the bond axis. LP(2) andLP(3) are the p-rich pi-type lone pairs (nF(7t) and nF(rf) 99.97% p-character), perpendicular to the bond axis. The lone pairs have occupancies slightly less than 2.000 00 (due to weak delocalization into Rydberg orbitals of the adjacent H), but overall, the correspondence with the elementary Lewis-structure description is excellent. 

Of the 17 kcal mol-1 total error, about half is estimated to arise from the single hp— h[ j sigma-type interaction shown in Fig. 2.8, while the remainder arises from weaker pi-type interactions (2-3 kcal mol-1 each). For example, we can carry out a partially localized variational calculation, similar to that described above but with only h prevented from delocalizing into tip (hisleads to a stabilization energy (at 7 = 1.6 A)... 

The unusual form of the C=C NBOs in Fig. 3.50 is clearly related to hypercon-jugative interactions with the nN lone pair. In (he 4> = 80° geometry, the nN has comparable delocalizations into both banana antibonds (stabilization energies of 8.41 and 7.06 kcal mol-1). The C=C NBOs revert rather abruptly to their usual sigma/pi form as nN is twisted to somewhat smaller

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Finally, we briefly compare the NBO donor-acceptor picture of geminal delocalization with Dewar s sigma-aromaticity concept and the associated pi-complex... 

Simple 2c/2e bonds to the transition metals commonly are weaker than the corresponding sigma bonds from the p-block elements, resulting in lower-lying acceptor ctml antibonds and increased electronic delocalization. 

In this part we employ the OEMO method in connection with bond MO s rather than delocalized MO s. There are three types of bond MO s n, pi and sigma. The interactions between bond MO s can be classified as follows ... 

An important point to be stressed before we proceed further with our analysis concerns the formal correspondence, of hybrid orbitals and delocalized group orbitals. For example, consider the model system HN=NH which can exist in a cis or a tram geometry. We can understand whether sigma interactions favor one or the other geometry be means of one of the following two approaches ... 

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