O-electron delocalization

Even a preliminary investigation of the properties of some inorganic pol5miers reveals that some of them have unexpected properties. Polythi-azyl is an anisotropic electrical conductor and shows conductivity that is comparable to metals. At 0.26 K this polymer becomes superconducting [4, 5]. Polysilanes which contain catenated silicon atoms in a polymeric chain have several unusual properties. These polymers have a o-electron delocalization. They are radiation sensitive and many of them are thermo-chromic. Many members of this family also show nonlinear optical behavior [17, 19]. 

The carbonyl carbon of a thioester is more susceptible to nucleophilic addition than is the carbonyl carbon of an oxygen ester, because electron delocalization onto the carbonyl oxygen, which reduces the carbonyl group s reactivity, is weaker when Y is S than when Y is O. Electron delocalization is weaker because less overlap occurs between the 3p orbital of sulfur and the 2p orbital of carbon than between the 2p orbital of oxygen and the 2p orbital of carbon. In addition, the tetrahedral intermediate formed from a thioester undergoes elimination more rapidly than the tetrahedral intermediate formed from an oxygen ester because a thiolate ion is a weaker base and is therefore easier to eliminate than an alkoxide ion. 

Cyranski MK (2005) Energetic aspects of cyclic o-electron delocalization Evaluation of the methods of estimating aromatic stabilization energies. Chem Rev 105 3773—3811... 

The trimethylsilylated positions in these molecules were metalated by reaction with t-BuOK, and further manipulation of the substiment pattern was thus possible [111, 120]. This method was also used to connect cyclo- and bicyclosilanes with oligosilanylene bridges to study their o-electron delocalization behavior [47]. 

Bande A, Michl J (2009) Conformational dependence of o-electron delocalization in linear chains permethylated oligosilanes. Chem Eur J 15 8504... 

One way to assess the relative stabilities of these various intermediates is to exam me electron delocalization m them using a resonance description The cyclohexadienyl cations leading to o and p mtrotoluene have tertiary carbocation character Each has a resonance form m which the positive charge resides on the carbon that bears the methyl group... 

TT-electron delocalization over the O—C—N system. No such delocalization is possible in the N-protonated form. 

Experimental pK data suggest that eyano substitution can exert substantial long-range effeets on phenol aeidity, but the reason for these effeets is not obvious. If ion-dipole interaetions were to blame, the effeet would fall off with increasing ion (0 ) - dipole (CN) separation. If electron delocalization were responsible, then the effeet would be aeeompanied by eharge transfer between the ionie site (O ) and other atoms in the moleeule. 

Almost all known inorganic heterocychc molecules, where N, O and S atoms with lone pair orbitals are donors while B atoms with vacant p orbitals are acceptors, are classified into discontinuous conjugation. The donors and the acceptors are alternately disposed along the cyclic chain. The thermodynamic stabilities are controlled by the non-cycUc electron delocalization or by the number of neighboring donor-acceptor pairs, but not by the number of % electrons [83]. In fact, both 4n % and 4n + 2% electron heterocycles are similarly known [84,85] (Scheme 33), contradicting the Hueckel rule. 

Fukui applied the orbital mixing rule [1,2, 59] to the orbital hybridization or the deformation of the LUMO of cyclohexanone to explain the origin of the Jt-facial selectivity in the reduction of cyclohexanone. Cieplak [60] proposed that electron delocalization occurs from the bonds into the o orbital of the incipient bonds at the transition state. 

The delocalization-polarization mechanism in the singlet state is more complicated than that in triplet. Similar to the triplet state, there also exists a cyclic - G- T - E- T - configuration or -7t-p-7t -q- (-o-p-o -q-) orbital interaction in the singlet (Fig. 6). In the singlet state, however, the radical orbital q is an electron-accepting orbital (A) for the a-spin electron (rather than the donating orbital in triplet). Thus, there is an additional path of a-spin electron delocalization, - G- T - Tj- T - or... 

Much of the Pt Mossbauer work performed so far has been devoted to studies of platinum metal and alloys in regard to nuclear properties (magnetic moments and lifetimes) of the excited Mossbauer states of Pt, lattice dynamics, electron density, and internal magnetic field at the nuclei of Pt atoms placed in various magnetic hosts. The observed changes in the latter two quantities, li/ (o)P and within a series of platinum alloys are particularly informative about the conduction electron delocalization and polarization. 

Evidently, stable nitrile oxides can be investigated by spectral and X-ray methods using ordinary procedures. As examples, X-ray diffraction studies of o-sulfamoylbenzonitrile oxides (20), 5-methyl-2-(methylsulfonyl)-3-thiophene-carbonitrile oxide (21), (),( >-diphenylacrylonitrile oxide (22), and (dimorpholino-phosphoryl) carbonitrile oxide (23) can be cited. It should be underlined that structures of the latter compounds differ from those of classical stable 0,0 -disubstituted arylcarbonitrile oxides and tert-alkylcarbonitrile oxides. Therefore, not only purely steric shielding of the CNO group but also electrostatic or donor-acceptor interactions between the atoms of the latter and adjacent polar substituents (21, 23) and also electron delocalization in it-systems (20, 22) enhance the stability of nitrile oxide. 

Figure 4.35 shows the CD and UV spectra for 48 in isooctane. The observation of the Cotton effect indicates that the chromophore in the polymer, which is a main-chain segment over which the silicon a and o orbitals are electronically delocalized, exists in a chiral structure a helix. 

The chemistry of nitroxide radicals has been extensively reviewed (e.g. Forrester et al., 1968 Rozantsev, 1970 Rozantsev and Scholle, 1971 Aurich and Heiss, 1976 Berliner, 1976 Keana, 1978). They are -radicals, the unpaired electron occupying a if orbital between the oxygen and nitrogen atoms [4]. Since there is also an N—O o-bond, and two electrons fill a -bonding orbital between these atoms, the effective N—O bond order is 1.5. The nitroxide function is frequently represented as a hybrid of the two structures [5a] and [5b]. The result of this electron delocalization is a relatively stable structure. An alternative view of the absence of any significant tendency... 

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