Electronic relationships

Three decades ago the preparation of oxepin represented a considerable synthetic challenge. The theoretical impetus for these efforts was the consideration that oxepin can be regarded as an analog of cyclooctatetraene in the same sense that furan is an analog of benzene. The possibility of such an electronic relationship was supported by molecular orbital calculations suggesting that oxepin might possess a certain amount of aromatic character, despite the fact that it appears to violate the [4n + 2] requirement for aromaticity. By analogy with the closely related cycloheptatriene/norcaradiene system, it was also postulated that oxepin represents a valence tautomer of benzene oxide. Other isomers of oxepin are 7-oxanorbornadiene and 3-oxaquadricyclane.1 Both have been shown to isomerize to oxepin and benzene oxide, respectively (see Section 1.1.2.1.). 

Let us first examine some of the structural and electronic relationships that exist for metal oxides and metal alkoxides. 

Kang, S., and Green, J. P. (1970) Steric and electronic relationships among some hallucinogenic compounds. Proc. Natl. Acad. Sci. USA, 67 62-67. 

The benzo[a] (19), benzo[/t (15) and benzo[c] (16) fused heterocycles are heterocyclic analogues of naphthalene, with the dibenzo heterocycles (17) bearing a similar electronic relationship to phenanthrene. Some of these compounds are still known by their trivial names indole (15 Z = NH), isoindole (16 Z = NH), carbazole (18) and indolizine (19). The names thianaphthene and pyrrocoline for (15 Z = S) and (19) respectively are now little used. Particular confusion can arise in consulting... 

The mechanism of this reaction involves an equilibrium between the 1- and 2-adamantyl cations established via intermolecular hydride transfers. Direct 1,2-hydride shifts on the adamantyl nucleus are inhibited by the unfavorable stereo-electronic relationship between the vacant orbital and the migrating group as discussed previously (see Fig. 1) 5 ). The 2-adamantyl cation, once formed, is trapped by water. The resulting 2-hydroxadamantane apparently then undergoes a disproportionation reaction with an adamantyl cation to give adamantanone and adamantane. The overall reaction is summarized in Scheme 15. 

Draw the electron-dot formulas that show all important contributors to a resonance hybrid and show their electronic relationship using curved arrows. 

In clusters composed solely of main-group elements, the following electronic relationship between closo, nido, and arachno structures has been found (12, 29) ... 

Of the 16 resonance forms of the triphenylmethyl cation shown in the solution for Problem 3(g), no two resonance forms place the positive charge on adjacent atoms. However, when looking at the 16 resonance forms of the phenylfluorenyl cation shown in the solution for Problem 3(h), there are multiple pairs of resonance forms (one of which is shown below) where the positive charge may be placed on adjacent atoms. This is a disfavored electronic relationship and is destabilizing to the cation itself. Thus, through charge distribution and delocalization, because the phenylfluorenyl cation possesses partial positive charges on two adjacent atoms, the triphenylmethyl cation [Problem 3(g)] is more stable. 

Bicyclic systems. The benzo-fused pyrrole heterocycles 13-15 are analogues of naphthalene with carba-zole 16 having an electronic relationship to phenanthrene. The systematic names show the position of the benzenoid ring but the common names indolizine 13, indole 14, isoindole 15, and carbazole 16 (Figure 1) are still widely used. The benzo derivatives of furan and thiophene are named in a similar manner, but their common names have largely fallen into disuse. For the benzo[7>] systems 17 and 20, the [b is commonly omitted. 

The unusual N—S—N bond system in 1,2,5-thiadiazoles poses interesting theoretical questions. Some insight into the structure and properties of 1,2,5-thiadiazoles was gained through studies of electron and X-ray diffraction, the microwave spectrum, and the Baman and infrared spectra of 4 and its derivatives. The iso-iT-electronic relationship between the 1,2,5-thiadiazoles and the pyrazines was examined in detail and a comparative study of the four isomeric thiadiazoles using the MO method in the LCAO approximation for small heterocyclic molecules was reported. 

We note that zeolites have also been used as hosts for a number of other intriguing "nanocomposites", for example in the field of encapsulated quantum-size semiconductor particles such as Se, CdS, CdSe, PbS, and GaP.3h32,33,34,35 phe encapsulation of metals such as Bi, Hg, Sn and Ga in zeolites has been described by Bogolomov.36 These studies demonstrate the enormous versatility of zeolite host systems for studies and control of structural/electronic relationships. 

In the second section this electron counting scheme is rationalized in terms of recent band structure calculations. The limitations of the ionic model as well as a variety of structural and electronic relationships for metal-rich halides are also discussed using these theoretical treatments. 

The structural factors that are fundamental to molecular organization are size, shape, electronic relationships, topology, and rigidity. The first three (size, shape, and electronic) combine to constitute the phenomenon of receptor/receptee... 

Hole-electron relationship in spin-orbit coupling... 

A modem tables the separated form types of elements. Modem periodic tables attempt to depict both chemical and electronic relationships. One such form is shown as Table 7.3. Note the following features ... 

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