Sigma electron

Hiickel-type systems (such as Hilcfcel pericyclic reactions and suprafacial sigmatropic shifts) obey the same rules as for sigma electron. The rationale for this observation is clear If the overlap between adjacent p-electron orbitals is positive along the reaction coordinate, only the peraiutational mechanism can... 

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)... 

Similar results are obtained also in sigma electron systems. Various examples can be given in regard to the Sn2 reaction of a methyl halide... 

F.E. Inscore, R. McNaughton, B.L. Westcott, M.E. Helton, R. Jones, I.K. Dhawan, J.H. Enemark, and M.L. Kirk, Spectroscopic evidence for a unique bonding interaction in oxo-molybdenum dithiolate complexes implications for sigma electron transfer pathways in the pyranopterin dithiolate centers of enzymes. Inorg. Chem. 38, 1401-1410 (1999). 

Alternatively, the promoted N2 configuration is obtained by two-electron Aufbau from C2 if one adds (a) two sigma electrons (tala ) to C2, or (b) two... 

One can also localize aromatic systems by applying the localization procedure to 7r electrons and a electrons simultaneously. The sigma electrons will then be localized in the regions of the single bonds. Since the localization energy to be gained from the n electron localizations is less than that from the sigma electron localization, the total localization will be dominated by the latter. 

Because of the ring strain in cyclopropane (Problem 9.6), there is less orbital overlap (Fig. 9-1) and the sigma electrons are accessible to attack by electrophiles. 

Inner electrons Sigma electrons n and pi electrons Conjugated systems Highly conjugated systems Vibrational and rotational Rotational... 

Some simple organometallics have been studied. Poly(silanes) and germanes exhibit very interesting behavior, since they are photochromic and appear to possess excitonic, charge delocalized excited states involving the sigma electrons of the organometallic backbone. 112) THG measured susceptibilities of up to 1 x... 

Recent studies of polymeric Group IV catenates (in particular, polysilanes and polygermanes, la,b have demonstrated that there is significant sigma electronic delocalization along the polymer backbone which is responsible for many of the curious electronic properties of this class of materials [3]. 

In polysilane polymers, the polymer backbone is made up entirely of silicon atoms. Therefore these materials differ from other important inorganic polymers, the siloxanes and phosphazenes, in which the polymer chain is heteroatomic. Structurally, they are more closely related to homoatomic organic polymers such as the polyolefins. However, because the units in the main chain are all silicon atoms, the polysilanes exhibit quite unusual properties. The cumulated silicon-silicon bonds in the polymer chain allow extensive electron delocalization to take place, and this delocalization of the sigma electrons in the Si-Si bonds gives the polysilanes unique optical and electronic properties. Many of the potential technical uses, as well as the remarkable properties, of polysilanes result from this unusual mobility of the sigma electrons. 

As explained in the introduction, the polysilanes (and related polygermanes and poly-stannanes) are different from all other high polymers, in that they exhibit sigma-electron delocalization. This phenomenon leads to special physical properties strong electronic absorption, conductivity, photoconductivity, photosensitivity, and so on, which are crucial for many of the technological applications of polysilanes. Other polymers, such as polyacetylene and polythiophene, display electron delocalization, but in these materials the delocalization involves pi-electrons. 

The Computer. AXIS is written in FORTRAN and runs on a GEC 4000 series multi-user mini computer equipped with a Graphics Option Controller (GOC) model 5250, manufactured by Sigma Electronic Systems Ltd. This device converts an alphanumeric visual display unit into a graphics terminal, allowing independent use of the alphanumeric and graphics screens. In addition, it provides cursor handling facilities and the ability to selectively erase part of the screen. All of these features are utilised by AXIS. 

Once the theory of pericyclic reactions was developed, it was recognized that the conversion of Dewar benzene to benzene is an electrocyclic reaction. This conversion involves two pairs of electrons one pair of pi electrons and one pair of sigma electrons of the Dewar benzene. (The third pair of electrons is located in exactly the same place in both the reactant and the product and so is not involved in the reaction.) An electrocyclic reaction involving two pairs of electrons must occur by a conrotatory motion if it is to be thermally allowed. However, the conrotatory opening of Dewar benzene is geometrically impossible, because it would result in a benzene with a trans double bond, a compound with too much angle strain to exist. 

Chelation involves coordination of more than one sigma - electron pair donor group from the same ligand to the same central atom . Note that in this quoted description the choice of the word "involves" appears to be deliberately more nebulous than what one expects from a formal definition of a term. 

The electrons in the pi bond are spread farther from the carbon nuclei and are more loosely held than the sigma electrons. 

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