Polysilanes bond energy

When polysilanes absorb energy in the ultraviolet region, electrons are promoted from the a valence band to the a conduction band.50 Absorption maxima for polysilanes fall between about 290 and 410 nm typical ultraviolet spectra for some polysilanes in solution are shown in Figure 5.5. Because this o-o transition is permitted, the electronic absorptions are intense, with extinction coefficients e between 5,000 and 10,000 per Si-Si bond. Consistent with the splitting portrayed in Figure 5.4, as the number of silicon atoms increases, the energy of the electronic transition decreases. The absorption wavelength therefore increases and eventually reaches a limit when... 

Group IVA starts with carbon, which is a nonmetal, followed by silicon and germanium, which are metalloids (semimetals), and tin and lead, which are metals. All of the group IVA elements have the ability to catenate however, as you move down the group, this ability decreases. Some of the more important inorganic polymers are polysilanes, in which there are repeat Si-Si units in the polymer backbone. However, the bond energy of Si Si bonds is 222 kJ/mol, while the... 

Another interesting feature of Si-Si bonds is the low energy level of the LUMO. The LUMO levels decrease with increasing chain length of polysilanes (Fig. 10). Because of the low lying LUMO, polysilanes can be reduced by both... 

An important property of the alkyl poly silanes is their electronic absorption at relatively low energy. This was particularly surprising because such molecules lack tt, d ot lone pair electrons they were the first substances containing only bonding cr-electrons to show such long-wavelength absorptions. The unusual electronic properties of the polysilanes have sparked considerable interest in the chemophysical properties of these molecules. The UV spectra of cyclic polysUanes follow a quite different pattern . 

Figure 25 Energy diagrams for orbital interactions between polysilane skeleton and pendant group. HOMOsi, HOMOp and LUMOs , LUMOp denote the HOMO and LUMO delocalized on the skeleton and a pendant group, respectively, osi-c and a si-c are bonding and antibonding Si-C a orbitals, respectively.

Polyolefins are, of course, usually synthesized by the catalyzed polymerization of alkenes. Why is an analogous route, polymerization of disilenes, not employed to prepare polysilanes The reason is paradoxical. The energy barrier to polymerization of doubly bonded silicon compounds is simply too low, so that in most cases they polymerize (or oligomerize) immediately when they are generated ... 

Robert West was bom in New Jersey and educated at Cornell University (B.A.) and Harvard University (A.M., Ph.D.). For the past 45 years he has been a faculty member in the chemistry department at the University of Wisconsin, where he is now E. G. Rochow Professor and Director of the Organosilicon Research Center. His many awards include the Frederick Stanley Kipping Award, the Wacker silicone prize, the Alexander von Humboldt Award, and the main group chemistry medal. He has published more than 600 scientific papers, mostly in the area of silicon chemistry. Major discoveries in his laboratories include the first soluble polysilanes (1978), the silicon-silicon double bond (1981), the first stable silylenes (1994), and electrically conducting organosilanes for high energy density batteries (2000). He is an airplane pilot and a mountaineer, with numerous first ascents in Canada and Alaska. 

The Si—Si bond is relatively weak, so homolytic decomposition readily occurs on application of thermal or electrical energy or radiation (light, neutrons, X-ray. .. )a. As expected from the coordinative unsaturated character of the Si atoms, the polysilanes are very reactive with nucleophilic agents as shown... 

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