Chalcogens pairs

Table 6.13. Energies (meV) of the first even-parity levels of neutral isolated chalco-gens and chalcogen pairs in silicon with respect to the CB. The optical ground state energies Ei0 is the same as that of the deepest level (first row)...

The response to stress of the deep chalogen-related donors in silicon shows some differences due to the large separation of the deepest Is state from the other EM-like Is states, and also to the introduction of degeneracy of the chalcogen pairs, discussed in the following section. 

Among the chalcogen-related centres in silicon, the chalcogen pairs (CI12) are well-characterized by electronic spectroscopy and ESR, and their atomic and electronic structures are well-established. The ESR results have shown that in silicon, they are oriented along a <111> crystal axis and their site symmetry is, therefore, I 2d when the two Ch atoms are the same and 6 [Pg.363]

There is practically no difference between the isolated chalcogen-donor and chalcogen-pair spectra for a stress along [100] as the four orientations of the pairs are equivalent with respect to that direction, but the existence of two families of the pairs with different ground-state energies for F// [110] and F// [111] produces for the np lines twice more split components than for the isolated chalcogens with Td symmetry, as can be seen in Fig. 8.12 for S°. 

These interactions are most commonly observed for divalent chalcogen atoms and the nitrogen atom (the electron donor D) lies within the X-E-Y (E = S, Se, Te) plane, preferably along the extension of one of the covalent bonds as in 15.3. This anisotropy is a clear indication that these short E N contacts have some bonding character, i.e., they are subject to the geometric restrictions of orbital overlap. Eor example, in the diselenide 15.4 the nitrogen lone pairs are clearly oriented towards the Se-Se linkage. ... 

A common interpretation of the interaction of chalcogens with nucleophiles considers donation of electron density from a lone pair on the donor atom into the o- (E-X) orbital (Figure 15.1). As the degree of covalency increases, a hypervalent three-centre four-electron bond is formed. Real systems fall somewhere between secondary interactions and hypervalent (three centre - four electron) bonds. The two extremes can be distinguished by the correlation of X-E and E D distances.In the hypervalent case both bond distances decrease simultaneously, whereas in the secondary bond the distances are anticorrelated. This concept has been applied in a study of selenoquinones 15.17 (R = Ph, Me) with short Se 0 contacts,for... 

The UPD of metals onto the surface of chalcogens can be performed not only electrochemically but chemically, i.e., through the use of appropriate redox pairs in solution. Such an approach has been demonstrated by the electroless UPD of bismuth(III) onto the Te surface using the titanium(III)/(IV) redox pair in aqueous solution [96]. 

Base Pair-Like Chalcogen-Halogen Arrays 865... 

This means that upon oxidation of neutral chalcogens giving the polychalcogen cations, electron density is removed from the occupied rap2 lone pair orbitals.15,16 From a molecular orbital (MO) view of such interactions... 

In this section we will describe some PES investigations on Group XIV element derivatives in which one or more sulphur or selenium atoms are bonded directly or indirectly to the metal120-132. The aspects investigated preferentially for these molecules were the electronic and conformational situation, the role played in the bonding by the chalcogen lone-pair orbitals and possible hyperconjugative interaction (n-cr or p -d ). 

This discrepancy may be ascribed to the differing electronic situation of both groups in that the chalcogens bear an additional occupied p-type lone pair orbital, which can be used to delocalize the positive charge over all atoms. A similar stabilization is impossible for the group 15 cations and may account for their rare occurrence as compounds in the bottle. 

The oxidations and reductions of organochalcogen compounds all involve the lone-pairs of electrons associated with the chalcogen atoms. These lone-pairs are stereochemically active, which provides well-defined geometries for many of the intermediate species described herein. Oxidized organochalcogen compounds also are stabilized by lone-pair donation from neighboring heteroatoms, which again leads to unusual structures with well-defined geometries as described herein. 

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