Valence alternation pairs

A more complicated model has been used with success by Cutler and others, in which selenium and tellurium chains are broken with increasing temperature, giving rise to dangling bonds in equilibrium with valence alternation pairs this is described in Section 6. 

Several consequences follow from the analogy with chalcogenide glasses. Valence alternation pairs pin the Fermi energy (see Section... 

Fig. 15.6 a Chain-like structure of amorphous Se. b Neutral undercoordinated Se atom (C ). Two Cj defects transform into a valence alternation pair consisting of a one-coordinated negative species C and a three-coordinated positive species Cj depicted in panel (c) and (d) respectively... 

Several investigators (28, 29, 45, 94) criticized as energetically unfavorable the Bjerrum concept of a valence defect pair formed by rotation of a rigid molecule in an otherwise undisturbed lattice. Alternative mechanisms were proposed. 

However, if we consider the alternative ionic structures, we see that SOCI2 and SO2CI2 contain three and four bonds to sulfur, respectively. In these cases, it s useful to recall that the sulfur shares one of its valence electron pairs with oxygen in SOCI2 and two of its valence electron pairs with the oxygens in SO2CI2, as follows ... 

Note that each LONE, BOND, or 3CBOND keyword list (if included at all) must be closed by a corresponding END keyword.] The Ic list (LONE... END) entries are integer pairs (AC, N,), each giving the atomic center number (AC ) and multiplicity of valence lone pairs (N,) on a lone pair bearing atom for example, the alternative formamide Lewis structure (5.24), with two valence lone pairs on Ni and one on O3, is specified by... 

An equation has been formulated to express the change in covalent radius (metallic radius) of an atom with change in bond number (or in coordination number, if the valence remains constant), the stabilizing (bond-shortening) effect of the resonance of shared-electron-pair bonds among alternative positions being also taken into consideration. This equation has been applied to the empirical interatomic-distance data for the elementary metals to obtain a nearly complete set of single-bond radii. These radii have been compared with the normal covalent... 

The alternative mechanism (Fig. 18.16, mechanism B) is based on the fully reduced [(dipor)Co2] state as the redox-active form of the catalyst. The redox equilibrium between the mixed-valence and fully reduced forms is shifted toward the catalytically inactive mixed-valence state, and hence controls the amount of catalytically active species in the catalytic cycle and contributes to the — 60 mV/pH dependence. The fully reduced form is known to bind O2 (probably reversibly) in organic solvents [LeMest et al., 1997 Fukuzumi et al., 2004], and the resulting diamagnetic adducts are typically viewed as a pair of Co ions bridged by a peroxide, which are of course quite common in the O2 chemistry of nonporphyrin Co complexes. To obtain the —60 mV/pH dependence of the catalytic turnover rate, a protonation step is required either prior to the TDS or as the TDS. Mechanism B cannot be extended to monometallic cofacial porphyrins or heterometallic porphyrins with a redox-inert ion, but there is no reason to assume that the two classes of cofacial porphyrin catalysts, with rather different catalytic performance (Fig. 18.15), must follow the same mechanism. 

Polydiacetylene crystals. The enhancement of x because of one-dimensional electron delocalization is strikingly corroborated in the polydiacetylene crystals. Their structure is that of a super alternated chain with four atoms per unit cell and the Huckel approximation yields four bands for the ir-electrons, two valence and two conduction bands. When depicted in the extended Jones zone, each pair can be viewed as arising by a discontinuity at the middle of the Brillouin zone of the polyene chain. The dominant contribution to X(2n 1) comes from the critical point at the edge of the extended Jones zone (initially at the center of the reduced B.Z.). The complete expressions are derived in (4,22) and calculated for different polydiacetylenes. We reproduce the values of x 2 for TCDU and PTS in table IV. The calculated values are in good agreement... 

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