Metallic chain

Polymerization occurs at active sites formed by interaction of the metal alkyl with metal chloride on the surface of the metal chloride crystals. Monomer is chemisorbed at the site, thus accounting for its orientation when added to the chain, and propagation occurs by insertion of the chemisorbed monomer into the metal—chain bond at the active site. The chain thus grows out from the surface (31). Hydrogen is used as a chain-transfer agent. Chain transfer with the metal alkyl also occurs. 

Packed column. A distillation column filled with an inert material to enhance vapor/liquid contact. The packing can be beads, pellets, Raschig rings, metal chains, or specifically shaped devices such as saddles, helices, or rings. 

Interest in the electrical conductivity of metal-chain complexes is increasing. Several studies have been made of the mixed valence complex K2Pt(CN)4BtQ 3 -2.3H2O. One reports that the conductivity in the direction of the Pt-atom chain is 100 times greater than at right angles to the chain. However, another study indicated considerable experimental scatter, which was attributed to variable water content from sample to sample. Small amounts of water were shown to increase the conductivity by a factor of up to It has also... 

Megnamisi-Belombe M (1977) Evidence for intrinsic electrical conduction in the linear metal-chain semiconductor bis(l,2-benzoquinonedioximato)platinum(II), Pt(bqd)2. J Solid State Chem 22 151-156... 

Mixed valency occurs in minerals (e.g. 63 ), metal-chain compounds, dimers and oligomers and metal complexes, and even in organic and biological systems (Brown, 1980 Day, 1981). Among the dimeric and oligomeric metal complexes exhibiting mixed valency, the pyrazine-bridged Ru (II, III) ammine complex. 

Day, P. (1976) Mixed valence chemistry and metal chain compounds. In Low... 

Peierls showed 74 [41,42] that an instability in a one-dimensional chain, with one electron per site, driven by electron-phonon interactions, can lead to a subtle structural distortion and to a first-order Peierls phase transition, at and below a finite temperature TP (the Peierls temperature) [42], For instance, at and below Tp either a dimerization into two sets of unequal interparticle distances d and d" (such that d + d" = 2d) or some other structural distortion must occur. The electronic energy of the metallic chain may also be lowered by the formation of a charge-density wave (CDW) of amplitude p(x) ... 

In the previous chapters the reactivity of metal ions with the monomer units of nucleic acids has been discussed. This section will deal with the binding of transition metals to the polynucleotides. There are also three types of complexes to be expected the metal-ring, the intermediate and the metal chain complex. The effect of the ribose or deoxyribose residue on the stability constants can be neglected since the reactivity of these sugars with cations is extremely low. However, as it will be seen later, the hydrolysis of polyribonucleotides is markedly facilitated by interaction of metal ions with the 2 —OH groups of the ribose. 

Peierls pointed out in 1955 that a one-dimensional metallic chain is not stable at T = 0 K, against a periodic lattice distortion of wave vector 2kF, as the result of electron-phonon coupling, opening a gap 2A at the Fermi level. From this fact a collective electronic state results called a charge density wave (CDW). In the limit where U, the intrasite Coulomb repulsion, is infinite, since a given k state cannot be occupied by more than one... 

Up to now we have discussed two extreme limits, the band picture on the one hand, and strong localization associated with interruptions in the metallic chains on the other. In fact, from work on thin metallic films and metallic glasses it is known that there is an intermediate region, that of weak localization. This occurs when the mean free path for elastic scattering (Lel) is only somewhat larger than, or comparable with, that for inelastic processes (Lin). In the first approximation there are corrections to the Boltzmann transport formula which depend on the ratio Lin/Lel in different ways for one-, two-, and three-dimensional materials. Weak localization... 

Hypothetical one-dimensional homoatomic transition-metal chain... 

Compare the DOS in Figures 6.12 and 6.14. It is the larger number of orbitals in the repeat unit of the metallic chain that generates the additional complexity. You will recall that it was often useful to discuss a complex MO in terms of its AO components. Similarly, the DOS can be analyzed more deeply by decomposing it into its AO components, i.e., by looking at the contributions of the individual AOs to the total DOS. These contributions are called DOS projections and Figure 6.14 shows the 4s, 4p and 3d projections. For the sake of simplicity we have separately... 

In Chapter 1 we saw that in moving from homonuclear to heteronuclear diatomics a new factor enters - the atom characters are distributed differently over the filled and unfilled MOs. As only the filled orbitals contribute to the atomic charges, the Mulliken charge distribution reflects the polarity of the molecule. Similar information for the HOMO and FUMO permitted us to discuss properties such as Lewis acidity and basicity in terms of frontier-orbital characteristics. As we were able to unravel the DOS of the metal chain in terms of AO type, we can also interrogate the DOS of a heteroatomic system for information on the distribution of atomic character over the total DOS. That is, we can reveal the contributions or character of a chosen atom to the DOS. We can begin to appreciate the power of this tool by... 

Figure 7.7. The Peierls distortion of a one-dimensional metallic chain, (a) An undistorted chain with a half-filled band at the Fermi level (filled levels shown in bold) has an unmodulated electron density, (b) The Peierls distortion lowers the symmetry of the chain and modulates the electron density, creating a CDW and opening a band gap at the Fermi level, (c) The Fermi surface nesting responsible for the electronic instability.

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