Two-electron jumps

Sometimes, spin-allowed bands are much weaker than otherwise expected. There can be many reasons for this, most of which require more detailed analysis than we are able to present here. One particular case, however, can be discussed. It is well illustrated by the spectra of octahedral cobalt(ii) species, an example being shown in Fig. 4-5. Three spin-allowed transitions are expected for these d complexes, namely Txg F)- T2g, - see Chapter 3. The bands in Fig. 4-5 are 

Of course, in real systems, the relative contributions of Coulomb and crystal-field effects are such as to place chromophores somewhere inbetween the weak-and strong-field limits. In that case, a real Txg F) A2g transition is not a pure two-electron jump, so that some intensity is observed. 

Absorption of one photon of light results in the relocation (with respect to space, spin or both) of one electron. It is possible, but extremely unlikely, that a second photon, together with its associated electronic rearrangement, can be absorbed before the ground state is reacquired upon expulsion of a photon. It s unlikelyhood is because the lifetime of the excited state is typically only 10 seconds or so. 

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Here we comment on the shape of certain spin-forbidden bands. Though not strictly part of the intensity story being discussed in this chapter, an understanding of so-called spin-flip transitions depends upon a perusal of correlation diagrams as did our discussion of two-electron jumps. A typical example of a spin-flip transition is shown inFig. 4-7. Unless totally obscured by a spin-allowed band, the spectra of octahedral nickel (ii) complexes display a relatively sharp spike around 13,000 cmThe spike corresponds to a spin-forbidden transition and, on comparing band areas, is not of unusual intensity for such a transition. It is so noticeable because it is so narrow - say 100 cm wide. It is broad compared with the 1-2 cm of free-ion line spectra but very narrow compared with the 2000-3000 cm of spin-allowed crystal-field bands. 

This treatment is based on the inadequate three-parameter theory of the free ion on the one hand and does not account for covalency effects on the other. Indeed, three main absorption bands are found, the third of which is of low intensity (two electron jump) and located in the UV-region. It is only observed in cases where it is not masked by charge-transfer transitions. In addition, some peaks of very weak intensity are found in the visible region around the first two main bands (Fig. 1, 2 Table 2). While A is fixed by the energetic position of the first main band, B may be calculated from the second (or third) main band by the formula ... 

Because each lithium atom has one valence electron and each molecular orbital can hold two electrons, it follows that the lower half of the valence band (shown in color in Figure 5) is filled with electrons. The upper half of the band is empty. Electrons near the top of the filled MOs can readily jump to empty MOs only an infinitesimal distance above them. This is what happens when an electrical field is applied to the crystal the movement of electrons through delocalized MOs accounts for the electrical conductivity of lithium metal. 

For reproducing as closely as possible diabatic conditions, we have fixed the Cl—Cl bondlength at its neutral equilibrium value. This way, the system depends on two parameters as shown in Figure 1. Previous experimental and theoretical studies on similar systems, [1,18] have shown that electron jump from Li to the acceptor molecule CI2, which has, once relaxed, a positive vertical electron affinity (see Table 1), is likely to take place at a distance d, (see the definition of this parameter in Figure 1) which is superior to the LiCl equilibrium distance (MP2 value 2.0425 A). The description of this phenomenon in terms of MO and states will be briefly recalled in the next section. 

Now the number of Ni2+ and Ni3+ ions is not changed, the electron jump is not connected with a change in energy, and this electron jump can be repeated in the same way with the next ion. Compounds containing two kinds of ions of the same metal are often very good electronic or metallic conductors. 

A related reaction that is known to proceed through acetyl-TDP is the previously mentioned bacterial pyruvate oxidase. As seen in Fig. 14-2, this enzyme has its own oxidant, FAD, which is ready to accept the two electrons of Eq. 14-22 to produce bound acetyl-TDP. The electrons may be able to jump directly to the FAD, with thiamin and flavin radicals being formed at an intermediate stage.1353 The electron transfers as well as other aspects of oxidative decarboxylation are discussed in Chapter 15, Section C. 

Electricity exists not only in homes, schools, and cars, it can be found in nature, too. Lightning, for example, is electricity. Lightning occurs when electrons move from cloud to cloud or from a cloud to the ground. Walking across a carpet and then touching something metallic can produce a nasty little shock, too. The shock happens when electrons jump from a persons finger to the metal that is touched. This is also electricity. Both of these examples involve static electricity. Static electricity occurs when two different... 

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