Close pair

All members of a diffuse series consist of compound doublets, as illustrated in Figure 7.7(b), but the splitting of the >5/2 states may be too small for the close pair of... 

Figure 30 shows the potential of the 10-ppm approach. The full-width HMBC spectra (left) do not allow a clear discrimination of all cross-peaks. For instance, the correlation between the proton resonance of CH3fi2 MeLeu 9 at 0.82 ppm and the close pair of carbons at 22.1 and 22.2 ppm remains ambiguous and it is not possible to attribute the resonance of the CF c) MeLeu 9. In the 10-ppm spectrum, the achieved resolution clearly shows that this correlation can be unambiguously attributed, and that the exact resonance of the CHyh MeLeu 9 is 22.2 ppm. 

From the relative intensities of the two diffracted waves one can deduce the absolute direction of the vector W —> Y with respect to the b axis. This type of reasoning was exploited by Nishikawa and Matsukawa (14) in 1928 and independently by Coster, Knol, and Prins (15) in 1930 to determine the absolute polarity of successive layers of zinc and sulfur in a polar crystal of zinc sulfide. In the zinc sulfide crystal, planes of zinc and sulfur alternate parallel to the face (111), as shown in Scheme 3 the distance between the close pairs of zinc and sulfur planes is one-quarter of the whole 111 spacing. 

The total pair-wise entanglement contribution is simply the sum of dS for strand pairs with all possible sets of internal coordinates. In a network of N strands there will be N(N — l)/2 such pairs. However, classification is only significant for pairs that are relatively close. Pairs separated by more than a few radii of gyration will belong to unentangled class exclusively. This assures that the total entropy change will be proportional only to N. 

As it is known, I centres are the most mobile radiation-induced radiation defects in alkali halides and therefore they play an essential role in low-temperature defect annealing. It is known, in particular, from thermally-stimulated conductivity and thermally-stimulated luminescence measurements, that these centres recombine with the F and F electron centres which results in an electron release from anion vacancy. This electron participates in a number of secondary reactions, e.g., in recombination with hole (H, Vk) centres. Results of the calculations of the correlated annealing of the close pairs of I, F centres are presented in Fig. 3.11. The conclusion could be drawn that even simultaneous annealing of three kinds of pairs (Inn, 2nn and 3nn in equal concentrations) results in the step-structure of concentration decay in complete agreement with the experimental data [82]. 

Therefore, the method of partial lightsums illustrates once more effects of the defect diffusion a number of close pairs increases, whereas that of distant pairs decreases, unlike the case of defect rotation. 

A simulation has been carried out [105, 106, 119] of the process of accumulation of the immobile Frenkel defects restricted by tunnelling recombination of dissimilar defects, as it is observed in many solid insulators. As follows from Chapter 3, in contrast to the ionic process of instant annihilation of close pairs of the vacancy-atom type, it is characterized by a broad spectrum of recombination times. Thus, the probability for a pair of chosen defects that lie at the relative distance r to survive for r seconds is... 

The most rapid bimolecular reactions must be the ion-molecular ones. Their duration can be limited only by the time of collision, thus being 10 13-10 12 s. The recombination time of radicals that have escaped from the cage depends on their concentration in the track. For close pairs of radicals the recombination may already begin in 10 us. From this moment on we can consider the chemical stage of radiolysis to have begun. 

The situation is different when donor and acceptor molecules are located at different interfaces, that are separated by a fatty acid monolayer of well defined thickness. There are no longer close pairs of donor and acceptor with a high probability of electron transfer as in the "contact" case. Consequently, no change in relative fluorescence intensity with increasing donor density is expected, contrary to the former case. Indeed, in systems with a spacer monolayer of... 

There are six P layers and a total of 12 (6 2) repeating layers total, giving the notation 6-2P /4 3/404/4. The spacing between P layers is 15 units for the NiAs regions (PoO/jP/iO/jPc) with close pairs of Ti atoms (both B or C positions), while spacing is 20 units between P layers (P0O/1 P/j) and these Ti atoms have no other close Ti atom. Other compounds with the BaTiOa structure are given in Table 5.9. 

In intermolecular PET processes, radical ions are formed either as close pairs or as free species from neutral molecules (Sch. 1) [2,6]. Most commonly, carbonyl compounds or related derivatives as for example enol ethers, cyclopropyl ketones, and siloxycyclopropanes are used for intramolecular cyclization reactions. With the exception of cycloadditions the ring-building key step is always an intramolecular bond formation. In PET... 

The Coulomb attraction between the donor and the defect leads to a non-random distribution, with an enhanced concentration of close pairs. The modified distribution is... 

The reconciliation of these two apparently conflicting results is quite interesting and is connected to the broad distribution of recombination lifetimes. Those electron-hole pairs which are created with small separations are more likely to result in geminate recombination than the more distant pairs. The close pairs are also more likely to contribute to the luminescence and the distant pairs to LESR. Thus the two experiments are selectively measuring different parts of the distribution. The density of geminate pairs of separation, R, is given by. 

Fig. 8.17 shows schematically that the transition occurs at a smaller pair separation as the excitation intensity increases. The LESR is dominated by the large separation pairs which are always nongeminate, while the luminescence is dominated by the close pairs which are mostly geminate. Geminate recombination is therefore seen in luminescence, but not in LESR. Furthermore, the excitation energy at which the transition occurs in the luminescence depends on the decay time being measured. 

FIGURE 11. An escapement mechanism is sometimes used to control the direction of electron transfer within a redox cluster (small box). Here electron transfers from the substrate (close pair of circles filled with electrons) to a redox center on the left which is effectively insulated by distance from other members of a redox chain (further left) so only one electron can be transferred. The radical intermediate can transfer electrons to the chain on the right. The thermally activated escapement motion of the redox center then carries an electron to the chain at the left, and finally reassembles the cluster in preparation for the next catalysis. 

On the other hand, the model is not without its drawbacks. It requires a certain structural association of 4 BChl molecules. Although a number of conceivable structures would be consistent with the tenets of the model, many otherwise plausible structures are excluded. This in itself is not a drawback the structural restrictions may be viewed simply as predictions of the model. Indeed, the idea of a basic dimer of closely paired BChls enters other models as well (see Section 3.2). However, problems may arise if in an actual structure more than two dimers are present and interact with comparable energies. In other words, the success of the Scherz-Parson model may depend on each dimer having only one nearest neighboring dimer, a somewhat unlikely supposition on the grounds of both symmetry and energy-transfer requirements (see Section 3.2.). 

Transistors are made of tiny slices of the crystals of semiconducting materials, such as silicon or germanium, to which wires are attached. In the early versions, two of the three electrical contacts—comparable to those in the vacuum tube, but miniaturized—were fine wires pressed into the surface of a semiconductor crystal in close pairs to create areas where current would flow only in one desired direction. 

Let us consider a reaction where the cage recombination occurs only through the singlet close pair but not through the triplet close pair as shown in Fig. 3-1. In such a reaction, Noyes [4] showed that the probability (f(r)) of the first re-encounter between t and r+dr for a pair, separating from an encounter at /=0, is given from the theory of random flights as... 

At the first collision (f=0) where the Q-D conversion is prohibited at the close pairs because of a large 7 value, the quartet and doublet pairs have equal population (p)... 

At the first collision, the T-quenching occurs at the close pairs from only the doublet pairs due to the fast processes (13-14c) and (13-14d). After the T-quenching, the population of the doublet pairs becomes smaller than that of the quartet pairs... 

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