Orientation parallel

Cjg H37 (CH3 )2CH2CH2CH2Si(OCH3 )3C1 , tend to orient Hquid crystals perpendicular to the surface (see Liquid crystalline materials) parallel orientation is obtained on surfaces treated with /V-methy1aminopropy1trimethoxysi1ane [3069-25-8] CH2NHCH2CH2CH2Si(OCH2)3 (25). 

In the bacterial reaction center the photons are absorbed by the special pair of chlorophyll molecules on the periplasmic side of the membrane (see Figure 12.14). Spectroscopic measurements have shown that when a photon is absorbed by the special pair of chlorophylls, an electron is moved from the special pair to one of the pheophytin molecules. The close association and the parallel orientation of the chlorophyll ring systems in the special pair facilitates the excitation of an electron so that it is easily released. This process is very fast it occurs within 2 picoseconds. From the pheophytin the electron moves to a molecule of quinone, Qa, in a slower process that takes about 200 picoseconds. The electron then passes through the protein, to the second quinone molecule, Qb. This is a comparatively slow process, taking about 100 microseconds. 

Epitaxy. There is often a sharp orientation relationship between a singlecrystal substrate and a thin-film deposit, depending on the crystal structures and lattice parameters of the two substances. When such a relationship exists, the deposit is said to be in epitaxy with the substrate. The simplest relationship is parallel orientation, and this is common in semiconductor heterostructures, but more complex relationships are often encountered. 

Cre is a bacterial recombinase (cre=causes recombination), which recognizes loxP sites of bacteriophage P. If two loxP (loxP= locus of x-ing over of bacteriophage P) sites have a parallel orientation, the DNA segment between these sites will be deleted by the action of the Cre recombinase. 

Recently it was pointed out by Zener7 that the atomic moments, in parallel orientation, might react with the electrons in the conduction band in such a way as to uncouple some of the pairs, producing a set of conduction electrons occupying individual orbitals, and with spins parallel to the spins of the atomic electrons. Zener assumed that the conduction band for the transition metals is formed by the 4.s orbitals of the atoms, and that there is somewhat less than one conduction electron per atom in iron, cobalt, and nickel. Like Slater, he attributed the atomic magnetic moments to the partially filled 3d subshell. 

An obvious refinement of the simple theory for cobalt and nickel and their alloys can be made which leads to a significant increase in the calculated value of the Curie temperature. The foregoing calculation for nickel, for example, is based upon the assumption that the uncoupled valence electrons spend equal amounts of time on the nickel atoms with / = 1 and the nickel atoms with J = 0. However, the stabilizing interaction of the spins of the valence electrons and the parallel atomic moments would cause an increase in the wave function for the valence electrons in the neighborhood of the atoms with / = 1 and the parallel orientation. This effect also produces a change in the shape of the curve of saturation magnetization as a function of temperature. The details of this refined theory will be published later. 

When A = B, the expression at Eq. (8) represents the usual interaction energy of permanent dipoles. When A < B, parallel orientations of dipoles are favoured and when A > B, the interaction energy has a minimum for antiparallel dipoles. 

Fig. 4.3 Triplex invasion by homopyrimidine PNA oligomers. One PNA strand binds via Watson-Crick base pairing (preferably in the antiparallel orientation), while the other binds via Hoogsteen base pairing (preferably in the parallel orientation). It is usually advanta-...

Above a critical temperature Tc, the Curie temperature, a ferromagnetic material becomes paramagnetic, since thermal motion inhibits the parallel orientation of the magnetic moments. The susceptibility then follows the Curie-Weiss law with a positive value of the Weiss constant, 0 > 0 (Fig. 19.6). 

What determines the way in which the spins couple Parallel orientation always occurs when the corresponding atoms act directly on one another. This is the case in pure metals like iron or nickel, but also in EuO (NaCl type). Antiparallel orientation usually occurs when two paramagnetic particles interact indirectly by means of the electrons of an intermediate particle which itself is not paramagnetic this is called superexchange mechanism. That is the case in the commercially important spinels and garnets. 

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