Crystals destabilization

In the simplest emulsions just described, the final separation is into two Hquid phases upon destabilization. The majority of emulsions are of this kind, but in some cases the emulsion is divided into more than two phases. One obvious reason for such a behavior is the presence of a material that does not dissolve in the oil or the water. One such case is the presence of soHd particles, which is common in emulsions for food, pharmaceuticals, and cosmetics. Another less trivial reason is that the surfactant associates with the water and/or the oil to form a colloidal stmcture that spontaneously separates from the two hquid phases. This colloidal stmcture may be an isotropic Hquid or may be a semisoHd phase, a Hquid crystal, with long-range order. 

As usual, this can be due both to thermodynamic and kinetic reasons. In fact, the presence of comonomeric units increases, in general, the energy content of all the crystalline forms, but, since the extent of increase may be different, it may destabilize some chain conformation or some kind of packing more than other ones. On the other hand, the influence of the comonomeric units on the polymorphic behavior of a polymer can be due to a change in the crystallization rates of the various forms. 

The crystal field energy level diagram for tetrahedral complexes. The d orbitals are split into two sets, with three orbitals destabilized relative to the two others. 

Antibody A52 with its epitope at residues 657-672 [129,139,274,275] inhibited the vanadate-induced crystallization of Ca " -ATPase and decreased the stability of preformed Ca " -ATPase crystals [285]. The vanadate-induced crystals arise by the association of the ATPase monomers into dimers (type A interaction), the dimers into dimer chains (type B interaction), and the dimer chains into 2-dimensional arrays (type C interaction). It is suggested that antibody A52 interferes with type B interactions, preventing the formation of dimer chains, without exerting major effect on the concentration of Ca -ATPase dimers in the membrane. The simplest interpretation of the destabilization of Ca -ATPase crystals by mAb A52 is that binding of the antibody to its antigenic site physically blocks the interaction between ATPase molecules [285]. Considering the large bulk of the antibody, such interference is not unexpected, yet only a few of the antibodies that bind to the Ca -ATPase in native sarcoplasmic reticulum interfered with crystallization. 

Multipole analysis with high-resolution X-ray data for [Ni(thmbtacn)]2+ was carried out to determine the electron configuration in the C3 symmetry-adapted orbitals of the Ni ion, confirming a higher occupancy of the crystal field-stabilized t2g orbitals relative to the destabilized eg orbitals. This is interpreted in terms of a predominantly ionic metal-ligand interaction.1424... 

The transition-state model of this reaction has been proposed as (1), based on X-ray analyses of single crystals prepared from Ti(OPr )4, (R,R)-diethyl tartrate (DET), and PhCON(OH)Ph and from Ti(OPr )4, and (R,R) N,/V -dibenzyltartramide.30-32 The Z-substituent (R2), located close to the metal center, destabilizes the desired transition state and decreases enantioselectivity (vide supra). When the Z-substituent is chiral, face selection induced by the substituent strongly affects the stereochemistry of the epoxidation, and sometimes reversed face selectivity is observed.4 In contrast, the. E-substituent (R1) protrudes into an open space and E -allylic alcohols are generally good substrates for the epoxidation. 

The crystal field effect is due primarily to repulsive effects between electron clouds. As we have already seen, the repulsive energy is of opposite sign with respect to coulombic attraction and the dispersive forces that maintain crystal cohesion. An increase in repulsive energy may thus be interpreted as actual destabilization of the compound. 

All the studies on the elucidation of the structure of diazoazoles lead unequivocally to a mesoionic structure in which the negative charge is localized in the azole ring. Of primary importance were C-NMR and IR spectra, as well as X-Ray crystal diffraction data. With regard to the stability in the solid state of diazoazoles and in solution in the pH range where they are not protonated, the presence of ortho ring nitrogens destabilizes the diazo compounds. 

The populations of the crystal-field-stabilized dxz, dyz, and dxy orbitals are in very good agreement with results of an Extended Hiickel (EH) calculation (third column of Table 10.7), which is perhaps unexpected, given the approximate nature of the calculation. Reasonable agreement is also obtained for dz2, but not for the destabilized dx2 y2 orbital, for which the EH method overestimates the population... 

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