Non-specific centers

A much more marked increase of the stereospecifity was observed in our laboratories 81) with a TiCl4/MgCl2 catalyst in combination with TEA (Fig. 34). In this case, however, the activity decreases so that the increase in isotacticity appears to be due to a preferential inhibition or deactivation of non-specific centers (Fig. 35). Similar results were reported by Pino and Rotzinger, 09). 

The increase in isotacticity seems to be essentially connected to the decrease of the initial rate, as practically no change in the isotacticity index with polymerization time was detected. Moreover, while the atactic productivity decreases monotonically with the EB/TEA ratio in both systems, the isotactic productivity has a more complex behavior with the binary catalyst it remains almost unchanged up to EB/TEA s 0.25 and then falls, whereas with the ternary catalyst it increases up to EB/TEA 0.2 and then rapidly drops. On the grounds of these results, Spitz suggested that the reversible adsorption on the catalytic surface of the TEA EB complex (which is supposed to be very fast) changes the non specific centers into stereospedfic, though less active, centers, while the slower adsorption of free EB reversibly poisons both types of sites. The differences between the binary and the ternary catalysts would arise mainly from the presence, in the latter, of a larger number of potential stereospecific sites. 

Thus it has been concluded that the donor is fixed on the catalyst surface, selectively poisoning the non-specific centers and simultaneously increasing the reactivity of the isospecific centers. 

From the above it is clear that the Cp and k values reported in the preceding section are only average values which do not reflect the real situation, although they are quite useful in understanding certain phenomena. The active species not only consist of isospecific and non-specific centers in the case of the propylene polymerization but, rather, by a plurality of species having different reactivities, which cannot be completely identified by kinetic studies or by catalyst poisoning. 

Moreover, application of the above law to the formation rates of isotactic and atactic fractions showed that the overall rate equation is the result of two equations characterized by different values of kA (200 1 mol-1 for the isospecific centers and 40 1 mol 1 for the non-specific centers). Thus, the kinetic behavior of the polymerization was rationalized on the basis of a two-center polymerization model. Furthermore, based on an approximate estimate of the partition function of the transition state involving propagating chain and coordinated monomer, monomer insertion was proposed as the rate determining step. 

Lipophilic ion exchangers traditionally used for polymeric membrane preparation are the anionic tetraphenylborate derivatives and the cationic tetraalkylammonium salts. The charges on both lipophilic ions are localized on a single (boron or nitrogen) atom, but the steric inaccessibility of the charged center, due to bulky substituents, may inhibit ion-pair formation in the membrane and provide, when necessary, non-specific interactions between ionic sites and sample ions. 

To answer this question, let us first consider a neutral molecule that is usually said to be polar if it possesses a dipole moment (the term dipolar would be more appropriate)1 . In solution, the solute-solvent interactions result not only from the permanent dipole moments of solute or solvent molecules, but also from their polarizabilities. Let us recall that the polarizability a of a spherical molecule is defined by means of the dipole m = E induced by an external electric field E in its own direction. Figure 7.1 shows the four major dielectric interactions (dipole-dipole, solute dipole-solvent polarizability, solute polarizability-solvent dipole, polarizability-polarizability). Analytical expressions of the corresponding energy terms can be derived within the simple model of spherical-centered dipoles in isotropically polarizable spheres (Suppan, 1990). These four non-specific dielectric in-... 

Cations and anions with a strong solvation shell retain their solvation shell and thus interact with the electrode surface only through electrostatic forces. Since the interaction is exclusively electrostatic, the amount of these ions at the interface is defined by the electrostatic bias between the sample and the counter electrodes and independent from the chemical properties of the electrode surface non-specific adsorption. Considering the size effect of their hydration shell, these ions are able to approach the electrode to a distance limited by the size of the solvation shell of the ion. The center of these ions at a distance of closest approach defined by the size of the solvation shell is called the outer Helmholtz layer. The electrode surface and the outer Helmholtz layer have charges of equal magnitude but opposite sign, resulting in the formation of an equivalent of a plate condenser on a scale of a molecular layer. Helmholtz proposed such a plate condenser on such a molecular scale for the first time in the middle of the nineteenth century. 

Ions with a weak solvation shell, anions in general, lose a part of or the complete solvation shell in the double layer and form a chemical bond to the metal surface. The adsorption is termed specific since the interaction occurs only for certain ions or molecules and is not related to the charge on the ion. The plane where the center of these ions are located is called the inner Helmholtz layer. In the specific adsorption, ions are chemically bound to the surface and the interaction has a covalent nature. In the case of non-specific adsorption, in which an electrostatic force binds ions to the surface, the coverage of ions is below 0.1 -0.2 ML due to electrostatic repulsion between the ions. In contrast, the coverage of specifically adsorbed ions exceeds this value, and a close-packed layer of specifically adsorbed ions is often observed. Specifically adsorbed ions are easily observed by STM [22], indicating that the junction between the electrode surface and the inner Helmholtz layer is highly... 

Figure 7.4. Schematic model of the Electrical Double Layer (EDL) at the metal oxide-aqueous solution interface showing elements of the Gouy-Chapman-Stern-Grahame model, including specifically adsorbed cations and non-specifically adsorbed solvated anions. The zero-plane is defined by the location of surface sites, which may be protonated or deprotonated. The inner Helmholtz plane, or [i-planc, is defined by the centers of specifically adsorbed anions and cations. The outer Helmholtz plane, d-plane, or Stern plane corresponds to the beginning of the diffuse layer of counter-ions and co-ions. Cation size has been exaggerated. Estimates of the dielectric constant of water, e, are indicated for the first and second water layers nearest the interface and for bulk water (modified after [6]).

The transport velocity of Li+ is faster than that of Na+ and K+ due to the size of the cation. The data are consistent with a hopping transport mechanism of the cations accompanied by a non-specific co-transport of the anions. The transport rates for N03 > Cl- > C104 are related to the adjacent hydrate shell and not yet fully understood. Anyway, a path in the center of the supramolecular tubes, where the crown ethers assemble, must exist and allow for the co-transport of the anions. By forming the membranes in the pores of track-etched membranes, the transport rates could be improved by an order of magnitude due to the orientation of the channels perpendicular to the membrane surface. 

The iron-ovotransferrin spectrum of Aasa et al. (7) consisted of a 3-component part around g = 4.1 and a low field part around g = 8.8 (Fig. 11). Some samples showed weak lines on the wings of the main line and these were thought to be due to non-specifically bound metal ions. No other lines were found. Windle et al. 137) independently found an asymmetric 3-component spectrum centered at g = 4.27 and attributed to Fe3+. 

In TiCl3, on the other hand, AlEt2Cl activates only the predominantly stereo-specific surface sites, while AlEt3 can disrupt the crystalline lattice of the catalyst thus forming non-stereospecific centers. 

The above-mentioned techniques have been employed in determining the total number of active centers Cp and, in the case of polypropylene, the number of iso-specific Cp, and non-specific Cp centers after polymer fractionation. However, none of these methods appears universal or completely reliable. It is known that the method based on quenching with ROT is complicated by secondary reactions with the aluminum alkyl and by isotopic effects. Even quenching with CO seems to give an underestimated value of the number of active centers 137,138) and, thus, an overestimation of the propagation rate constant as determined according to the correlation ... 

The activity ratio is Eu/Rac = 1 in this case both isomers are equipotent and no stereoselectivity is observed. This can be explained by the assumption (a) that the compounds act through a non-specific mechanism, (b) that the active compound and the receptor make only a two-point contact with the chiral center, (c) that the chiral center is not involved in the contact (is located in a silent region ). 

In our earlier study (2) we measured the concentration of ions required to inactivate the enzyme and tried to determine whether the inactivation could be changed by competition with the normal ions, Mg2+, Na+, and K From these studies we assigned the inactivation effects of some cations to actions at specific sites. The cations that could not be associated by the demonstration of competitive inhibition with Mg2+, Na+, or K+ sites were classed as 4 non-specific inhibitors. These cations act at relatively low concentrations, and the concentrations of ions giving 50% inhibition of enzyme activity are correlated with the oxidation potential of the ion and with the binding constants to ethylenediamine, histidine, and imidazole. These results suggest that the non-specific ion inhibitors may react at one site—a histidine-like residue near the active center of the enzyme. 

Two diverse views of non-specific adhesion processes form the bases for contemporary theories introduced to rationalize observations of colloidal stability and flocculation in solutions of macromolecules (see 16-18 for general reviews). The first view is based on adsorption and cross-bridging of the macromolecules between surfaces. Theories derived from this concept indicate a strong initial dependence on concentration of macromolecules there is a rapid rise in surface adsorption for infinitesimal volume fractions (32) followed by a plateau with gradual attenuation of surface-surface attraction because of excluded volume effects in the gap at larger volume fractions (19-20). The interaction of the macroinolecule with the surface is assumed to be a snort range attraction proportional to area of direct contact. The second - completely disparate - view of non-specific adhesion is based on the concept that there is an exclusion or depletion of macromolecules in the vicinity of the surface, i.e. no adsorption to the surfaces. Here, theory shows that attraction is caused by interaction of tne (depleted) concentration profiles associated with each surface which leads to a depreciated macrornolecular concentration at the center of the gap. The concentration... 

As has been shown in Figures 4.2.-4.4., adsorption of NaBr on non-specific Ni-centers can enhance ee in this hydrogenation as a result of blocking of "racemic" centers, which produce racemic MHB and result in increases in optical yields... 

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