Secondary-overlap effect

Both primary and secondary extinction effects may usually be avoided by powrdering a crystal. For this and other reasons the intensities of the arcs on powder photographs are likely to be more reliable than those of other types of photograph but in practice, in structure determination it is only possible to use powder intensities alone for very simple structures for complex crystals reflections from different planes overlap seriously. 

The additional stabilizing secondary electronic effect in the Z form might be larger than that (=1.4 kcal/mol) observed in acetals, because the carbonyl bond in esters is a more polarized bond than the C—OR bond in acetals and therefore, the antibonding orbital of the o C—0 bond should be of lower energy allowing increased overlap. It has been postulated on that basis (1-3) that the greater stability of the Z form (=3 kcal/mol) by comparison with the form in esters would be due mainly to this secondary electronic effect. 

Furthermore, the oxygen atom of the carbonyl group in the amide function has an electron pair oriented antiperiplanar to the polar C-N bond there is therefore an electronic delocalization caused by the overlap of that oxygen electron pair orbital with the antibonding orbital of the C-N sigma bond (o ) as shown in two dimensions by structure 5 and in three dimensions by structure . This additional n-o delocalization is referred to here as a secondary electronic delocalization. Thus, amides are similar to esters because they both have the primary electronic effect and one secondary electronic effect. This is in contrast with Z esters which have two secondary electronic effects besides the primary electronic effect. 

In the anti conformation, the second electron pair on the oxygen atom (cf. 49 and 51 ) is oriented anti peri planar to the polar C—N bond, so this electron pair orbital can overlap with the antibonding orbital (o ) of the C —N sigma bond. Thus, contrary to the syn isomer, anti imidates have in addition to the primary electronic effect, one secondary electronic effect (n-o ). This additional electronic delocalization should stabilize the anti form relative to the syn form. 

Electroviscous Effect Any influence of electric double layer(s) on the flow properties of a fluid. The primary electroviscous effect refers to an increase in apparent viscosity when a dispersion of charged colloidal species is sheared. The secondary electroviscous effect refers to the increase in viscosity of a dispersion of charged colloidal species that is caused by their mutual electrostatic repulsion (overlapping of electric double layers). An example of the tertiary electroviscous effect would be for polyelectrolytes in solution where changes in polyelectrolyte molecule conformations and their associated effect on solution apparent viscosity occur. 

In concentrated suspensions, the motion of particles is cmcially affected by hydrodynamic interaction between neighbouring particles, which strongly depends on the interparticle distances, i.e. on the suspension structure (cf. Overbeck et al. 1999 Watzlawek and Nagele 1997, 1999). This structure is clearly influenced by the inteiparticle forces, in particular by the forces that occur when the EDL of two particles overlap (e.g. Russel 1978 Quemada and Berli 2002). When a suspension contains only a single particulate component, such a double layer overlap leads to repulsions and, thus, decreases the particle mobility and increases the suspension viscosity (Fig. 3.5). This effect is called secondary electroviscous effect. Its... 

Fig. 3.5 Secondary electroviscous effect strong repulsion between similarly charged particles reduces their mobility since the overlap of EDL (indicated by dotted lines) would require additional energy and, as a result, the suspension is not randomly ordered the decisive parameter is the double layer thickness, which can be controlled via the electrolyte concentration...

This method is especially useful for the study of photo-oxidized surfaces where different oxy groups, e.g. C—O and C—O—O, have similar shifts or may overlap due to secondary substituent effects. 

The limits of pore size corresponding to each process will, of course, depend both on the pore geometry and the size of the adsorbate molecule. For slit-shaped pores the primary process will be expected to be limited to widths below la, and the secondary to widths between 2a and 5ff. For more complicated shapes such as interstices between small spheres, the equivalent diameter will be somewhat higher, because of the more effective overlap of adsorption fields from neighbouring parts of the pore walls. The tertiary process—the reversible capillary condensation—will not be able to occur at all in slits if the walls are exactly parallel in other pores, this condensation will take place in the region between 5hysteresis loop and in a pore system containing a variety of pore shapes, reversible capillary condensation occurs in such pores as have a suitable shape alongside the irreversible condensation in the main body of pores. 

Recently Stamhuis et al. (33) have determined the base strengths of morpholine, piperidine, and pyrrolidine enamines of isobutyraldehyde in aqueous solutions by kinetic, potentiometric, and spectroscopic methods at 25° and found that these enamines are 200-1000 times weaker bases than the secondary amines from which they are formed and 30-200 times less basic than the corresponding saturated tertiary enamines. The baseweakening effect has been attributed to the electron-withdrawing inductive effect of the double bond and the overlap of the electron pair on the nitrogen atom with the tt electrons of the double bond. It was pointed out that the kinetic protonation in the hydrolysis of these enamines occurs at the nitrogen atom, whereas the protonation under thermodynamic control takes place at the -carbon atom, which is, however, dependent upon the pH of the solution (84,85). The measurement of base strengths of enamines in chloroform solution show that they are 10-30 times weaker bases than the secondary amines from which they are derived (4,86). 

The electroviscous effect present with solid particles suspended in ionic liquids, to increase the viscosity over that of the bulk liquid. The primary effect caused by the shear field distorting the electrical double layer surrounding the solid particles in suspension. The secondary effect results from the overlap of the electrical double layers of neighboring particles. The tertiary effect arises from changes in size and shape of the particles caused by the shear field. The primary electroviscous effect has been the subject of much study and has been shown to depend on (a) the size of the Debye length of the electrical double layer compared to the size of the suspended particle (b) the potential at the slipping plane between the particle and the bulk fluid (c) the Peclet number, i.e., diffusive to hydrodynamic forces (d) the Hartmarm number, i.e. electrical to hydrodynamic forces and (e) variations in the Stern layer around the particle (Garcia-Salinas et al. 2000). 

This being the case, a purely sweet molecule may be expected to interact with structurally suitable features distributed over the areas SB and SB (see Fig. 35,ii) interactions in the area of overlap SB do not produce a secondary bitter note, but are essential for the generation of the sweet taste. A bitter taste can be produced only when suitable features in the area of the overlap and in the whole of the bitter area (SB) interact, as in Fig. 35,iii (pure bitter). Fig. 35,iv (bitter-sweet), and Fig. 35,v (sweet with a secondary bitter taste). Therefore, as long as the interactions are distributed over the whole area representing a modality, that modality will be identified by higher centers, provided that its intensity is not subliminal. If interactions are distributed over a part of a modal area (see Fig. 35,vi), identification of the modality will not result, so that no response will be produced. Interactions in the bitter or in the sweet area have a low efficiency, or they may be effective but involve only a small fraction of the sites. This can only affect the intensity of the generated response. 

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