Orienting effects

Face-centered cubic crystals of rare gases are a useful model system due to the simplicity of their interactions. Lattice sites are occupied by atoms interacting via a simple van der Waals potential with no orientation effects. The principal problem is to calculate the net energy of interaction across a plane, such as the one indicated by the dotted line in Fig. VII-4. In other words, as was the case with diamond, the surface energy at 0 K is essentially the excess potential energy of the molecules near the surface. 

Mixtures of polymers at surfaces provide the interesting possibility of exploring polymer miscibility in two dimensions. Baglioni and co-workers [17] have shown that polymers having the same orientation at the interface are compatible while those having different orientations are not. Some polymers have their hydrophobic portions parallel to the surface, while others have a perpendicular disposition. The surface orientation effect is also present in mixtures of poly(methyl methacrylate), PMMA, and fatty acids. 

The polarization model suggests strongly that orientational effects should be present in multilayers. As seen in Section X-6, such perturbations are essential to the explanation of contact angle phenomena. 

Kozack R E, d Mello M J and Subramaniam S 1995 Computer modeling of electrostatic steering and orientational effects in antibody-antigen association Biophys. J. 68 807-14... 

Likewise, Grieco, while working with amphiphile-like reactants, observed an enhanced preference for endo-adduct in aqueous solutions, which he attributed to orientational effects within the micelles that were presumed to be present in the reaction mixture ". Although under the conditions used by Grieco, the presence of aggregates cannot be excluded, other studies have clearly demonstrated that micelle formation is not the reason for the improved selectivities . Micellar a peg tes even tend to diminish the preference for endo adduct. ... 

Jaeger and co-workers studied the regioselectivity of the reaction of a surfactant diene with a surfactant dienophile in micellar media" ". The orientational effects in the aggregates could result in an increase in the regjoselectivity in aqueous solutions of these compounds as compared to the reaction in organic media. 

The halogen substituents ( — 7 +M) owe their o p-orientating effect, achieved in spite of the deactivation, to polarisability by the conjugative process. The strength of the inductive deactivation is seen in the sequence of the two ratios quoted. 

The presence of activating substituent on the carbocyclic ring can, of course, affect the position of substitution. For example, Entries 4 and 5 in Table 14.1 reflect such orientational effects. Entry 6 involves using the ipso-directing effect of a trimethylsilyl substituent to achieve 4-acetylation. 

Tacticity of products. Most solid catalysts produce isotactic products. This is probably because of the highly orienting effect of the solid surface, as noted in item (1). The preferred isotactic configuration produced at these surfaces is largely governed by steric and electrostatic interactions between the monomer and the ligands of the transition metal. Syndiotacticity is mostly produced by soluble catalysts. Syndiotactic polymerizations are carried out at low temperatures, and even the catalyst must be prepared at low temperatures otherwise specificity is lost. With polar monomers syndiotacticity is also promoted by polar reaction media. Apparently the polar solvent molecules compete with monomer for coordination sites, and thus indicate more loosely coordinated reactive species. 

Crystallization. Acidified aluminum sulfate solutions can be supercooled 10 °C or more below the saturation point. However, once nucleation begins, the crystallization rate is rapid and the supersaturated solution sets up. The onset of nucleation in a gentiy stirred supersaturated solution is marked by the appearance of silky, curling streamers of microscopic nuclei resulting from orientation effects of hydraulic currents on the thin, platelike crystals. Without agitation, nucleation in an acidified solution, in glass tubes, can yield extended crystalline membranes of such thinness to exhibit colors resulting from optical interference. 

Substituents with -I,+M effects such as halogen show a similar orientation effect to that of alkyl groups. If the substituent is in the 2-position, substitution occurs at C-5 if it is in position 3, substitution occurs preferentially at C-2 or if steric requirements of the group or electrophile intervene, then substitution occurs at C-5. 

Despite some recent discoveries, free radical reactions are still very much less common in azole chemistry than those involving electrophilic or nucleophilic reagents. In some reactions involving free radicals, substituents have little orienting effect however, rather selective radical reactions are now known. 

This dispersion interaction must be added to the dipole-dipole interactions between molecules, such as HCl, NH3 and H2O which have a permanent dipole, fi. The magnitude of die dipole moment depends on tire differences in electronegativity of the atoms in the molecule. Here again, the energy of interaction varies as (orientation effect). 

As examples of the relative magnitudes of these contributions, only tire dispersion effect applies to monatomic gases, and in tire case of HCl (/ = 12.74eV, fjL — 1.03 debye), tire dispersion effect predominates, in NH3 (/ = 10.2eV, ijl — 1.49d) these effects are about equal, and in H2O (I — 12.6eV, IJL — 1.85 d), the orientation effect predominates. 

Anisotropic behaviour is also exhibited in optical properties and orientation effects can be observed and to some extent measured by birefringence methods. In such oriented materials the molecules are in effect frozen in an unstable state and they will normally endeavour to take up a more coiled conformation due to rotation about the single bonds. If an oriented sample is heated up the molecules will start to coil as soon as they possess sufficient energy and the mass will often distort. Because of this oriented materials usually have a lower heat distortion temperature than non-oriented polymers. 

Biaxial orientation effects are of importance in the manufacture of films and sheet. Biaxially stretched poly(ethylene terephthalate) (e.g. Melinex),... 

In Chapter 3 it was pointed out that certain rod-like polymers showed many of the attributes of liquid crystals in the melt. In particular, these molecules were oriented in shear to such an extent that interchain entanglement was small and the melts had a low viscosity. On cooling of the melt these rod-like molecules remained oriented, effectively self-reinforcing the polymer in the direction of flow. The essential differences in the properties of liquid crystal polymers... 

This characteristic of RAIR can be observed experimentally. Fig. 8 shows the transmission spectrum of polydimethylsiloxane (PDMS) while Fig. 9 shows the RAIR spectrum of a thin film of PDMS spin-coated onto a chromium substrate. It can be observed that the bands near 1024 and 1095 cm have similar intensities in the transmission spectra but the band at higher frequencies is clearly much more intense in the RAIR spectrum. This change in relative intensity when PDMS is deposited onto a reflecting substrate is related to optical effects and is not related to orientation effects. 

These long fibres give better product performance although injection moulding machine modifications may be necessary to prevent fibre damage and reduce undesirable fibre orientation effects in the mould. 

Based on Figs. 7-9, another interesting observation can be made concerning orientational effects in the confined film. As shown above on the... 

FIGURE 16.15 Orientation effects in intramolecular reactions can be dramatic. Steric crowding by methyl groups provides a rate acceleration of 2.5 X 10 for the lower reaction compared to the upper reaction. (Adaptedfrom Milstien,. S., and Cohen, L. A., 1972. Stereopopnlation control I. Rate enhancements in the laetonization of o-hyelroxyhyeJroeinnamie acid. Journal of the American Chemical Society 94 9158-9165.)... 

Clearly, proximity and orientation play a role in enzyme catalysis, but there is a problem with each of the above comparisons. In both cases, it is impossible to separate true proximity and orientation effects from the effects of entropy loss when molecules are brought together (described the Section 16.4). The actual rate accelerations afforded by proximity and orientation effects in Figures 16.14 and 16.15, respectively, are much smaller than the values given in these figures. Simple theories based on probability and nearest-neighbor models, for example, predict that proximity effects may actually provide rate increases of only 5- to 10-fold. For any real case of enzymatic catalysis, it is nonetheless important to remember that proximity and orientation effects are significant. 

The ratio of the amounts of isomeric addition products will depend on the orienting effect of the hetero atom in the nucleus. 

Those reactions of halogenopyridines with potassium amide and lithium piperidide which proceed via 3,4-pyridyne form the 3- and 4-substituted pyridine derivatives in ratios of 1 2 and 1 1, respectively (see Section II, A, 1). It appears that the ring nitrogen atom has an orienting effect on these additions, but the quantitative divergence of the addition of ammonia and piperidine is not understood at present. 

The addition ratios observed for 46 and 47 can be associated with a wieto-orienting effect of the ethoxy group, analogous to that observed... 

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