Orientation preferential

If the medium is sufficiently basic to generate the arabident anion 31. mixtures of products resulting from N-nng and N-exocyclic reactivity are observed. Here again steric effects can preferentially orient the whole reaction toward one of the two nitrogens. A general study clearly delineating the rules of behavior for 31 accordine to the nature of R. the... 

At elevated temperatures (250-400°C) bromine reacts with thiazole in the vapor phase on pumice to afford 2-bromothiazole when equimolecu-lar quantities of reactants are mixed, and a low yield of a dibromothiazole (the 2,5-isomer) when 2 moles of bromine are used (388-390). This preferential orientation to the 2-position has been interpreted as an indication of the free-radical nature of the reaction (343), a conclusion that is in agreement with the free-valence distribution calculated in the early application of the HMO method to thiazole (Scheme 67) (6,117). 

Figure 7.14a illustrates the insertion of a propylene monomer into an edge vacancy in a crystal adjacent to an alkylated titanium atom. In Fig. 7.14b a cross-sectional view of the same site shows how the preferential orientation of the coordinated monomer is dictated by constraints imposed by the protuberances on the crystal surface. 

Cyclo ddltion. Ketenes are ideal components ia [2 + 2] cycloadditions for additions to the opposite sides of a TT-system as shown ia the cyclobutane product (2) ia Figure 1. Electron-rich double bonds react readily with ketenes, even at room temperature and without catalysts. In conjugated systems, ketenes add ia a [2 + 2] fashion. This is illustrated ia the reaction foUowiag, where the preferential orientation of L (large substituent) and S (small substituent) is seen (40). This reaction has been used ia the synthesis of tropolone [533-75-5]. 

Textile technology is used to mechanically or aerodynamicaHy arrange textile fibers into preferentially oriented webs. Fabrics produced by these systems are referred to as dry-laid nonwovens. Dry-laid nonwovens are manufactured with machinery associated with staple fiber processing, such as cards and gametts, which are designed to manipulate preformed fibers in the dry state. Also included in this category are nonwovens made from filaments in the form of tow, and fabrics composed of staple fibers and stitching filaments or yams, ie, stitchbonded nonwovens. 

Compared to PAN and pitch-based carbon fiber, the morphology of VGCF is unique in that the graphene planes are more preferentially oriented around the axis... 

Usually, dilute polymer solutions are isotropic systems, i.e. macromolecular chains can exist in these solutions independently of each other with a random distribution of orientations of the long axes of coils. The solutions of flexible-chain polymers remain isotropic when the solution concentration increases whereas in concentrated solutions of macromolecules of limited flexibility the chains can no longer be oriented arbitrarily and some direction of preferential orientations of macromolecular axes appears, i.e. the mutual orientations of the axes of neighboring molecules are correlated. This means that... 

For a biaxially drawn polymer with orthorhombic symmetry, the chain axis is preferentially oriented with respect to the XtX2 plane of the sample, and the direction normal to the chain axis is also preferentially oriented. Infra-red measurements enable the determination of the quantities Pf20 and PL202 and P222, which define these orientation effects as discussed in case (iv) above. 

The biaxially oriented PET sheets have been extensively studied with regard to their mechanical anisotropy and all nine independent elastic constants have been determined by a variety of experimental techniques 38,39). The complete set of compliances for a one-way drawn sheet of draw ratio 5 1 is shown in Table 7. It is interesting to note that these compliances clearly reflect the two major structural features, the high chain axis orientation and the preferential orientation of the benzene rings... 

For Hg, the temperature coefficient of Ea=0 was determined by Randies and Whiteley78 and found to be equal to 0.57 mV K l.On the basis of a simple up-and-down molecular model for water,79 this positive value has been taken to indicate a preferential orientation, with the negative end of the molecular dipole (oxygen) toward the metal surface. While this may well be the case, the above discussion shows that the analysis of the experimental value is far more complex. 

An aspect that is difficult to treat is the nature of the boundary between the adsorbate layer and the bulk of the solution. Solvent molecules are now in contact with an organic layer and the kind of interaction is expected to differ substantially from that with a bare metal surface. The layers of solvent molecules in the immediate proximity of the adsorbate might exhibit some preferential orientation, which is not explicitly accounted for in Eq. (36), and this adds some additional ambiguity to the physical interpretation of the results. 

A negative temperature coefficient of the inner-layer potential drop was observed, -0.8 mV KT1. Estimates of dipole potential drops due to solvent molecules22,23,29,30 gave much larger values for DMSO than for H20, which can be explained by a strong preferential orientation of DMSO at the Hg surface.26,8,304... 

A controversy exists over the interpretation of such a correlation. According to the simple two-state model for water at interfaces, the higher the preferential orientation of one of the states, the higher the value of BEa=Q/BT. If the preferentially oriented state is that with the negative end of the dipole down to the surface, the temperature coefficient of Ev is positive (and vice versa). Thus, in a simple picture, the more positive BEa=0/BTt the higher the orientation of water, i.e., the higher the hydro-philicity of the surface. On this basis, Silva et al.446 have proposed the... 

Fig. 2. (continued)—(d) an aggregate of microcrystallites whose long axes are parallel, but randomly oriented (left), diffracts to produce a series of layer lines (right) and (e) a polycrystalline and preferentially oriented specimen (left) diffracts to give Bragg reflections on layer lines (right). The meridional reflection on the fourth layer line indicates 4-fold helix symmetry. 

Chitosan scaffolds were reinforced with beta-tricalciiun phosphate and calcium phosphate invert glass [177]. Along the same line, composites of Loligo beta-chitin with octacalcium phosphate or hydroxyapatite were prepared by precipitation of the mineral into a chitin scaffold by means of a double diffusion system. The octacalciiun phosphate crystals with the usual form of 001 blades grew inside chitin layers preferentially oriented with the 100 faces parallel to the surface of the squid pen and were more stable to hy-... 

Fig. 4.2 (Left) XRD patterns (CuK ) of equally thick ZnSe (ca. 1 xm) deposits prepared on H (a) and on CdSe/Ni (b), at a deposition potential of -0.7 V vs. SHE from a typical acidic (pH 3) solution. The reflection intensities for ZnSe(l 11) are denoted in the figure (right) XRD patterns within the low-angle region showing the (111) reflections of ZnSe/CdSe heterostructures (A). Overlayers of ZnSe were deposited on CdSe films of various (111) texture intensities. The substrate features are shown in (B) in full-intensity scale. Evidently, the preferential orientation of the ZnSe crystallites increases with that of the CdSe substrate, up to a constant limiting value. (Reprinted from [16], Copyright 2009, with permission from Elsevier)...

A similar one-step process was employed successfully [66] to prepare well-crystallized CdS thin films of optical quality on Au(lll) from an aqueous solution of CdSOa, EDTA, and Na2S at room temperature. A phase transition from cubic (zinc blende) to hexagonal (wurtzite) CdS structure was observed with decreasing pH below 5, while highly preferential orientation along [11.0] directions for the... 

Solla-Gullon J, Solla-Gullon J, Vidal-Iglesias FJ, Herrero E, Feliu JM, Aldaz A. 2006. CO monolayer oxidation on semi-spherical and preferentially oriented (100) and (111) platinum nanoparticles. Electrochem Commun 8 189-194. 

---