The Different Structures

We have described the thickened state of 8CB (i.e., its smectic phase), obtained at temperatures below 29°C, as being composed of liquid layers stacked up on top of each other. We also mentioned that the long thin molecules making up the layers were on average disposed normal to those layers. Such a phase will hereafter be referred to as a smectic A phase, or Sa- We shall see that this is not the only possible arrangement. Let us take our microscope and observe the behaviour of a few milligrams of TBPA, i.e., terephthal-bis-pentylaniline, which has formula 

At temperatures above 232°C, we observe the colours of birefringent layers as previously, together with the threads characteristic of nematic phases. Below 213.5°C, the threads disappear and we observe the focal conic texture which typifies the smectic in Fig. 9.0c. In fact, certain regions of our preparation appear dark when viewed under crossed polarisers, whatever their disposition. Such an observation could only be possible if the optical properties of our smectic phase were the same as those of the uniaxial nematic described in the last section, and if this optical axis were parallel to the microscope axis. This implies that the smectic layers are parallel to the microscope slide in these regions of the preparation, and also that molecules are on average normal to the layers. Cooling further, at temperatures below 182.5°C, the dark regions become coloured and reveal a threadlike texture very similar to that of the nematic phases (see Fig. 9.10). 

Intensity fluctuations are also clearly visible. However, the practiced eye will notice that only disclinations of whole number rank are present in the preparation. Focal conic zones remain unchanged, up to some small details. But where only one colour was observed previously, we now often find two colours separated by a thin wall. All these observations are compatible with the simple idea that molecules are tilted relative to the plane of the layer, as shown in Fig. 9.11a. When there is no external field, the tilt direction remains indeterminate, just as we found for the directions of optical axes in nematic phases, which gave rise to their threadlike texture. However, the absence of disclinations of whole number rank is characteristic of a layered structure. The two colours can be understood as being due to occurrence of positive and negative tilts in thin preparations (see Fig. 9.11b). These arguments are corroborated by crystallographic studies. We have thus discovered a second type of layered liquid crystal, called the smectic C phase, or Sc- Note that this tilting does not preclude a liquid-type order within layers (a kind of 2-dimensional nematic phase). 

Cooling the preparation down to 153.5°C, a low-contrast front crosses the sample and we observe a texture almost identical with that of the smectic C phase. As the temperature falls even further below 153.5°C, disclination cores open up into magnificent five-pointed stars, whose description goes beyond the context of the present introduction. The sample is still in the smectic state, for the focal conics are still clearly visible. The exact nature of this phase is more subtle than those described previously. Layers are organised in a different way. Although it is still liquid and molecules are still tilted, the local order is very 

If we had chosen other compounds, we could have observed phases in which the tilt angle alternates from one layer to another, and many other smectic phases too. It is not difficult to convince ourselves, at least theoretically, that there must be infinitely many possibilities  

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The structure coding depends strongly on the properties which arc to be modeled. For an explanation of the different structure codes and their applications. sec Chapter 8,... 

Fig. 11.6. Room temperature microstructures in slowly cooled steels of different carbon contents, (a) The proportions by weight of the different phases, (b) The proportions by weight of the different structures.

Critoph and Turner [15] carried out similar direct measurements for ammonia and 208C (coconut shell based) carbon manufactured by Sutcliffe Speakman Carbons. The bed conductivity was found to be around 0.165 W/mK at concentrations less than 20% and to rise to 0.19 W/mK at 25% concentration. The corresponding grain conductivities rose from 0.85 to 1.25 W/mK respectively. The higher grain conductivity than that found by Gurgel and Grenier may reflect the different structures present within the extmded and nut shell carbons. 

As regards the general behaviour of polymers, it is widely recognised that crystalline plastics offer better environmental resistance than amorphous plastics. This is as a direct result of the different structural morphology of these two classes of material (see Appendix A). Therefore engineering plastics which are also crystalline e.g. Nylon 66 are at an immediate advantage because they can offer an attractive combination of load-bearing capability and an inherent chemical resistance. In this respect the arrival of crystalline plastics such as PEEK and polyphenylene sulfide (PPS) has set new standards in environmental resistance, albeit at a price. At room temperature there is no known solvent for PPS, and PEEK is only attacked by 98% sulphuric acid. 

All 12 compounds are well known and are available commercially their physical properties are summarized in Table 13.6 Comparisons with the corresponding data for NX3 (p. 438) and PX3 (p. 496) are also instructive. Trends in mp, bp and density are far from regular and reflect the differing structures and bond types. 

Figure 2. The structural energy difference (a) and the magnetic moment (b) as a function of the occupation of the canonical d-band n corresponding to the Fe-Co alloy. The same lines as in Fig. 1 are used for the different structures. In (b) the concentration dependence of the Stoner exchange integral Id used for the spin-polarized canonical d-band model calculations is shown as a thin dashed line with the solid circles. The value of Id for pure Fe and Co, calculated from LSDA and scaled to canonical units, are also shown in (b) as solid squares.

The sometimes contradictory results from different workers in relation to the elements mentioned above extends to other elements . Some of these differences probably arise from variations in test methods, differences in the amounts of alloying additions made, variations in the amounts of other elements in the steel and the differing structural conditions of the latter. Moreover, the tests were mostly conducted at the free corrosion potential, and that can introduce further variability between apparently similar experiments. In an attempt to overcome some of these difficulties, slow strain-rate tests were conducted on some 45 annealed steels at various controlled potentials in three very different cracking environments since, if macroscopic... 

Draw the different structures for an octahedral cobalt compiex containing four NHj and two N02 groups. 

The following example provides information on designing of plastic structural products to take static loads. It is a structural problem common to a number of different structures to show how the different structural requirements will affect the choice architectural designers has to make. The design problem will be a roof section which may be used for anything from a work shed,... 

It is intriguing that upon emersion the value of A0 changes up to about 0.3 V compared with the immersed state.41 This has been attributed42,51 to the different structure of the liquid interfacial layer in the two conditions. In particular, the air/solvent interface is missing at an emersed electrode because of the thinness of the solvent layer, across which the molecular orientation is probably dominated by the interaction with the metal surface. 

The elucidation of the factors determining the relative stability of alternative crystalline structures of a substance would be of the greatest significance in the development of the theory of the solid state. Why, for example, do some of the alkali halides crystallize with the sodium chloride structure and some with the cesium chloride structure Why does titanium dioxide under different conditions assume the different structures of rutile, brookite and anatase Why does aluminum fluosilicate, AljSiCV F2, crystallize with the structure of topaz and not with some other structure These questions are answered formally by the statement that in each case the structure with the minimum free energy is stable. This answer, however, is not satisfying what is desired in our atomistic and quantum theoretical era is the explanation of this minimum free energy in terms of atoms or ions and their properties. 

Figure 1. Crystallographic relation (schematic) between the structure types of RhB (anti-NiAs type), TaFeB (ordered Fe2P type) and ZrIrjB4 type. Numbers given indicate heights in projection along [001]. Large circles are metal atoms, small circles are B atoms. Metal sublattice of RhB and different modes of filling the voids (squares) generate the different structure types (see text).

Table 3. Geometrical Relationship Between the Different Structure Types MreMt4B4 (My = Co, Ru, Rh, Os, Ir)...

Simple geometric relations exist for the packing mode of the M 4 units within the different structure types MRgMT4B4 for details see also Table 3 and Fig. 3... 

AFM is used in the surface analysis. Figure 16 is the AFM topography of the monolayer and the multilayer L-B films. It shows that the monolayer L-B film is well packed and highly ordered on the mica surface. The surface of the monolayer film (shown in Fig. 16(a)) has a higher packing density than that of the four-layer L-B film (shown in Fig. 16(b)). This is because the molecules form the different structures in the monolayer film from those in four-layer... 

Fe(TPP)(CCl2)(H20)] is not significantly displaced from the porphyrin plane. Despite the different structures of [Fe(F20-TPP)CPh2], [(Melm)Fe(F2o-TPP) CPh2], and [Fe(TPP)(CCl2)(H20)], the average Fe-N(pyrrole) distances in these three complexes are similar. 

Changing the substrate from gold to silver has been shown to strongly affect the structure of the first few layers of CdS grown by ECALE. STM measurements carried out on the first CdS layer on Ag(lll) revealed a much less compact structure than the one found on Au(lll). This disparity was tentatively attributed to the different structure of the first S layer on Ag(l 11), as obtained by oxidative UPD from sulfide ion solutions, due to a higher affinity of sulfur for silver than for gold. The Cd layers were attained on S by reductive UPD from cadmium ion solutions. Precursors for both elements were dissolved in pyrophosphate/NaOH at pH 12 [43 5],... 

If a product is to be exported to different countries, an understanding of the different structures and requirements set by the European Union and United States regulatory authorities is critical. 

In 2000, novel thiazolidine derivatives were prepared from L-cysteine by Guan et al, showing good enantioselectivities of up to 90% ee when used as ligands in the addition of ZnEt2 to aldehydes. The results collected in Scheme 3.6 show that the catalytic efficiency of the thiazolidine derivatives was influenced by the different structures of the thiazolidine rings and the bulkiness of the moiety in... 

Molecules having the same composition but different structures are called isomers. The corresponding phenomenon for crystalline solids is called polymorphism. The different structures are the modifications or polymorphic forms. Modifications differ not only in the spatial arrangement of their atoms, but also in their physical and chemical properties. The structural differences may comprise anything from minor variations in the orientation of molecules up to a completely different atomic arrangement. 

Fig. 18. Comparison between the different structures of Me2Si(NCMe3)2Sn2NCMe3 (81), (Me3CN)4-Sn3H2 (87) and (Me3CN)4Sn4 (82), resp. I, II and III of Scheme 3 10)...

In this zone, the quantity of extracted oil is generally sufficient to obtain the distribution of the different structural groups (SARA analysis) except for oil A (Fig. 6 to 9) For oil B (Fig. 6), for the first two samples, the amount of extracted products is too low and the analysis is uncertain. It can only be noticed that the asphaltene content is null. On the contrary, just beyond the coke zone (samples III-IV), the asphaltene content respectively reaches 12.9 and 5 4 whereas the asphaltene content of the initial oil is only 0.3. This effect is also observed for oil C (10 versus 6.3%) (Fig. 7), D 24% versus 13.8 ) (Fig. 8), E (24 4 versus 8.1 ) (Fig. 9) For all the oils, the amount of resins+asphaltenes generally remains constant and the amount of saturates increases... 

State-of-the-art polymeric materials possess property distributions in more than one parameter of molecular heterogeneity. Copolymers, for example, are distributed in molar mass and chemical composition, while telechelics and macromonomers are distributed frequently in molar mass and functionality. It is obvious that n independent properties require n-dimensional analytical methods for accurate (independent) characterization of the different structural parameters. 

Let us assume that system 0 can be transformed to system 1 through the continuous change of some parameter A defined such that Ao and Ai correspond to systems 0 and 1, respectively. This parameter could be a macroscopic variable - viz. the temperature, a parameter that transforms J o to -A j, or a generalized coordinate (e.g., a torsional angle or an intermolecular distance) that allows the different structural states of the system to be distinguished. It follows that ... 

The TBP structures of 9 I2 and 11 2I2 are noteworthy, since iodine adducts of selenides are predicted to be MC based on the general rule (x(Se) (=2.48) > xCO (=2.21)). This must be the reflection of the high ability of the imidazoline ring to donate electrons to Se by the formation of the stable cyclic 6ji system. It will decrease xeff of Se in 9 and 11 to give TBP with iodine. Since the electronic conditions in 10 must not be so different from those in 9 and 11, they should not be responsible for the different structures of the adducts. This working hypothesis is supported by DFT calculations.38 Therefore, the crystal packing effect must play a crucial role in determining the structures of the iodine adducts of 9-11. The reactions are followed by spectroscopic and conductometric methods.37... 

Fig. 16. General reactivity of the ruthenium(II)-arenes. Hydrolysis of the Ru—Z bond gives the more reactive aqua species. The pKa of the coordinated water molecule is important, as the hydroxido complex is less reactive. The different structures are exemplified by the reactivity of [Ru(ri6-bip)Cl(en)]+ (10) for which Z = Cl.

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