Pseudo-crystals

Lest I leave the erroneous impression here that colloid science, in spite of the impossibility of defining it, is not a vigorous branch of research, I shall conclude by explaining that in the last few years, an entire subspeciality has sprung up around the topic of colloidal (pseudo-) crystals. These are regular arrays that are formed when a suspension (sol) of polymeric (e.g., latex) spheres around half a micrometre in diameter is allowed to settle out under gravity. The suspension can include spheres of one size only, or there may be two populations of different sizes, and the radius ratio as well as the quantity proportions of the two sizes are both controllable variables. Crystals such as AB2, AB4 and AB13 can form (Bartlett et al. 1992, Bartlett and van... 

The importance of these pseudo-crystals is that their periodicities are similar to those of visible light and they can thus be used like semiconductors in acting on light beams in optoelectronic devices. 

The formation of a 3D lattice does not need any external forces. It is due to van der Waals attraction forces and to repulsive hard-sphere interactions. These forces are isotropic, and the particle arrangement is achieved by increasing the density of the pseudo-crystal, which tends to have a close-packed structure. This imposes the arrangement in a hexagonal network of the monolayer. The growth in 3D could follow either an HC or FCC struc-... 

Walsh, W. M., Jr., L. W. Rupp, and B. J. Wyluda Paramagnetic resonance studies of magnetic ions bound in nucleic acid pseudo crystals. Proc. First Intern. Conf. Paramagnetic Resonance 2, 836 (1963). 

Fig. 14.12. A molecular decoration a computer-generated pseudo-crystal structure for chloro-corannulene in space group fw3w, with 48 molecules in the unit ceU.

The crystal stmcture of PPT is pseudo-orthorhombic (essentially monoclinic) with a = 0.785/nm b = 0.515/nm c (fiber axis) = 1.28/nm and d = 90°. The molecules are arranged in parallel hydrogen-bonded sheets. There are two chains in a unit cell and the theoretical crystal density is 1.48 g/cm. The observed fiber density is 1.45 g/cm. An interesting property of the dry jet-wet spun fibers is the lateral crystalline order. Based on electron microscopy studies of peeled sections of Kevlar-49, the supramolecular stmcture consists of radially oriented crystaUites. The fiber contains a pleated stmcture along the fiber axis, with a periodicity of 500—600 nm. 

Appearance of the metallic structure of CNT is based on the crossing of the highest occupied (HO) and the lowest unoccupied (LU) bands (see, e.g.. Fig. 3), each accompanying pseudo rt-type crystal orbital. Note that pseudo n-type orbital, particularly when all the valence atomic orbitals (AO) are taken into consideration, implies that its main AO component is normal to the cylindrical CNT surface. The band crossing mentioned above is possible when these two... 

Figure 10.2 Crystal structure of GeF2 (a) projection along the chains, and (b) environment of Ge (pseudo trigonal bipyramidal). The bond to the unshared F is appreciably shorter (179 pm) than those in the chain and there is a weaker interaction (257 pm) linking the chains into a 3D structure.

Turning finally to compounds with bonds from the heavier Group 14 elements to heavier Group 15 elements we may note compounds such as [Sn C(PMe2)3)2] which has the pseudo trigonal bipyramidal structure (6). This complex, which has Sn bonded exclusively to four P atoms, is formed as yellow crystals by the... 

Conversely, when A-alkyl tryptophan methyl esters were condensed with aldehydes, the trans diastereomers were observed as the major products." X-ray-crystal structures of 1,2,3-trisubstituted tetrahydro-P-carbolines revealed that the Cl substituent preferentially adopted a pseudo-axial position, forcing the C3 substituent into a pseudo-equatorial orientation to give the kinetically and thermodynamically preferred trans isomer." As the steric size of the Cl and N2 substituents increased, the selectivity for the trans isomer became greater. A-alkyl-L-tryptophan methyl ester 42 was condensed with various aliphatic aldehydes in the presence of trifluoroacetic acid to give predominantly the trans isomers. ... 

Solid solutions, Lii.xMe03.xFx, where Me = Nb or Ta, are formed in the LiMe03 - Me02F system and crystallize in a pseudo-ilmenite, LiNb03 type structure, as reported by Guelin, Ravez and Hagenmuller [246]. 

The value of Emin in the pseudo-stable state has been found to increase in the sequence (111) (100)single-crystal faces in aqueous solutions. It has been concluded that the Au-DMSO interactions vary much more with the atomic structure of the gold surface than the Au-H20 interactions and that the Au-DMSO interactions are stronger than forH20.477 Following Trasatti s relation,7 the values of A(<5 m - have been obtained for different planes of Au. It has been found that the difference (<5/M - S/s) for Au( 110) and Au( 111) planes is greater than 0.5 V.477 It should be noted that the same order of Au( 111) and Au(210) has been found in a 2.5 x 10-3 M KPF6 + AN solution.63,392,477... 

The crystal structure of 2-bromo-l,4-phenylenediyl bis(tran5-4-n-prop-ylcyclohexanoate) was determined by Hartung and Winter [114]. The molecules exhibit pseudo-centrosymmetry in consequence of a special kind of disorder within the crystal lattice. The peculiarity of the crystal structure is the disorder of the molecules with respect to the position of their bromine atoms which occupy the 2- or 5-position of the phenyl ring in a statistical manner. 

The three iridium complexes 72d, 72f and 72g were analyzed by X-ray diffraction. Unfortunately the iridium complex 72a, the most efficient in many reactions, failed to give suitable crystals for analysis but the corresponding crystalline rhodiiun complex 73 coifld be analyzed. According to the results obtained, the coordination sphere of the Ir atom and of the Rh atom can be described as pseudo-square planar (Fig. 12). 

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