Substitutions in crystals

Vishnimurthy, K., Row, T. N. G. and Venkatesan, K. (1996). Studies in crystal engineering effect of fluorine substitution in crystal packiing and topological photodimerization of styryl coumarins in the solid state. J. Chem. Soc., Perkin Trans. 2, 1475-8. [236]... 

Alexandrite, like ruby, contains Cr ions but they are substituted in the lattice of chrysoberyl, BeAl204. The chromium ions occupy two symmetrically non-equivalent positions which would otherwise be occupied by aluminium ions. In this environment the 2 ground state of Cr is broadened, compared with that in ruby, by coupling to vibrations of the crystal lattice. 

The elegant genetic studies by the group of Charles Yanofsky at Stanford University, conducted before the crystal structure was known, confirm this mechanism. The side chain of Ala 77, which is in the loop region of the helix-turn-helix motif, faces the cavity where tryptophan binds. When this side chain is replaced by the bulkier side chain of Val, the mutant repressor does not require tryptophan to be able to bind specifically to the operator DNA. The presence of a bulkier valine side chain at position 77 maintains the heads in an active conformation even in the absence of bound tryptophan. The crystal structure of this mutant repressor, in the absence of tryptophan, is basically the same as that of the wild-type repressor with tryptophan. This is an excellent example of how ligand-induced conformational changes can be mimicked by amino acid substitutions in the protein. 

X-Ray analyses and solid-state IR spectra were recorded for a number of 1,4-and 1,6-dihydropyrimidines, demonstrating the dependency of the tautomeric composition in the crystal on the substitution in the pyrimidine ring and on the ability of these compounds to form intermolecular hydrogen bonds. Thus,... 

One of the most important parameters that defines the structure and stability of inorganic crystals is their stoichiometry - the quantitative relationship between the anions and the cations [134]. Oxygen and fluorine ions, O2 and F, have very similar ionic radii of 1.36 and 1.33 A, respectively. The steric similarity enables isomorphic substitution of oxygen and fluorine ions in the anionic sub-lattice as well as the combination of complex fluoride, oxyfluoride and some oxide compounds in the same system. On the other hand, tantalum or niobium, which are the central atoms in the fluoride and oxyfluoride complexes, have identical ionic radii equal to 0.66 A. Several other cations of transition metals are also sterically similar or even identical to tantalum and niobium, which allows for certain isomorphic substitutions in the cation sublattice. 

The lowest coordination number of tantalum or niobium permitted by crystal chemistry formalism is 6, which corresponds to an octahedral configuration. X Me ratios that equal 3, 2 or 1 can, therefore, be obtained by corresponding substitutions in the cationic sub-lattice. A condition for such substitution is no doubt steric similarity between the second cation and the tantalum or niobium ion so as to enable its replacement in the octahedral polyhedron. In such cases, the structure of the compound consists of oxyfluoride octahedrons that are linked by their vertexes, sides or faces, according to the compound type, MeX3, MeX2 or MeX respectively. Table 37 lists compounds that have a coordination-type structure [259-261]. 

In more recent times it is used as a thickener in salad dressings, cosmetics, and sauces as an agent in ice cream that prevents ice crystals from forming and as a fat substitute. In pastry fillings, it prevents syneresis (weeping of the water in the filling), keeping the pastry crust crisp. 

Probably the high calculated inter-atomic distances in the oxides are due to our method of using the crystal radii. The substitution in Equation 13 of z = 4 for the cation and z = 2 for the anion, instead of z = V8 for each, would lead to high calculated values in case the anion is much smaller than the cation, as in the rutile type crystals and anatase. 

The presence of isotopic impurities causes clear effects in the vibrational spectra. Almost all modes studied so far show frequency shifts on S/ S substitution [81, 107]. The average shift of the internal modes is ca. 0.6 cm , and of the external modes it is 0.1-0.3 cm (Tables 3, 4 and 5). Furthermore, the isotopomers which are statistically distributed in crystals of natural composition can act as additional scattering centers for the phonon propagation. Therefore, in such crystals the lifetime of the phonons is shortened in comparison with isotopically pure crystals and, as a conse-... 

With regard to the molecular structure of CCHn, both cyclohexyl rings having a chair conformation are substituted in the equatorial positions and the alkyl chain is completely extended in the all-trans conformation. The cyclohexyl rings are nearly coplanar. The crystal structures of the investigated CCHn show that various types of molecular overlapping are present in the crystal. The molecular packing in the crystalline state is quite different in all three compounds. 

Differences in behavior between polycrystalline and single-crystal CdSe electrodes in polysulfide PEC involving the short- and long-term changes in photovoltage and photocurrents have been discussed by Cahen et al. [88], on the basis of XPS studies, which verified the occurrence of S/Se substitution in these electrodes when immersed in polysulfide solution, especially under illumination. The presence of a thin (several nanometers) layer of CdS on top of the CdSe was shown to influence... 

Hird, M. and Toyne, K.J. (1998) Fluoro Substitution in Thermotropic Liquid Crystals. Molecular Crystals and Liquid Crystals, 323, 1-67. 

Attempts have been made to lower the temperature of appearance of the sub-lattice motions. It was found that substitution in the I- sub-lattice of Agl, e.g. by WOj", stabilizes this structure up to rather low temperatures crystals of (AgI)1 JC(Ag2W04)JC show, for = 0.18, an Ag+ conductivity of 0.065 S/cm at 20°C. Addition of cationic species, for instance in Ag2HgI4, Ag4RbI6, and Ag7[N(CH3)4]I8 has a similar effect. 

Chromium has a similar electron configuration to Cu, because both have an outer electronic orbit of 4s. Since Cr3+, the most stable form, has a similar ionic radius (0.64 A0) to Mg (0.65 A0), it is possible that Cr3+ could readily substitute for Mg in silicates. Chromium has a lower electronegativity (1.6) than Cu2+ (2.0) and Ni (1.8). It is assumed that when substitution in an ionic crystal is possible, the element having a lower electronegativity will be preferred because of its ability to form a more ionic bond (McBride, 1981). Since chromium has an ionic radius similar to trivalent Fe (0.65°A), it can also substitute for Fe3+ in iron oxides. This may explain the observations (Han and Banin, 1997, 1999 Han et al., 2001a, c) that the native Cr in arid soils is mostly and strongly bound in the clay mineral structure and iron oxides compared to other heavy metals studied. On the other hand, humic acids have a high affinity with Cr (III) similar to Cu (Adriano, 1986). The chromium in most soils probably occurs as Cr (III) (Adriano, 1986). The chromium (III) in soils, especially when bound to... 

Smith, C.A., Anderson, B.F., Baker, H.M., and Baker, E.N. 1992. Metal substitution in transferrins the crystal structure of human copper-lactoferrin at 2.1-A resolution. Biochemistry 31 4527 -533. 

Similar to the above-mentioned alkoxy, alkylthio groups are also excellent peripheral substitutes. In 2003, Cook [58] determined the crystal structure of substituted Pc complexes PbnPc(HT)s (58) with eight hexylthio groups attached to the... 

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