Crystals stack structure

Study of the Crystalline Regions. According to our morphological studies, the crystalline structure of UHMWPE pseudo-gel was different under different sample preparations. For example, spherulites and lamellar single crystal stacks were observed when the pseudo-gel was prepared under quiescent conditions, shish-kebab crystals under stirring conditions, and a mixture of single and shish-kebab crystals under uncontrolled conditions (10). 

Neutral square coplanar complexes of divalent transition metal ions and monoanionic chelate or dianionic tetrachelate ligands have been widely studied. Columnar stack structures are common but electrical conductivities in the metal atom chain direction are very low and the temperature dependence is that of a semiconductor or insulator. However, many of these compounds have been shown to undergo partial oxidation when heated with iodine or sometimes bromine. The resulting crystals exhibit high conductivities occasionally with a metallic-type temperature dependence. The electron transport mechanism may be located either on predominantly metal orbitals, predominantly ligand re-orbitals and occasionally on both metal and ligand orbitals. Recent review articles deal with the structures and properties of this class of compound in detail.89 90 12... 

Conducting halogenated linear metal atom chain systems were first reported in 195095 and are now known for Ni and/or Pd complexes of glyoxime (GLY), DPG and BQD. The compounds are usually produced by treating a hot solution of the neutral complex in o-C6H4C12 with I2 and lustrous coloured crystals of the NIOS compound are obtained on cooling the solution. The limited range of compounds studied is shown in Table 3 and all have metal-over-metal stacked structures. 

Figure 7.3 Analysis of the 4H four-way RNA junction of the human U1 snRNA by comparative gel electrophoresis (Duckett et al., 1995). The central sequence of the junction is shown. The A G pair at the center was retained in this analysis, although changing it to a Watson—Crick pair did not alter the global shape of the junction. The six long—short species can be considered to be derived from a junction with four arms of 40 bp. The central 20 bp comprises RNA, and the outer arms are DNA. The junction species were electrophoresed in an 8% polyacrylamide gel, in 90 mM Tris—borate (pH 8.3) and 1 mM Mg2+. The mobility pattern of the six species is slow, slow, fast, fast, slow, slow. The simplest interpretation (shown on the right-hand side) is that of a stacked structure based on A on D and B on C coaxial stacking, with the axes nearly perpendicular. The pattern would also be consistent with a rapid exchange between nearly equal populations of parallel and antiparallel forms. However, a recent crystal structure has found a perpendicular stacked structure for this RNA junction (Pomeranz-Krummel et al., 2009).

When the temperature is below 193 K, the orthorhomic form further transforms to the hexagonal system in space group D h - P63/mmc. The O2- and Ba2+ ions together form close packed layers and stack in the sequenceABCACB and the Ti4+ ions occupy the octahedral interstices composed only of O2-anions. In this layer stacked structure, the A layers have horizontal mirror symmetry as shown in Fig. 10.4.2(d). Two-thirds of the TiC>6 octahedra share faces to form the Ti2C>9 group, in which the Ti-Ti distance is 267 pm, as represented by a line. This centrosymmetrical crystal is non-ferroelectric. 

The transition-metal component of [Pt(bipy)2][TCNQ]3 is also diamagnetic. The structure86 consists of stacks of trimers, [T CNQ], surrounded by non-interacting Pt(bipy)2+ cations. Adjacent trimers are closely spaced with an interplanar separation of only 3.33 A as compared with 3.23 A within the trimer. The magnetic properties of this system have been studied much less extensively than those of [TMPD]2[Ni(mnt)2]. Single crystal EPR spectra do show, however, that spins are interacting along the TCNQ stacks. Structurally, this compound is very different from the 1 2 compound [Pt(bipy)2][TCNQ]258> (Sect. 2) which forms Da stacks with the two TCNQ moieties <7-bonded to each other.. ... 

Crystals of achiral amino acid derived NDI 8 suitable for X-ray diffraction study were obtained from slow diffusion of water in DMSO. In this hydrogen bond acceptor solvent, the NDIs are arranged in a 7t-stacked structure, with the NDI cores n and n + 1 tilted at ca. 30° with respect to each other, while n and n + 2 are parallel to each other (dihedral angle 0.6°, Fig. 9). 

Stabilization energies of the hydrogen-bonded and stacked structures of nucleic acid base pairs in the crystal geometries of CG, AT, and AC DNA steps and in the NMR geometry of the 5 -d(GCGAAGQ-3 hairpin Complete basis set calculations at the MP2 and CCSD(T) levels130... 

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