Space groups poly

The cubic UB, 2-type boride structure with space group Fm3m can be described on the basis of a B,2-cubooctahedron (see Fig. 1) . The association of the B,2-poly-hedra by oriented B—B bonds gives rise to a three-dimensional skeleton with boron cages. Formally, the arrangement of the B,2-units and of the metals atoms is of the NaCl-type. Each metal is located in the center of a B24-cubooctahedron. 

Crystallographic study of 2,5-DSP and poly-2,5-DSP demonstrated that the polymerization proceeded with retention of the space group (Pbca) of the starting 2,5-DSP crystal (Sasada et al., 1971 Nakanishi et al., 1972a). The result was the first evidence of an organic reaction which proceeded in the crystal lattice. 

Optical compensation for polymers with chiral monomeric units may also occur when the racemic polymer consists of crystallites, each composed only of the rectus chains or only of the sinister polymer chains, and a same amount of optical antipode crystallites is present. This intercrystallite optical compensation211 has been found, for instance, in isotactic poly(propylene sulfide),212 poly ((3 -methy lpropiol ac (one),213 and poly(isopropylethylene oxide),214 where isochiral 2/1 helical chains are included in orthorhombic unit cells according to the space group P2 2 2. ... 

Figure 3. Fiber diffraction from a statistically disordered fiber of the sodium salt of poly d(GC) poly d(GC). The molecules in this structure form an unusual left-handed DNA duplex in which the dinucleotide pCpG is the molecular asymmetric unit. The unit cell is trigonal with a = b = 1.91 nm c = 4.35 nm. The space group is probably P226s. The molecular symmetry is itself 226s, and the statistical structure arises from a random choice of a molecular diad to a point along a particular direction.

The electron diffraction data have been reevaluated in support of a new proposal in a more recent paper by Murthy et al. [188]. A hexagonal structure, meanwhile presented for Li- and Na-doped polyacetylene as well as for Na-doped poly(pora-plienylene vinylene), is found to describe the diffraction patterns of PPP-Na rather well, as illustrated in Figure 1,24, The ions reside in channels within columns of three-fold symmetry (Figure 1,25). A compatible space group is P62m with a = 8.6 A, but the symmetry may be lower, for instance due to non-coplanarity of the rings. This may remain hidden in disorder. The authors pay considerable attention to the implications of the hexagonal structure... 

The crystal structure of PHB is a orthorhombic form with unit cell parameters fl = 0.576 nm, = 1.320 nm, and c(fiber axis)=0.596 nm, and space group P2,2,2, (Alper et al. 1963 Okamura and Marchessault 1967). The conformational analysis by intermolecular energy calculation has indicated that the PHB molecule has a left-handed 2j helix conformation (Comibert and Marchessault 1972 Yokouchi et al. 1973 Bruckner et al. 1988). The crystal structure of random copolymers of 3HB and 3HV has been investigated extensively (Bloembergen et al. 1986 Kamiya et al. 1991 VanderHart et al. 1995). A structural characteristic of poly(3HB-co-3HV) is isodimorphism, i.e., cocrystallization, of the two monomer units. In contrast, the cocrystallization of 3HB with 3HH or (7 )-6-hydroxyhexanoate (6HH) does not occur. 

The combination of motifs and repetition schemes is the second topic of this section. The space group of the lattice is the scheme that acts on motifs placed into the unit cell and generates the crystal stracture. The space group is determined by X-ray diffraction or, less frequently, by electron or neutron diffraction. Typically one to 18 repeating units of a macromolecule exist within one unit cell, but helices with long translational repeats may place many more repeating units into one unit cell. For example, poly(m-methylstyrene)s with a 2 40/11 helix form a four-chain unit cell with... 

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