Pseudo-symmetry, space-group

Yet another type of canal structure has been reported for the urea inclusion compound of 1,4-dichlorobutane 51). Even though the canals are pseudo-hexagonal in dimension, there is a significant difference in their symmetry. The host lattice is orthorhombic, space group Pbcn. The difference lies in the directions of the six pseudo-3j helices of host molecules around the walls of each canal the sequence is cyc/o-RRRLLL, as opposed to cyc/o-RRRRRR in the hexagonal inclusion compounds and cvc/o-RLRLRL in the rhombohedral. This orthorhombic host structure probably occurs also in the urea inclusion compounds with 1,5-dichloropentane and 1,6-dibromohexane 51). 

Almost all unit cells shown in Table I are either orthorhombic or pseudo-orthorhombic, with a majority of space groups P2] 2] 2] and P2. Only a few structures exhibit higher symmetry and none shows lower symmetry. All structures have an antiparallel packing of chains (however, see A- and B-amyloses). On the other hand, a large variety of helix characteristics are evident, in addition to the variability in the unit cell dimensions. Some of the features useful for classifying amylose structures are shown in Table II. The distance between the two nearest antiparallel-... 

The structure of Th2(P04)2P030H(H20) has recently been determined in space group P2 [31], This compound has also been reported to have orthorhombic s mimetry [32] and it is likely that it is actually pseudosymmetric. The matrix 102/100/010] transforms the monoclinic P cell reported in the stmcture refinement to a pseudo-orthorhombic C setting, a 21.36, b 6.70, c 7.02 A, p 89.8°, which is extremely similar to the cell reported solely from powder data a 21.37, b 6.70, c 7.02 A, p 90° [32]. In contrast, the uranium analog U2(P04)2P030H(H20) has cell dimensions similar to its Th analog in the orthorhombic setting, but monoclinic symmetry a 21.15 b 6.61 c 6.99 A, p 91.7° its stmcture has not been refined [35]. 

Several metaphosphates of the tetravalent actinides have been described (Table 6), and the stmcture of U(P03)4 has been refined in the orthorhombic space group Pbcn [92]. However, in this model, five of the nine symmetrically independent oxygen positions are disordered and partially occupied. It is likely that the actinide metaphosphates with stoichiometry An(P03)4 actually adopt the ordered monoclinic Zr(P03)4 stmcture [96]. The unit cell of Zr(P03)4 is pseudo-orthorhombic and nearly four times the volume of the actinide metaphosphates a 13.50, b 28.80, c 8.66 A, (3 90.04(2)°. The matrix [200/0-20/00-1] transforms the primitive orthorhombic cell of U(P03)4 to a larger C-centered cell comparable to that of Zr(P03)4. It is probable that the unit cell data reported for the actinide metaphosphates with stoichiometry An(P03)4 in Table 6 correspond to subcells, and that their tme cells are four times larger and have C-centered monoclinic symmetry. 

Of particular relevance to the measurability of crystal-field effects is a recent study that analyzed the experimental BCP properties of 9-ethynyl-9-fluorenol (crystallizes in the centrosymmetric space group C2/c with two molecules in the asymmetric unit) in relation to the extent to which constraints (local pseudo-symmetry and chemical similarity) were imposed on the pseudoatom model... 

The symmetry of the pseudo structure is that of the space group there are eight amino-acid... 

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