Perfluoro-n-eicosane (n-C 2o F 42 ) passes through three different solid modifications, called M(T < 146 K), 1(146 < T <200 K), and R( T> 200 K). Structures of the phases M and R were characterized by x-ray scattering experiments. Crystals are built up by rectangular layers. In modification M layers are set up by arrays of right-handed and left-handed (15/7-) helices in alternate succession. The unit cell is monoclinic (a = 9.65 A, b = 5.70 A, c = 28.3 A, and /3 = 97"; space group Pa). In modification R molecules are highly mobile. There is no ordering with regard to the helix handedness. Molecules resemble cylindrical objects and are packed in a hexagonal dense mode. Regular layer stacking results in crystals with rhombohedral symmetry (a = b = 5.70 A, c = 85.00 A, r = 120°; space group R 3m).
The defect structure in the rhombohedral modification R of perfluoro-n-eicosane has been analyzed in a temperature-dependent x-ray and neutron scattering study. The analysis was based on an evaluation of Bragg-reflection intensities and of the diffuse scattering. Different components of disorder were detected and characterized. At the transition from the low temperature phase to modification R, rotation of the molecules about their long axes sets in. As shown by the azimuthal distribution of the diffuse scattering, rotations occur cooperatively with correlations between neighbors. The Debye–Waller factors measured for the layer reflections are indicative for a moderate roughening of the interfaces due to a restricted longitudinal diffusive motion. Molecules are perturbed by the formation of helix reversal defects. Evaluation of the diffuse scattering yields the associated finite azimuthal correlation length. There is also evidence for a continuous slow untwisting of the helix. With increasing temperature, it approaches the all-trans form.
Defect structures in the high temperature modifications of oligomer crystals can generally be discribed as a superposition of chain rotations, longitudinal shifts of the molecules and the occurrence of conformational defects. Applying a suitable combination of experimental techniques it is possible to discriminate between the different components of disorder. This is demonstrated in a discussion of results obtained on n-alkanes, cycloalkanes and perfluoro-n-alkanes.
The main effect of pressures up to 6 kbar on the Raman spectra of liquid nalkanes and perfluoro-n-alkanes is a reduction of line widths indicative for a slowing-down of rotational motions about the long axes. There is no evidence for a change in the conformer populations.
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