Rotator-phase solids

Solid-solid transitions are observed especially for short alkanes. Odd numbered n-paraffins with more than 9 carbon atoms, and even numbered ones with more than 22 and up to about 45 carbons, undergo the transition to the rotator phase. Solid-state NMR spectra give evidence for this transition at the molecular level. Moller et al. [16] studied the temperature dependences of several n-paraffins and polyethylene systematically. Figure 9.4 shows the... 

Regarding Case 1, it is known that the rotator phases of -paraffins are three dimensionally ordered solids with molecules in aW-trans conformation, their specific volumes, Vh, and specific entropies, Sh, are closer to corresponding values and for the orthorhombic phases than they are to those for the melt and Sm In contrast,... 

With concern to the high internal mobility of the molecules in the high temperature solid state phase, some parallelism to n-alkanes can be stated. In the pseudohexagonal (rotator) phase the latter are also characterized by fast molecular motions. For discrimination and according to Pfitzer 14) and Dale 13) in the following the term pseudorotator phase is used for the mobil crystalline state of cyclic molecules. 

Plastic crystal phases in organic materials have been known since the time of Timmermans [1], and these phases are often reached via a solid-solid transition below the final melting point of the crystal. These transitions often represent the onset of rotational motions of the molecules within the crystalline lattice and the resultant phases are sometimes referred to as rotator phases [18-23]. Timmermans proposed a general rule that plastic phases have a low final entropy of fusion (A5f < 20 J K mol because the rotational component of the entropy of fusion of the fully ordered phase is already present in the plastic phase. The bulk of the... 

It is generally immaterial which phase, solid or fluid, is assumed to be at rest, and it is the relative velocity between the two that is important. An exception to this is met in some situations when the fluid stream has been previously influenced by solid walls and is in turbulent flow. The scale and intensity of turbulence then may be important parameters in the process. In wind tunnels, for example, where the solid shape is at rest and the stream of air is in motion, turbulence may give different forces on the solid than if the solid were moving at the same relative velocity through a quiescent and turbulence-free mass of air. Objects in free fall through a continuous medium may move in spiral patterns or rotate about their axis or both again the forces acting on them are not the same as when they are held stationary and the fluid is passed over them. 

Figure 4. Mean longitudinal magnetic field strength (Be) of p CrB vs. phases of stellar rotational period. Solid circles Be measurements by Wade [47] using the bulk of spectral lines. Open triangles up are Crimean measurements obtained using Fe I 6136.615 A spectral line. Open circles are our5j. measurements obtained using Fe I 6137.692 A spectral line. Open triangles down are our Be measurements obtained using Ca I 6162.173 A spectral line.

Population distributions for butane conformers in the gas phase (solid line) and in water (dashed line) at 25°. The units of the vertical axis are relative mole fraction per degree of rotation. (Adapted from reference 41.)... 

In these simple molecules, the dielectric relaxation curves conform to the simple Debye form and hence it is relatively straightforward to determine activation energies from the variation of the relaxation frequency with temperature (Table 4.1). A surprising feature of the rotator phase is that in certain cases the activation for dipolar relaxation is smaller in the solid than it is in the liquid phase, when the solid conforms to a face-centred cubic lattice. When the solid melts, the local force field loses its symmetrical form and the result is that the activation energy for free rotation is observed to increase slightly (Figure 4.5). For most polar liquids, the activation energy for viscosity flow and for dipole... 

Brot et al. (1968) studied the far infrared spectra of liquid and solid tertiary butyl chloride. The study is of interest in connection with rotations in solid phases. The lattice vibrations of thiourea and its deuterated analog were studied by Takahashi and Schrader (1967), who also carried out calculations of the optical mode frequencies and achieved good agreement with infrared and Raman measurements. 

Solid-state C-NMR spectroscopy has proven to be also a valuable tool for studying the properties of buckminsterfullerene. In Fig. 4.33 a series of spectra of a solid sample of C o at different temperatures is reproduced. The single, relatively narrow signal observed at room temperature indicates that under these conditions the molecules reorient rapidly and isotropically in the solid. However, as the temperature goes down, a broad pattern characteristic for the chemical shift anisotropy of powder samples is observed. The evolution of the spectrum described in Fig. 4.33, corresponding to the transformation of a situation of rapid molecular rotation into one characterized by static molecules, points to a phase equilibrium between a mobile phase called the rotator phase ... 

Figure 21.24. (a) Magnitude (dashed line) and phase (solid line) spectra of ferricyanide and ferrocya-nide with hexacyanoferrate adsorbed on the electrode surface (b) in-phase spectrum after phase rotation of solution-phase ferricyanide and ferrocyanide bands into the quadrature channel. 10 mM ferrocyanide 1 Hz modulation frequency potential modulation limits, 0.02 to 0.42 V versus SCE. (Reproduced from [20], by permission of the American Chemical Society copyright 1997.)... 

Nonadecane undergoes a phase transition at 295.2 K from an ordered solid state to a Ri "rotator" phase. In the low temperature crystalline state, C19H40 crystallises in an orthorhombic layered structure composed of molecules parallel to each other and in their "all trans" conformation. One can distinguish, in a horizontal section of the layers, two sub-lattices A and B according to two different orientations of the chains, about 85° apart. The a/b ratio of the lattice parameters increases markedly in the Ri phase and tends to the "hexagonal" value V3, only reached in the Rn phase of longer chain compounds. ... 

The accordance between experimental and calculated data in Figure 9.2 is rather satisfactory. At the same time. Figure 9.2 clearly shows the tendency of ATj values, calculated on the basis of Equation 9.24, to overestimate the experimentally measured shifts of surface freezing temperature. Evidently, this results from using the values of n, evaluated for the bulk solid (rotator phase) and liquid states, instead of the unavailable corresponding characteristics for surface monolayers. 

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