Crystalline plastic

For the experiments in type C catalysts, the pellets were overfilled with cyclohexane and initially cooled to 230 K. They were then reheated in steps of 1 K and allowed to equilibrate for 10 min before each measurement. The signal was determined from 32 accumulations with an echo sequence of 20 ms echo time to ensure that the signal from the plastically crystalline phase of cyclohexane had decayed fully. The typical heating curves of cyclohexane in the fresh and coked catalyst are displayed in Figure 3.3.3(a) As the temperature is increased, larger and... 

As = 4.8 J mol K l), at 122°C, is also characteristic of a plastic crystalline phase, as is the softness of the crystals and their sensitivity to macroscopic mechanical damage. Variable temperature X-ray powder diffraction shows a change in pattern from monoclinic to cubic upon heating the diffraction lines, sharp and numerous for the "rigid" crystals, become few and less well-defined for the plastic crystal. 

Figure 7. Crystalline (LT) and plastic (HT) phases of MjPy and MjPu. Key a, structure of the orthorhombic LT-NajPj as an example for the ordered crystalline low-temperature modifications (See Table I. The relation of the LT-phase to the HT-phase is indicated by a dotted line.) b, structures of the cubic HT-MjPj and HT-M3P11 as examples for the plastically crystalline phases c, temperature-dependent Raman spectra of Na Py and d, typical electron density in the plastically...

Because of the orientational freedom, plastic crystals usually crystallize in cubic structures (Table 4.2). It is significant that cubic structures are adopted even when the molecular symmetry is incompatible with the cubic crystal symmetry. For example, t-butyl chloride in the plastic crystalline state has a fee structure even though the isolated molecule has a three-fold rotation axis which is incompatible with the cubic structure. Such apparent discrepancies between the lattice symmetry and molecular symmetry provide clear indications of the rotational disorder in the plastic crystalline state. It should, however, be remarked that molecular rotation in plastic crystals is rarely free rather it appears that there is more than one minimum potential energy configuration which allows the molecules to tumble rapidly from one orientation to another, the different orientations being random in the plastic crystal. 

Existence of a high degree of orientational freedom is the most characteristic feature of the plastic crystalline state. We can visualize three types of rotational motions in crystals free rotation, rotational diffusion and jump reorientation. Free rotation is possible when interactions are weak, and this situation would not be applicable to plastic crystals. In classical rotational diffusion (proposed by Debye to explain dielectric relaxation in liquids), orientational motion of molecules is expected to follow a diffusion equation described by an Einstein-type relation. This type of diffusion is not known to be applicable to plastic crystals. What would be more appropriate to consider in the case of plastic crystals is collision-interrupted molecular rotation. 

The possible transitions of plastic and condis crystal-forming materials are shown in Fig. 4. For plastic crystals, this diagram is fully based on information on low molecular weight materials. No flexible, linear macromolecules which resemble plastic crystalline behavior have been reported (see Sect. 5.2.3). Similarly, little attention has been paid in the past to conformationally disordered mesophases in small molecules. In fact, some of the plastic crystals of larger organic molecules may actually be condis crystals (see Sects. 5.2,2 and 5.3.3). Since the positional order is preserved in both plastic and condis crystals, the possible phase relations are similar. The major difference from the liquid crystals is the possibility of partial mesophase formation. 

In the solid state the translational motion of the molecules is slow and the molecules are arranged with long-range orientational and positional order. However, for compounds with long hydrocarbon chains the molecules may rotate in their lattice sites at the same time as they maintain full positional order, forming so-called plastic crystals (Evans and Wennerstrom, 1994, p. 412). The stability of these plastic crystalline phases (cy-forms) increases with chain length and with the presence of impurities (e.g., broad chain-length distributions) (Larsson, 1994, p. 27). 

Carbon tetrafluoride. Carbon tetra-fluoride, which undergoes a transition to a plastically crystalline (orientationally disordered) phase, has been investigated by the Parrinello-Rahman molecular dynamics method under constant-pressure conditions (6). A simple intermolecular potential model of the Lennard-Jones form was derived by taking into account the experimen-... 

K. J. McGrath and R. G. Weiss, Rate of chair-to-chair interconversion of cyclohexane-d in the neat plastic crystalline phase.. Phys. Chem., 1993, 97(11), 2497-2499. 

J. N. Sherwood, ed., The Plastically Crystalline State, Wiley, New York, 1979. 

J. Sherwood, The Plastically Crystalline State Orientationally Disordered Crystals, Wiley, 1979. 

The M3P11 phases transform into plastically crystalline modifications like the M3P7 phases. In the high-temperature form (/3), the phosphides match the structure of the intermetallic Li3Bi. The Bi positions are surrounded by the cages of P and Pn with orientational disorder. 

Figure 25 (a) Raman spectra of solid M3P7 (M = Li, Na, K, Rb, Cs) and (b) temperature-dependent Raman spectra of NasP , indicating the first-order phase transition at Tc from the low-temperature (LT) crystalline a-phase to the high-temperamre (HT) plastically crystalline /3-phase. Note the disappearing of the sharp external lattice modes just above Tc... 

FIGURE 13.20. Differential scanning calorimetry trace. The heating curve for 1,1,2-trichloro-2,3,3-trifluorocyclobutane. The larger peak is due to the transition from the anisotropic crystalline phase to the plastic crystalline phase the smaller peak is due to the transition from the plastic crystalline pheise to the liquid phase. (Courtesy V. B. Pett and David L. Powell, The College of Wooster, Ohio.)... 

Leadbetter A.J. and Lechner R.E. "Neutron Scattering Studies" in "The Plastically Crystalline State" Edited by J.N. Sherwood J. Wiley and Sons, 285 (1979)... 

Phase effects in the oligomerization of l-aIIyl-l,2-dicarba-doso-dodecaborane by electron radiolysis have been investigated. Liquid state radiolysis indicates an activation energy of ca. 5 kcal moP for the formation of oligomer, and ca. 0 kcal mol for the formation of unsaturated dimers. For the plastic crystalline phases, negative activation energies were observed, indicating the existence of a complex reaction sequence." ... 

---