Plastic Crystalline Phase Research Articles

Calorimetric Investigations of the Liquid Crystalline Material Heptyloxybenzylidene-Heptylaniline (70.7)

Abstract Using an adiabatic scanning calorimeter we have studied the thermal behavior of the liquid crystalline material heptyloxybenzylidene-heptylaniline in the temperature range between 52°C and 86°C. In this temperature range a rich variety of liquid crystalline and plastic crystalline phases and phase transitions could be studied. The isotropic to nematic (NI) transition and the nematic to smectic A (NA) transition (only 0.3°C below TNI) are both first-order with a latent heat of 2.1 (± 0.1) kJ/mol and 2.2 (± 0.1) kJ/mol respectively. The first-order nature of the NA transition is in accordance with recent observations for systems with similarly small nematic ranges. The smectic A to smectic C (AC) transition is second-order within the experimental resolution. The observed anomaly in the specific heat Cp for the AC transition is not consistent with the theoretically expected XY behavior. It can, however, be described rather well with a mean field theory. The transition from the smectic C to the plast...

Phase transitions crystalline to plastic-crystalline and the specific heat of M 3P 7 compounds (M = Li, Na, K, Rb, Cs)

Heat capacities of the trialkaliheptaphosphides M 3P 7 (M=Li…Cs) have been measured in the range 120–770K in closed alumina pans Temperatures T c, enthalpies ΔH c, and entropies ΔS c of the crystalline to plastic-crystalline phase transitions have been determined. The nature of the first order transition is proofed by a hysteresis in T c, a jump in specific heat, no space group - subgroup relationship and a volume jump of 8% at T c. The melting points T m, the melting enthalpies ΔH m and entropies ΔS m have been estimated from DTA measurements in closed quartz ampoules. The T c values are related to binding energies. For all compounds the ΔS c values are larger than ΔS m, which is in agreement with theoretical considerations and proof the nature of the phase transitions.

The vibrational spectra of 1-chloro-2,2-dimethylpropane

Infrared and Raman spectra in the region 5000-20 cm−1 were recorded at low temperature and high pressure of gaseous, liquid and crystalline solid 1-chloro-2,2-dimethylpropane. Quantitative Raman polarization data were obtained. The fundamental frequencies of vibration were assigned according to Cs symmetry and compared with the results of a normal coordinate analysis.Under high pressure a plastic crystalline phase and two anisotropic crystalline phases were observed.

NMR and X-ray Investigations of a Plastic Crystalline Phase in MBBA

Wideline and high resolution NMR studies have been carried out on MBBA in its isotropic, nematic and solid phases. Isotropic and nematic phase spectra correspond to what has been reported earlier. In the solid phase, contrary to expectations, very intense narrow signals similar to signals of the isotropic phase have been observed for the first time at temperatures close to the solid \leftrightarrow nematic phase transition temperature. This indicates rapid reorientational or translational motion in the system. The X-ray results however confirm the existence of translational order. The results are interpreted as indicative of the existence of a plastic crystalline phase in MBBA.

ChemInform Abstract: DIELECTRIC AND ELECTRON SPIN RESONANCE SPECTROSCOPIC STUDIES OF GLASSY CRYSTALLINE STATES OF ORGANIC COMPOUNDS

Dielectric studies of the glassy crystalline states of cyclohexanol, cyclohexanone, and camphor obtained by upercooling the plastic crystalline phase demonstrate the presence of characteristic a- and p-relaxations. The parameters of the a-relaxation fit the Vogel-Tammann-Fulcher (VTF) equation. ESR spin-probe studies of the glassy crystalline phase of cyclohexanol show that there is a marked decrease in the correlation time above the glasslike transition temperature. The present studies suggest the similarity between glassy crystals having long-range orientational disorder and glasses which are known to betra nslationally disordered.

Electron diffraction studies of supersonic jets. VII. Liquid and plastic crystalline carbon tetrachloride

Carbon tetrachloride at 3–11 mol % in neon carrier produces clusters of CCl4 molecules in flow through a Laval nozzle. Electron diffraction patterns of the clusters formed at a total pressure below 2.5 bar are intermediate between x-ray diffraction patterns of bulk liquid CCl4 and neutron patterns of the amorphous material formed by condensation at 10 K. Strong, sharp interference features at low angle imply appreciable long range order, perhaps beyond 50 Å, in the clusters, whose temperatures are about 210 K. When carrier partial pressure is increased beyond 2.6 bar, clusters begin to exhibit crystallinity. Crystallites belong to the rhombohedral plastic-crystalline phase Ib, a=14.27 Å and α=90.0°.

Glassy crystals III : X-ray determination of the structures of the plastic and glassy crystalline phases of cyclohexanol

Single-crystal X-ray diffraction data were collected and interpreted for plastic (I) and glassy crystalline (I' G ) phases of cyclohexanol. For phase I, the unit cell is face-centred cubic, the space group is Fm3m, (Z=4), with parameter a=8.809 A at 275 K. A Monte Carlo method coupled with a rigid group least squares refinement led to a structural model (weighted residual WR=1.05%) in which both cyclohexanol conformers have equal occupancy factors: each isomer can occupy 48 equiprobable orientations at each crystalline site. The mass centres are not fixed, the molecular symmetry plane does not coincide with a symmetry element of the space group and each conformer is characterized by the oxygen atom position in the crystalline cell. The quality of phase I' G single crystals is not as good as for phase I. But the same structural model fits the X-ray data (WR=2.6%) with a=8.608 A at 121 K. Comparison of these results with NMR data suggests that the same large amplitude motions exist in phases I and I' G , with much lower frequencies in the glassy crystal L'etude par diffraction RX montre que la maille cristalline de la phase plastique est cubique a faces centrees, de groupe Fm3m avec a=8,809 A. Description d'un modele structural ou les deux conformeres du cyclohexanol ont le meme taux d'occupation, chaque isomere pouvant occuper 48 orientations equiprobables en chaque site cristallin. Le meme modele semble convenir a la phase cristalline vitreuse. La comparaison des resultats a ceux obtenus par RMN suggere que les memes mouvements de grande amplitude existent dans les deux phases, meme si leurs frequences sont bien inferieures dans le cristal vitreux

Dielectric and electron spin resonance spectroscopic studies of glassy crystalline states of organic compounds

Mise en evidence de relaxations dielectriques α et β pour les etats «cristallins vitreux de cyclohexanol, cyclohexanone et camphre obtenus par superrefroidissement de la phase cristal plastique. Etude RPE du cyclohexanol: diminution du temps de correlation au-dessus de la temperature de transition. Parente entre ces cristaux vitreux, a desordre d'orientation a grande distance, et les verres, a desordre translationnel

A molecular dynamics simulation study of the plastic crystalline phase of sulphur hexafluoride

A molecular dynamics simulation study of the plastic crystalline phase of SF6 is reported. Various equilibrium properties have been measured and, where possible, compared with corresponding experimental measurements. The form of the molecular orientational distribution function has been obtained and is found to be in good agreement with that determined in a recent neutron scattering study. Information concerning the single-molecule dynamics has been obtained from calculations of linear and angular velocity autocorrelation functions, and it has been shown that the SF6 molecules undergo hindered rotation. The existence of coupling between the translational and rotational motions has been demonstrated, and it is shown how the behaviour of the plastic crystal is related to the form of the intermolecular potential. In addition, a transition to a trigonal low-temperature crystal phase at 80K has been observed, and the structure of this phase has been identified and found to be in good agree with experimental results. The calculations have been performed using a large parallel-array processor, and features of the simulation algorithms that reflect the architecture of such a computer are discussed.

Plastic and glassy crystalline states of methane: A Monte Carlo simulation study

A Monte Carlo simulation study has been carried out on the glassy crystalline phases of methane obtained by annealing or quenching the plastic (orientationally disordered) phase. Different cooling rates lead to different states of the glass. Temperature variation of the reorientational parameter suggests the presence of a transition between the plastic and glassy crystalline phases.

Temperature dependence of 13C 1H one-bond coupling constants of methyl groups in plastic crystals

The temperature dependence of the one-bond 13C 1H coupling constant of the methyl groups in pivalic acid, tert-butyl chloride and hexamethylethane has been studied in the liquid and plastic crystalline phases. A steady decrease in the coupling constant with falling temperature in the plastic crystalline phase has been observed for these organic solids. A maximum change in the 13C 1H coupling constant of 25 Hz has been found after deduction of the effect of overlap of the broadened lines in the methyl quartet. The CNDO/2 calculations indicate that the temperature dependence of the coupling constant is not caused by intramolecular transitions. The significant reduction of the 13C 1H coupling constant is largely attributed to intramolecular dipole-dipole interactions due to a slight anisotropic tumbling of the molecules in the plastic phase.

Phases of Decaphenylcyclopentagermane (Ph2Ge)5 [1] / Phases of Decaphenylcyclopentagermane (Ph2Ge)5 [1

Abstract (Ph2Ge5) can be obtained from solutions in five crystalline phases, two pure and three including solvent of crystallization. This is lost at 70 °C. At 190 °C all phases transform into a plastic crystalline phase. For this tetragonal phase a gear wheel model for librations of the molecules is developed. The crystal structure of triclinic (Ph2Ge)5 has been determined and refined to R = 0.050. The molecule has a flexible conformation in accordance with a better solubility than exhibited by the homologous rings (Ph2Ge)4 and (Ph2Ge)6 containing rigid normal conformations. The arrangement of phenyl groups is either parallel or perpendicular for both cases intra- and intermolecular. The compound (Ph2Ge)6 attains a plastic crystalline state at 346 °C.

A Raman spectroscopic study of the plastically crystalline state of organic compounds

The depolarized and polarized components of internal Raman bands of succinonitrile, t-butyl chloride and t-butyl bromide in their plastic crystalline and liquid phases have been analyzed to obtain rotational and vibrational correlation functions. The rotational correlation times are continuous through the plastic-liquid transition and show an Arrhenius behaviour, from which activation energies for rotation have been calculated. Vibrational correlation times are nearly constant in the range of temperature investigated, except for a small change near the plastic-liquid phase transition temperature.

Vibrational spectra and conformational behaviour of 1,2,2,3-tetrachloropropane and 1,3-dichloro-2,2-dimethylpropane

The IR and Raman spectra of 1,2,2,3-tetrachloropropane and 1,3-dichloro-2,2-dimethylpropane are recorded as liquids and as solids both at low temperature and at high pressure. Both compounds exist as a mixture of at least 2 conformers in the liquid phase and crystallize in the GG conformation at low temperature. Under high pressure the first compound also crystallizes in the GG conformation, but the second crystallizes in the AG conformation. The second compound is shown to have a plastic crystalline phase, but no such phase is evident for the first. Vibrational assignments for the various conformers are made, supported by normal coordinate calculations.

Vibrational relaxation of SiD4 in the liquid and plastic crystalline phases

The spontaneous Raman spectra of the ν1 band of SiD4 in the liquid and plastic crystalline phases have been modeled by a memory-function procedure. It was found to be necessary to go to third order in the memory-function procedure. In order to get agreement between the ν2 reorientational relaxation times and the deuteron magnetic resonance correlation times, anisotropic rotation has been postulated. The anisotropic rotation accounts for an additional slow variable in the vibrational dephasing dynamics. The ν1 vibration is apparently modulated by the rotations. The vibrational–correlation function is reported out to 10 ps.

Molecular motions in plastic crystals:orientational relaxation in camphor derivative compounds

The frequency spectrum of the Rayleigh depolarised light scattered by dl-camphene, dl-borneol,dl-bornyl chloride and dl-camphor in their plastic crystalline phases was analysed at high resolution,using a piezoelectric scanned Fabry-Perot interferometer. The entire spectrum was found to be fitted by a single Lorentzian plus a broad back- ground. The origin of this spectrum is attributed to molecular reorientation of the permanently anisotropic molecules of the systems under study. The temperature dependence of the relaxation times for the reorientation in the plastic crystalline phase of those solids is found to obey an Arrhenius law on the basis of which are deduced the activation energies associated with the molecular reorientational process:8.6 KJ. mol −1 for dl-camphene; 8.8 KJ.mol. −1 for dl-bornyl chloride; 20.5 KJ.mol. −1 for dl-bor-neol; and 14.6 KJ.mol. −1 for dl-camphor. An interpretation of these results on the basis of the inter- and intramolecular interactions is discussed.

Brillouin scattering in Li2SO4: Liquid and plastic crystalline phases

Abstract The hypersonic velocity in Li 2 SO 4 was measured through the liquid-plastic transition for an arbitrary crystalline orientation. The velocity showed a sharp discontinuity at the phase transition. The phonon frequency was a periodic function of crystalline orientation.

Carbon-13 NMR studies of camphene in the plastic crystalline phase

The 13C NMR of camphene in the solid plastic crystalline phase was studied at room temperature. Chemical shifts, spin-lattice relaxation times ( T 1), nuclear Overhauser enhancement factors and 13C- 1H coupling constants were measured and compared with the corresponding values obtained in solution.

Calorimetric Study of the Glassy State. XIV. Calorimetric Study on Unusual Glass Transition Phenomena in CFCl2–CFCl2

Abstract The heat capacities of the glassy crystalline, plastic crystalline, and liquid phases of CFCl2–CFCl2 were measured by an adiabatic calorimeter over a temperature range from 13 to 310 K, with a sample of 99.93% purity. The triple point was determined to be 297.91 K, the enthalpy and entropy of fusion being 3.666 kJ mol−1 and 12.30 J K−1 mol−1, respectively. Three kinds of relaxation phenomena were found during the heat capacity measurement centered at around 60, 90, and 130 K, respectively. The first was attributable to the β relaxation. The second was for primary glass transition, where overall rotation or reorientation of the molecules might be frozen. The third was analyzed by ascribing it to the freezing of conversion between the trans and the gauche conformers of the molecule. Activation enthalpy for the internal rotation and energy difference between the two conformers were determined to be 41 kJ mol−1 and (790±60) J mol−1, respectively. This experiment provides the first example in which three different freedoms of molecular motion are frozen successively at different temperatures as the crystal is cooled down. An effort to get a possible low-temperature ordered crystalline phase was partially successful by long annealing of the crystal. The extremely sluggish nature of the phase transition into the ordered phase will be related with the slow rate of the internal rotation of the molecule below the transition temperature.

Thermal conductivity and heat capacity of cyclopentane in the range 100–300 K and up to 2·1 GPa

The thermal conductivity, λ, and the specific heat capacity per unit volume, ρC p, of cyclopentane were determined in the range 100–300 K and up to 2·1 GPa. The transient hot-wire technique was used, and the experiments were carried out in a piston-cylinder apparatus. The λ values for the liquid and the two plastic crystalline phases are quite similar, while there is a strong increase corresponding to the plastic →normal crystal transition. In the normal crystal phase a T -1 dependence is observed, in contrast to the small variations of λ with temperature in the other phases. The pressure dependence of λ for the two plastic crystal phases shows a behaviour similar to that of the compressed liquid. Approximate ρC p data are reported. An extended phase diagram for cyclopentane is given.