Study of thermodynamic stabilities of polytypes of n-C36H74 by solubility measurements and incoherent inelastic neutron scattering

The thermodynamic properties of the two polytypes of n-hexatriacontane (n-C36H74), single-layered structure Mon and double-layered structure Orth II have been investigated by means of solubility measurements and incoherent inelastic neutron scattering. The solubility measurements reveal that Orth II is more stable than Mon by 1.2 kJ/mol because of the advantage of larger entropy. The neutron scattering measurements show that the vibrational modes of Orth II shift to the lower frequencies compared with those of Mon in the frequency region below 120 cm(-1). The advantage of Orth II in vibrational entropy due to the low-frequency shifts is estimated to be 9.6 J K(-1)/mol at 288 K under the harmonic approximation, which nearly agrees with the entropy difference of 6.8 J K(-1)/mol between Mon and Orth II determined by solubility measurements. These results suggest that the difference in vibrational entropy due to low-frequency modes mainly contributes to the relative thermodynamic stabilities of polytypic structures of long-chain compounds. From the frequency of methyl torsional mode, it is suggested that the cohesive force at the lamellar interface is stronger in Mon than in Orth II.     

The influence of polytypic structures on the M011-->M101 solid-solid phase transition of n-C36H74: an application of the oblique infrared transmission method

The molecular displacements on the M011-->M101 phase transition of n-hexatriacontane (n-C36H74) have been investigated with an IR microscope designed for the oblique infrared transmission method. It has been clarified that two polytypic structures of the M011 modification, single-layered structure (Mon) and double-layered one (Orth II), both transform to the M101 modification of single-layered structure with their respective mechanisms. On the M011(Mon)-->M101 transition, the inclination direction of molecular axis is changed by 90 degrees through an intermediate state in which the molecular chain is set perpendicular to the basal plane of the single crystal. On the other hand, a polymorphic-polytypic composite structural change on the M011(Orth II)-->M101 transition is accomplished through two kinds of molecular displacements occurring alternately along the stacking direction of molecular layers.     

Infrared spectroscopic study of polytypic effects on the crystal-growth mechanism of n-hexatriacontane (n-C36H74)

The solution-crystallization mechanism was investigated for two polytypes in the M011 modification of n-hexatriacontane (n-C36H74), single-layered structure Mon, and double-layered one Orth II. The crystal growth under controlled supersaturation was followed with a micro- Fourier-transform-infrared spectrometer equipped with an optical system for oblique transmission measurements. Supersaturation dependence of growth behavior was significantly different between Mon and Orth II. Although the Mon crystal continued growing at a supersaturation of 0.27, the overgrowth of Orth II on the (001) face of the Mon crystal was confirmed at supersaturations below 0.21. Such a polytypic transformation was not observed for the Orth II crystal at any supersaturation below 0.30. The growth rate of Mon showed a quadratic dependence on supersaturation, while that of Orth II was approximately linear, suggesting spiral growth and two-dimensional-nucleation mechanisms for Mon and Orth II, respectively.     

Development of a simultaneous measurement system of x‐ray diffraction and raman spectra: Application to structural study of crystalline‐phase transitions of chain molecules

The development of a bench‐top‐type system for simultaneous measurement of X‐ray diffraction and Raman spectra has been made to investigate structural changes in the phase transitions of chain molecules such as polyethylene, n‐alkane, and so forth from various viewpoints. For the X‐ray diffraction measurement an imaging plate or a charge‐coupled device camera was used as a two‐dimensional detector. For the Raman spectral measurement a miniature Raman spectrometer was used with optical fibers for the irradiation of incident laser beams and collection of scattered signals. For example, in the heating process of the n‐C₃₀H₆₂ sample, the phase transition from orthorhombic‐to‐hexagonal lattices could be detected clearly by the X‐ray and Raman measurements. By comparing these two different types of data in detail, an intimate relationship between conformational disordering and rotational motion of molecular chains is detected more clearly than before. Also, similar discussion can be made for the orthorhombic‐to‐hexagonal phase transition of the geometrically constrained polyethylene sample occurring immediately below the melting point. Another example concerns the structural change in the photoinduced solid‐state polymerization of cis,cis‐diethylmuconate single crystal. From the shifts in the X‐ray reflection position and Raman frequency characteristic of the produced polymer, it was found that the molecular deformation of the polymer chains and lattice strain was induced in the early stage of the polymerization reaction. For the ferroelectric‐phase transition of vinylidene fluoride copolymer, the simultaneous measurement was made for collecting triple information of small‐angle and wide‐angle X‐ray scatterings and Raman spectra to know the relationship between the structural change in the crystal lattice and the morphological change in the lamellar stacking mode. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 495–506, 2002; DOI 10.1002/polb.10112     

Die Kristall- und Molekülstrukturen von (Me2SiNCN)4 (1) und Me3SiNCNSiMe3 (2)

Crystal and Molecular Structures of (Me₂SiNCN)₄ (1) and Me₃SiNCNSiMe₃ (2) Synthesis, spectroscopic characterization (NMR, IR, Raman and MS) and crystal structure of the novel compound 1 are described. The molecules form almost planar 16 membered rings with four SiMe₂-groups connected to four NCN groups. The corresponding compound 2 exhibits a phase transition from the HT phase 2a to the LT phase 2b at 131 K. Consecutive X-ray structure determinations of the molecular structures of both modifications were performed on a crystal, grown in situ on the diffractometer at 231 K.     

A new polymorph of triphenylmethylamine: the effect of hydrogen bonding

Crystallization of the hexane reaction mixture after treatment of LiGe(OCH₂CH₂NMe₂)₃ with Ph₃CN₃ gives rise to a new triclinic (space group P) polymorph of triphenylmethylamine, C₁₉H₁₇N, (I), containing dimers formed by N—H...N hydrogen bonds, whereas the structure of the known orthorhombic (space group P2₁2₁2₁) polymorph of this compound, (II), consists of isolated molecules. While the dimers in (I) lie across crystallographic inversion centres, the molecules are not truly related by them. The centrosymmetric structure is due to the statistical disordering of the amino H atoms participating in the N—H...N hydrogen‐bonding interactions, and thus the inversion centre is superpositional. The conformations and geometric parameters of the molecules in (I) and (II) are very similar. It was found that the polarity of the solvent does not affect the capability of triphenylmethylamine to crystallize in the different polymorphic modifications. The orthorhombic polymorph, (II), is more thermodynamically stable under normal conditions than the triclinic polymorph, (I). The experimental data indicate the absence of a phase transition in the temperature interval 120–293 K. The densities of (I) (1.235 Mg m⁻³) and (II) (1.231 Mg m⁻³) at 120 K are practically equal. It would seem that either the kinetic factors or the effects of the other products of the reaction facilitating the hydrogen‐bonded dimerization of triphenylmethylamine molecules are the determining factor for the isolation of the triclinic polymorph (I) of triphenylmethylamine.     

Introduction to bioenergetics: Thermodynamics for the biologist — a learning program for students of the biological and medical sciences | by N. Christensen and Richard A. Cellarius,W.B. Saunders Company,Philadelphia, 1972, pp. xii + 223, price £2.35

The infrared and Raman spectra of the light blue modification of anhydrous copper(II) formate, Cu(HCOO)₂, and copper(II) formate-d₂, Cu(DCOO)₂, are reported, as well as the Raman spectra of copper(II) formate tetrahydrate Cu(HCOO)₂ · 4H₂O and copper(II) formate tetrahydrate-d₈ Cu(HCOO)₂ · 4D₂O over a wide range of temperatures. In the latter two compounds, the fundamental formate modes, active in the Raman spectra, showed splittings when the phase transition temperature was traversed. These low-temperature Raman spectra were interpreted in terms of a P2₁ space group and prove that the phase transition not only involves an ordering in the orientation of the water molecules, but also displacements of the heavy atoms. Only a limited number of weak translational modes of the water molecules could be identified in the Raman spectra of the copper(II) formate tetrahydrate, and it is not possible therefore to determine exactly how ordering affects the Raman-active lattice modes of these molecules.     

Polymer single crystals of poly(4-hydroxybenzoate). II. A contribution to crystal structure and polymorphism

Electron diffraction has been used to investigate the structure of a wide range of as-polymerized crystals of poly(4-hydroxybenzoate) [systematic name: poly(1,4-oxybenzoyl)]. The chemical composition and the degree of polymerization (DP) have been varied and some samples have been thermally treated. At room temperature two crystalline modifications with orthorhombic unit cells coexist. The chains adopt a 2₁ helical conformation in both forms, but there are differences for oligomer and polymer crystals. Oligomers of low DP have an extended chain-conformation, whereas in polymers a shortening of the repeat distance along the chain is observed as a function of both the DP and the crystallization conditions. From the most extensive data sets we have derived the lattice parameters a = 7.52, b = 5.70, and c = 12.49 Å for polymer crystals of phase I, and the subcell parameters for oligomer crystals of phase II a = 3.77, b = 11.06, and c = 12.89 Å. Both phases contain two chains per unit cell. In addition to modifications I and II several defect structures exist the unit cells of which contain more than two chains. At temperatures which depend on the degree of polymerization, a phase transition to a third modification takes place. The large difference between the densities of phase III as compared to both phase I and II suggests that torsional degrees of freedom exist in phase III which allow a certain mobility of the phenyl and ester groups. This mobility enables the end groups of adjacent layers in interlamellar regions of oligomer crystals to undergo transesterification reactions and therefore to increase the molecular weight of the samples.     

Pressure-induced structural changes of the tetragonal Bi2CuO4

The tetragonal compound Bi₂CuO₄ was investigated at high pressures by using in situ Raman scattering and X-ray diffraction (XRD) methods. A pressure-induced structural transition started at 20 GPa and completed at ∼37 GPa was found. The high pressure phase is in orthorhombic symmetry. Raman and XRD measurements revealed that the above phase transition is reversible.     

Laser-excited Raman scattering in polybutene-1

Raman spectra are described for the three crystalline modifications of polybutene-1. The Raman frequencies are compared with infrared results and give good agreement. A normal-coordinate analysis for A-mode vibrations is presented for five possible helical conformations. The behavior of several calculated helix-sensitive bands is characterized. These results are applied to the experimental Raman frequencies and lead to the prediction of a 10₃ helix for the orthorhombic form III modification.     

Raman and X-ray scattering studies of high-pressure phases of urea

Single-crystal and polycrystalline urea samples were compressed to 12 GPa in a diamond-anvil cell. Raman-scattering measurements indicate a sequence of four structural phases occurring over this pressure range at room temperature. The transitions to the high-pressure phases take place at pressures near 0.5 GPa (phase I --> II), 5.0 GPa (II --> III), and 8.0 GPa (III --> IV). Lattice parameters in phase I (tetragonal, with 2 molecules per unit cell, space group P42(1)m (D3(2d))) and phase II (orthorhombic, 4 molecules per unit cell, space group P2(1)2(1)2(1) (D2(4))) were determined using angle-dispersive X-ray diffraction experiments. For phases III and IV, the combined Raman and diffraction data indicate that the unit cells are likely orthorhombic with four molecules per unit cell. Spatially resolved Raman measurements on single-crystal samples in phases III and IV reveal the coexistence of two domains with distinct spectral features. Physical origins of the spatial domains in phases III and IV are examined and discussed.     

Correlations of the thermodynamic quantities with the frequency shifts of a lattice mode close to the I–II transition in NH4Br

We correlate here the specific heat C_p with the frequency shifts (1/V) (?V/?T)_p and the thermal expansivity α_p with the (1/ν) (?V/?P)_T close to the I–II transition in NH₄Br. This correlation is performed for the Raman mode of ν_s (140 cm^?1) using the molar volume data for NH₄Br. It is shown here that spectroscopic modifications of the Pippard relations are applied satisfactorily to the I–II phase transition by using a lattice mode of NH₄Br.     

Raman study of phase transitions in (n-C4H9NH3)2BiCl5

The Raman spectra of (n-C₄H₉NH₃)₂BiCl₅ were recorded and analysed from 4 K to 390 K. The phase transition (α → β) at 370 K to the metastable form is manifested by changes in the low-frequency Raman spectra, indicating the changes in the anionic structure of the crystal. The phase transition at 215 K is clearly manifested by the temperature evolution of the internal modes of the butylammonium cation. The phase transition is likely to be due to a reorientational motion of the cation.     

Potassium Aluminate Crystalline Structure and Electroconduction

The crystalline structure of low- and high-temperature modifications of KAlO₂ is studied by a neutron diffraction method combined with a full-profile Rietveld analysis. At low temperatures KAlO₂ has an orthorhombic structure (spatial group Pbca), which turns tetragonal (spatial group P4₁2₁2) at 540°C. During a phase transition along the c axis there open new conduction channels with a large cross-section of voids, which is a probable reason for the conduction jump. The presumed anisotropy of conduction in the low-temperature modification disappears during the phase transition.     

Anisotropy of the local field of the light wave in cholesteric liquid crystals

The experimental values of the L _j components of the Lorentz tensor have been obtained for the first time for the quasinematic layer of the cholesteric phase and in the smectic phase A for homologs of cholesteric fatty ethers using the dispersion dependences of the refraction indices for the planar texture of cholesteric liquid crystals (CLCs). The dependence of the L _j components on the homolog number, mesophase temperature, the magnitude of birefringence, and the change in the orientational ordering of molecules in the cholesteric phase and at the cholesteric-smectic A phase transition was determined. Isotropization of the Lorentz tensor L and the local field tensor f was found for CLCs when the birefringence of LCs and the anisotropy of molecular polarizability decreased simultaneously. The anisotropy Δf was found to be negative for the quasinematic layer of CLCs and the smectic phase in the visible range of the spectrum. The values of L _j , obtained with known local field models for CLCs and smectics A, gave positive Δf irrespective of the chemical structure of molecules, optical anisotropy of LCs, and the spectral region, which contradicts to the experiment.     

Synthesis,mesomorphic, photophysical and computational studies of new achiral four-ring unsymmetrical bent-core mesogens and their Copper(II) complexes

New achiral four-ring unsymmetrical bent-core mesogens derived from 2,5-dihydroxybenzaldehyde and their copper(II) complexes have been synthesised as a new design with an imine and ester linkage. These new bent-core molecules resemble hockey-stick shape, which possesses 4-n-alkyloxy chain (4-n-hexyloxy and 4-n-decyloxy) at one end and methyl or methoxy group at the other end of the molecule. The synthesis, spectroscopic characterisation, phase transition temperature and characterisation of phase behaviour are reported. The bent-core molecules exhibited monotropic nematic and smectic A phase depending on the terminal chain length. Interestingly, copper(II) complexes of bent-core molecules displayed monotropic nematic phase. This is the first report on copper(II) complexes of bent-core molecules that exhibited nematic phase. The four-ring bent-core molecule exhibited fluorescence with large stoke shift. The density functional theory calculations of bent-core molecules and their copper(II) complexes are carried out using Gaussian 09 program at B3LYP level to obtain the stable molecular conformation, dipole moment, highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO) energies and bending angle of the compounds. The natural atomic charges and electronic configurations of the atoms of free ligands as well as the complexes have been evaluated.     

Vibrational spectroscopic study on polymorphism and molecular aggregation states in polyoxymethylene

Polarized Raman spectra of the moth-shaped single-crystal of orthorhombic polyoxymethylene (metastable phase) have been investigated by means of the Raman microprobe technique in the frequency range covering both molecular and lattice vibrations. The observed polarizations of the Raman bands are found to be strongly influenced by the optical alignment in the microscope, mainly by the polarization scrambling effect. By taking the optical effect into account, the orientation of the crystal axes in the single-crystal is investigated. Normal mode analysis of the orthorhombic crystal is performed based on the newly obtained polarization data. Raman spectra of two crystalline samples of trigonal polyoxymethylene (stable phase) differing in their morphology and in their structural orderliness are compared. In the spectrum of the needle-like crystal consisting of the fully extended molecules, measured at low temperature, eight Raman bands due to the E₂ symmetry species are newly detected     

Pressure-induced structural transformations in lanthanide titanates: La2TiO5 and Nd2TiO5

The structure of orthorhombic rare earth titanates of La₂TiO₅ and Nd₂TiO₅, where Ti cations are in five-fold coordination with oxygen, has been studied at high pressures by X-ray diffraction (XRD), Raman scattering measurements, and quantum mechanical calculations. Both XRD and Raman results indicated two pressure-induced phase transitions during the process. An orthorhombic super cell (a×b×2c) formed at a pressure between 6 and 10 GPa, and then transformed to a hexagonal high-pressure phase accompanied by partial decomposition. The hexagonal high-pressure phase is quenchable. Detailed structural analysis indicated that the five-coordinated TiO₅ polyhedra remain during the formation of super cell, but the orthorhombic-to-hexagonal phase transition at high pressures is a reconstructive process, and the five-fold Ti-O coordination increased to more than 6. This phase transition sequence was verified by quantum mechanical calculations.     

Coumarin based emissive rod shaped new schiff base mesogens and their zinc(II) complexes: synthesis,photophysical, mesomorphism,gelation and DFT studies

New homologous series of coumarin Schiff base derived from 6-aminocoumarin and their zinc(II) complexes have been synthesised. The spectroscopic characterisations, photophysical properties, phase transition temperature, characterisation of phase and gelation behaviour are reported. The ligand is non-mesogenic at lower 4-n-alkoxy chain length (n = 4) and mesogenic for longer chains (n > 4). For small chains (n = 5, 7, 8), ligand displays monotropic nematic or nematic-smectic A phase sequences, whereas longer homologues (n = 12, 14, 16 and 18) display only enantiotropic smectic A phase. The ligands and their Zn(II) complexes are fluorescent in nature. Interestingly, ligands exhibit gelation property only in polar solvents, whereas Zn(II) complexes discourage gelation. The effect of 4-n-alkoxy chain length on the gelation properties was also discussed. Density functional theory calculations show broad agreement with observed molecular conformation, dipole moment, molecular orbitals and polarisability of the coumarin Schiff base molecules and their Zn(II) complexes.     

Raman spectra and phase transition of the phenothiazine crystal

Polarized Raman spectra of single crystals of phenothiazine which undergoes a phase transition around 250 K and is ferroelastic in the low-temperature phase were measured in the lattice-vibrational region for temperatures ranging from 89 to 300 K. The spectra of the high- and low-temperature phases obey the selection rules required for the orthorhombic and monoclinic structures, respectively. Anomalous temperature dependences are observed in the frequency, intensity and linewidth of a band appearing in the lowest-frequency region. This band is attributed mainly to a librational motion of the molecule, and is important in the evaluation of this phase transition. A gradual change of the molecular orientation is inferred to occur over a wide temperature interval in the low-temperature phase along the vibrational coordinate of the above libration. Another strong, low-frequency band, which is characteristic of the phenothiazine spectra, is suggested to arise from mixing between intramolecular and lattice vibrations. This mode is considered to play some role in the phase transition too. The interaction between the low-frequency optical modes and the acoustic modes is briefly discussed in connection with the ferroelasticity of the low-temperature phase. The transition temperature depends on the quality of the specimen; the correct transition temperature is found to be 248.8 K and slightly lower than the previously reported value.