N. B. Chanh

USE OF THE "SHAPE FACTORS" AS AN EMPIRICAL METHOD TO DETERMINE THE ACTUAL CHARACTERISTIC TEMPERATURES OF BINARY PHASE DIAGRAMS BY DIFFERENTIAL SCANNING CALORIMETRY

A typical problem with differential scanning apparatus used in thermal analysis is the determination of the actual characteristic temperatures of thermodynamic equilibrium states. An empirical method based on the notion of “shape factors” is given here. It allows the determination of all the characteristic temperatures of a binary phase diagram.

Molecular alloys; Status and opportunities

Abstract The paper points out the great richness of the organic material and specially of the molecular alloys. Two particular points have been developed : on the one hand, the importance of the preparation method for alloys in the case of compounds with strongly directed intermolecular bonds, on the other hand, the wide diversity presented by the solid-liquid phase diagrams of organic systems, which is mostly linked with solid-solid polymorphism phenomena.

Transitions de Phase en Series Mesogenes:: Les di[(n-alkyl)-4 phényl] TTF et les di[(n-alkyloxy)-4 phényl]TTF

Abstract The polymorphism of the di(4-alkylphenyl) and di(4-alkoxyphenyl) tetrathiafulvalene (TTF) is investigated by optical, crystallographic and calorimetric methods. All the studied derivatives are mesomorphic. The first mesophase is always a smectic G phase; the second one is a nematic N phase for short chains and a smectic C phase for long chains. The entropies corresponding to the passage high-temperature crystalline phase C1 → SG and to the “melting” (C1 → mesophases → isotropic phase I) increase with the length of the chain and with the presence of the oxygen atom in the radical.

A Characteristic Order-Disorder Molecular Phase Transition in 2-Chloronaphthalene

Abstract An order-disorder phase transition has been studied in 2-Chloronaphthalene by both calorimetry and X-Ray diffraction. The transformation is reversible and does not destroy the single crystal. It extends over a large temperature range 253-309 K and can be considered as a “high order” transition: the high temperature form corresponds to a totally disordered phase and the low temperature form to a semi-ordered one.

X-Ray kinetic study of glassy crystal formation in adamantane derivatives: TTT curves and crystal size effect

Abstract The kinetics of polymorphic solid-state transformation in mixed adamantane compounds (CN1−x Cl x ADM: x = 0 and x = 0.25) have been studied by X-ray scattering. The classical form of the time-temperature-transformation TTT curves has been directly observed for the first time for the ordering supercooled plastic phases. For both compounds a considerable effect of crystal size on the kinetics has been observed. For x = 0.25 it leads to a continuous transition from Avrami to nucleation behaviour. These observations help us to understand the factors controlling nucleation and growth as well as to establish better operating conditions in order to form a glassy crystal.

A neutron diffraction study of the high pressure structural phase transition in (CD3ND3)2MnCl4

Abstract High-pressure neutron diffraction experiments have been performed at room temperature on a powdered sample of the perovskite type-layer compound (CD3ND3)2MnCl4. A phase transition from the orthorhombic room-temperature phase (ORT) to a new high-pressure phase (HP) is demonstrated at 20.5 ± 0.2 kbars. A monoclinic unit cell with lattice parameters a = 6.824 (5) A; b = 7.409 (8) A c = 17.126 (12) A and β = 82.94(9)° has been inferred for the HP phase, consistent with a two-dimensional perovskite-type structure. The HP phase appears to be much more compact than ORT; it is characterized, in particular, by an important compression (≃10%) of the inter-layer distance. Space groups P2/c or P21/c consistent with the experimental data have been deduced for the HP phase, after group theoretical considerations based on shear transformation and order-disorder mechanisms.

Thermal expansion of the disordered form of 2-fluoronaphthalene (form I)

The thermal-expansion tensor of the disordered form of 2-fluoronaphthalene (C10H 7F) was calculated from the changes of the lattice constants with temperature (273–323 K) as measured with X-ray and neutron diffraction. The results show that the thermal-expansion anisotropy is significant. The structural interpretation of this property is not trivial.

Isomorphism and mixed crystals in 2-R-naphthalenes: evidence of structural subfamilies and prediction of metastable forms

Solid–liquid binary phase diagrams and isothermal unit-cell parameters as a function of composition are given for the high-temperature form (P21/a, Z = 2) mixed crystals generated by 2-fluoronaphthalene, naphthalene and 2-naphthol with four other β derivatives of naphthalene. The study leads to the distinction between two high-temperature forms (or types of packing). The first one (type 1) is taken by five 2-R-naphthalenes (R = F, Cl, Br, SH, CH3; the first subfamily), the second one (type 2) by naphthalene itself and 2-naphthol (R = H, OH; the second subfamily). The crystallographic data also allow an estimation of unit-cell parameters for the metastable forms of naphthalene (type 1, a = 8.0, b = 5.95, c = 8.6 A, β = 116°) and 2-fluoronaphthalene (type 2, a = 8.336, b = 5.915, c = 9.000 A, β = 122.23°), supporting an interpretation, in terms of crossed isodimorphism, of the observed two-phase regions in systems in which the components do not belong to the same subfamily.

Phase transitions in a two-dimensional molecular complex NH3-(CH2)4-NH3 CuCl4

Abstract The phase transitions occurring in the perovskite type two-dimensional molecular composite NH3-(CH2)4-NH3 CuCl4, have been studied by D. S. C., X-ray diffraction, EPR and magnetic susceptibility measurements. This compound exhibits a reversible first-order structural phase transition at T = 325 K, between two monoclinic phases III (P2/c) and II (P21/c). This transition is mainly characterized by an important increase of the interlayer metallic distance (+0.137nm) and a shortening of the intralayer metallic distance Cu-Cu (−0.017 nm). These features are due to a conformational change of the molecular part [NH3-(CH2)4-NH3]2+ from left-handed to all-trans conformation and affect noticeably the magnetic properties of the compound. At higher temperature (T = 423 K), a second-order phase transition leading to an orthorhombic phase I (Pncm) is observed by powder X-ray experiments.

A simple conversion of tryptophan to a yohimbane derivative

DL-Tryptophan in 0,1 N HCl reacts with ninnydrin (triketoindane hydrate) at room temperature to give a brown precipitate; washing the precipitate with cold methanol leaves a yellow compound which is crystallized from boiling methanol (C20H14O4N2, 1 CH3OH). X-ray diffractometry showed that the crystals belong to the monoclinic system, with P21/n as a spatial symmetry group; the unit cell (a = 12.016 A; b = 13.336 A; c = 12.128 A; s = 113.45°) contains four molecules, each one formed by the association of a molecule of methanol with 5-carboxy-14-hydroxy-,(3,14,15,16,17,18,19,20) octadehydro-yohimban-21-one, a hitherto not described compound possessing the same ring skeleton as yohimbine and reserpine, two major indole alkaloids. The structure of a spiro derivative of 2,3,4,5-tetrahydro-s-carboline, previously proposed [16], can thus be eliminated.