E. Estop

The thermal properties of three (Cl-, Br-, I-) para-halotoluenes. Formation of glassy crystals

Abstract The heat capacities of three para-halotoluenes, p -chlorotoluene, p -bromotoluene and p -iodotoluene, were measured with an adiabatic calorimeter. All three compounds showed the formation of glassy crystals. The glass transition temperatures are 220, 242 and 250 K respectively. The temperature and enthalpies of fusion are: for p -chlorotoluene, 280.69 ± 0.01 K and 13554 J mol −1 ; for p -bromotoluene, 299.94 ± 0.01 K and 15127 J mol −1 ; and for p -iodotoluene, 306.70 ± 0.05 K and 14963 J mol −1 . The heat capacity of p -iodotoluene was measured from 5 K; the derived thermodynamic properties are given. Those of the other compounds were measured from 80 K.

Dynamic properties of p-diiodobenzene investigated by solid-state 2H and 13C nuclear magnetic resonance spectroscopy

p-Diiodobenzene exists in two crystalline phases, both of which are orthorhombic, with space groups Pbca(α phase) and Pccn(β phase) respectively. The α phase is the stable form at ambient temperature, and a phase transition to the β phase occurs at ca. 326 K. In this paper, we present variable-temperature solid-state 2H NMR studies of perdeuteriated p-diiodobenzene. Over the temperature range 293–320 K, the 2H NMR spectrum of the α phase is a rigid lattice powder pattern, with static quadrupole coupling constant 176 ± 1 kHz and static asymmetry parameter 0.04 ± 0.01. Over the temperature range 350–370 K, the 2H NMR spectrum of the β phase shows features characteristic of 180° jumps of the aromatic ring about the C(1)–C(4) axis, and the jump frequency (κ) for this motion has been determined as a function of temperature. On the assumption of Arrhenius behaviour for the temperature dependence of κ, the activation energy for this jump motion is estimated to be (90 ± 10) kJ mol–1. The temperature-dependent structural properties of perdeuteriated p-diiodobenzene have also been investigated by high-resolution solid-state 13C NMR spectroscopy, and results from these investigations are reported and discussed in the light of the dynamic information determined from the 2H NMR studies.

Molecular alloys in the series of para disubstituted benzene derivatives: Part III. The para-bromochlorobenzene-para-dibromobenzene system

Abstract This paper discusses the binary system between para-bromochlorobenzene and para-dibromobenzene. The phase diagram and crystalline parameters of the molecular alloys of this system have been determined by DSC and X-ray diffraction. The miscibility versus temperature study has shown a miscibility in all proportions in the solid state between room temperature and the melting points. The evolution of the alloy crystalline parameters with the composition shows that the deformation of the para-bromochlorobenzene (pBCB) by introducing para-dibromobenzene (pDBB) takes place mainly in the a direction.

Molecular alloys in the series of paradisubstituted benzene derivatives. IV. Parabromochlorobenzene-parachloroiodobenzene system

Abstract The stability of mixed crystals between parabromochlorobenzene (pBCB) and parachloroiodobenzene (pCIB), in all proportions, from 293 K to melting point, is established. The alloys between both components differ slightly from linear behaviour, from a thermal and crystallographic point of view, according to variations in their composition. The greater isotropic nature of the crystalline expansion caused by the introduction of the bulkier component (pCIB) into that of less reticular volume (pBCB) is revealed, with respect to the alloys that only contain Br and Cl as benzene substituents.

Interactive PDF files with embedded 3D designs as support material to study the 32 crystallographic point groups

Crystallography and X-ray diffraction techniques are essential topics in geosciences and other solid-state sciences. Their fundamentals, which include point symmetry groups, are taught in the corresponding university courses. In-depth meaningful learning of symmetry concepts is difficult and requires capacity for abstraction and spatial vision. Traditionally, wooden crystallographic models are used as support material. In this paper, we describe a new interactive tool, freely available, inspired in such models. Thirty-two PDF files containing embedded 3D models have been created. Each file illustrates a point symmetry group and can be used to teach/learn essential symmetry concepts and the International Hermann-Mauguin notation of point symmetry groups. Most interactive computer-aided tools devoted to symmetry deal with molecular symmetry and disregard crystal symmetry so we have developed a tool that fills the existing gap. Display Omitted PDF is a self-contained format that supports 3D designs and multi-layer structure.We use embedded 3D designs in PDF files to illustrate point groups.We use crystallographic notation to set a structure of interactive buttons in PDF files.

Non-stable mixtures of molecular alloys The p-bromoiodobenzene/p-diiodobenzene phase diagram

Abstract The p-bromoiodobenzene/p-diiodobenzene phase diagram has been determined from samples prepared by quenching from the melt, from room temperature to the melting. The solid state miscibility between the monoclinic p-bromoiodobenzene and the orthorhombic p-diiodobenzene is extensive and it is only interrupted by a narrow region of demixing derived from a peritectic invariant. In the course of this investigation non-stable mixtures of two molecular alloys have been identified from X-ray diffraction experiments on samples of the central compositions prepared by precipitation from solution. The phenomenon can be easily confused with a region of demixing when observed by X-ray diffraction, particularly in the case of two pure compounds with very close crystalline unit cells. This finding could be the key for the interpretation of the contradictions found in the 10 binary systems’ group of the monoclinic p-dihalobenzenes. In five of these systems, demixing phenomena not predicted by thermodynamic analysis were deduced from X-ray diffraction experiments. The thermodynamic analysis of the experimental p-bromoiodobenzene/p-diiodobenzene phase diagram confirms the absence of any demixing phenomenon for the central compositions where the non-stable mixtures were found. The coefficient of crystalline isomorphism e M is more indicative of the observed solid state miscibility than Kitaigorodskys coefficient e K of molecular shape similarity. A detailed formalism is presented for extracting the characteristic temperatures of the DSC signals by means of two shape-factors.

Behaviour during thermal cycling of molecular alloys for the storage of thermal energy. II: Alloys with P-chloroiodobenzene and P-bromoiodobenzene

The p-chloroiodobenzene and p-bromoiodobenzene compounds and molecular alloys between p-dichlorobenzene, p-bromochlorobenzene, p-dibromobenzene, p-chloroiodobenzene and p-bromoiodobenzene have been characterized by thermal cycling. DSC thermal analysis have been used to determine T[sub fusion], [Delta]H[sub fusion] in order to evaluate their performance in the thermal cycling.

From all-atom vision to molecular pairwise-interactions: the mean-field approach in molecular alloys

The pairwise-interaction model for molecular alloys has been improved by using all-atom potential functions. Both descriptions have been correlated under a more general approach, the mean-field. Expressions for the excess internal energy have been obtained. Two sources of non-thermodynamical regularity arose: the differences in the geometry and chemical nature of the molecules and the non-transferability of the interaction potentials from pure components to alloys. The p-dichlorobenzene (1,4-C6H4Cl2)/p-dibromobenzene (1,4-C6H4Br2) molecular system has been used to illustrate the theoretical treatments.