N.B. Chanh

Thermal conformational changes in a bidimensional molecular composite material: A thermodynamic and crystallographic study of NH3—(CH2)4—NH3 CdCl4

Abstract In the series of molecular composites of perovskite-type layer structures NH3—(CH2)n—NH3 MCl4, the derivative with n = 4 and M = Cd appears as a particular case, compared to other homologous compounds (M = Mn). The distance between two successive perovskite layers, which is dependent on the molecular chain length, appears very short in the crystalline initial form. The polymorphism has been studied by thermal and crystallographic investigations on both powder and single-crystal materials. Five crystalline forms V, IV, III, II and I have been evidenced. The irreversibility or reversibility of the different phase transitions has been established. Structure determinations of the “limit” phases V (initial room-temperature form), III (high-temperature form) and I (new room-temperature form) allow us to correlate the different phase transitions of this complex to the conformational evolutions of the organic part [NH3—(CH2)4—NH3]2+. The results show that a temperature increase on form V leads to an irreversible transformation from gauche to all-trans conformation in the organic chain which keeps the definitive conformation in further phase transitions. Orientational disorder of the chain appears in the high-temperature phase and its evolution with decreasing temperature has been analysed from entropy change measurements.

Structures and thermal behavior in the series of two-dimensional molecular composites NH3(CH2)4NH3MCl4 related to the nature of the metal M. Part 1: Crystal structures and phase transitions in the case M = Cu and Pd

Abstract X-ray diffraction and differential scanning calorimetry studies have been undertaken on the layered NH3ue5f8(CH2)4ue5f8NH3 MCl4 complexes with M = Cu and Pd. Both complexes are structurally very similar at room temperature (monoclinic P2 1 c :a = 0.9270(3) nm , b = 0.7600(3) nm, c = 0.7592(3) nm, β = 103.14(4) ° for M = Cu; a = 0.9087(2) nm, b = 0.7699(2) nm, c = 0.7792(2) nm, β = 103.82(2) ° for M = Pd). The organic layers are composed of [NH3ue5f8(CH2)4ue5f8NH3]2+ cations with a left-handed conformation at both ends; the mineral layers are composed of [MCl4]2− square planar anions. The structure cohesion is achieved via Nue5f8H---Cl hydrogen bonds. The copper complex exhibits a structural phase transition at T = 328 K, characterized by an increase of the interlayer distance (+0.137 nm) from powder diffraction results and which can be interpreted as due to a change in the molecular conformations, from left-handed to all-trans. This assumption is confirmed by single crystal structure determination. (High temperature phase M = Cu, monoclinic P2 1 c :a = 1.0420(3) nm , b = 0.7442(1) nm, c = 0.7225(5) nm, β = 93.46(4) °.) The palladium complex is stable up to its decomposition temperature (490 K). However, a detailed thermal expansion analysis shows that a virtual structural phase transition is expected above 490 K. The correlation between the nature of the metal, the strength of the hydrogen bonds and the occurrence of left-handed conformations and related phase transitions, is discussed.

Polymorphism in the Bidimensional compound (n-C16H33NH3)2CdCl4

Abstract Four crystalline phases have been observed in the perovskite-type layer compound ( n -C 16 H 33 NH 3 ) 2 CdCl 4 using X-ray diffraction, differential scanning calorimetry and vibrational analysis. Phase IV, stable at low temperature, has a monoclinic ordered structure. The two intermediate phases III and II are orthorhombic and the high temperature form I is tetragonal. In the last case, the diffraction pattern shows the presence of a diffuse scattering phenomenon corresponding to the bidimensional melting of only the organic part of the structure (C 16 H 33 NH 3 chains). Entropy values of the order-disorder process are best interpreted by a disorder similar to that of the molten state of the n -paraffins. The largest enthalpies are measured at the (IV-III) and (II-I) transitions; they correspond to the two discontinuities of the interlayer spacing and are related to a modification of the organic chain length due to the occurrence of kink defects ( GT 2 n +1 G ′), and a decrease of the number of trans forms between the G and G ′ bonds, respectively.

Thermal study and crystal structure of a perovskite-type unsaturated molecular composite: Propargylamine and cadmium chloride complex salt

Abstract The propargylamine-cadmium chloride complex (HC ue5fc C-CH2NH3)2CdCl4 belongs to the family of bidimensional perovskite-type compounds which are able to show photoreactivity phenomena under irradiation (γ, X, u.v…) leading to a possible polymerization in the solid state due to the presence of triple bonds in the organic layer. The crystal structure of the complex, before reaction, has been solved by the X-ray diffraction method (monoclinic, C2 c , a = 23.624 (6) A, b = 7.367 (2) A, c = 7.353 (4) A, β = 98.89 (1)°, Z = 4). The distances between adjacent triple bonds (3.44–3.48 A) are in the range of a possible formation of polyene structure initiated by a 7-6 addition and oriented to the b and/or →c directions. On the other hand, the complex exhibits, under the heating process, an exothermic phenomenon corresponding to the irreversible formation of a “high temperature” phase: the crystallographic characteristics of this new phase have been determined.

Transitions de phase destructive et non destructive dans le cristal moleculaire bromo-2 naphtalene

Abstract The polymorphism of 2-bromonaphthalene has been studied by both crystallographic and energetical methods. Three crystalline forms have been characterized, respectively denoted Br III , Br II and Br I . The first one is the stable form up to 319 K. At this temperature a first order phase transition occurs which breaks the single crystal and leads to a disordered structure Br I isotypic with the naphthalene one. The Br II form is a metastable one which appears when cooling the Br I , form; when so formed, the Br II phase can give again the Br I one by annealing; so that the Br I -Br II transition is reversible. This second order transition extends over a large temperature range (275–319 K), it does not destroy the single crystal and corresponds to a substructural modification along the c axis of the monoclinic cell. The anomalies of the Cp curve and the variations of the principal components of the thermal expansion tensor suggest that there are two competitive temperature dependent molecular reorientations. The transition Br II →Br III which leads to the stable ordered form is spontaneous at room temperature, it takes place sluggishly and destroys the single crystal. Cell parameters of the Br III form have been determined from powder data using an automatic indexing method: a = 9.578(1) A, B = 9.601(1) A, c = 10.303(1) A, α = 100.79(1)°, β = 109.06(1)°, γ = 101.77(1)°. (Figure of merit M 20 = 86.)

Polymorphism of β R-substituted naphthalene derivatives—Review and comparative study

Abstract The polymorphic behaviour of seven β R-substituted naphthalene derivatives (R = H, F, OH, Cl, CH3, SH and Br) is reviewed and complementary accurate data are given concerning particularly the structural evolution aspects. Sixteen phases have been characterized. It is shown that they can be classified into three categories, respectively labelled I, II and III. All the derivatives exhibit the high temperature form I isostructural with naphthalene ( P2 1 a , Z = 2). Form II ( P2 1 n , Z = 4) corresponds to a unidirectional substructure of form I along the c axis (CII = 2C1) as R = Cl or Br. Form III which is encountered for R = F, OH, CH3, SH or Br does not show any evident relations. The I–II phase transitions are of second order, all the others (I–III, II–III, III-III) belong to the first order. Form III (as far as known) is ordered and forms II and I are disordered. The II-I transition corresponds to a progressive modification of the symmetry of the molecular site. The statistical molecular entity becomes centrosymmetrical in the high temperature form. It seems that two mechanisms govern this modification. In form I, a dynamical disorder is hypothesized with two types of motion: reorientations of the molecule in its own plane and diffusion jumps of the molecule from site to site. In form II these motions persist at high tempeature but become progressively weaker as the temperature lowers so that finally the disorder is quite static.

Etablissement du diagramme binaire hydroxy.2 naphtalene-chloro.2 naphtalene par differentes analyses thermiques

The phase diagram of the 2-chloronaphthalene-2-hydroxynaphthalene binary system has been established in three thermal investigations (calorimetric, crystallographic and optical methods). This diagram is fairly complicated. The main feature is the presence of three invariants: a eutectoid atTe=309 K, a peritectic atTp=333 K and a metatectic atTm=364 K.ZusammenfassungDas Phasendiagramm des binären Systems 2-Chlornaphthalen-2-Hydroxynaphthalen wurde fürT⩾293 K mittels drei thermischer Analysenmethoden (kalorimetrische, kristallographische und optische Methoden) aufgestellt. Das Diagramm ist verhältnismäßig kompliziert. Der wichtigste Wesenszug ist das Vorliegen von drei Invarianten: einer eutektoidischen beiTe=309 K, einer peritektischen beiTp=333 K und einer metatektischen beiTm=364 K.РезюмеКалориметрическим, к ристаллографически м и оптическим методами изучена фазовая диаграмма дв ойной системы 2-хлорнафталин-2-оксина фталин при температу реT ⩾ 293 K. Диаграмма являетс я относительно сложн ой и для которой характерным является наличие тре х инвариантных систе м: эвтектики приT3=309 K, перитектики пр иTn=333 K и метатектики приTM=364 K.

Polymorphism in organo-metallic bidimensional structures: The BIS (n-octylammonium) tetrachlorocadmate

Abstract Raman spectra, X-ray diffraction diagrams and differential scanning calorimetry show the existence of two structural phase transitions in crystalline (C8H17NH3)2 CdCl4. The solid stable at low temperature is ordered, the alkylammonium chains are in extended configuration with probably a gauche bond in the proximity of the NH3 group. The intermediate phase is characterized by a confonnational disorder and a modification of the crystalline structure. The number of internal rotations increases in the high temperature form; however the total transition entropy is lower than the value expected for a total “melting” of the chains.

Phase transitions in 2-bromonaphthalene. The structure of the low temperature phase from X-ray powder diffraction

Abstract The crystal structure of the low temperature stable form of 2-bromonaphthalene has been solved ab initio from laboratory X-ray powder diffraction data. Crystal data: a = 9.572(2), b = 9.599(2), c = 10.307(1) A , α = 100.75(1), β = 109.04(1), γ = 101.76(1) ° , P 1 space group, C 10 H 7 Br, Z = 4, 298 K. 2-bromonaphthalene occurs in three polymorphic varieties. While the crystal structures of the high temperature form and of the metastable form at 25 °C were already known, the structure of the low temperature form remained unsolved due to the difficulty of growing single-crystals. In spite of its complexity (two molecules in the asymmetrical unit), the structure has been solved from powder data at moderate resolution (2.2 A) combining direct with search methods. The subsequent Rietveld refinement with the bromonaphthalene molecules as rigid groups converged to R p = 7.6% and R wp = 10.3%. The structure is almost ordered and built of molecular layers stacked along [001] with the long molecular axis normal to the layer. The orientation of the naphthalenes within the layers is nearly the same as in the structure of the unsubstituted naphthalene. The probable mechanism of the transition from metastable to low temperature form has been established.

Caractérisation du comportement polymorphique du fluoro-2 naphtalene

Abstract The use of various techniques such as X-ray and neutron diffraction, Raman scattering, diffusion and calorimetry has allowed us to characterize the polymorphic behaviour of 2-fluoronaphthalene. Under atmospheric pressure, three crystalline forms can be detected between 80 K and the melting temperature. The high temperature one, F I , which is stable from 272 K to the melting point, is isomorphous with naphthalene (space group P 2 1 / a,Z = 2); by cooling it can be preserved as a metastable form down to 154 K, the 272-154 K range being the stability domain of the form F III . Below 154 K, a new form, called F III appears. The kinetics of the transformation F I → F III seem to be dependent upon the size of the crystals. The Raman spectra of f I and F III are identical, which suggests similar structural disorder in both phases. This is also supported by the low value of the entropy associated with the transition F III → F I (0.55 J K −1 mole −1 ). The unit cell parameters of the F III and F III forms have been determined from data using automatic indexing methods. The two solid-solid transitions are first order, the F III – F III one being “explosive”.