Madeleine Rey-Lafon

Discussion des modes normaux des groupements amides cis et trans à partir des champs de force du σ-valérolactame et du N methylacetamide

Valence force fields of σ-valerolactam and N methylacetamide are determined. The normal modes of characteristic vibrations of cis and trans amide groups are specially discussed. This comparison shows the existence of an interaction between oxygen and hydrogen in the amide cis group.

Analyse des spectres de vibration de la diméthylnitramine et du dérivé perdeutérié. Détermination des champs de force de valence et des modes normaux de vibration du nitrométhane et de la diméthylnitramine

Abstract Infrared and Raman spectra of N-nitrodimethylamine and its perdeuterated derivative have been recorded between 3100 and 80 cm−1. An interpretation is proposed. Valence force fields of nitromethane and N-nitrodimethylamine are determined.

X-ray and neutron studies of a displacive phase transition in N,N-dimethylnitramine (DMN)

N,N-Dimethylnitramine has a phase transition at 107 K for the hydrogenated compound and 111 K for the deuterated compound, associated with a soft mode previously observed by Raman spectroscopy at low temperature. The purpose of this work is to determine the structural modifications at the transition. Powder diffraction patterns (120, 95, 15 K) and rotating-crystal and Weissenberg photographs (155, 95 K) obtained with neutrons show the existence of a superlattice corresponding to a doubling of the lattice below the transition temperature. The structures at 300, 180, 125 K (X-rays) and 125, 91, 85 K (neutrons) have been determined. The structural modifications between the high-temperature form (space group P2~/m, Z = 2, planar molecule) and low-temperature form (P2~/c, Z = 4, deformed molecule) can essentially be described by a rotation of the whole molecule (2 4 o at T t - 20 K) plus an intramolecular deformation: the N--N bond and the C-N-C plane form an angle of about 10 ° at Tt-- 20 K.

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.

Vibrational study of p, p′-disubstituted N-benzylideneanilines

Identification des spectres IR et Raman de molecules isoelectroniques: trans-stilbene, trans-azobenzene et N-benzylideneanilines

Spectroscopic Study of a Disc-Like Liquid Crystal: Hexa-N-Nonanoyloxytriphenylene Structure and Dynamics of the Alkyl Chains

Abstract The structure of the alkyl chains bonded to the aromatic core in hexa-n-nonanoyloxytriphenylene is studied as a function of temperature. In the crystal phase, the spectra suggest a disorder of the chains due to angular fluctuations about the carbon-carbon bonds in the vicinity of trans states. Numerous conformational defects appear at the first phase transition.

Vibrational and calorimetric study of phase transitions in n-C6F14 and n-C8F18 crystals

Abstract Heat capacities of n -perfluorohexane and n -perfluorooctane were measured from 4.2 K to room temperature. One solid—solid transition is found for n -C 6 H 14 ,two for n -C 8 F 18 . Spectra show that the molecules have the same conformations in the two forms of n -C 6 F 14 . Several isomers are present in the high-temperature forms of n -C 8 F 18 ;thus transitions in this compound have order—disorder character.

Raman study of N‐nitrodimethylamine single crystal. Evidence for a structural phase transition

Low frequency Raman spectra of oriented single crystals of N‐nitrodimethylamine and its perdeuterated derivative have been recorded using polarized light. The observed frequencies have been assigned to the different symmetry species of the unit‐cell factor group. The study of the temperature dependence of the spectra has shown the occurrence of a structural phase transition. This later involves a very weak entropy of transition, about 0.04 cal deg−1 mole−1, and is accompanied by a soft mode, which implies a displacive character. The soft mode has been observed in the Raman spectrum of the low temperature phase only; the analysis of the spectra corresponding to the different elements of the polarizability tensor suggests the existence of a superstructure in the low temperature phase.

X-ray diffraction, differential scanning calorimetry, and spectroscopic studies of phase transitions in FeOCl-n-alkylamine intercalation complexes

Abstract β /ag α Phase transitions in the layered compound FeOClC n H 2 n +1 NH 2 ( n = 10, 12, 14, 16, and 18) have been studied by X-ray diffraction, differential scanning calorimetry (DSC), and IR. The transition temperature ( T c ) is approximately 20°C higher than the melting point of each corresponding amine and increases with the alkyl chain length. The enthalpy of transition has been estimated from DSC measurements as ca. 6 kJ/mole. The conformational changes of the alkyl chains during the β /ag α transition could be confirmed by infrared studies.

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.