J. Gallier

1H-NMR, dielectric and calorimetric studies of molecular motions in m-nitroaniline crystal

Abstract Spin-lattice relaxation time, T 1 , spin-lattice relaxation time in the rotating frame, T 1 ρ , and the second moment of the resonance line measurements at 80 MHz and over the 190–380 K temperature range are reported for protons in the optically nonlinear material m -nitroaniline ( m -NA). T 1 has also been measured for samples irradiated by low energy and low intensity radiation. The real and imaginary parts of electric permittivity as well as tanu2008 δ have been recorded in the 80–380 K temperature range at frequencies ranging from 100 Hz to 1.0 MHz. DSC measurements have been performed in the 110–387 K temperature range. Two phase transitions have been found: A glassy to rotative transition at 160 K and a plastic to plastic transformation at 365 K. The main feature of the m -NA crystal is that its plasticity continues to grow as the temperature increases. The reorientations of phenyl rings, the –NH 2 group proton 180° jumps, the lattice distortions caused by anisotropic thermal expansion and the cooperative reorientations of big molecular aggregates are thought to be the reasons for phase transitions and for the subsequent intermolecular charge transfer. The results are discussed with respect to optical second harmonic generation and near-IR photochemical reaction found in the m -NA crystal.

Low-frequency motions in an alkali phosphate glass studied by 7Li and 31P NMR

Abstract A 31P and 7Li NMR study of the phosphate glass Na0.5Li0.5PO3 leads to the following dynamical picture. Below Tg, only the Li ions diffuse slowly in the glass structure. This motion seems unaffected by the glass transition and presents a linear frequency dependent spectral density in the two probed frequency ranges. This behaviour, classical in glasses, could be explained by soft potential modes occuring with a high density in this disordered material. Above Tg, the phosphate units of the polymer chains begin to move isotropically giving rise to the so-called secondary relaxation in glasses while the primary or structural relaxation is only effective on the NMR parameters near 600 K, well above Tg.

Proton magnetic resonance study of some crystalline p-polyphenyls: Evidence of relaxation by slow molecular motions above room temperature

Abstract A NMR study of three p -Polyphenyls ( p terphenyl C 18 DH 14 , Selectively deuterated p -terphenyl C 18 D 10 H 4 and p quaterphenyl C 24 H 18 ) has been performed from about room temperature to their melting temperatures. The second moments and the spin-lattice relaxation times indicated similar dynamical processes for the three compounds: evidence of two slow molecular jump processes governed by high energy activations (about or larger than 20 kcal/mole). The geometry of these motions is described by large amplitude rotational jumps (≈ 180°) of respectively the external and the central phenyl rings of the molecules, along their long molecular axis.

N. M. R. Investigations of Phase Transitions in Pure and Mixed Crystals of Pentachlorophenol

Abstract The nuclear magnetic resonance line width, second moment and the relaxation times T 1 and T 1D have been measured for protons in a pentachlorophenol (PClPh)-hexachlorobenzene (HClB) mixed crystal (70% in concentration of PClPh), from 120 K to the melting point. The results have been interpreted in terms of the various molecular motions occurring in this crystal. We first observed, as the temperature increased, the fast jump of the proton around the C–O bond, even at low temperature. The activation enthalpy for this motion is 7.5 ± 0.2 KJ mole−1 and its correlation time of ≃ 2.3.10−8 s at ambient temperature. Before the transition (410 K), a slow motion occurs, molecular self diffusion or tumbling, which exhibits a discontinuity in its correlation time at the transition. The hindering enthalpy for this process is found to be 100 ± 20 KJ mole−1 above the transition. The associated correlation frequency is estimated from T 1d to be 50 s−1 near the melting point. These resultscan be extended to other...

PMR study of some quinol clathrates

Abstract The motion of trapped molecules in some deuterated quinol clathrates is studied by proton magnetic resonance. Theoretical calculations of second moments and line widths are made for several reorientation assumptions of the trapped molecules.

Local Properties in Disordered Mixed M1-xPx Monomer Polymer Crystal : A D-NMR Study

Abstract The influence of the polymerization on the structural phase transition in a mixed monomer-polymer pTS-D system is studied by D-NMR. Results are compared with previous neutron and Raman scattering results.

Experimental Study of the Liquid-Glass Transition in an Inorganic Polymer Li 0.5 Na 0.5 PO 3

New experimental results obtained with various techniques on a less-studied glass-forming system are presented. At low frequency, a secondary β slow -process, decoupled from the viscous flow, is observed by 3 1 P NMR. Raman scattering spectra and coherent neutron scattering spectra has been obtained in wide frequency and temperature ranges showing the same qualitative features for the Boson peak while the quasielastic contribution seems to differ markedly.

Motions by 1H NMR in Crystalline Phases of β-Naphthol and β-Naphthylamine

Abstract For β-naphthol, where the measurements have been made above the ambient temperature, we observed the fast hydroxyl proton jump. Its mean correlation time is ∼5 10−7 s at ambient temperature. Around 350 K, a low temperature transition seems correlated with a frequency independent relaxation contribution and leaves the above motion unaffected. Above this transition, a very slow motion appears (20 s −1 at 365°K) which is modified by the high temperature transition (385 K). For β-naphthylamine, only one motion is present between 200 K and the melting point: the 180° jumps of the amino group around its axis. A large distribution of correlation times can explain the relaxation features.

Molecular Motion in Solid Pentachlorobenzenethiol by N. M. R.

Abstract The nuclear magnetic resonance line width, second moment and the relaxation times T1 and T1p have been measured for protons in a pentachlorobenzenethiol (PCIBT) crystal, from 120 K to the melting point. The results have been interpreted in terms of the various molecular motions occurring in this compound. We observed successively as the temperature increases: the fast jump of the proton around the C-S bond. even at low temperature. The associated activation enthalpy is 7.95 ± 0.2KJ mole−1 and correlation time at 125K is about ∼ 2.8 10−1s. Then appears the in plane molecular reorientation around the pseudo hexad axis, with activation enthalpy equal to 48.5 ± 2 KJ mole−1. and correlation time of 1.610−7s. Then appears the in plane molecular reorientation around the pseudo hexad axis, with activation enthalpy equal to 48.5 ± 2KJ mole−1 and correlation time of 1.610−7s at ambient temperature. These last results are in good agreement with those obtained by Brot by dielectric methods. Before the transi...

Ferroelastic lattice instability during the thermally enhanced reaction of MSE

A p-dimethylaminobenzenesulfonate (MSE) polycrystalline powder is converted to the zwitterionic product p-trimethylammoniumbenzenesulfonate (ZWT) by a chemical thermally induced reaction at T= 55,5°C. Experimental results obtained by differential scanning calorimetry and 13C solid-state NMR indicate that reaction-induced stress gives rise to a ferroelastic lattice instability during the thermally enhanced solid state reaction of methyl p-dimethylaminobenzenesulfonate (MSE).