Experimental and numerical signatures of dynamical crossover in orientationally disordered crystals

By means of NMR experiment and MD computer simulation we investigate the dynamical properties of a chloroadamantane orientationally disordered crystal. We find a plastic-plastic dynamical transition at T_x ~ 330 K in the pico-nanosecond regime. It is interpreted as the rotational analogue of the Goldstein crossing temperature between quasi-free diffusion and activated regime predicted in liquids. Below T_x, NMR experimental data are well described by a Frenkel model corresponding to a strongly anisotropic motion. At higher temperatures, a drastic deviation is observed toward quasi-isotropic rotational diffusion. Close to T_x, we observe that two-step relaxations emerge. An interpretation which is based on the present study of a specific heat anomaly detected by a recent calorimetric experiment is proposed.Comment: 4 pages, 4 figures; changed abstract and references; corrected figure

Onset of slow dynamics in difluorotetrachloroethane glassy crystal

Complementary Neutron Spin Echo and X-ray experiments and Molecular Dynamics simulations have been performed on difluorotetrachloroethane (CFCl2-CFCl2) glassy crystal. Static, single-molecule reorientational dynamics and collective dynamics properties are investigated. The orientational disorder is characterized at different temperatures and a change in nature of rotational dynamics is observed. We show that dynamics can be described by some scaling predictions of the Mode Coupling Theory (MCT) and a critical temperature $T_{c}$ is determined. Our results also confirm the strong analogy between molecular liquids and plastic crystals for which $\alpha$-relaxation times and non-ergodicity parameters are controlled by the non trivial static correlations as predicted by MCT

Polymorphism and Dynamics of Nifedipine: a Combined Dielectric and Calorimetric investigation

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Dynamics NMR investigations of a glassy crystal confined in porous materials

The temperature dependence of the proton relaxation rates has ben studied in molecular glassy crystals cyano-adamantane (C10H15CN) confined in the porous space of a silica glass of 60Å and 150Å nominal pore sizes. In the temperature range corresponding to the bulk plastic phase, the NMR proton lineshape of the confined materials revealed a two phase system consisting of a liquid-like component and a crystalline solid bulk-like component. As for the bulk, the [Math] process, related to the dipolar axis motion, controlled both T1z and T1p in the largest pores. From the NMR results we concluded that the dynamics of this [Math]-relaxation process is slightly enhanced by the confinement giving rise to a negative shift of the glass transition temperature. On the contrary, the confinement has no influence on the relaxation rates in the glassy state

Polymorphism and Dynamics of Nifedipine: a Combined Dielectric and Calorimetric investigation

International audienc

Dielectric and NMR Investigations of a Pharmaceutical Complex Compound

International audienc

Molecular motions in glassy crystal cyanoadamantane : a proton spin-lattice relaxation study

Cyanoadamantane C$_{10}$H$_{15}$CN exhibits four different solid phases : two cubic plastic (I and I$'$), one cubic glassy (I$_{\rm g}$) and one monoclinic ordered (II). In cubic plastic phases (I, I$'$) three types of motion coexist : a uniaxial rotation of the molecule around its C—C$\equiv$N axis, a tumbling reorientation of this dipolar axis between the $\langle 001\rangle$ directions and a vacancy self-diffusion. In the cubic glassy state (I$_{\rm g}$) the tumbling motion is frozen and therefore only the uniaxial rotation survives. In the ordered phase (II), the molecules only perform a 3-fold uniaxial rotation among identical positions. These different molecular motions in the four solid phases have been studied by the analysis of the $T_{1 z}$ and $T_{\rm 1 \rho}$ spin-lattice relaxation times in $^1$H-NMR. The derived residence time are compared, when possible, to values previously deduced from quasi-elastic neutron scattering, dielectric relaxation and second moment of the $^1$H-NMR lineshape.Le cyanoadamantane C$_{10}$H$_{15}$CN possède quatre phases solides différentes : deux plastiques cubiques (I et I$'$), une vitreuse cubique (I$_{\rm g}$) et une ordonnée monoclinique (II). Dans les phases plastiques cubiques (I, I$'$) trois types de mouvements coexistent : une rotation uniaxiale de la molécule autour de son axe C—C$\equiv$N, un basculement de cet axe dipolaire entre les directions $\langle 001\rangle$ et une diffusion moléculaire. Dans l'état vitreux cubique (I$_{\rm g}$), le mouvement de basculement est gelé et seule la rotation uniaxiale subsiste. Enfin dans la phase ordonnée (II), les molécules effectuent une rotation uniaxiale d'ordre 3 entre positions indiscernables. Ces différents mouvements dans les quatre phases solides ont été évalués par l'analyse des temps de relaxation spin-réseau $T_{1 z}$ et $T_{1 \rho}$ en $^1$H-RMN. Les temps de résidence qui en sont déduits sont comparés (lorsque cela est possible) aux valeurs correspondantes déduites précédemment par diffusion quasi-élastique des neutrons, par relaxation diélectrique et par mesure du second moment de la raie RMN

Dielectric and NMR Investigations of a Pharmaceutical Complex Compound

International audienc

Modelling of dynamical processes in a molecular crystal by NMR

In this contribution we report on the plastic crystal 1-chloroadamantane dynamics via conventional frequency dependent ( 1H and 13 C) and field cycling NMR measurements. A suitable microscopic dynamical model, worked out from from X-ray analysis is developed and the molecular motions are interpreted in terms of: self diffusion and dipolar molecular axis combined with uniaxial rotation. In the rotator phase the molecules execute a bimodal reorientation process whereas the uniaxial rotation solely persists in the low temperature phase. In both phases, the residence times exhibit an Arrhenius temperature dependence. The results confirm the existence of a dynamic crossover transition predicted by molecular dynamics simulation. Copyright EDP Sciences/Società Italiana di Fisica/Springer-Verlag 200776.60.-k Nuclear magnetic resonance and relaxation, 61.50.-f Crystalline state, 76.60.Es Relaxation effects,