18 research outputs found
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
is determined. Our results also confirm the strong analogy between
molecular liquids and plastic crystals for which -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
International audienc
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 60Ă
and 150Ă
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 CHCN exhibits four different solid phases : two cubic plastic (I and I), one cubic glassy (I) and one monoclinic ordered (II). In cubic plastic phases (I, I) three types of motion coexist : a uniaxial rotation of the molecule around its CâCN axis, a tumbling reorientation of this dipolar axis between the directions and a vacancy self-diffusion. In the cubic glassy state (I) 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 and spin-lattice relaxation times in 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 H-NMR lineshape.Le cyanoadamantane CHCN possĂšde quatre phases solides diffĂ©rentes : deux plastiques cubiques (I et I), une vitreuse cubique (I) et une ordonnĂ©e monoclinique (II). Dans les phases plastiques cubiques (I, I) trois types de mouvements coexistent : une rotation uniaxiale de la molĂ©cule autour de son axe CâCN, un basculement de cet axe dipolaire entre les directions et une diffusion molĂ©culaire. Dans l'Ă©tat vitreux cubique (I), le mouvement de basculement est gelĂ© et seule la rotation uniaxiale subsiste. Enfin dans la phase ordonnĂ©e (II), les molĂ©cules effectuent une rotation uniaxiale d'ordre 3 entre positions indiscernables. Ces diffĂ©rents mouvements dans les quatre phases solides ont Ă©tĂ© Ă©valuĂ©s par l'analyse des temps de relaxation spin-rĂ©seau et en H-RMN. Les temps de rĂ©sidence qui en sont dĂ©duits sont comparĂ©s (lorsque cela est possible) aux valeurs correspondantes dĂ©duites prĂ©cĂ©demment par diffusion quasi-Ă©lastique des neutrons, par relaxation diĂ©lectrique 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/SocietĂ Italiana di Fisica/Springer-Verlag 200776.60.-k Nuclear magnetic resonance and relaxation, 61.50.-f Crystalline state, 76.60.Es Relaxation effects,