Intra-cage dynamics of molecular hydrogen confined in cages of two different dimensions of clathrate hydrates.

In porous materials the molecular confinement is often realized by means of weak Van der Waals interactions between the molecule and the pore surface. The understanding of the mechanism of such interactions is important for a number of applications. In order to establish the role of the confinement size we have studied the microscopic dynamics of molecular hydrogen stored in the nanocages of clathrate hydrates of two different dimensions. We have found that by varying the size of the pore the diffusive mobility of confined hydrogen can be modified in both directions, i.e. reduced or enhanced compared to that in the bulk solid at the same temperatures. In the small cages with a mean crystallographic radius of 3.95 Å the confinement reduces diffusive mobility by orders of magnitude. In contrast, in large cages with a mean radius of 4.75 Å hydrogen molecules displays diffusive jump motion between different equilibrium sites inside the cages, visible at temperatures where bulk H2 is solid. The localization of H2 molecules observed in small cages can promote improved functional properties valuable for hydrogen storage applications.

Structural and dynamical properties of reconstituted myelin sheaths in the presence of myelin proteins MBP and P2 studied by neutron scattering.

The myelin sheath is a tightly packed, multilayered membrane structure wrapped around selected nerve axons in the central and the peripheral nervous system. Because of its electrical insulation of the axons, which allows fast, saltatory nerve impulse conduction, myelin is crucial for the proper functioning of the vertebrate nervous system. A subset of myelin-specific proteins is well-defined, but their influence on membrane dynamics, i.e. myelin stability, has not yet been explored in detail. We investigated the structure and the dynamics of reconstituted myelin membranes on a pico- to nanosecond timescale, influenced by myelin basic protein (MBP) and myelin protein 2 (P2), using neutron diffraction and quasi-elastic neutron scattering. A model for the scattering function describing molecular lipid motions is suggested. Although dynamical properties are not affected significantly by MBP and P2 proteins, they act in a highly synergistic manner influencing the membrane structure.

BH4− self-diffusion in liquid LiBH4.

The hydrogen dynamics in solid and in liquid LiBH4 was studied by means of incoherent quasielastic neutron scattering. Rotational jump diffusion of the BH4- subunits on the picosecond scale was observed in solid LiBH4. The characteristic time constant is significantly shortened when the system transforms from the low-temperature phase to the high-temperature phase at 383 K. In the molten phase of LiBH4 above 553 K, translational diffusion of the BH4- units is found. The measured diffusion coefficients are in the 10(-5)cm2/s range at temperatures around 700 K, which is in the same order of magnitude as the self-diffusion of liquid lithium or the diffusion of ions in molten alkali halides. The temperature dependence of the diffusion coefficient shows an Arrhenius behavior, with an activation energy of Ea = 88 meV and a prefactor of D0 = 3.1 × 10(-4)cm2/s.

Guest dynamics in endohedrally doped fullerenes.

We use molecular dynamics to calculate the vibrational density of states of several endohedral mono-metallofullerenes and compare the results with the experimental values. The vibrational patterns depend weakly on the charge transfer from the metal to the fullerene cage, with the largest variations of approximately 2 cm(-1) observed upon switching off completely the Coulomb interactions, and more strongly on the cage isomer, with variations of up to approximately 13 cm(-1). Analysis of the metal motions inside the cage shows that they can be chaotic. The origin of such chaotic behavior is discussed.