CHARACTERIZING NANOSCOPIC DISORDER USING QUANTUM MOLECULAR TOPS.

Researchers at the NIST Center for Neutron Research (NCNR) have recently carried out the first U.S. studies by cold neutron spectroscopy of molecular disorder induced by nanoscale confinement. In this work inelastic neutron scattering was used to probe the rotational dynamics of molecules adsorbed into porous glass disks with pores ranging from 2.5 nm to 7.5 nm.

In bulk molecular solids at [approximate]5 K, the molecular rotors are classically forbidden from rotating due to the surrounding molecular field. However, quantum mechanics allows the molecules to rotate by tunneling through the barrier. The differences observed between the quantum tunneling in the bulk and confined molecular solids provide a powerful probe of the structure of the confined molecular solid. This information, which is key to useful properties of nanostructured materials, is very difficult to obtain using conventional structural methods. Using neutron inelastic scattering, NCNR scientists have measured the effect of both pore size and the surface chemistry on the degree of disorder in the local molecular environment for molecules such as [CH.sub.3]I confined in porous glasses.

Thus, novel cold neutron techniques that probe the dynamics of the adsorbed solid rather than the static structure could prove very useful in characterizing the next generation of engineered nanoporous materials, with applications including chemical separation.