DYNAMICS OF LINEAR ALKANES OF DIFFERENT CHAIN-LENGTH IN NANOTUBES

A solid state characterization of inclusion compounds containing isola ted molecules of linear alkanes and polyethylene was performed by Nucl ear Magnetic Resonance. The chemical shifts in the C-13 high resolutio n experiments, the quadrupolar H-2 powder spectra and the relaxation e xperiments provided us a detailed description of the conformation and mobility of linear polymethylene chains of different length, once isol ated in a crystalline hydrocarbon environment. The chain-length depend ence of the internal methylene chemical shifts was observed for the fi rst time, giving an evidence of the departure of the internal conforma tions from the trans-planar arrangement. Since the high relaxation rat es are associated to fast reorientations of the C-H or C-D vectors, as referred to the nanotube walls, the departure from the trans-planar a rrangement was interpreted according to a dynamic model. The longer is the chain the less frequent are those defects or smaller is the ampli tude of the oscillations, indicating that the defects are associated t o the presence of the chain-ends at a distance of several bonds. This is a suggestion for a concerted mechanism of motion, associated to a l ong correlation length, instead of a local dynamic process due to ther mal excitations.