Dynamics of an inclusion compound of alkyl chains in an organic matrix

The nitroxide CHNO forms channel inclusion compounds with linear hydrocarbons. In particular, alkane molecules CH (if n>6) can be enclosed end to end in parallel channels (c.a. 5 Å diameter). At room temperature, disorder of the chains is revealed by diffuse planes on x-ray photographs. Lock-in transitions occur at low temperature: one if n<14 and two for greater length. Inclusion compounds with shorter chains, heptane and octane, were extensively analyzed. Besides other techniques, neutron experiments were carried out with selectively deuterated samples, in order to access the individual dynamics of the alkane or of the matrix. Advantage was taken of the characteristics of different spectrometers: IN6 and IN5 at Institut Laue-Langevin in Grenoble, MIBEMOL in Saclay, and IRIS at the Rutherford Appleton Laboratory. We propose a general model for the dynamics of this system. The motion of the alkyl chains combines kink defects responsible for jumps of the CH around the channel axis, together with a local fast motion of the H atoms, originating from the torsions of the chains and with a component parallel to the channel axis. Moreover, the reorientations of the alkyl chains are clearly related to an inversion of the cycles of the matrix molecules around the channels. Their displacements are evident. © 1992 American Institute of Physics.