Molecular dynamics in Nanoporous Crystals

In thiourea organic intergrowth crystals (also called inclusion compounds), thiourea molecules, by means of an extensively hydrogen-bonded arrangement, form a host sub-structure constiotuted of infinite and parallel channels, within which suitable guest molecules can be included (see figure below). The aim of our research on these materials is to elucidate the phase transition mechanisms involving the guest molecules dynamics in these composite crystals.

View of a thiourea channel (0.7nm diameter) filled with cyclohexane.

The symmetry properties of the various thermotropic phases are identified and analysed by means of calorimetric measurements, X-ray diffraction and Raman scattering. The Rietveld refinement of the crystalline structure enable molecular dynamics to be performed. These theoretical results are then used as a guide for the interpretation of single-crystal deuterium NMR and incoherent quasi-elastic neutron scattering experiments. This multi-technique approach combining experiments (Raman scattering, X-ray or neutron diffraction, neutron scattering, NMR ...) and theoretical calculations (molecular dynamics) allows to propose ordering mechanisms, to identify the order parameters driving the phase transitions and, on the basis of group theory and Landau theory considerations, to develop phenomenological models reproducing the transitional sequences.

Examples of results obtained on cyclohexane / thiourea inclusion compound can be found in the references [ref 1,2,3,33]. The case of a polar guest molecules has been treated by the study of the chlorocyclohexane / thiourea inclusion compound [ref 8,12].