Dynamic properties of cyclohexane guest molecules constrained within the zeolite H-ZSM-5 host structure: A wide-line solid state H-2 NMR investigation

Dynamic properties of perdeuterated cyclohexane (C6D12) molecules included within the zeolite H-ZSM-5 host structure have been studied by variable-temperature wide-line solid-state H-2 NMR spectroscopy. Three types of rapid motion have been identified for the C6D12 molecules in the temperature range 93-233 K as follows: motion A, rapid reorientation about the C-3 symmetry axis of the C6D12 molecule; motion B, rapid restricted wobbling of the C-3 symmetry axis, described as precession of the C-3 symmetry axis on the surface of a cone (the half angle of which increases with increasing temperature); motion C, a rapid four-site jump motion, the geometry of which is dictated by the orientations of the tunnel axes at the intersection between the zigzag and straight tunnels of the H-ZSM-5 host structure. The temperature dependence of the quadrupole echo H-2 NMR line shape in the temperature range 263-368 K is interpreted in terms of cyclohexane ring inversion occurring in the intermediate motion regime, in addition to motions A-C, which are in the rapid motion regime. From the H-2 NMR line shape analysis, the activation energy for the cyclohexane ring inversion process is estimated to be ca. 48 kJ mol(-1). The dynamic properties are compared with those of cyclohexane in other solid host structures, and the constraints imposed by the H-ZSM-5 host structure on the dynamic properties of the cyclohexane guest molecules are assessed.