Dynamics of isobutane inside zeolite ZSM-5. A study with deuterium solid-state NMR

Perdeuterated isobutane-d(10), when it is adsorbed on zeolite ZSM-5, has been shown to exhibit a superposition Of two H-2 NMR Pake-powder patterns with quadrupole constants C-Q = 2.35 kHz and asymmetry parameters eta = 0.17 for the CD3 groups and C-Q = 23.0 kHz and eta = 0.20 for the CD group at 153-373 K. The narrowed Pake-powder patterns have been interpreted in terms of the motional behavior of isobutane molecules located at the channel intersections of the zeolite framework. The dynamic behavior of isobutane is complex and includes probably both fast intramolecular rotations and large amplitude hops of the molecule as a whole. For example, the motion of the CD group may be effectively described as consisting of the combination of the precession of the CD group around some axis Z, which makes up the angle gamma approximate to 47degrees with the direction of the C-D bond, and fast hops of the axis Z itself between four nonequally populated orientations, determined by the geometry of the zeolite channel segments. Simultaneously, fast rotation of the methyl groups occurs around two different C-3 axes. The activation energies and rotational correlation times at 373 K have been estimated to be 13.5 kJ/mol and 41 ps for the methyl groups and 17.7 kJ/mol and 1200 ps for the methene group, respectively.