Quasi-free methyl rotation in zeolitic imidazolate framework-8.

Using neutron inelastic scattering and diffraction, we have studied the quantum methyl rotation in zeolitic imidazolate framework-8 (ZIF-8: Zn(MeIM)(2), MeIM = 2-methylimidazolate). The rotational potential for the CH(3) groups in ZIF-8 is shown to be primarily 3-fold in character. The ground-state tunneling transitions at 1.4 K of 334 +/- 1 mueV for CH(3) groups in hydrogenated ZIF-8 (H-ZIF-8) and 33 +/- 1 mueV for CD(3) groups in deuterated ZIF-8 (D-ZIF-8) indicate that the barrier to internal rotation is small compared to almost all methylated compounds in the solid state and that methyl-methyl coupling is negligible. A 2.7 +/- 0.1 meV scattering peak assigned to the ground-state to first-excited-state, hindered rotational transition for H-ZIF-8, combined with a approximately 3 meV activation energy for methyl-group 3-fold jump reorientation estimated by quasi-elastic neutron scattering, suggests a very low methyl rotational barrier of approximately 7 meV. Results are compared to the CH(3) rotational amplitude at 3.5 K derived from neutron diffraction data, which are also consistent with a small 3-fold barrier and a very low energy rotational oscillation.