Methyl-Group Rotational Tunneling Perturbations in Zeolitic Imidazolate Framework-8

Using neutron inelastic scattering and diffraction techniques, we have extended our studies of the quantum methyl-group rotations in the metal organic framework: zeolitic imidazolate framework-8 (ZIF-8: Zn(MeIM)$_{2}$, MeIM=2-methylimidazolate). The framework-bonded methyl groups are oriented toward the large cavities of the nanoporous ZIF-8 structure and thus exhibit very rapid one-dimensional rotations. Indeed, the rotational potential was previously shown$^{ }$[1] to be primarily 3-fold in character with a very low rotational barrier of $\approx $7~meV and a ground-state tunneling energy of 334~$\mu $eV at 1.4~K. In this talk, we discuss the observed changes to this potential upon various perturbations to the ZIF-8 system, including Co substitution for Zn; site-specific adsorption of H$_{2}$, D$_{2}$, and CD$_{4}$; and high-pressure ($\le $10 kbar) He infiltration. Depending on the perturbation, the tunneling energy was found to vary by more than an order of magnitude, with values ranging from 408~$\mu $eV to $<$30~$\mu $eV. [1] W. Zhou, H. Wu, T. J. Udovic, J. J. Rush, and T. Yildirim, J. Phys. Chem. A \textbf{112}, 12602 (2008).