Quantum Mechanical Interpretation of the Ultra-Low Energy Methyl-Rotation Dynamics in Porous Metal-Organic Frameworks Probed by Low-Frequency Vibrational Spectroscopy and ab initio Simulations

Understanding the nature of the interatomic interactions present within the pores of metal-organic frameworks

is critical in order to design and utilize advanced materials

with desirable applications. In ZIF-8 and its cobalt analogue

ZIF-67, the imidazolate methyl-groups, which point directly

into the void space, have been shown to freely rotate - even

down to cryogenic temperatures. Using a combination of ex-

perimental terahertz time-domain spectroscopy, low-frequency

Raman spectroscopy, and state-of-the-art ab initio simulations,

the methyl-rotor dynamics in ZIF-8 and ZIF-67 are fully charac-

terized within the context of a quantum-mechanical hindered-

rotor model. The results lend insight into the fundamental

origins of the experimentally observed methyl-rotor dynamics,

and provide valuable insight into the nature of the weak inter-

actions present within this important class of materials.

Abstract