Vibrational Modes and Terahertz Phenomena of the Large-Cage Zeolitic Imidazolate Framework-71

The zeolitic imidazole framework
ZIF-71 has the potential to outperform
other well-studied metal–organic frameworks due to its intrinsic
hydrophobicity and relatively large pore size. However, a detailed
description of its complex physical phenomena and structural dynamics
has been lacking thus far. Herein, we report the complete assignment
of the vibrational modes of ZIF-71 using high-resolution inelastic
neutron scattering measurements and synchrotron radiation infrared
spectroscopy, corroborated by density functional theory (DFT) calculations.
With its 816 atoms per unit cell, ZIF-71 is the largest system yet
for which frequency calculations have been accomplished employing
the CRYSTAL17 DFT code. We discover low-energy terahertz dynamics
such as gate-opening and shearing modes that are central to the functions
and stability of the ZIF-71 framework structure. Nanoscale analytical
methods based on atomic force microscopy (near-field infrared spectroscopy
and AFM nanoindentation) further unravel the local chemical and mechanical
properties of ZIF-71 single crystals.