Detecting Molecular Rotational Dynamics Complementing the Low-Frequency Terahertz Vibrations in a Zirconium-Based Metal-Organic Framework

TL;DR

This study combines experimental and computational methods to investigate low-frequency terahertz vibrations and rotational dynamics in a zirconium-based MOF, revealing soft modes and shear dynamics that influence its mechanical properties.

Contribution

It provides the first detailed experimental and theoretical analysis of THz rotational and shear dynamics in a Zr-based MOF, linking these motions to mechanical anomalies.

Findings

Identified soft trampoline-like vibrational modes.

Observed coordinated shear dynamics destabilizing the crystal.

Linked low-energy vibrations to anomalous mechanical behavior.

Abstract

We show clear experimental evidence of co-operative terahertz (THz) dynamics observed below 3 THz (~100 cm-1), for a low-symmetry Zr-based metal-organic framework (MOF) structure, termed MIL-140A [ZrO(O2C-C6H4-CO2)]. Utilizing a combination of high-resolution inelastic neutron scattering and synchrotron radiation far-infrared spectroscopy, we measured low-energy vibrations originating from the hindered rotations of organic linkers, whose energy barriers and detailed dynamics have been elucidated via ab initio density functional theory (DFT) calculations. For completeness, we obtained Raman spectra and characterized the alterations to the complex pore architecture caused by the THz rotations. We discovered an array of soft modes with trampoline-like motions, which could potentially be the source of anomalous mechanical phenomena, such as negative linear compressibility and negative thermal expansion. Our results also demonstrate coordinated shear dynamics (~2.5 THz), a mechanism which we have shown to destabilize MOF crystals, in the exact crystallographic direction of the minimum shear modulus (Gmin).