Compared with conventional porous materials, metal organic frameworks (MOFs) have a greater diversity of framework properties which can give rise to interesting structural responses and dynamic behaviors toward external stimuli. The structural responses of these “flexible” MOFs can significantly improve performances in a variety of applications including gas storage, sensing, and separations. However, in order to turn potential into reality, it is necessary to understand the physical structure of MOFs at a fundamental level and how this determines properties on a macroscopic scale. The structural changes that certain MOFs can undergo are thought to be related to collective vibrations of the crystal lattice. Spectroscopic investigation of the lattice modes of MOFs can help to provide insight into their framework properties and other physical phenomena that MOFs can exhibit. The MIL-53 family of MOFs has been found to display breathing behavior of its framework upon application of an external stimuli. A combination of experimental and theoretical techniques were used to investigate the lattice modes of four MIL-53 MOFs that were synthesized by varying the metal node (Al3+, Ga3+, Cr3+, and Fe3+) . Lattice modes present &lt;300 cm-1 were measured using low frequency Raman and far-IR spectroscopy and were then correlated to vibrational analysis from ab inito density functional theory calculations using CRYSTAL17. A number of low energy lattice vibrations pertaining to the breathing phenomenon are observed and assigned.

Probing the connection between low-frequency vibrational modes and macroscopic structural behavior of metal organic frameworks

2019 Spring National Meeting

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