Elucidating the Vibrational Fingerprint of the Flexible Metal–Organic Framework MIL-53(Al) Using a Combined Experimental/Computational Approach

In
this work, mid-infrared (mid-IR), far-IR, and Raman spectra
are presented for the distinct (meta)­stable phases of the flexible
metal–organic framework MIL-53­(Al). Static density functional
theory (DFT) simulations are performed, allowing for the identification
of all IR-active modes, which is unprecedented in the low-frequency
region. A unique vibrational fingerprint is revealed, resulting from
aluminum-oxide backbone stretching modes, which can be used to clearly
distinguish the IR spectra of the closed- and large-pore phases. Furthermore,
molecular dynamics simulations based on a DFT description of the potential
energy surface enable determination of the theoretical Raman spectrum
of the closed- and large-pore phases for the first time. An excellent
correspondence between theory and experiment is observed. Both the
low-frequency IR and Raman spectra show major differences in vibrational
modes between the closed- and large-pore phases, indicating changes
in lattice dynamics between the two structures. In addition, several
collective modes related to the breathing mechanism in MIL-53­(Al)
are identified. In particular, we rationalize the importance of the
trampoline-like motion of the linker for the phase transition.