Properties of mixed-metal
MOFs depend on the distribution of different
metals within their frameworks. Determination of this distribution
is often very challenging. Using an example of aluminum- and iron-containing
MIL-100, we demonstrate that
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Al NMR spectroscopy, when
combined with first-principles calculations and magnetic, X-band electron
paramagnetic resonance, Fe K-edge extended X-ray absorption fine structure,
and Mössbauer measurements, enables one to accurately determine
the arrangement of Al and Fe within the metal trimers, which are the
basic building units of MIL-100. In this particular material, the
incorporation of Fe and Al on the framework metal sites is random.
Crucial for deciphering the arrangement is detecting NMR signals,
shifted because of the strong hyperfine interaction between the
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Al nuclei and the unpaired electronic spins of Fe
3+
ions, assigning the shifted signals aided by first-principles calculations
of hyperfine couplings, and quantitatively evaluating the NMR intensities
and the measured effective magnetic moment.