Improved resolution and simplification of the spin-diffusion-based NMR method for the structural analysis of mixed-linker MOFs.

Nuclear magnetic resonance spectroscopy combined with modeling represents a powerful tool for the structural analysis of heterogeneous materials. In this contribution we describe an upgraded method, particularly suited for the structural analysis of mixed-linker metal-organic framework materials, which is based on the measurement and modeling of proton spin diffusion among constituents. We tested the method on a UiO-66-type metal-organic material, in which the organic building units were 1,4-benzenedicarboxylate and trans-1,4-cyclohexanedicarboxylate anions distributed within the framework in an unknown manner. We showed that resolution of the signals of different building units could be significantly enhanced by the carbon-detected version of the proton spin-diffusion measurement. Because this kind of measurement is much more time consuming than the proton-detected measurement and because one has to carry out several two-dimensional measurements to extract spin-diffusion curves, we inspected the possibility of reducing the number of such measurements. This could be done by limiting the analysis to short mixing times, for which, as shown in this contribution, linear approximation is valid. When working in the linear regime, only a few experimental points are needed to determine the slope of spin-diffusion curves. Usage of short spin-diffusion mixing times significantly shortened the total measurement time and also markedly simplified the modeling of spin-diffusion curves.