Exploring Structural Disorders in Aluminum-Containing Metal-Organic Frameworks: Comparison of Solid-State Al-27 NMR Powder Spectra to DFT Calculations on Bulk Periodic Structures
Abstract
We use solid-state Al-27 NMR spectroscopy to test four aluminum-based metal-organic framework (MOF) materials-ICR-2, ICR-4, ICR-6, and ICR-7-against their structural models obtained from electron and X-ray diffraction. The lineshape MOF ICR analysis of the Al-27 NMR spectra reveals varying degrees of disorders in the Al coordination environment, depending on the NICIF ICR sample, which we were able to parameterize in terms of the extended Czjzek model (ECM) of the electric field gradient (EFG) distribution.
The model's degree-of-disorder parameter epsilon was found to correlate inversely with the crystallite size of the MOFs. This suggests that the EFG distribution is related to a high surface-to-volume ratio of the particles and can be ascribed to the deviation of molecular arrangements near the surface from the bulk crystalline order.
The ECM's parameters for a fixed part of the EFG tensor, a quadrupolar coupling constant C-Q,C-0 and asymmetry parameter eta(0), were compared to the EFG parameters calculated on fully periodic models using density functional theory (DFT). The DFT-calculated C-Q was found to exceed C-Q,C-0 by 25-60%, depending on the MOF.
Although this result finds support in the literature, an explanation of such apparent spectral narrowing is still lacking.