Probing Acidity in Metal–Organic Frameworks via Pyridine Adsorption

Semilocal density functional theory (DFT) and semiemprical GFN1/GFN2-xTB, as well as selected DFTB/3ob calculations, were performed to examine acidity in metal–organic frameworks (MOFs) using pyridine (Py) as a probe molecule. We studied Py adsorption on both Lewis and Bro̷nsted acid sites in ten distinct MOF structures. Our study provides detailed insight into their structures, thermodynamic properties, and vibrational spectra in selected ranges. All geometrical and energetic parameters obtained at the GFN1/GFN2-xTB levels were compared with those calculated using DFT. The results suggest that GFN1-xTB can only be used as a low-cost substitute for DFT for some systems. The adsorption energy prediction usually leads to unsatisfactory findings. GFN2-xTB and DFTB/3ob perform better than GFN1-xTB. Importantly, we identified areas where the GFN1/GFN2-xTB methods could be improved to yield more accurate results. By analyzing the density overlap region indicator (DORI) versus the adsorption energies, we revealed the nature and type of interactions in Py@MOFs.