Water Adsorption on Coordinatively Unsaturated Sites in CuBTC MOF

We report a theoretical study of water adsorption on coordinatively unsaturated sites (cus's) in a metal−organic framework (MOF) compound CuBTC. The reliability of the density functional theory (DFT)-based methods and dispersion-corrected DFT-D schemes for the description of cus sites was investigated with respect to the accurate reference CCSD(T)/CBS data. The accuracy of both DFT and DFT-D methods was found to be insufficient. The proposed DFT/CC correction scheme gave the results in excellent agreement with the reference CCSD(T)/CBS data. DFT/CC calculations performed for the periodic CuBTC model gave RCu−OH2 = 2.19 Å and −ΔHads= 49 kJ mol−1, both in very good agreement with available experimental data. The interaction of the first water molecule with the paddle-wheel unit is about 5 kJ mol−1 stronger than the interaction of the second water molecule with the same paddle-wheel unit. The DFT/CC scheme provides an accurate description of the extended MOF systems, and the results obtained with periodic DFT/CC model can be used for the testing and improvement of the force fields for classical simulations.