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 R Cu−OH2 = 2.19 Å and −ΔH ads= 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.

A novel method, designated as the density functional theory/coupled-cluster with single and double and perturbative triple excitation [DFT/CCSD(T)] correction scheme, was developed for precise calculations of weakly interacting sp(2) hydrocarbon molecules and applied to the benzene dimer. The DFT/CCSD(T) interaction energies are in excellent agreement with the estimated CCSD(T)/complete basis set interaction energies. The tilted T-shaped structure having C(s) symmetry was determined to be a global minimum on the benzene-dimer potential energy surface (PES), approximately 0.1 kcal/mol more stable than the parallel-displaced structure. A fully optimized set of ten stationary points on the benzene-dimer PES is proposed for the evaluation of the reliability of methods for the description of weakly interacting systems.

The interaction of the water molecule with benzene, polycyclic aromatic hydrocarbons, graphene, and graphite is investigated at the density-functional/coupled-cluster (DFT/CC) level of theory. The accuracy of the DFT/ CC method is first demonstrated by a comparison of the various interaction energies on the potential energy surface of water-benzene, water-naphthalene, and water-anthracene complexes with the data calculated at the coupled-cluster level at the basis set limit. The potential energy surface of water-graphene and water-graphite is relatively flat with diffusion barriers of about 1 kJ/mol. The structure with both hydrogen atoms of water pointing toward the graphene plane (denoted as a circumflex structure) above the center of the six-member ring is the global minimum characterized with an electronic interaction energy of -13 and -15 kJ/mol for graphene and graphite, respectively. The OH · · · π complexes (with one OH pointing toward the surface and the other OH being oriented along the surface) are up to 2 kJ/mol less stable than the circumflex complexes of water on graphene/graphite, depending on the position of the oxygen atom.

A scaling method based on the linear correlation between the CO bond length and the CO stretching frequency has been applied to the CO molecule adsorbed on the Cu-exchanged MFI zeolite. Effects of anharmonicity, cluster size, unit cell size, and the Madelung potential were investigated. Interaction of CO with zeolite framework was described at the combined RI−BLYP/IPF level. The inner part of the combined model (RI−BLYP description) consisted of up to 23 TO4 tetrahedra. The effect of the Madelung potential on CO stretching frequencies was negligible. All Cu+ sites on the channel intersection and on the wall of the main channel are characterized by the CO stretching frequencies in the narrow range of 2159−2164 cm-1 in excellent agreement with experimental data. The Cu+ sites on the wall of the zigzag channel show slightly higher frequencies (3−6 cm-1). These sites are populated, however, only when the framework Al atoms are at T4 or T10 positions. The proposed computational scheme provides essentially the same level of accuracy as obtained from CCSD(T) calculations for small copper carbonyl species with overall error smaller than 5 cm-1.