Interaction of small gases with the unsaturated metal centers of the HKUST-1 metal organic framework

The interactions of CO, CO₂, OCS, SO₂, NO, NO ₂, N₂O, NH₃, PH₃, and other small molecules with the undercoordinated metal centers of the HKUST-1 metal organic framework are studied by means of density functional theory. These molecules are potentially harmful for humans and the environment and are widely studied because of their spectroscopic properties. In this work, the energetic and vibrational characteristics of the adsorbed species are calculated. Adsorption energies on the Cu²⁺ sites of the paddlewheel have been calculated, and the order is: NH₃ > H₂O > PH₃ > H₂S > SO₂ > CO ∼ OCS ∼ CO₂ ∼ N_yO_x > N₂ > O₂. The results show that the interactions can be classified into three categories: (1) weak physisorption, (2) polarization and electrostatics, and (3) strong acid-base. Moreover, interesting vibrational properties are calculated especially for carbonyl sulfide and dinitrogen monoxide, which can be bound via two different configurations on the metal atoms. The vibrational modes are shifting in different directions depending on the binding way of the molecule; e.g., the symmetric stretching of OCS is shifted by +17 or -16 cm^-1 when bound via the oxygen or the sulfur atom, respectively.