The behavior of water molecules in the nanocavities of the HKUST-1 framework: A combined quantum mechanical and vibrational spectroscopy investigation

We have investigated the host/guest molecular interactions and the H-bonding clustering in the H2O/HKUST-1 system. In-situ FTIR spectroscopy was the experimental technique used to gather information at molecular level. Measurements collected under sorption equilibrium conditions and in a dynamic regime were interpreted with the aid of a detailed QM vibrational analysis of the host/guest adduct and of a series of H2O clusters. The local arrangement of the adduct formed by H2O and the coordinatively unsaturated sites was characterized in terms of structure and interaction energy, and signatures for this adduct were predicted by theory in the ν(OH) range and experimentally confirmed. Within the MOF nanocavities six-membered water clusters having specific conformations were found to be the prevailing species. Cyclic trimers were also identified, their stabilization being ascribed to nanoconfining effects. The analysis of the δ(HOH) range elucidated the nature of the complex bandshape and confirmed the results obtained in the ν(OH) interval. Low-temperature measurements allowed us to exclude the crystallization of the water clusters in the nanocavities.