Hydrogen bonding structure of confined water templated by a metal-organic framework with open metal sites*

While the properties of bulk water are difficult to understand, water in confinement is even more poorly understood. Metal-organic frameworks (MOFs) are unique a class of materials that display high adsorption properties. Recently, the MOF Co₂Cl₂BTDD has been shown to adsorb water at low relative humidity, attracting interest for applications in harvesting water from air. Here, we investigate structural and dynamical properties of water adsorbed in Co₂Cl₂BTDD as a function of relative humidity using many-body molecular dynamics simulations with the MB-pol model. Comparisons of the experimental and theoretical infrared spectra allow us to elucidate the mechanism of pore filling, with water initially binding to the open metal sites and forming one-dimensional chains along the interior of the framework. These chains nucleate pore filling, establishing a three-dimensional hydrogen-bond network. As the pore fills, the spectroscopic features and orientational dynamics indicate a gradual transition from “ice-like” to “liquid-like” properties modulated by the heterogenous confinement, with individual water molecules exhibiting distinct behavior depending on their specific location inside the pores. Nat. Commun. 2019, 10, 1-7.