Neither Solid Nor Liquid Nor Vapor: Hydrogen Bonding Structure of Water Confined in Metal-Organic Frameworks with Open Metal Sites

Water in confinement exhibits properties significantly different from bulk water due to frustration in the hydrogen-bond network induced by interactions with the substrate. Here, we combine infrared spectroscopy and advanced molecular dynamics simulations to probe the structure of confined water as a function of relative humidity within a metal-organic framework containing cylindrical pores lined with an ordered array of cobalt open coordination sites. Building upon the quantitative agreement between experimental and theoretical spectra, we demonstrate that water at low relative humidity initially binds to the open metal sites and subsequently forms disconnected one-dimensional chains of hydrogen-bonded water molecules bridging between the cobalt sites. Upon further increase in relative humidity, these water chains nucleate pore filling, with water molecules occupying the entire pore interior before the relative humidity reaches 30%. Systematic analysis of the rotational and translational dynamics indicates heterogeneity in this pore-confined water, with water molecules displaying distinct levels of mobility as a function of the distance from the pore surface.