The O chemical shielding reference value for these cluster calculations was taken from a linear fit (δ iso = σ iso -σ ref ) of a range of experimental ). Periodic plane-wave DFT calculations were performed in CASTEP 91 using the default on-the-fly pseudopotentials and Perdew-Burke-Ernzerhol (PBE) functional with a G06 van der Waals correction. The magnetic shielding tensors and electric field gradient tensors were computed in CASTEP, which uses density functional perturbation theory and the GIPAW method. The periodic MFU-4l structure was generated from the crystal structure of MFU-4l-Cl from Denysenko et al.. This unit cell was reduced to a rhombohedral unit cell (a = 21.961 Å, α = 60 • ) of 162 atoms. The chlorine atoms were replaced by -OH groups followed by geometry optimisation. Subsequently, a singular -OH group was replaced by varied -HCO 3 starting binding configurations from which the two mode-A and mode-B (Fig. ) structures where derived through further geometry optimisation, in agreement with previous literature cluster calculations. Geometry optimisation tolerances used were: force tolerance = 0.05 and 0.075 eV Å-1 , displacement tolerance = 0.002 and 0.003 Å for the -OH and -HCO 3 modes, respectively, and an energy tolerance = 2 × 10 -5 eV atom -1 was used for both.