The final stage of fitting involved confirming the established fit with multi-field NMR data at 20.0 T. The fit parameters from Fig. were then varied, from an initial fixed position, sequentially and cumulatively in integral, δ iso , C Q and η Q . This gave the two final fits as used in Fig. and Fig. . , with a multi-field fit as performed in ssNake. The peak ratios, from left to right, are 1.2 : 1 : 0.7 and 0.6 : 1 : 1.3, for 23.5 T and 20.0 T, respectively (see Figs. S37-S38 for quantitative regime establishment). Although the ratios change across the field strengths, the overall ratio between the assigned carbonate oxygens (purple) and bicarbonate oxygens (blue + green) remains constant across the two field strengths (≈ 1 : 1.8-1.9). This is assignable to a smaller absolute frequency difference between the bicarbonate carbonyl (blue) and the 'dynamic hydroxyl' (green) at 20.0 T that are hypothesised to be involved in slow dynamic exchange, thus affecting peak intensities. Table : Explicit tabulation of which KHCO 3 -CD-MOF samples were used in each of the NMR spectra present in this work.