Amine Structure O parameters for dmpn-Mg 2 (dobpdc). DFT calculations are for the mixed carbamic acid -ammonium carbamate adsorption structure. The experimental fit was obtained by simultaneously fitting data on two independent samples at two field strengths (20.0 and 23.5 T) using ssnake software , see Figure . We estimate that the experimental errors are ± 3 ppm, ± 0.5 MHz, and ± 0. ), suggesting that the DFT-proposed models are inaccurate. The models for (ee-2)-Mg 2 (dobpdc) and (i-2)-Mg 2 (dobpdc) (based on single crystal diffraction structures for the analogous zinc frameworks) show important differences in their hydrogen bonding arrangements, with a hydrogen bond formed from the ammonium to the "free" carbamate oxygen for the ee-2 variant (Figure ), and the metal bound carbamate oxygen for the i-2 variant (Figure ). In the DFT model for (e-2)-Mg 2 (dobpdc) additional hydrogen bonding interactions are also present. The findings that (i) the experimental O NMR parameters for (i-2)-Mg 2 (dobpdc) and (e-2)-Mg 2 (dobpdc) show poor agreement with the DFT values, and (ii) the spectra closely resemble that for (ee-2)-Mg 2 (dobpdc), challenge these alternative hydrogen bonding arrangements and suggests that all structures instead have hydrogen bonds solely between the ammonium proton and the "free" oxygen as in (ee-2)-Mg 2 (dobpdc) (Figure ). DFT parameters for an improved (e-2)-Mg 2 (dobpdc) model are shown in Figure and Table , with our findings further highlighting the excellent ability of O NMR experiments to differentiate subtly different CO 2 adsorption products.