As a final test of O NMR as a probe of carbon capture mechanisms in MOFs, we examined the capture mode of (ii-2)-Mg 2 (dobpdc) for the first time (ii-2 = N,Ndiisopropylethylenediamine). We initially assumed this material would form ammonium carbamate chains upon CO 2 adsorption, as in the related material (ee-2)-Mg 2 (dobpdc). Excitingly, the O MAS NMR spectrum acquired at 23.5 T (Figure ) instead reveals a clear carbamic acid resonance (δ iso = 125.8 ppm, C Q = 7.99 MHz, η Q = 0.43, see carbamic acid OH groups in Figure ). This peak was also seen in a spectrum on an independent sample at 20.0 T (Figure ). Furthermore, the integral of the carbamic acid peak relative to the rest of the peaks is 1:3.34, likely indicating the presence of four oxygen environments which would be expected in a mixed ammonium carbamate -carbamic acid adsorption structure. The NMR spectrum seen is different from that of dmpn-Mg 2 (dobpdc) (Figure ), hinting that this is a different mixed mechanism to that previously reported (Figure ). The left-hand overlapped feature is harder to deconvolute, likely consisting of three oxygen environments at similar shifts. Investigating this adsorption mechanism further, a C MAS NMR spectrum (Figure ) showed two chemisorbed CO 2 resonances at 163.8 and 160.1 ppm, assignable to ammonium carbamate and carbamic acid, respectively, and consistent with the observations from O NMR. The 13 C peaks had relative intensities of 1:0.9 (quantitative NMR) further supporting a mixed adsorption mechanism consisting of two different CO 2 environments. Support for the C peak assignments is provided by a 2D 1 H- C heteronuclear correlation experiment with a short contact time (Figure ), which reveals 1 H- C correlations for hydrogens nearby the carbons of the chemisorbed CO 2 . Most importantly, the C resonance at 160.1 ppm shows a strong correlation with a 1 H resonance at 10.3 ppm, assigned to a carbamic acid COOH group. Strong N-H correlations are observed for both resonances supporting reaction with CO 2 at the primary amine in both cases. The 163.8 ppm C resonance also shows a weak correlation with an ammonium group at 14.6 ppm, with the 1 H chemical shift of this species comparable to those observed previously for tertiary ammonium groups in ammonium carbamate chains for related materials. Finally, an adsorption structure was proposed to explain the above results (Figure ). The proposed structure features a mixture of ammonium carbamate chains and carbamic acid chains, with CO 2 insertion occurring between the metal-amine bond in both cases. The relative signal ratios from O and C NMR suggests that these two chain variants are present in similar proportions and hence have similar free energies. To check the proposed model, NMR parameters were obtained from DFT (Table ) and showed a good agreement for the OH peak corresponding to carbamic acid, however further work is needed to confidently assign the environments of the other resulting three peaks. Importantly, ii-2 is only the second diamine variant (after dmpn) that has been shown to form carbamic acid. In common with dmpn, ii-2 possesses bulky alkyl groups, suggesting that steric bulk is an important handle for tuning adsorption chemistry in amine-functionalised metal-organic frameworks. This may suggest that a range of diamines which contain bulky alkyl groups can access this new adsorption mechanism and it will be important to understand the kinetics of this mechanism and how it relates to the application of MOF carbon capture systems. O parameters of (ii-2)-Mg 2 (dobpdc). DFT calculations are for the mixed carbamate chain structure described in Figure . 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 . Overall, the work shown here demonstrates that O NMR spectroscopy is a powerful tool for understanding CO 2 adsorption mechanisms in MOFs. To highlight the versatility of this approach we further applied this technique to amine-grafted silica materials, another important adsorbent class for CO 2 separations. The CO 2 binding modes in amine-grafted silicas remain widely debated and an array of adsorption products have been proposed, including, ammonium carbamate, carbamic acid, ammonium bicarbonate, and surfacebonded silyl carbamates. Here, two amine-grafted silicas, propylamine-SBA15 (Pr-Si) and triamine-SBA15 (Tri-Si) (Table ) were synthesised and investigated by O MAS NMR spectroscopy at 23.4 T (Figure ). The O MAS NMR spectra show broad signals at 177.5 ppm and 28.7 ppm for Tri-Si, and 175.5 ppm and 33.3 ppm for Pr-Si, with no signal corresponding to physisorbed CO 2 observed in these samples. The left hand signals at ~175 ppm are consistent with ammonium carbamate oxygens, see Figure and additional DFT calculations for amine-grafted silicas in Table and Figure , suggesting that ammonium carbamate is the major adsorption product for the two materials.