We hypothesised that O NMR spectroscopy would be a powerful probe of CO 2 adsorption modes as the two oxygen atoms per CO 2 molecule can have significantly different local environments depending on the adsorption product formed. The investigated adsorption products have a total of four main types of oxygen environment (Figure ). Ammonium carbamate chains have two oxygen environments with one of these bound to a metal ion, carbamic acid pairs have two oxygen environments corresponding to the carbonyl and hydroxyl oxygens, and the mixed adsorption mode features all four of these oxygen environments. Importantly, as O is a spin 5/2 nucleus, the resulting NMR spectra will be affected not only by the familiar chemical shift, (δ iso ), but also by the quadrupolar interaction, i.e., the interaction of the nuclear quadrupole moment with the surrounding electric field gradient, which is defined by the parameters C Q and η Q . C Q is the quadrupolar broadening defined as C Q = eQV zz /h and gives the magnitude of the interaction whilst η Q measures the asymmetry of the interaction as η Q = (V yy -V xx /V zz ), where V xx , V yy and V zz are the principal components of the electric field gradient tensor, e is the electronic charge, Q is the nuclear quadrupole moment, and h is the Planck constant. By performing a O MAS NMR experiment, δ iso , C Q and η Q values can be obtained, therefore providing more information than The calculated O NMR parameters (Figure , Table ) show that, broad clusters of data points are found for the various adsorption environments. Excitingly, the OH oxygens in carbamic acid species are differentiated from the other oxygen environments by a lower O chemical shift, a higher C Q and a lower η Q (marked with a yellow box). This differentiation is unambiguous compared to that found by C NMR (Figure ). For the non-protonated oxygen present in carbamic acids (green), two distinct groupings are seen, corresponding to oxygen in the mixed and carbamic acid pair environments, with the former having a higher shift and C Q but a lower η Q . Interestingly, the data also show that metal-bound carbamate oxygens (blue) are differentiated from free carbamate oxygens (red) by their generally lower chemical shifts. We note that some outliers are seen amongst the carbamate oxygens (blue and red), which correspond to ammonium carbamate chain structures with different hydrogen bonding arrangements. One of these is for (R,R)-dach-Mg 2 (dobpdc) (dach = trans-1,2diaminocyclohexane) where additional hydrogen bonding is seen between pairs of carbamate chains and another outlier is for (i-2)-Mg 2 (dobpdc) (i-2 = n-isopropylethylenediamine) where the proposed structure has a hydrogen bond between an amine and the oxygen next to the metal. Overall the DFT calculations show that O NMR should provide good differentiation between adsorption products, especially for carbamic acid species.