The right hand resonances at ~ 30 ppm in the two spectra are harder to assign, however, it is clear that these signals arise from an interaction between the CO 2 and the amine-grafted silica material given the lack of any O resonances in the control experiment without CO 2 (Figure ). We considered the previously proposed carbamic acid, ammonium bicarbonate and silylpropylcarbamate species as candidate products (Figure , Table ), but DFT NMR calculations gave chemical shifts that differed significantly from the experimental values, suggesting that none of these species are present in any significant quantity (Table ). This new resonance is therefore suggestive either of a new adsorption mode in amine-grafted silica materials, or of isotopic enrichment of oxygen atoms in the silica backbone. Overall our experiments on amine-grafted silicas highlight the broad applicability of O NMR measurements as a probe of CO 2 capture modes, and we envisage that these techniques will lead to the discovery of new CO 2 chemistry in a wide range of materials.