The activated amine functionalised adsorbents were packed into 4 mm or 3.2 mm NMR rotors inside a nitrogen-filled glovebag. Each sample was then evacuated for a minimum of 10 min in a home-built gas manifold, as described previously. O-enriched CO 2 gas (20 at. % 17 O) was then used to dose the samples with gas at room temperature, before sealing the rotors inside the gas manifold with a mechanical plunger. (ee-2)-Mg 2 (dobpdc) was dosed for 0.5 h with a final gas pressure of 896 mbar. The (dmpn)-Mg 2 (dobpdc) sample for measurements at 20.0 T was dosed for 15 h with a final gas pressure of 1253 mbar, and the second independent (dmpn)-Mg 2 (dobpdc) sample for measurements at 23.5 T was dosed with 17 O-enriched CO 2 for 15 h with a final gas pressure of 1113 mbar. (e-2)-Mg 2 (dobpdc) was dosed for 1 h, and the final gas pressure was 448 mbar. (i-2)-Mg 2 (dobpdc) was dosed for 0.5 h, and the final gas pressure was 1116 mbar. The (ii-2)-Mg 2 (dobpdc) sample for measurements at 20.0 T was dosed for 0.75 h with a final gas pressure of 1015 mbar, and the second independent (ii-2)-Mg 2 (dobpdc) sample for measurements at 23.5 T was dosed with 17 O-enriched CO 2 for 0.5 h with a final gas pressure of 1039 mbar. For activated Mg 2 (dobpdc) (i.e,. with no amines), activation was first carried out by heating in flowing nitrogen gas at 180 ºC for 15 h. This sample was then packed in an NMR rotor (as above) and dosed with gas for 0.5 h, and the final gas pressure was 1075 mbar. For activate silica grafted amines, samples were dosed inside 3.2 mm NMR rotors for 0.5 h with final gas pressures for Pr-Si and Tri-Si of 1083 mBar and 993 mBar, respectively. with a 20.0 T wide-bore and 23.5 T standard bore magnets, corresponding to a 1 H Larmor frequencies, ν 0 , of 850 MHz and 1 GHz. For experiments at 20.0 T, a Bruker Avance III spectrometer was used, alongside a Bruker 4mm low-ɣ HX double resonance probe, and experiments were performed with an MAS frequency, ν R , of 14 kHz. O MAS spectra were acquired using a spin-echo pulse sequence with radiofrequency field strength, ν 1 , of ~50 kHz and a recycle delay of 0.05 s. NMR parameters were optimised experimentally and therefore the spin-echo experiments cannot be considered quantitative. MQMAS experiments were acquired using a z-filter pulse sequence with triple-quantum excitation/conversion pulses with ν 1 ≈ 50 kHz and a central-transition selective π/2 pulse at ν 1 ≈ 11 kHz. All MQMAS spectra are shown after shearing using the convention described in Ref. 35. For experiments at 23.5 T, a Bruker Avance NEO spectrometer equipped with a Bruker 3.2 mm HX double resonance probe was used with a MAS rate of 20 kHz. O MAS spectra were acquired using a spin-echo pulse sequence, with ν 1 = 25 kHz, and with a recycle delay of 0.05 s. Chemical shifts are given in ppm, and are referenced relative to liquid H 2 O at 0 ppm.