The 17 O NMR spectrum of the CD-MOF dosed with C 17 O 2 (Fig. ), shows three peaks clearly resolved at δ obs = -12, 68 and 140-180 ppm. Physisorbed CO 2 and chemisorbed CO 2 are readily assignable at 68 and 140-180 ppm, respectively, in line with MFU-4l and with previous work. The peak at negative ppm bares similarity to the Zn-OH peak in MFU-4l and is in a similar region to minor peaks previously assigned as framework defects in amine-based MOFs. However, here we assign the peak as partially desolvated H 2 O within the MOF pore. This species will likely be in equilibrium with hydroxide, (bi)carbonate and CO 2 (see equ (2-3)) explaining both the O enrichment and the similarities (negative δ obs ) to the Zn-OH peak in MFU-4l.