With the expected theoretical parameters established, the application of O NMR spectroscopy to hydroxide-based CO 2 capture materials was initially assessed using the well-known MFU-4l (Zn 5 (OH) 4 (btdd) 3 ) MOF, Fig. . MFU-4l has well defined, isolated monodentate Zn-hydroxide sites situated within cubic pores (Fig. ). These hydroxide sites are thought to react in a 1 : 1 ratio with CO 2 to form individual bicarbonate type structures via an insertion mechanism. The 17 O NMR spectrum of a C 17 O 2 dosed sample of the MOF showed 4 clear resonances (Fig. ).