It is noted that for this final assignment, although some Zn-OH sites remain apparently unsaturated at 0.8 bar CO 2 , the appearance of signal (given the low natural abundance of 17 O) must be a result of reaction and exchange with the 17 O isotope labels in the dosed CO 2 . This indicates that there is enrichment through dynamic bond forming and breaking at some Zn-OH sites that do not strongly bind CO 2 . This supports previous observations that binding of sequential equivalents of CO 2 leads to weaker binding energies. Although the chemical shift of the carbonyl peak agrees well with the cluster DFT calculations in Fig. , comparison to the calculated MFU-4l values is poor. The asymmetric Zn-oxygen bonds (Fig. ) predicted by DFT, and resulting asymmetric polarisation, leads to two calculated C=O peaks separated by 90-120 ppm (Fig. ), with Zn-binding leading to lower chemical shifts.