The energy of mode at 107.9 cm -1 exhibits the strongest dependence on the occupied volume upon adsorption, reaching a maximum displacement of 27 cm -1 when 2 CO per f.u. are adsorbed. Interestingly, when 1 SO2 (point c, 23.8 cm 3 /mol) and 1.5 CO per f.u. (point d, 24.2 cm 3 /mol) are considered, the displacement of this mode differs considerably by 9.7 cm -1 even though the occupied volume is similar in the two cases. This indicates a dependence not only on the volume but also on the nature of the gas and the associated interactions established within the cavity. For the three modes under peak A (137.8, 138.5 and 139.2 cm -1 ) we find overall a smaller shift upon adsorption and the correlation between energy shift and occupied volume is weaker, in particular for the mode at 137.8 -1 (black data) which exhibits the weakest correlation with the gas volume.