Signature at 100 cm -1 . Upon CO and CO2 adsorption, the peak at 96 cm -1 blue-shifts giving rise to a broader and less intense peak. The gas adsorption results in a larger shift for CO (37 cm -1 ) than CO2 (20 cm -1 ) and the previously studied SO2 molecule (29 cm -1 , see ref. 48). This is well reproduced by the computed S(ω) (see Fig 6). In this region, the calculations predict two peaks centered at ca. 107 cm -1 and 138 cm -1 for the bare MOF. The first one corresponds to a libration of the pyrazine ligand around the z-axis, while the second, named A in Figure , contains three vibrational modes: (i) a collective mode involving in-plane and out-ofplane movements of the CN groups and a libration of the pyrazine around the z-axis at 137.8 cm -1 , (ii) a rigid out-ofplane twisting of the Fe(CN)4N2 octahedra at 138.5 cm -1 and (iii) an out-of-plane vibration of the cyanide together with a small libration of the pyrazine at 139.2 cm -1 . The first peak at 107.9 cm -1 blue-shifts to 134.3 cm -1 (under peak A ′ ) upon CO adsorption and to 116.6 cm -1 (under peak A ′′ ) upon CO2 adsorption. In the new mode, the initial pyrazine libration is coupled with a libration of the CO and with a small translation of the CO2, respectively. The shift is substantially larger for CO. We attribute the blue-shift to the hindered rotation of the pyrazines. As shown in more detail in the next section, this mode is the most sensitive to both the amount of adsorbed gas and the type of molecule. The band A ′ that arises upon CO adsorption includes two additional modes at 131.0 and 135.1 cm -1 . The first one results from a pure libration of the CO. The second one is the result of the red-shift of the mode located at 137.8 cm -1 in peak A, i.e. mode (i) described above. A similar situation is found for CO2: the mode at 137.8 cm -1 under peak A red-shifts to 130.9 cm -1 , under peak A ′′′ . Because the shift of the mode at 107 cm -1 is significantly smaller for CO2 (9.6 cm -1 ) than for CO (27.4 cm -1 ), two separate peaks (A ′′ and A ′′′ ) are observed for CO2 and one for CO (peak A ′ ). The peak A ′′ includes as well two additional translational modes of CO2 at 122.5 and 114.4 cm -1 . The other two vibrational modes under peak A, i.e. (ii) 138.5 and (iii) 139.2 cm -1 , undergo a blueshift to 146.6 (142.3) and 151.5 cm -1 (141.3), upon CO adsorption (CO2), respectively, resulting in peak B ′ (B ′′ ). For these modes, a larger blueshift is found for CO (8 and 12.3 cm -1 ) than CO2 (3.8 and 2.1 cm -1 ). This blue-shift can be attributed to the increase in the cyanide out-of-plane bending force constant due to steric hindrance. For the bare MOF, the two vibrational modes under peak B, at 147.9 and 152.2 cm -1 , correspond to an in-plane movement of the Pt atoms together with a rigid back-and-forth movement of the pyrazines and Fe atom. Upon CO adsorption, we observe a small blue-shift to 148.1 and 154.4 cm -1 , respectively, (peaks B ′ and C ′ ). In the case of CO2, the first red-shifts by 0.8 cm -1 (peak B ′′ ) and the second blue-shifts by 0.7 cm -1 (beginning of peak C ′′ ). Finally, under peak C, two vibrational modes are found at 159.7 and 161.2 cm -1 for the bare MOF. These are similar to the modes of peak B but in this case the rigid displacement of the pyrazine together with the Fe atoms occurs in-plane. A shift to lower energies is found upon CO (CO2) adsorption from 159.7 to 156.7 (156.5) cm -1 and from 161.2 to 157.5 (159.6) cm -1 under peak C ′ (C ′′ ). In the case of CO adsorption, a new vibrational mode located at 160.9 is found under peak C ′ . This corresponds to a collective mode consisting of a libra-tion of the pyrazines and the CO and an in-plane movement of the Pt.