The CO2 molecule adsorbs on the Cu site via a lone pair electron interaction with the distance between Oc and Cu of 1.98 Å (Figure 2c) and the Cu•••O–C angle of 147.7°. This interaction shortens the Cu–O1 distances to 1.94 Å, while the Cu–O2 distances are lengthened from 1.97 to 2.07 Å. The linear structure of CO2 is slightly distorted from its isolate molecule with the O–C–O angle of 178.4°. In addition, the C-Oc bond length is slightly lengthened from 1.17 to 1.18 Å, while the length of the C–Oc′ bond is shortened by 0.01 Å. The trends of these structural results are comparable to those observed in the previous theoretical studies of CO2 adsorption on alkali-metal exchanged zeolites,(56, 57) Mg-MOF-74,(58) and also on Cu-BTC.(59) The adsorption complex provides the stretching and bending vibration of CO2 at 2502.9 and 624.0 cm–1. There are slight blue shifts by 4.2 cm–1 for the stretching mode and the red -shift by 35.4 cm–1 for the bending mode with respect to corresponding vibration in the isolated CO2 molecule (2498.7 and 659.4 cm–1 for stretching and bending, respectively). These slightly changed vibrational frequencies are clearly the result of some distortion of the CO2 structure in the adsorption complex. Moreover, the electron transfer from CO2 to the Cu active site leads to the decrease in the positive charge on the Cu site from +0.771 to +0.720|e|, while the total charge of the CO2 molecule is more positive by +0.039|e| (cf. Table 1). However, the partial charge of Oc compared with the isolated molecule is more negative (from −0.514 to −0.599|e|) by reason of the electrons induced from C and Oc′ to Oc. The adsorption energy of CO2 on Cu-MOF-5 is calculated to be −13.5 kcal/mol. This is higher than the theoretical values of CO2 adsorbed on Cu-BTC (∼8 kcal/mol),(59, 60) which is probably due to the more available Cu active site on Cu-MOF-5. This result shows that the Cu-MOF-5 is one of the suitable materials for CO2adsorption.