A first principles study of CO₂ adsorption is presented for a group of metal–organic frameworks (MOFs) known as CPO-27-M, where M = Mg, Mn, Fe, Co, Ni, Cu, and Zn. These materials consist of one-dimensional channels with a high concentration of open metal sites and have been identified as among the most promising MOFs for CO₂ capture. In addition, extensive, high-pressure, experimental adsorption results are reported for CO₂, CH₄, and N₂ at temperatures ranging from 278 K to 473 K. Isosteric heats of adsorption were calculated from the variable-temperature isotherms. The binding energies of CO₂ calculated using an MP2-based QM/MM method are in good agreement with those obtained from experiments. The relative CO₂ binding strengths for the different transition metals can be explained by the relative strength of electrostatic interactions caused by the effective charge of the metal atom in the direction of the open metal site induced by incomplete screening of 3d electrons. The Mn, Fe, Co, Ni, and Cu versions of CPO-27 are predicted to be anti-ferromagnetic in their ground states. Selectivities for CO₂ over CH4 or N₂ were calculated from the experimental isotherms using ideal adsorbed solution theory.