Characteristic Features of CO2 and CO Adsorptions to Paddle-Wheel type Porous Coordination Polymer

Adsorptions of CO, N-2, NO, and CO2 in a paddle-wheel type porous coordination polymer (PCP) [Cu(aip)] (aip = 5-azidoisophthalate) were investigated with ONIOM[MP4(SDQ):omega B97XD] method using a model system consisting of two [Cu-2(O2CC6H4-R)(4)] units (R = H and Me) and one [Cu-2(O2CC6H4R)(4)] unit, namely, dimer and monomer models. The experimental CO adsorption position was reproduced well by the present calculation with the dimer model. For adsorptions of CO, N-2, NO, and CO2 in the dimer model, the position of gas molecule deviates from the normal one that is found in the monomer model and becomes more distant from the surrounding phenyl group(s) of the neighbor [Cu(aip)] unit. For all of these gas molecules, the calculated binding energy (BE) at the deviating adsorption position is larger than that at the normal one against our expectation that the normal position is the best for the gas adsorption. The deviation of gas adsorption position arises from the interaction between the organic linker (O2CC6H4-R moiety) and gas molecule. For all cases, the exchange repulsion with the organic linker decreases to a larger extent than the attractive electrostatic and dispersion interactions decrease when going from the normal position to the deviating one. To enhance the binding energy of gas molecule, the introduction of electron-donating substituent on phenyl moiety is computationally recommended for this PCP.