Theoretical perspective on the interaction of CO2 and H2O molecules with functionalized magnesium and scandium phthalocyanines

Abstract

This paper investigates about the theoretical study on center and edge site interaction of carbon dioxide and water molecules with amine (NH2)-, methyl (CH3)-, hydroxyl (OH)-, aldehyde (CHO)- and carboxyl (COOH)- functionalized Magnesium (Mg) and Scandium (Sc) phthalocyanines. The study shows that the gas molecules are physisorbed over the Pcs through electrostatic interactions, while weak interactions occur at the edges. The perpendicular orientation of CO2 shows stronger interaction while the adsorption is greater for H2O having out-of-plane orientation. The binding of CO2 molecule stems via a simultaneous charge transfer from the metal center to the oxygen atom of the CO2 molecule in addition to a donation from the N–C bond of the Pc to the Lewis acidic carbon atom of the CO2 molecule. The peak splitting in bending frequency of the adsorbed CO2 molecule is maximum, and the asymmetric stretching frequency is blueshifted in the case of (CHO)16 Sc-Pc and (COOH)16 Sc-Pc in which the interaction energy is higher. The interaction of H2O molecule increases the stability of phthalocyanines thereby imparting practicality in using them under humid conditions. The interaction of Pcs with water is more feasible and spontaneous, whereas Sc metal induces feasibility to adsorb CO2 as well. In the case of functionalized MgPcs with CO2, the interactions are possible below the room temperature. The adsorption capacity shows that a lesser number of gas molecules adsorb at the metal center while more number of weakly bound gas molecules adsorb at the edges. Overall, the study concludes that scandium substitution at center and functionalization of acceptor molecules such as CHO and COOH at the edges in M-Pcs shows greater ability toward gas adsorption.