Formation of large clusters of CO2 around anions: DFT study reveals cooperative CO2 adsorption

The structure and energetics of the interaction of CO2 molecules with anions F-, Cl-, Br-, CN-, NC-, OH-, ClO-, NH2-, and NO2-, have been studied at the M06L/6-311++G** level of density functional theory. The maximum number of CO2 molecules (n(max)) adsorbed by the anions to saturate the first shell of coordination varies from 8 to 12 in different complexes. The anionMIDLINE HORIZONTAL ELLIPSISCO2 distance (d(int)) in F-(CO2), NC-(CO2), ClO-(CO2), HO-(CO2) and H2N-(CO2) is 1.533, 1.527, 1.468, 1.456, and 1.470 angstrom, respectively, which indicates covalent bond formation between carbon and the anion, which is confirmed from the interaction energy (E-int) values of these complexes 29.0, 14.7, 23.2, 41.7, and 48.1 kcal mol(-1), respectively. The Cl-, Br-, CN- and NO2- interact always non-covalently with the carbon center of CO2 with d(int) in the range of 2.5-2.9 angstrom. With the adsorption of each CO2, an average increment of 5.9-6.7 kcal mol(-1) is observed in the E-int value of the complexes. The E-int per CO2 (E-int/CO2) is nearly a constant for all the non-covalent complexes, even up to n(max) number of CO2 adsorbed. Though the primary anionMIDLINE HORIZONTAL ELLIPSISCO2 interaction gets weaker with the increasing size of the CO2 cluster, a steady increase in the secondary OMIDLINE HORIZONTAL ELLIPSISC interaction between adsorbed CO2 molecules helps the systems to maintain a constant value for E-int/CO2. The electron density data of non-covalent bond critical points in quantum theory of atoms in molecules (QTAIM) analysis are used to partition the total interaction energy data into primary anionMIDLINE HORIZONTAL ELLIPSISC and secondary OMIDLINE HORIZONTAL ELLIPSISC interactions. Furthermore, the multicenter charge delocalization in the anionic complexes is explained using the molecular electrostatic potential (MESP) analysis. This study proves that the anions possess a remarkable ability to interact with a large number of CO2 molecules due to cooperativity resulting from the secondary OMIDLINE HORIZONTAL ELLIPSISC interactions which compensate for the weakening of the primary anionMIDLINE HORIZONTAL ELLIPSISC interactions. This property of the anion-CO2 interactions can be exploited for developing anionic or anion-incorporated materials for CO2 storage.