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

Abstract

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 anion⋯CO₂ 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 Å, 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 Å. 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 anion⋯CO₂ interaction gets weaker with the increasing size of the CO₂ cluster, a steady increase in the secondary O⋯C 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 anion⋯C and secondary O⋯C 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 O⋯C interactions which compensate for the weakening of the primary anion⋯C interactions. This property of the anion–CO₂ interactions can be exploited for developing anionic or anion-incorporated materials for CO₂ storage.