Anionic behavior and single-molecule crystal in fullerene confinements: A contribution from DFT energy decomposition and cooperativity analyses

The recently proposed systems of various anions (A) confined inside C₆₀ , A^? @ C₆₀ , which in turn behave as large and stable anions, (A @ C₆₀)^? , can find potential applications in various fields. On the other hand, it has earlier been shown that from the dihalogens (X₂ ) encapsulated C₆₀ , X₂ @ C₆₀ , only F₂ @ C₆₀ can be introduced as a system in which the cage acts as a cation C₆₀⁺ and interacts with an endohedral anion, F₂^? , forming the F₂^? @ C₆₀⁺ as a single-molecule crystal compound. In this work, two density functional theory energy decomposition analysis (EDA) schemes, where in one of them the noninteracting kinetic, electrostatic, and exchange-correlation energies come into play while another scheme, called as EDA-SBL, includes the steric, electrostatic, and quantum effects as essential ingredients (S. Liu, J. Chem. Phys. 2007 , 126, 244103), are utilized to find out what energetic components govern the unique characteristics of the (A @ C₆₀)^? and X₂ @ C₆₀ confinements. It is shown that the noninteracting kinetic energy and steric energies have important contributions to the total interaction energies for the considered systems. However, there are other confinements for which the electrostatic and exchange-correlation contributions play also imperative roles. Furthermore, we find reasonable correlations between interaction energies and their components as well as the energetic components themselves, leading to an alternative EDA scheme including the noninteracting kinetic, steric, and electrostatic energies for investigations on other endohedral fullerenes. Extending our analyses to large size confinements, Cl^? @ C_n with n up to 90 as illustrative examples, the quantitative cooperativity concept is also explored, where the positive and negative cooperativity profiles unveil a specific size of the anionic confinements to form the most stable large anion.