Computational Investigation of the Structural, Electronic, and Topological Properties of Quinizarin and Quinizarin–Lithium Ion (Li⁺) Complexes

Abstract

The development of new technologies for the monitoring and sustainable recovery of this metal becomes essential. In this context, this work theoretically investigates the interaction between quinizarin (1,4-dihydroxyanthraquinone) (Qz) and the lithium ion (Li + ) through computational simulations based on Density Functional Theory (DFT). Geometry optimizations, vibrational frequency calculations, and electronic structure analyses were performed using the ωB97X-D functional with the 6–31 + G(d, p) basis set, and solvent effects were treated with the SMD implicit solvation model. The Quantum Theory Atom in Molecule (QTAIM) was used to evaluate the topological properties of the interactions. Lithium coordination promotes significant structural changes in quinizarin, evidenced by the elongation of carbonyl bonds (from 1.243 to 1.244 Å to 1.252 Å) and the shortening of adjacent C-C bonds (down to 1.450 Å). Frontier Molecular Orbital (FMO) analysis revealed a considerable increase in the energy gap (from 1.63 eV in Qz to 6.50 eV in Qz-1Li and 6.41 eV in Qz-2Li) after complexation. QTAIM analysis characterized the O–Li interactions as strong and predominantly electrostatic/ionic, between the oxygen centers and the metal, with positive Laplacians (∇ 2 ρ(r) > 0), positive total energy densities (H(r) > 0), and energy density ratio -G(r)/V(r) = 1.37. The Binding energy (ΔE Bind up to -20.57 kcal mol − 1 ), Gibbs energy (ΔG up to -6.98 kcal mol − 1 ), and enthalpy (ΔH up to -21.45 kcal mol − 1 ) values showed that the complexation occurs spontaneously and releases heat. Overall, these findings support quinizarin as a promising matrix for adsorptive recovery of lithium, while highlighting that DFT approach defines the scope and limitations of the present computational study.