Experimental and Theoretical Charge Density Studies of 8-Hydroxyquinoline Cocrystallized with Salicylic Acid

The experimental electron density distribution (EDD) in 8-hydroxyquinoline cocrystallized with salicylic acid, <b>1</b>, has been determined from a multipole refinement of high-resolution X-ray diffraction data collected at 100 K. The experimental EDD is compared with theoretical densities resulting from high-level ab initio and BHandH calculations using Atoms in Molecules theory. <b>1</b> crystallizes in the triclinic crystal system, and the asymmetric unit consists of a neutral salicylic acid molecule, a salicylate anion, and an 8-hydroxyquinolinium cation exhibiting a number of inter- and intramolecular hydrogen bonds and π–π interactions. Topological analysis reveals that π–π interactions are of the “closed-shell” type, characterized by rather low and flat charge density. In general, the agreement of the topological values (ρ_bcp and ∇²ρ_bcp) between experiment and theory is good, with mean differences of 0.010 e Å^–3 and 0.036 e Å^–5, respectively. The energetics of the π–π interactions have been estimated, and excellent agreement is observed between the relative energy and the strength of π-stacking derived from the Espinosa approach, with an average difference of only 4.4 kJ mol<sup>–1