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, 1, 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. 1 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 pi-pi interactions. Topological analysis reveals that pi-pi interactions are of the closed-shell type, characterized by rather low and flat charge density. In general, the agreement of the topological values (rho(bcp) and Delta(2)rho(bcp)) between experiment and theory is good, with mean differences of 0.010 e angstrom(-3) and 0.036 e angstrom(-5), respectively. The energetics of the pi-pi interactions have been estimated, and excellent agreement is observed between the relative. energy and the strength of pi-stacking derived from the Espinosa approach, with an average difference of only 4.4 kJ mol(-1).