*Article* **Oxalic Acid, a Versatile Coformer for Multicomponent Forms with 9-Ethyladenine**

**Mónica Benito 1,\*, Miquel Barceló-Oliver 2, Antonio Frontera <sup>2</sup> and Elies Molins 1,\***


**\*** Correspondence: mbenito@icmab.es (M.B.); elies.molins@icmab.es (E.M.)

**Abstract:** Six new multicomponent solids of 9-ethyladenine and oxalic acid have been detected and characterized. The salt screening has been performed by mechanochemical and solvent crystallization processes. Single crystals of the anhydrous salts in 1:1 and 2:1 nucleobase:coformer molar ratio were obtained by solution crystallization and elucidated by single-crystal X-ray analysis. The supramolecular interactions observed in these solids have been studied using density functional theory (DFT) calculations and characterized by the quantum theory of "atoms in molecules" (QTAIM) and the noncovalent interaction plot (NCIPlot) index methods. The energies of the H-bonding networks observed in the solid state of the anhydrous salts in 1:1 and 2:1 nucleobase:coformer are reported, disclosing the strong nature of the charge assisted NH···O hydrogen bonds and also the relative importance of ancillary C–H··O H-bonds.

**Keywords:** nucleobases; multicomponent solids; crystal engineering; DFT; H-bonding
