particularly, how it changes when it participates in noncovalent interactions are studied combiningexperimental observations and theoretical analysis using the nucleus independent chemical shift (NICS). Moreover, the ability of the squaric acid and its derivatives to establish hydrogen bonds, π–π stacking, and other π-interactions (anion−π, lone pair−π, and C–H/π) is related to the increase in the aromaticity
solid state structures of four new squaric acidderivatives, i.e. three polymorphs of 3,4-bis((2-(dimethylamino)ethyl)amino)cyclobut-3-ene-1,2-dione (1a–c) and a co-crystal of compound 1 and resorcinol (2). All structures form interesting supramolecular assemblies in the solid state which have been analyzed using high level DFT calculations and molecular electrostatic potential (MEP) surface calculations
experimental observation (X-ray characterization) of an anion–anion complex (anion = hydrogen fumarate) stabilized by H-bonds that is trapped in a secondary squaramide receptor. High level abinitiocalculations indicate that the anion–anion complex is thermodynamically unstable but kinetically stable with respect to the isolated anions.