Structural features of pyridylcinnamic acid dimers and their extended hydrogen-bonded aggregations

• E-3-(x-pyridyl)propenoic acids (x = 2, 3 or 4) were synthesized.
• The structure forming abilities were investigated.
• Energies and bond lengths of CH⋯N H-bonds were determined.
• Conformational behaviour of the acids and the zwitterionic forms were studied.

The conformational as well as the structure-forming properties of E-3-(x-pyridyl)propenoic acids (x = 2, 3 or 4) have been studied with a combination of computational and spectroscopic methods. IR spectroscopy revealed that in the solid state the zwitterionic species predominate, while NMR measurements showed that dimers, kept together by strong CO⋯HO hydrogen bonds, were formed in a dipolar aprotic solvent (DMSO). In concentrated solution, extended aggregation occurred through the cooperative effect of (aromatic) CH⋯N weak hydrogen bonds. Conformational search was performed at the HF/6-31G(d,p) level of theory. Comparison with experimental values as well as benchmarking calculations at several different levels of theory to probe the performance of the methods, B3LYP/6-31G++(d,p) method was found to be able to provide reasonable geometries as well as quantitative formation energies for the dimers and the tetramers, too.