Stable Molecular Complex of Squaric Acid with 2-(Quinuclidinium)propionate

Squaric acid (3,4-dihydroxy-3-cyclobuten-1,2-dione, H(2)SQ) forms a complex with 2-(quinuclidinium)propionate (QNPr). In crystals, the molecules of H(2)SQ and zwitterions of QNPr are bridged by two strong non-equivalent O-H center dot center dot center dot O hydrogen bonds of 2.476(2) and 2.482(1) angstrom. The complex is investigated by X-ray diffraction, FTIR, and NMR techniques, and the results are supported by density functional theory calculations. The solid-state aggregation is consistent with the NMR results, recorded for an aqueous solution, and is also reproduced for the structure optimized at the B3LYP/6-311++G(d,p) level of theory. The calculated IR frequencies for the optimized structure have been used for the assignment of the experimental FTIR spectrum, where the broad absorption at similar to 2400 cm(-1) corresponds to the short asymmetric OH center dot center dot center dot O bonds.