Hydrogen bonds and electrostatic interactions in the 1:1 complex of DABCO di-betaine with squaric acid: Crystallographic, theoretical and spectroscopic studies

The molecular structure of the 1:1 complex of DABCO di-betaine (1,4-dicarboxymethyl-1,4-diazabicyclo[2.2.2]octane inner salt) with squaric acid (1) has been characterized by single-crystal X-ray diffraction, infrared spectroscopy and DFT calculations. Crystals 1 are monoclinic, space group P21/c. One proton of squaric acid is transferred to one of the carboxylate groups of DABCO di-betaine. The mono-protonated DABCO di-betaine cation is further engaged in the COOH⋯OOC hydrogen bond of 2.526(2) Å with the neighboring cations, linking them into a zigzag chain. The hydrogen squarate anion interacts with the oxygen atom of the carboxylate group of DABCO di-betaine cation, through the OH⋯OC hydrogen bonds of 2.600(2) Å. The structures of monomer 1a, dimer 2 and cation 3 have been optimized at the B3LYP/6-31G(d,p) level of theory. The FTIR spectrum is consistent with the X-ray results. The second-derivative spectrum of 1 and calculated frequencies for the optimized structure 1a are used to explain the frequencies in the experimental FTIR spectrum.