OH⋯O and OH⋯N hydrogen bonds in the complex of DABCO mono-betaine with p-hydroxybenzoic acid studied by X-ray diffraction, DFT calculations and spectroscopic methods

DABCO mono-betaine (1,4-diazabicyclo[2.2.2]octane-1-acetate) forms a complex with p-hydroxybenzoic acid (HBA) at the 1:1 ratio. The crystals are monoclinic, space group P21/m. The HBA and DABCO mono-betaine molecules are linked into infinite chains through the COOH⋯OOC and OH⋯N hydrogen bonds of 2.604(1) and 2.771(2) Å, respectively. The DABCO mono-betaine is located on a mirror plane and the HBA molecule lies on this plane. In the structure of the 1:1 complex optimized by the B3LYP/6-31G(d,p) approach, the DABCO ring has a planar conformation, while in the chains formed by two complexes, the DABCO moieties assume the propeller conformations. The FTIR spectrum shows several bands attributed to the OH, OH⋯O, OH⋯N, CO and COO stretching vibrations. 1H and 13C chemical shifts assignments were confirmed by the magnetic isotropic shielding constants, calculated by the GIAO/B3LYP/6-31G(d,p) approach.

► p-Hydroxybenzoic acid (double proton-donor) and DABCO mono-betaine (double proton-acceptor) from a 1:1 complex.
► Molecules are linked into linear chain through the COOH·OOC and OH·N hydrogen bonds.
► DABCO ring assumes a planar conformation in crystals, while a propeller one in the optimized structure of dimer.