Molecular structure of hydrated complex of trigonelline with l(+)-tartaric acid

Crystal structure of the 1:1:1 complex of trigonelline with l(+)-tartaric acid and water has been determined by X-ray diffraction. The crystals are monoclinic, space group P21. Trigonelline is protonated and it is linked with the semi-tartrate anion by the COOH⋯OOC hydrogen bond of 2.475(2) Å. The semi-tartrate anions form infinite chains through the COOH⋯OOC hydrogen bonds of 2.599(2) Å. Water molecules play a role of double donors and double acceptors of hydrogen bonds with the semi-tartrate anions and link them into a three-dimensional net. In the optimized structure of the title complex at the B3LYP/6-31G(d,p) level of theory trigonelline is linked with l(+)-tartaric acid by the COO⋯HOOC hydrogen bond of 2.473 Å. The solid-state FTIR spectrum is consistent with the X-ray results. The 1H and 13C NMR spectra elucidate the structure of the complex investigated in aqueous solutions. The value of pKa of trigonelline has been determined by the potentiometric titration of its hydrochloride.