Three hydrogen-bonded complexes of trigonelline with squaric acid: Crystallographic, spectroscopic and theoretical studies

Trigonelline (N-methyl-3-carboxypyridinium inner salt, TRG) forms with squaric acid (H(2)SQ) and water three complexes with TRG:H(2)SQ:H2O ratios of 3:3:1 (1), 2:2:1 (2) and anhydrous 1:1 (3), which stoichiometry depends on the solvent used for recrystallization. Their structures have been determined by X-ray diffraction, DFT calculations and characterized by FTIR and NMR spectroscopy. Crystals 1 contain three hydrogen-bonded TRG-H(2)SQ complexes and one water molecule in the asymmetric unit of triclinic space group P (1) over bar. Crystals 2 are monoclinic P2(1)/c space group and contain two hydrogen-bonded TRG-H(2)SQ complexes and one water molecule. In complexes 1 and 2 water molecule play a role of proton-donors and proton-acceptors in hydrogen bonds. Crystal 3, obtained from 98% ethanol belongs to the monoclinic P2(1)/n space group and has one TRG-H(2)SQ complex in the asymmetric unit. In the structures of 1-3 complexes there are several short asymmetric O-H center dot center dot center dot O hydrogen bonds (2.428(1)-2.542(1) angstrom). FTIR spectra are dominated by a broad and intense absorption in the 1600-400 cm(-1) region, typical of such short hydrogen bonds. The structures of hydrate 2a and anhydrous 3a have been optimized at the B3LYP/6-31G(d,p) level of theory. (C) 2011 Elsevier B.V. All rights reserved.