Crystal structure and spectroscopic properties of the complex of trigonelline hydrate with p-hydroxybenzoic acid

The crystal structure of the 1:1:1 complex of trigonelline (N-methyl-3-carboxypyridinium, TRG) water and p-hydroxybenzoic acid, HBA, has been determined by X-ray diffraction. The crystals are monoclinic, space group P2(1)/c. The carboxylic group of HBA is linked to the carboxylate group of TRG by the COOH center dot center dot center dot OOC hydrogen bond of 2.632(2) angstrom, while the hydroxyl group interacts, as a proton-donor, with the water molecule via the O-H center dot center dot center dot O hydrogen bond of 2.639(2) angstrom. The water molecule plays a role of the double proton-donor to two TRG molecules by the O-H center dot center dot center dot OOC hydrogen bonds of 2.772(2) and 2.751(2) angstrom. This net of hydrogen bonds forms in the crystal a layer with hydrophilic internal area and hydrophobic external area. Four structures, with different order of TRG, water and HBA in the complex, have been optimized by the B3LYP/6-31G(d,p) approach. The experimental solid-state FTIR spectrum of the complex investigated have been compared with the calculated one by the B3LYP/6-31G(d,p) approach. The assignments of the H-1 and C-13 chemical shifts are based on the H-1-H-1 and H-1-C-13, and HMBS experiments. The isotropic magnetic shielding constants, calculated by the GIAO/B3LYP/6-31G(d,p) approach, have been used to predict the H-1 and C-13 chemical shifts. (c) 2010 Elsevier B.V. All rights reserved.