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-carboxy-pyridinium, 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⋯OOC hydrogen bond of 2.632(2) Å, while the hydroxyl group interacts, as a proton-donor, with the water molecule via the O–H⋯O hydrogen bond of 2.639(2) Å. The water molecule plays a role of the double proton-donor to two TRG molecules by the O–H⋯OOC hydrogen bonds of 2.772(2) and 2.751(2) Å. 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 1H and 13C chemical shifts are based on the 1H– 1H and 1H– 13C, and HMBS experiments. The isotropic magnetic shielding constants, calculated by the GIAO/B3LYP/6-31G(d,p) approach, have been used to predict the 1H and 13C chemical shifts.