Conformational analysis of bis(trigonelline) hydrochloride, perchlorate and their monohydrates by the B3LYP calculations, X-ray diffraction and vibrational spectra

In the crystal structure of bis(trigonelline) hydrochloride monohydrate, (TRG)2H·Cl·H2O, the trigonelline units are hemiprotonated and form homoconjugated cations through short asymmetric O·H·O hydrogen bonds of 2.456(4) Å in length. The H-bonded proton is closer to TRG which has shorter contacts with Cl− anion. The water molecules and Cl− anions are linked alternatively by hydrogen bonds of lengths 3.247(5) and 3.354(4) Å into planar zigzag chains along the [z] direction. The Cl− anion additionally displays electrostatic interactions with two positively charged nitrogen atoms of the neighboring TRG molecules. The two most stable conformers of (TRG)2H·Cl·H2O, three of (TRG)2H·ClO4·H2O and eight others were analyzed by the B3LYP/6-31G(d,p) calculations in order to determine the influence of the anions and water molecule on the hydrogen bond in the homoconjugated TRG·H·TRG unit. The FTIR spectrum of (TRG)2H·Cl·H2O shows a broad and intense absorption in the 1500-400 cm−1 region, typical of short hydrogen bonds. The band at 3383 cm−1 confirms a medium-strong hydrogen bonds between the water molecules and Cl− anions. In the complex crystallized from CH3OD this broad absorption is replaced by two bands at 2500 and 1900 cm−1, which are similar to those in the spectra of TRG·D2O and TRGD·Cl, respectively.