Hydrogen bonds and electrostatic interactions in 1:1 and 2:1 complexes of homarine with mineral acids studied by NMR, FTIR, DFT and X-ray diffraction

The 2:1 complexes of homarine (HOM) with HCl, HBr crystallized as monohydrates, while with HI, HNO3 and HClO4 as anhydride. In the crystal structure of bis(homarine) hydrochloride monohydrate, (HOM)2H·Cl·H2O, the homarine units are hemiprotonated, and form a nonplanar hemiconjugated cation linked by a short asymmetric O·H·O hydrogen bond of 2.449(3) Å. The COO groups are inclined by 82.6(1)° and 89.1(1)° to the plane of the pyridine rings. The H-bonded proton is closer to this HOM which has shorter contacts with the Cl− anion. The water molecules and Cl− anions are linked alternatively by hydrogen bonds of 3.168(3) and 3.210(3) Å into zigzag planar chains along the [c] axis. The Cl− anion additionally interacts electrostatically with two positively charged nitrogen atoms of the neighboring HOM molecules. Two most stable conformers of (HOM)2H·Cl·H2O and six others were analysed by the B3LYP/6-31G(d,p) calculations in order to determine the influence of the anion and water molecule on the hydrogen bonds in the homoconjugated HOM·H·HOM unit. Homarine with mineral acids also forms 1:1 complexes, which during recrystallization slowly convert into the 2:1 ones. The FTIR spectra of the 2:1 complexes show a broad and intense absorption in the 1500–400 cm−1 region, typical of short hydrogen bonds. In the 1:1 complexes a proton is transferred from the acid to HOM and the anions are bonded with the carboxylic group, COOH⋯X−. The absorption due to the νOH vibration depends on the anion and appears in the 3100–1850 cm−1 region. The proton and carbon-13 chemical shifts of the complexes in D2O have been determined and discussed.