Hydrogen bonding and molecular association in 2-(quinuclidinium)-butyric acid bromide hydrate studied by X-ray diffraction, DFT calculations, FTIR and NMR spectroscopy, and potentiometric titration

The structure of 2-(quinuclidinium)-butyric acid bromide hydrate (QNBu·H2O·HBr, 3) has been determined by X-ray diffraction, DFT calculations and characterized by FTIR and NMR spectroscopy. Crystals of 3 are monoclinic, space group P21. The water molecule interacts with the carboxylic group of 2-(quinuclidinium)-butyric acid and with the bromide anion by the COOH⋯OH2 and HOH⋯Br hydrogen bonds of 2.575(3) and 3.293(2) Å, respectively. The structures of monomer (4) and dimeric cation (5) of the title complex have been optimized by the B3LYP/6-31G(d,p) approach, yielding conformations consistent with this in the crystal. The solid-state FTIR spectra of 3 and its deuterated analogue have been measured and compared with the theoretical spectrum of 4. The assignments of the observed and predicted bands have been proposed. The molecule of 3 has a chiral center at the C(9) atom, which is responsible for the non-magnetically equivalence of the α-ring and C(11)H2 methylene protons in 1H NMR spectrum. The values of pKa of quinuclidinium-acetate (quinuclidine betaine), 2-(quinuclidinium)-propionate and 2-(quinuclidinium)-butyrate have been determined by the potentiometric titration of their hydrohalides.