Strong hydrogen bonds in 1:1 and 2:1 complexes of pyridine betaine with strong acids

Abstract

The crystal structure of bis(pyridine betaine) hydrochloride- d 1 monohydrate- d 2 has been determined by X-ray analysis. The carboxylate groups of a pair of pyridine betaine molecules are bridged by a deuteron to form a centro-symmetric dimer featuring a very strong hydrogen bond of length 2.444(4) Å. The geometric mass effect (Δ R ≈ 0.008 Å) is well within the range observed for this type of hydrogen bond. The FT-IR spectra of polycrystalline 1:1 and 2:1 complexes of pyridine betaine with HNO 3, HCl, HBr, HI, HO 3SCF 3, HClO 4, HBF 4, and H 2SO 4 have been investigated in the 4000–200 cm −1 range. In the 1:1 complexes a proton is transferred from the acid to the betaine molecule, C 5H 5N +CH 2COOH · A −, and both the νOH and νCO frequencies vary with the proton acceptor properties of the anion. The spectra of the 2:1 complexes show broad and intense O · H · O stretching absorptions in the 1500–200 cm −1 range which are slightly affected by the anion and are similar to that for type A acid salts of carboxylic acids. The skeletal vibrations of the betaine residue were identified by second derivative spectroscopy. Evidence based on the νCO vibration and deuteration suggests that the hydrogen bonds in [C 5H 5NCH 2COO · H · OOCCH 2NC 5H 5] +A − are described by single minimum potentials; ν H = 940 cm −1, ν H/ν D = 1.2. As betaines are widely distributed in plants and animal tissue and form complexes with strong hydrogen bonds, such bonds should be formed in biological systems.