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

Article abstract of DOI:10.1016/0022-2860(94)87047-0

The crystal structure of bis(pyridine betaine) hydrochloride-d1, monohydrate-d2 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 centrosymmetric dimer featuring a very strong hydrogen bond of length 2.444(4) Angstroem.The geometric mass effect (ΔR ca. 0.008 Angstroem) 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 HNO3, HCl, HBr, HI, HO3SCF3, HClO4, HBF4, and H2SO4 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, C5H5N⁺CH2COOH*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 ⁺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.