STRONG HYDROGEN-BONDS IN 1 1 AND 2/1 COMPLEXES OF PYRIDINE BETAINE WITH STRONG ACIDS/

The crystal structure of bis(pyridine betaine) hydrochloride-d1 monohy drate-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 centro-symmetric symmetric dimer featuring a very strong hydrog en bond of length 2.444(4) angstrom. The geometric mass effect (DELTAR almost-equal-to 0.008 angstrom) 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 r ange. In the 1:1 complexes a proton is transferred from the acid to th e betaine molecule, C5H5N+CH2COOH . A-, and both the nuOH and nuC=O fr equencies vary with the proton acceptor properties of the anion. The s pectra of the 2:1 complexes show broad and intense O . H . O stretchin g absorptions in the 1500-200 cm-1 range which are slightly affected b y the anion and are similar to that for type A acid salts of carboxyli c acids. The skeletal vibrations of the betaine residue were identifie d by second derivative spectroscopy. Evidence based on the nuC=O vibra tion and deuteration suggests that the hydrogen bonds in [C5H5NCH2COO. H . OOCCH2NC5H5]+A- are described by single minimum potentials; nu(H) = 940 cm-1, nu(H)/nu(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.