Crystal and molecular structure, hydrogen bonding and electrostatic interactions of bis(pyridine betaine) perchlorate

Bis(pyridine betaine) perchlorate, (PB)₂H·ClO₄, has been characterized by X-ray diffraction, B3LYP and MP2 calculations, FTIR, Raman and NMR spectroscopy and DSC. The complex crystallizes in space group C2/c. A pair of PB molecules is bridged by a short symmetric O·H·O hydrogen bond of the length 2.456(5) Å. Three conformers (I-III), with respect to the N⁺?O distances, of (PB)₂H cation have been analyzed at the DFT and MP2 level of theory and compared with the C(1)?O distances in the (PhCH₂COO)₂H anion. The calculated O?O distances for the analyzed cations are slightly shorter than that in the crystal. The type of hydrogen bond depends on the cation conformation and the method of calculations. The FTIR spectra of (PB)₂H·ClO₄ and (PBd₅)₂H·ClO₄ (bis(pyridine-d₅ betaine) perchlorate) show a broad and intense absorption in the 1500-400 cm⁻¹ range, which is similar to the type A acid salts of carboxylic acids and other 2:1 betaine complexes with mineral acids. In the Raman and second FTIR-derivative spectra (d²) the broad absorption is absent. A linear unit-slope correlation between d² negative band and Raman band is found. This confirms that derivative spectrometry can be used to estimate frequencies of the narrow bands covered by the broad absorption due to the O·H·O stretching and bending vibrations.