Structure and vibrational spectra of pyridine betaine hydrochloride

The crystal structure of pyridine betaine hydrochloride (PBET . HCl) was determined by X-ray diffraction to be monoclinic, space group P2(1)/c with a=8.533(2) Angstrom, b=9.548(2) Angstrom, c=10.781(2) Angstrom, beta=107.228(3)degrees and Z=4. Betaine is protonated and the carboxyl group forms a hydrogen bond with the chloride ion: O ... Cl(-) distance is 2.928(3) Angstrom. The interaction of pyridine betaine (PBET) with HCl was examined by ab initio self-consistent field (SCF), second-order Moller-Plesset (MP2) and density functional theory (DFT) methods using the 6-31G(d,p) basis set. Two minima are located in the potential surface at the SCF level (PBET-H(+)... Cl(-) and PBET ... H-Cl, with the latter being 1.2 kcal mol(-1) lower in energy) and only one minimum (PBET ... H-Cl) at the MP2 and DFT levels. The molecular parameters of PBET-H(+)... Cl(-), computed by the SCF method, reproduce the corresponding experimental data. The computed vibrational frequencies of PBET-H(+)... Cl(-) resemble correctly the experimental vibrational spectrum in the solid state. The root-mean-square (r.m.s.) deviations between the experimental and calculated SCF frequencies are 65 cm(-1) for all bands and 15 cm(-1) without the nu Cl-H band. All measured IR bands were interpreted in terms of the calculated vibrational modes. (C) 1997 Elsevier Science B.V.