Structure and vibrational spectra of pyridine betaine hydrochloride

Abstract

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) A ̊ , b = 9.548(2) A ̊ , c = 10.781(2) A ̊ , β = 107.228(3)° 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) Å. The interaction of pyridine betaine (PBET) with HCl was examined by ab initio self-consistent field (SCF), second-order Møller-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 νClH band. All measured IR bands were interpreted in terms of the calculated vibrational models.