Pyridine carboxylic acids (picolinic, nicotinic and isonicotinic) on methylation are converted into inner salts (betaines, zwitterionic molecules): homarine (2-carboxy-1-methylpyridinium inner salt), trigonelline (3-carboxy-1-methylpyridinium inner salt) and 4-carboxy-1-methylpyridinium inner salt. The first two betaines, homarine and trigonelline have been isolated from various plants, seeds and animal tissues. All these betaines form 1:1 and 2:1 crystalline complexes with mineral acids. Most of the 2:1 complexes crystallize as monohydrate.

Crystal structures of the 3 inner salts and their 1:1 and 2:1 complexes with HCl were determined by X-ray diffraction and analysed by DFT calculations. FTIR spectra of the 2:1 complexes show a broad and intense absorption in the 1600-400 cm^-1 region, similar to these in the spectra of acid salts of carboxylic acids. This type of absorption is typical for short hydrogen bonds (O···O distance ≤ 2.5 Å). In Raman spectra the broad absorption is absent in agreement with weak intensities of OH vibrations.

The experimental solid-state vibrational spectra of the investigated betaines and their 1:1 complexes with HCl have been assigned on the basis of the MP2/ccpVDZ calculated frequencies and intensities. Analysis of the effect of quaternization of pyridine carboxylic acids on the chemical shifts of the ring carbons and protons revealed correlations between the experimental carbon-13 and proton chemical shifts and the computed magnetic isotropic screening tensors.

The discussion will be focused on the effect of hydrogen bond and electrostatic (Coulombic) interactions between the oppositely charged groups on structure, stability and conformation of complexes investigated in the crystal and isolated molecules. The obtained results will be compared with data of acid salts of carboxylic acids.

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