X-ray and ab initio studies of the structure and vibrational spectra of 4-carboxy-1-methylpyridinium chloride

The effects of hydrogen bonding, inter- and intramolecular electrostatic interactions on the structure of 4-carboxy-1-methylpyridinium chloride (1-Me-isonicotinic acid chloride, 1-methyl-isonicotinium chloride), MINH·Cl, in the crystal and isolated molecule have been studied by X-ray diffraction, FTIR, Raman, 1H and 13C NMR spectra, B3LYP, MP2 and MP3 theoretical methods. In the crystal, the complex structure is stabilized by the O–H⋯Cl− hydrogen bond of 2.940(3) Å, and N+⋯Cl− interionic electrostatic interactions. In the isolated molecule, according to the B3LYP, MP2 and MP3 calculations, the Cl− anion is engaged in a shorter O–H⋯Cl− hydrogen bond of 2.931, 2.856 and 2.880 Å, respectively, and forms one distinct intramolecular electrostatic contact. The calculated bond lengths and bond angles are in good agreement with the X-ray data, except for the conformation of the COOH group, which is cis (syn) in the crystal and trans (anti) in the isolated molecule. The experimental solid-state vibrational spectra of MINH·Cl and MIND·Cl have been tentatively assigned on the basis of the MP2/ccpVDZ calculated frequencies and intensities. Analysis of the effects of quaternization of isonicotinic acid 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.