X-Ray, MP2 and DFT studies of the structure and vibrational spectra of homarinium chloride

The effects of hydrogen bonding, inter- and intramolecular electrostatic interactions on the structure of homarinium chloride, HOMH·Cl, in the crystal and its isolated molecule have been studied by X-ray diffraction, FT-IR, Raman, ¹H and ¹³C NMR spectroscopies, and by the MP2 and DFT theoretical methods. In the crystal, the Cl⁻ anion is connected with protonated homarine via the O–HCl⁻ hydrogen bond of the length of 2.937(4) Å, and two N⁺Cl⁻ intermolecular electrostatic interactions. In the isolated molecule, according to the MP2 and B3LYP calculations, the Cl⁻ anion is engaged in a shorter hydrogen bond (O–HCl⁻ of 2.811–2.861 Å) and in one type of intramolecular electrostatic interactions. The calculated bond lengths and bond angles at the MP2 and B3LYP levels of theory are in good agreement with the X-ray data, except the conformation of the COOH group, which is cis (syn) in the crystal and trans (anti) in the isolated molecule. The tentative assignments for the experimental solid state vibrational spectra of HOMH·Cl and HOMD·Cl have been made on the basis of the B3LYP/6-31G(d,p) calculated frequencies and intensities. The effect of quaternization of picolinic acid on the chemical shifts of the ring protons and carbons is analyzed.