Intermolecular Li+ and Na+ bonds in N-(1,6-dihydro-6-oxopyrimidin-2-yl) amino acids: cation polarizabilities studied by Fourier-transform infrared spectroscopy

Abstract

The tetrabutylammonium, Li⁺ and Na⁺ complexes of six N-(1,6-dihydro-6-oxopyrimidin-2-yl) amino acids (CAA) were studied by FTIR spectroscopy and compared with results obtained earlier with isomeric amino acids. In all amino acids an intramolecular OH⋯ON ⇌ O^–⋯H⁺ON hydrogen bond with large proton polarizability has been observed. In the tetrabutylammonium salts of the amino acids the carboxylic acid group forms a strong asymmetrical NH⋯O^– bond. In all Li⁺ compounds intramolecular Li⁺ bonds with double-minimum proton potentials and large ‘Li⁺ polarizability’ are found. In the case of the Na⁺ bonds flat, single-minimum Na⁺ potentials are present in the intramolecular Na⁺ bonds. The amplitude of the fluctuation, and hence the ‘Na⁺ polarizability’, is however small owing to the large mass of the Na⁺ ions.