IR spectroscopy studies of sodium salts of some aminobenzoic acid derivatives

Resumo

Products of a reaction of four aromatic amino acids—para-aminobenzoic (p-ABA), ortho-aminobenzoic (o-ABA), methyl-bis-anthranilic (MBAA), and 4,6-diamino-isophthalic (DAPhA)—with sodium hydroxide have been studied by Fourier-transform IR spectroscopy. It has been shown that acid groups do not undergo complete conversion to salt groups because a decrease in the solution pH during reaction leads to a shift of the tautomeric equilibrium toward the formation of zwitterions. In the solid phase, the zwitterions remain in compounds in which they are present in the acids (MBAA and o-ABA) and appear in the DAPhA salt owing to the electron density redistribution in the aromatic ring. It has been found that the p-ABA salt molecules are in the canonical form. The formation of the salts leads to a change in the supramolecular organization of the molecules in the solid phase. The most energetically favorable intermolecular hydrogen bond of carboxylic acid dimers is replaced by the hydrogen bond between the oxygen atoms of the carboxylate anions and the protons of the amino groups.