ESR STUDY OF COPPER(II) COMPLEXES OF α-AMINO ACIDS. COORDINATION MODES AND METAL-LIGAND BONDS IN FROZEN AQUEOUS SOLUTIONS

Abstract

Abstract Various copper(II) complexes of potentially terdentate α-amino acids and, for comparison, those of ethylenediamine and histamine have been investigated. The ESR spectra at 1 : 1 and 1 : 4 metal-ligand concentration ratios over different pH regions have been recorded in frozen aqueous solutions. The parallel and perpendicular components of the g and A tensors, the nitrogen superhyperfine coupling constants in the different directions, and the numbers of equatorial nitrogen donor atoms have been determined by computer simulation of the spectra. The energies of the d-d electronic transitions have been determined by Gaussian analysis of the visible absorption bands. Molecular orbital coefficients characteristic of the metal-ligand bonds have been derived for an effective D 4h local symmetry. Near the freezing point, the equilibrium for the complexes involving bis(glycinato) coordination is shifted towards the trans isomer. The amino groups of R,S-2,3-diaminopropionic acid and R,S-2,4-diaminobutyric a...