A density functional theory study of complex species and reactions of Am(III)/Eu(III) with nitrate anions

Complex behaviours of americium(III) and europium(III) with nitrate anions have been studied using density functional theory and small-core quasi-relativistic effective core potential. The structural parameters of a series of hydrated and nitrate complexes of Am(III) and Em(III) are probed and compared to available EXAFS studies. The thermodynamic stability of these species is calculated by simulating related complexation reactions both in the gas phase and aqueous solution. For taking into account the solvent effect, both the solvation model density (SMD) implicit solvent model and the conductor-like screening model (COSMO) are adopted for comparison. From our calculations, the octa-aqua and non-aqua species of Am(III) and Eu(III), [M(H2O)(8,9)](3+) (M=Am, Eu), form square antiprism and trigonal prism structures, respectively. In the presence of rich-enough nitrate ions, at most three nitrate anions can chelate to Am(III)/Eu(III) ions in a bidentate fashion in the first coordination shell, owing to the strong Coulomb interactions. From the changes of the Gibbs free energy for complexation reactions, [M(H2O)(8)](3+) -> [M(NO3)(x)(H2O)(8-y)](3-x), the bidentate complexes are more favourable than the monodentate ones with the same coordination numbers. The comparison of ionic exchange reactions further suggests a stronger coordination ability and harder acidity of Eu(III) than Am(III). The SMD and COSMO predict the same trend in solvent effect, whereas the latter seems to give obviously lower changes of the Gibbs free energy.