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Proceedings of the First ACSEPT International Workshop
Lisbon, Portugal, 31 March – 2 April 2010
Theoretical Modelling of Europium(III) and Americium(III)
Complexes with BTBP Ligands W.P. Oziminski and J. Narbut
Abstract – Tetra-N-dendate derivatives of bis([1,2,4]-triazin-3-yl)-2,2’-biyridine
(BTBP) selectively extract tervalent actinides over lanthanides from nitric acid
solutions. A hypothesis was tested that the selectivity is due to a greater
contributions into the metal-ligand bonding from the 5f orbitals than from the 4f
ones. Molecular modelling of Eu(III) and Am(III) complexes with BTBP ligands was
carried out, and the electron population of the bonding MOs in the complexes was
examined. The cationic complexes of the stoichiometries 1:1 and 1:2, as well as
the neutral [M(BTBP)(NO3)3] species, were modelled. The calculations were done
by using Gaussian 03 package. Becke three-parameter hybrid functional B3LYP
was used. Stuttgart-Dresden SDD small-core ECP pseudo-relativistic basis set was
employed for metal atoms and standard all-electron Pople 6-31G(d) double
valence basis set – for other atoms. Frequency calculations proved that the
obtained stationary points had been the true minima on the potential energy
surface. Thermodynamic functions of the complex formation were calculated,
related to the hydrated metal ions. The Atoms in Molecules (AIM) and Natural
Bond Orbitals (NBO) theories were used to analyse the differences in the metalligand bonding. The effect of addition of four ethyl chains to the BTBP molecule on
the bonding energies was also analysed after full optimization of geometry of the
complexes. The results indicate larger covalent metal-ligand interactions in the
case of the Am(III) than Eu(III) complexes. The electron density at bond critical
points (AIM), and bond orders (NBO) indicate stronger bonding of Am than Eu to
BTBP. Charge transfer from the ligands to the Am(III) ion is also larger. The
analysis of canonical symmetry-adapted orbitals (HOMO, HOMO–1 and other) and
of semi-localized Natural Localized Molecular Orbitals (NLMOs) supports these
observations. Larger contribution to bonding from 5f orbitals of Am than from 4f
orbitals of Eu has been evidenced. The contribution from other valence subshells
(7s/6s and 6d/5d) has also been discussed.
ACSEPT (Actinide reCycling by SEParation and Transmutation)