Solvation effects on isomeric preferences of curium(iii) complexes with multidentate phosphonopropionic acid ligands: CmH(2)PPA(2+) and CmHPPA(+) complexes.

ABSTRACT

We have carried out both time-resolved laser fluorescence spectroscopic and computational studies on the complexes of curium(III) with multidentate Phosphonopropionic (PPA) acid ligands. A number of complexes of Cm(III) with these ligands, such as CmH(2)PPA(2+), CmHPPA(+), Cm[H(2)PPA](2)(+), and Cm[HPPA](2)(-) have been studied. Our computational studies focused on all possible isomers in the gas phase and aqueous solution so that the relative binding strengths of carboxylic versus phosphoric groups can be assessed in these multidentate systems. The solvation effects play an important role in the determination of the preferred configurations and binding propensities of carboxylate versus phosphate sites of the ligands. Our computations assess the relative strengths of single and multidentate complexes in solutions for these systems. The computed free energies of solvation explain the experimentally observed fluorescence spectra and the lifetimes of these complexes in that as more water molecules are displaced from the first hydration sphere by the ligands that bind to Cm(III), the fluorescence lifetime increases. We have found that the most stable complex for CmH(2)PPA(2+) in the aqueous phase exhibits a monodentate complex where the curium(III) is bound to the deprotonated phosphate oxygen atom. Our computations support the observed longer fluorescence lifetime of CmH(2)PPA(2+) (112 mus) compared to the free Cm(III) aquo ion (65 mus), suggesting a greater degree of H(2)O displacement from the hydration sphere. For the Cm-HPPA(+) complex, we find a tridentate form as the most stable structure which supports the observed fluorescence lifetime for the CmHPPA(+) complex (172 mus), confirming the removal of up to six water molecules from the inner hydration sphere. The relative stabilities of the complexes are found to vary substantially between the gas phase and solution, indicating a major role of solvation in the relative stabilities of these complexes.