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Spectroscopic and computational investigation of actinium coordination chemistry - PubMed

  • ️Fri Jan 01 2016

Spectroscopic and computational investigation of actinium coordination chemistry

Maryline G Ferrier et al. Nat Commun. 2016.

Abstract

Actinium-225 is a promising isotope for targeted-α therapy. Unfortunately, progress in developing chelators for medicinal applications has been hindered by a limited understanding of actinium chemistry. This knowledge gap is primarily associated with handling actinium, as it is highly radioactive and in short supply. Hence, Ac(III) reactivity is often inferred from the lanthanides and minor actinides (that is, Am, Cm), with limited success. Here we overcome these challenges and characterize actinium in HCl solutions using X-ray absorption spectroscopy and molecular dynamics density functional theory. The Ac-Cl and Ac-OH2O distances are measured to be 2.95(3) and 2.59(3) Å, respectively. The X-ray absorption spectroscopy comparisons between Ac(III) and Am(III) in HCl solutions indicate Ac(III) coordinates more inner-sphere Cl(1-) ligands (3.2±1.1) than Am(III) (0.8±0.3). These results imply diverse reactivity for the +3 actinides and highlight the unexpected and unique Ac(III) chemical behaviour.

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Figures

Figure 1
Figure 1. Solution phase AcIII and AmIII L3-edge XANES.

(a) XANES spectra for Am-Aquo (0.11 M HO3SCF3) in blue trace. (b) XANES spectra for Am-HCl (11 M HCl) in orange trace. (c) XANES spectra for Ac-HCl (11 M HCl) in red trace. The actinium and americium spectra were calibrated in situ to RbCl pellet (15,203.81 eV) and Zr foil (17,998 eV) respectively.

Figure 2
Figure 2. Solution phase AcIII and AmIII L3-edge EXAFS.

(a) EXAFS k3χ(k) spectra for Am-Aquo (0.11 M HO3SCF3) in blue trace. (b) EXAFS k3χ(k) spectra for Am-HCl (11 M HCl) in orange trace. (b) EXAFS k3χ(k) spectra for Ac-HCl (11 M HCl) in red trace. Fits to the data have been provided as dashed black traces.

Figure 3
Figure 3. Fourier Transform of the AcIII and AmIII k3-EXAFS.

(a) Fourier Transform of k3-EXAFS spectra from Am-Aquo (0.11 M HO3SCF3) in blue trace and its fit in dashed black trace. (b) Oxygen path contribution in the fitted data in dashed purple. (c) Fourier Transform of k3-EXAFS spectra from Am-HCl (11 M HCl) in red trace and its fit in dashed black trace. (d) Oxygen and chlorine paths contribution in the fitted data in dashed purple and green solid trace, respectively. (e) Fourier Transform of k3-EXAFS spectra from Ac-HCl (11 M HCl) in red trace and its fit in dashed black trace. (f) Oxygen and chlorine paths contribution in the fitted data in dashed purple and green solid trace, respectively.

Figure 4
Figure 4. MDDFT calculations.

The calculated mean M–OH2O (solid red traces) and M–Cl (solid blue traces) distances for O and Cl atoms within 4 Å of the metal (Am or Ac) have been compared with the values obtained from the experimental EXAFS analyses (M–OH2O dashed red and M–Cl dashed blue traces with their associated errors bars represented as shaded boxes). Experimental errors bars were 0.01 or 0.03 Å. See Table 1. (a) Data for Am-Aquo, (b) Data for Am-HCl and (c) Data for Ac-HCl.

Figure 5
Figure 5. Ionic radii of the early to mid-actinides.

Scheme showing the ionic radii (Å) of trivalent actinides from actinium to californium drawn to scale for six-coordinate ions.

Figure 6
Figure 6. Cartoon describing 227Ac radiochemical purification.

Scheme showing the procedure followed to separate 227Ac from its daughters 229Th and 223Ra using anion exchange resin and DGA resin, respectively.

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