There is continuing interest in the study of actinide elements for over six decades because of their fundamental role in the nuclear industry. Despite the extensive chemistry reported for these elements, little is known about their relaxivity properties. The purpose of this chapter is to arouse the interest of the reader for unusual relaxation behaviors in an often overlooked family of elements. Potential applications of relaxivity to practical problems involving actinides will also be stressed upon. The many experimental difficulties encountered when working with radioactive nuclides is of course at the origin of the dearth of nuclear magnetic resonance studies on the actinides and on their compounds. Relaxometry has received even less attention than NMR spectroscopy. There is, however, a wealth of information on the electronic structure of the actinide ions that has been collected by electronic paramagnetic resonance, magnetic susceptibility, fluorescence, and electronic spectroscopy. Relaxometric studies on solutions of actinide ions should become an interesting complement to these investigations. The actinide elements result from the filling up of the 5f orbitals and are the analogs of the lanthanides (1). There is a close similarity between the trivalent 4f and 5f ions. However, the actinides differ from the lanthanides in many respects as follows: - Relativistic effects are more pronounced for the actinides because of their higher nuclear charge. As a result, the s and p orbitals screen the charge of the nucleus better and the d and f orbitals expand, and are destabilized (2,3). The shielding of the 5f orbitals by filled outer s and p orbitals is thus not as effective, and actinide ions form more covalent bonds and are found in higher oxidation states, at least at the beginning of the 5f series. - The spin-orbit coupling constants of the actinides are about twice as large as those of the corresponding lanthanides and the interelectronic repulsion parameters are approximately 30% smaller. Covalency effects can cause a reduction in the orbital momentum but no quenching unlike in the case of transition metal ions. There is a considerable departure from the Russell-Saunders coupling scheme with a tendency towards a distribution of electronic levels that is expected for jj coupling. However, neither schemes allows an accurate computation of the electronic levels, and models must be based on intermediate couplings (4). The calculation of energy levels is thus much more complicated than for lanthanides (5). - The ground state of the lighter actinides could vary depending on the nature of the ligands. Although often mentioned in text books (6,7), there are not many clear examples of a change of ground state with the crystal field. One interesting case has been reported for an organometallic derivative of trivalent thorium that was shown to be a 6d1 rather than a 5f1 compound as expected (8,9). Other differences between the lanthanides and the actinides are of particular relevance for relaxivity studies. - All actinides are radioactive and many can only be handled in glove-boxes located in specially designed laboratories. At present, the only field-cycling relaxometer installed in a "hot" environment is at the University of Liége (10). Also, one is rarely welcome with a highly radioactive solution in a NMR spectroscopy laboratory, and it is thus difficult to measure relaxation time at high fields. This problem may be alleviated thanks to ACTINET-6, a Network of Excellence in the Sixth Framework Program of the European Atomic Energy Community (EURATOM) that will give access to high-field NMR spectrometers in governmental nuclear facilities. - Relaxivity measurements are more easily performed with long-lived actinide nuclides that do not cause too much radiation damage (11). This is particularly true for curium whose most common nuclide 244Cm (t1/2 = 18.11 y) causes extensive radiolysis. It would be much more convenient to work with 248Cm (t1/2 = 3.4 105 y) for instance but only minute quantities of this nuclide are available in Europe. The situation is of course even more difficult with heavy actinides (Bk-Lr) which are available only at the trace level and there is thus no hope of being able to study the relaxivity of the whole actinide family as has been done for the lanthanides (12). - Many oxidation states of the actinides are poorly stable or stable only under certain conditions. Great care must thus be taken in preparing samples for relaxometry studies. Working under the same chemical conditions with different actinides in the same oxidation state is sometimes impossible. Plutonium is particularly noteworthy because it is the only element in the Mendeleev table that can exist simultaneously in solution in four different oxidation states. This unusual situation stems from the fact that the ions Pu4+ and PuO+2 have a tendency to undergo dismutation according to:2Pu4++2H2O⇌Pu3++PuO +2+4H+2PuO2++ 4H+⇌Pu4++PuO22++ 2H2O. while the reactions involving the formation and the rupture of Pu-O bonds are much slower than simple electron transfers such as:Pu3++PuO22+⇌Pu4++P uO2+. Plutonium solutions are also slowly reduced under the action of the α radiations produced by the isotopes 239Pu or 240Pu (11). Finally, PuO2+2 is reduced by some organic ligands (13). Since this involves a lot of difficulties, the reader will not be surprised that there are only a limited number of studies devoted to the relaxation properties of the actinides. The present chapter starts with a survey of the relaxometric measurements performed so far on actinide ions. These ions are divided into two groups depending on whether they are of the simple Men+ type or if they are combined with oxygen atoms in the form of MeO2n+ species. A second part of this chapter is devoted to studies of practical interest in nuclear chemistry that have been carried out by relaxometry using Gd3+ as a model of the trivalent actinides. The processing of nuclear wastes before disposal relies on solvent extraction procedures and the NMRD technique yields interesting information on the dynamic behavior of metal complexes with extracting agents. The distribution of actinides in nature is another field of great interest in radiochemistry. Humic acids form complexes with the actinide ions and play an active role in the migration of these ions. NMRD throws some light on the speciation of metal humates. © 2005 Elsevier Inc. All rights reserved.
