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The complexation of 1,4,7,10-tetrazacyclodecane-1,4,7,10-tetraacetic acid (DOTA) ligand with two trivalent actinides (Am and Pu) was investigated by UV-visible spectrophotometry, NMR spectroscopy, and extended X-ray absorption fine structure in conjunction with computational methods. The complexation process of these two cations is similar to what has been previously observed with lanthanides(III) of similar ionic radius. The complexation takes place in different steps and ends with the formation of a (1:1) complex [(An(III)DOTA)(HO)], where the cation is bonded to the nitrogen atoms of the ring, the four carboxylate arms, and a water molecule to complete the coordination sphere. The formation of An(III)-DOTA complexes is faster than the Ln(III)-DOTA systems of equivalent ionic radius. Furthermore, it is found that An-N distances are slightly shorter than Ln-N distances. Theoretical calculations showed that the slightly higher affinity of DOTA toward Am over Nd is correlated with slightly enhanced ligand-to-metal charge donation arising from oxygen and nitrogen atoms.

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Future efforts involving the technique presented here will involve improvement of resolution using a smaller slit, improvement in alignment between slit and electrode, and the use of diffracted light to examine events close to the electrode. In its present form, the technique allows direct observation of concentrations of electroactive materials as a function of both time and distance with unprecedented sensitivity and selectivity. The long pathlength, selectivity gained from wavelength selection, and spatial resolution will allow much greater definition of the solution adjacent to an electrode surface. In addition to observing fundamental aspects of electrochemical mass transport, the technique will allow improved characterization of homogeneous reactions accompanying charge transfer. ACKNOWLEDGMENTThe authors thank Robert Fagan for helpful discussion. LITERATURE CITED(1) Winograd, N.; Kuwana, T. "Spectroetectrochemlstry at Optically Transparent