The fundamental properties of proton, fluoride, and chloride binding of the octaazacryptand L have been investigated by potentiometry and NMR spectroscopy. Successive protonation constants of L are 11.18(16), 9.43(08), 7.59-(07), 5.78(02), and 4.39(33), with the last value corresponding to a two-proton step. 'H NMR spectra at 250 MHz indicate that proton exchange is slow on the NMR time scale in the pH region where the two-proton step occurs. It is proposed that this results from disruption of an intemal hydrogen bond network and heightened electrostatic repulsions present in tetraprotonated L. NMR data also suggest that protonation occurs predominantly at the secondary amines over the pH range studied. Triprotonated L binds fluoride significantly (log K = 3.6(4)) near neutral pH, and further protonation of L increases the fluoride complex stability (log K = 11.2(5) for the hexaprotonated fluoride complex). 'H NMR data show that the rate of fluoride exchange decreases with solution pH. The magnitude of the aqueous fluoride binding constant, the substantial NMR shifts, and the published solid state structure suggest fluoride is bound inside the cavity of protonated L. MM2 calculations predict the strain energy of L increases by 10.2 kcal/mol when the expected nitrogen-halide hydrogen bond length increases from that of fluoride to that of chloride. Size selectivity of L is proposed to explain the fact that fluoride is bound over lo7 times stronger than chloride.

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In the context of the demonstration of the UREX (uranium extraction) process, a separation of uranium and technetium using an anion exchange resin was performed on a simulant solution containing 98.95 g/L of 238U and 130.2 mg/L of 99Tc. After sorption on the resin, TcO4 − was eluted with NH4OH, the eluting stream was treated, and the technetium converted to Tc metal (yield=52.5%). The purity of the compound was analyzed: it contains less than 23.8 μg of 238U per gram of 99Tc. Tc metal was characterized by X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS) spectroscopy; EXAFS analysis clearly confirms the hexagonal structure of the metal obtained after treatment. Uranium was converted to ammonium diuranate and to U3O8 in a yield of 88.2%, analysis indicates that the compound contains less than 0.16 μg of 99Tc per gram of ammonium diuaranate.