MULTINUCLEAR NMR, RAMAN, EXAFS, AND X-RAY-DIFFRACTION STUDIES OF URANYL CARBONATE COMPLEXES IN NEAR-NEUTRAL AQUEOUS-SOLUTION - X-RAY STRUCTURE OF C(NH2)(3)](6)[(UO2)(3)(CO3)(6)]CENTER-DOT-6.5H(2)O

C-13 and O-17 NMR and Raman spectroscopies were used to monitor the fr actions of UO2(CO3)(3)(4-) (1) and (UO2)(3)(CO3)(6)(6-) (2) in aqueous carbonate solutions as a function of pH, ionic strength, carbonate co ncentration, uranium concentration, and temperature. The multinuclear NMR and Raman data are consistent with the formation of (UO2)(3)(CO3)( 6)(6-). The pH dependence of the C-13 NMR spectra was used to determin e the equilibrium constant for the reaction 3UO(2)(CO3)(3)(4-) + 3H(+) reversible arrow (UO2)(3)(CO3)(6)(6-) + 3HCO(3)(-), log K = 18.1(+/- 0.5) at I-m = 2.5 m and 25 degrees C, and corresponds to log beta(36) = 55.6(+/- 0.5) for the reaction 3UO(2)(2+) + 6CO(3)(2-) reversible ar row (UO2)(3)(CO3)(6)(6-) under the same conditions. Raman spectra show ed the uranyl nu(1) stretching band at 831.6 cm(-1) for monomeric 1 an d at 812.5 cm(-1) for trimeric (UO2)(3)(CO3)(6)(6-) (2). EXAFS data fr om solid [C(NH2)(3)](6)[(UO2)(3)(CO3)(6)] and a solution of (UO2)(3)(C O3)(6)(6-) suggest that the same uranium species is present in both th e solid and solution states. Fourier transforms of the EXAFS spectra o f both solid and solution samples revealed five well-resolved peaks co rresponding to nearly identical near-neighbor distances for solid and solution- samples of 2. Fitting of these peaks yields U-O(uranyl) = 1. 79, U-O(carbonate) = 2.45, U- -C = 2.90, U- -O(terminal carbonate) = 4 .16, and U- -U = 4.91 Angstrom for the solid and similar distances for the solution sample. A peak at 4.75 Angstrom in both Fourier transfor ms (uncorrected for phase shift) corresponds to a U- -U interaction at 4.91 Angstrom, a conclusion which is supported by the absence of this peak in the Fourier transform of the crystalline monomeric K-4[UO2(CO 3)(3)] Multiple scattering along the uranyl vector is believed to play a significant role in the EXAFS of all three systems. The EXAFS data are consistent with the trimeric uranyl carbonate species indicated by NMR spectroscopy. Single crystals of [C(NH2)(3)](6)[(UO2)(3)(CO3)(6)] . 6.5H(2)O were obtained from a solution that contained stoichiometric amounts of uranyl nitrate and guanidinium carbonate and an excess of guanidinium nitrate at pH 6.5 under a CO2 atmosphere. The solid state molecular structure of [C(NH2)(3)](6)[(UO2)(3)(CO3)6]. 6.5H(2)O contai ns a planar D-3h trimetallic (UO2)(3)(CO3)(6)(6-) anion, the structure that Aberg and co-workers originally proposed for the trimeric soluti on species. The trimetallic anion contains three uranium atoms and all six carbonate ligands in the molecular plane with three uranyl oxygen atoms above and three below the plane. Uranyl U=O distances average 1 .78(1) Angstrom, while U-O distances to the carbonate oxygen atoms ave rage 2.41(1) Angstrom for terminal and 2.48(1) Angstrom for bridging l igands. Particularly significant is the average nonbonding U- -U dista nce of 4. 97 Angstrom which compares favorably to the 4.91 Angstrom di stance seen in the EXAFS analysis. The molecule crystallizes in the tr iclinic space group P ($) over bar 1, with a = 6.941(2) Angstrom, b = 14.488(2) Angstrom, c = 22.374(2) Angstrom, alpha = 95.63(2)degrees, b eta = 98.47(2)degrees, gamma = 101.88(2)degrees, R = 0.0555, R(w), = 0 .0607, V = 2158.5 Angstrom(3),d(cal), = 2.551 g cm(-3), and Z = 2.