The dynamics of the carbonate and the uranyl exchange in aqueous solutions of uranyl(VI) carbonate has been studied in the pH-range 6 < pH < 9 and in the temperature range -5 degrees C < t < +85 degrees C using C-13 and O-17 NMR line broadening, selective magnetization transfer and 2-dimensional exchange spectroscopy (2D-EXSY). This extensive experimental approach was necessary in order to cover the vastly varying time scales for the different exchange processes. Two uranyl complexes dominate the investigated chemical system: the MONOnuclear UO2(CO3)(3)(4-) and the TRINuclear (UO2)(3)(CO3)(6)(6-) complex. In the MONO complex, all the three ligands are bidentately coordinated to uranium. The TRIN complex comprises two different types of carbonate ligands: three TERMinal carbonates with bidentate coordination to the uranium atoms and three CENTral carbonates with a rare mu(3)-coordination to two uranium atoms. In addition, FREE carbonate can be present in the studied solutions as CO32-/HCO3- or CO2(aq), and participate in exchange reactions. The proton exchange for CO32-/ HCO3- is known to be very fast, but it is very slow for HCO3-/CO2(aq). Thus, the exchange between four carbonate sites had to be considered: MONO, CENT and TERM, and FREE. Carbonate exchange for the MONO complex has been been found to proceed according to two parallel pathways. The first is a dissociative (''water-assisted'') process, UO2(CO3)(3)(4-) (k(1)) reversible arrow UO2(CO3)(2)(2-) + CO32-, With a first-order rate equation: rate = k(1)[MONO], where k(1) = 11(3) s(-1) at 298 K, which dominates at high pH-values. This pathway was proposed in our previous paper and is confirmed here for a much wider uranyl concentration range. The second is a proton-catalyzed reaction with the following rate law: rate = k(2) .[MONO].[H+], where k(2) = 2.32(0.03) x 10(9) M(-1) s(-1), Delta H double dagger = 52(1) kJ mol(-1), and Delta S double dagger = 110 J mol(-1) K-1, which dominates the carbonate exchange for pH < 8.6 and shows no dependence on the free carbonate concentration. This reaction is almost 2 times faster in D2O than in H2O. It is proposed that this is a dissociative process with fast protonation of the non-coordinated carbonate oxygen in MONO, followed by a water-assisted dissociation of the carbonate ligand. The rate of carbonate exchange for the TRIN complex is independent of the free carbonate concentration and is much larger for the TERMinal ligands, k(obs) = 42(3) s(-1) at pH = 6.1, than for the CENTral ligands, k(obs) = 1.4(0.3) s(-1) at the same pH. Whereas the former reaction (i.e. TERM double left right arrow FREE) represents a similar type of exchange as that for the MONO complex, the CENT reaction proceeds through a decomposition of the entire TRIN complex (i.e. TRIN double left right arrow MONO). This has been proved by the simultaneous line broadening of the O-17 signal of the TRIN uranyl group and the C-13 signal of CENT carbonate at increasing temperature, in combination with O-17 NMR magnetization-transfer experiments and the observation that after 5 s O-17-enriched uranyl oxygens of the TRIN complex were already equilibrated with the nonenriched uranyl oxygens of MONO. The mechanism of carbonate and uranyl exchange processes is discussed in the framework of the structure of the interacting species. The O-17 and C-13 NMR spin-lattice relaxation times, T-1, of the different species have been determined.
