Structural and Kinetic Studies on Uranyl(V) Carbonate Complex Using 13C NMR Spectroscopy

We have measured 13C NMR spectra of uranyl(V) carbonate complex in D2O solution containing 1.003 M Na2^13CO3 at various temperatures. Two singlet signals corresponding to free and coordinated CO3^2- were observed at 169.13 and 106.70 ppm, respectively. From the peak area ratio, the structure of the uranyl(V) carbonate complex was determined as [UVO2(CO3)3]^5-. Furthermore, kinetic analyses of the exchange reaction of free and coordinated CO3^2- in [UVO2(CO3)3]^5- were carried out using 13C NMR line-broadening. As a result, the first-order rate constant at 298 K and the activation parameters for CO3^2- exchange reaction in [UVO2(CO3)3]^5- were evaluated as 1.13 × 10^3 s^-1 and (delta)H(double dagger) = 62.0 (plus-minus) 0.7 kJ·mol^-1, (delta)S(double dagger) = 22 (plus-minus) 3 J·mol^-1·K^-1, respectively. We suggest that the exchange follows a dissociative mechanism as in the corresponding [UVIO2(CO3)3]^4- complex.