Do perchlorate and triflate anions bind to the uranyl cation in an acidic aqueous medium? A combined EXAFS and quantum mechanical investigation

Structural properties of uranyl cations in acidic aqueous perchlorate and t riflate solutions were investigated using uranium L-1-edge extended X-ray a bsorption fine-structure spectroscopy (EXAFS) in conjunction with quantum m echanical calculations of gas-phase model complexes. EXAFS spectra were mea sured in aqueous solutions of up to 10 M triflic and 11.5 M perchloric acid , as well as mixtures of perchloric acid and sodium perchlorate. In no case is the perchlorate anion coordinated to UO22+. The number of equatorial wa ter molecules bound to UO22+ is always about five. In the case of the 10 M CF3SO3H solution, an inner-sphere complexation of the triflate is observed with a U-S radial distance of 3.62 Angstrom. These results are in qualitati ve agreement with quantum mechanical calculations of model uranyl complexes , according to which the interaction energies of anions follow the order pe rchlorate < triflate much less than nitrate.