Oxygen-17 NMR Study of the Uranyl Ion. III. Factors Influencing 17O Chemical Shifts of Uranyl Oxygens in Aqueous Perchlorate Media

Effects of concentrations of perchlorate ion, nitrate ion, uranyl ion, and acid, isotopic substitutions, and temperature on 17O chemical shifts of oxygen in the uranyl ion (uranyl oxygens hereafter) have been studied in aqueous perchlorate media. Insensitivity of the 17O chemical shifts to [ClO4−] leads to the conclusion that the perchlorate ion is not coordinated to the uranyl ion. The stability constant (0.10±0.04 m−1 at 25 °C and ionic strength 5.0 m; m=mol kg−1) of a UO2NO3+ complex, has been obtained from the dependence of the 17O chemical shifts on [NO3−]. The 17O resonances shift toward the lower field with increasing concentrations of uranyl ion and acid, indicating the possibilities of the dimer formation of the uranyl ion and the protonation of uranyl oxygen atoms in concentrated solution, respectively. Isotope shifts in 17O nuclear magnetic shielding in the uranyl ion are interpreted in terms of the change of mean separation between the ground and lowest excited electronic states upon oxygen and uranium isotopic substitutions. Temperature dependence of 17O chemical shifts is discussed in connection with the isotope effects.