PNNL

Abstract

Reactions of laser-ablated U atoms and H2O2 molecules produce UO2, H2UO2, and UO2(OH)2 as major products and U(OH)2 and HU(O)OH as minor products. Complementary information is obtained from similar reactions of U atoms with D2O2, with H2+O2 mixtures, and with water in excess argon. Through extensive relativistic density functional theory (DFT) calculations we have determined the geometry structures and ground states of these uranium species with a variety of oxidation states U(II), U(IV), U(V), and U(VI). The calculated vibrational frequencies, infrared (IR) intensities, and isotopic frequency ratios are in good agreement with the experimental values, thus supporting assignments of the observed matrix IR spectra. We propose that the reactions proceed by forming an energized [U(OH)4]* intermediate from reactions of the excited U atom with two H2O2 molecules. Due to the special stability of the U(VI) oxidation state this intermediate decomposes to the UO2(OH)2 molecule, which reveals a distinctive difference between the chemistries of uranium and thorium, where the major product in analogous thorium reactions is the tetrahedral Th(OH)4 molecule owing to the stable Th(IV) oxidation state.