A Density Functional Study of the Various Forms of UN₄O₁₂ Containing Uranyl Nitrate

In this paper we report the computational results of a density functional study of 73 UN₄O₁₂ isomers containing uranyl nitrate, UO₂(NO₃)₂, as a component. The isomers are grouped into three categories and 19 types. Forty-four isomers of 14 types are dinitrogen tetroxide adducts of uranyl nitrate, UO₂(NO₃)₂·N₂O₄, 22 are nitrosonium salt adducts of uranyl nitrate, NO⁺UO₂(NO₃)₃^-, NO⁺UO₂(NO₃)₂O(NO₂)^-, NO⁺UO₂(NO₃)₂(ONOO)^-, or (NO⁺)₂UO₂(NO₃)₂O₂^2-, and 7 are bis(nitrogen dioxide) adducts of uranyl nitrate, UO₂(NO₃)₂·2NO₂. The 22 most stable isomers in solution, representing the 20 most stable gas-phase isomers, were selected for analysis. Of these selected structures only two categories and six types were represented. Structures, frequencies, gas-phase and solution energetics, atomic charges, dipole moments, and the bonding within the N₂O₄ unit and between NO⁺ and UO₂(NO₃)₃^- components have been analyzed in detail. On the basis of relative Gibbs free energy calculations five isomers (the N₂O₄ adducts <b>a1</b>, <b>a2</b>, and <b>a3</b> and the nitrosonium salts <b>b1</b> and <b>b2</b>) were identified as strong candidates to exist and possibly predominate in the gas phase, with <b>a1</b> and <b>a2</b> being the strongest candidates. Similarly, four isomers (<b>a6</b>, <b>a5</b>, <b>a8</b>, and <b>a1</b>, all of them N₂O₄ adducts) were identified as strong candidates to exist and possibly predominate in a nonaqueous solution of nitromethane/dinitrogen tetroxide. Of these, <b>a6</b> was determined to be the most likely candidate to predominate in solution. The possibility of dissociation in solution has been addressed briefly. In addition, computational evidence for the existence of four new N₂O₄ isomers <b>20</b>, <b>22</b>, <b>27</b>, and <b>28</b> in both the gas and the solution phases is presented for the first time.