Density functional theory investigation of the geometric and electronic structures of [UO2(H2O)m(OH)n]2−n (n + m = 5)

Abstract

Gradient corrected density functional theory has been used to calculate the geometric and electronic structures of the family of molecules [UO₂(H₂O)_m(OH)_n]²⁻ⁿ (n + m = 5). Comparisons are made with previous experimental and theoretical structural and spectroscopic data. r(U–O_yl) is found to lengthen as water molecules are replaced by hydroxides in the equatorial plane, and the _sym and _asym uranyl vibrational wavenumbers decrease correspondingly. GGA functionals (BP86, PW91 and PBE) are generally found to perform better for the cationic complexes than for the anions. The inclusion of solvent effects using continuum models leads to spurious low frequency imaginary vibrational modes and overall poorer agreement with experimental data for _sym and _asym. Analysis of the molecular orbital structure is performed in order to trace the origin of the lengthening and weakening of the U–O_yl bond as waters are replaced by hydroxides. No evidence is found to support previous suggestions of a competition for U 6d atomic orbitals in U–O_yl and U–O_hydroxide π bonding. Rather, the lengthening and weakening of U–O_yl is attributed to reduced ionic bonding generated in part by the σ-donating ability of the hydroxide ligands.