Equatorial and apical solvent shells of the $UO22+$ ion

First principles molecular dynamics simulations of the hydration shells surrounding $UO22+$ ions are reported for temperatures near $300K$⁠. Most of the simulations were done with 64 solvating water molecules $(22ps)$⁠. Simulations with 122 water molecules $(9ps)$ were also carried out. The hydration structure predicted from the simulations was found to agree with very well-known results from x-ray data. The average $UO$ bond length was found to be $1.77Å$⁠. The first hydration shell contained five trigonally coordinated water molecules that were equatorially oriented about the O–U–O axis with the hydrogen atoms oriented away from the uranium atom. The five waters in the first shell were located at an average distance of $2.44Å$ (⁠$2.46Å$⁠, 122 water simulation). The second hydration shell was composed of distinct equatorial and apical regions resulting in a peak in the U–O radial distribution function at $4.59Å$⁠. The equatorial second shell contained ten water molecules hydrogen bonded to the five first shell molecules. Above and below the $UO22+$ ion, the water molecules were found to be significantly less structured. In these apical regions, water molecules were found to sporadically hydrogen bond to the oxygen atoms of the $UO22+$⁠, oriented in such a way as to have their protons pointed toward the cation. While the number of apical waters varied greatly, an average of five to six waters was found in this region. Many water transfers into and out of the equatorial and apical second solvation shells were observed to occur on a picosecond time scale via dissociative mechanisms. Beyond these shells, the bonding pattern substantially returned to the tetrahedral structure of bulk water.