Force field representation of the UO22+ cation from free energy MD simulations in water. Tests on its 18-crown-6 and NO3- adducts, and on its calix[6]arene(6-) and CMPO complexes

We report a theoretical study of the UO22+ cation in aqueous solution, using an empirical 1-6-12 representation of the nonbonded interaction energies. Based on free energy perturbation FEP calculations, we first derive new parameters for UO22+ which account for its hydration free energy, compared with the Sr2+ cation (Delta G = 13.6 kcal mol(-1)). With the new parameters, we simulate the UO22+.2NO(3)(-) salt and the UO22+.18-crown-6 adduct which both dissociate in water, and the CMPO.UO2(NO3)(2) complexes of 1:1 and 1:2 stoichiometries in which the ligand remains bound to UO22+. The complex of UO22+ with the calix[6]arene(6-) hexanion remains inclusive, and predicted by free energy simulations to be more stable than the Sr2+ complex.