Département de chimie moléculaire

Optimizations at the BLYP and B3LYP levels are reported for mixed uranyl-water/acetonitrile complexes [UO2(H2O)5-n(MeCN)n]2+ (n = 0-5), in both the gas phase and a polarizable continuum modeling acetonitrile. Car-Parrinello mol. dynamics (CPMD) simulations have been performed for these complexes in the gas phase, and for selected species (n = 0, 1, 3, 5) in a periodic box of liq. acetonitrile. According to structural and energetic data, uranyl has a higher affinity for acetonitrile than for water in the gas phase, in keeping with the higher dipole moment and polarizability of acetonitrile. In acetonitrile soln., however, water is the better ligand because of specific solvation effects. Anal. of the dipole moment of the coordinated water mol. in [UO2(H2O)(MeCN)4]2+ reveals that the interaction with the second-shell solvent mols. (through fairly strong and persistent O-H···N hydrogen bonds) causes a significant increase of this dipole moment (by more than 1 D). This cooperative polarization of water reinforces the uranyl-water bond as well as the water solvation via strengthened (UO2)OH2···NCMe hydrogen bonds. Such cooperativity is essentially absent in the acetonitrile ligands that make much weaker (UO2)NCMe···NCMe hydrogen bonds. Beyond the uranyl case, this study points to the importance of cooperative polarization effects to enhance the Mn+ ion affinity for water in condensed phases involving Mn+-OH2···A fragments, where A is a H-bond proton acceptor and Mn+ is a hard cation. [on SciFinder(R)]