title: Investigation of uranyl nitrate ion pairs complexed with amide ligands using electrospray ionization ion trap mass spectrometry and density functional theory.
abstract: Ion populations formed from electrospray of uranyl nitrate solutions containing different amides vary depending on ligand nucleophilicity and steric crowding at the metal center. The most abundant species were ion pair complexes having the general formula [UO(2)(NO(3))(amide)(n=2,3)](+); however, singly charged complexes containing the amide conjugate base and reduced uranyl UO(2)(+) were also formed as were several doubly charged species. The formamide experiment produced the greatest diversity of species resulting from weaker amide binding, leading to dissociation and subsequent solvent coordination or metal reduction. Experiments using methyl formamide, dimethyl formamide, acetamide, and methyl acetamide produced ion pair and doubly charged complexes that were more abundant and less abundant complexes containing solvent or reduced uranyl. This pattern is reversed in the dimethylacetamide experiment, which displayed lower abundance doubly charged complexes, but augmented reduced uranyl complexes. DFT investigations of the tris-amide ion pair complexes showed that interligand repulsion distorts the amide ligands out of the uranyl equatorial plane and that complex stabilities do not increase with increasing amide nucleophilicity. Elimination of an amide ligand largely relieves the interligand repulsion, and the remaining amide ligands become closely aligned with the equatorial plane in the structures of the bis-amide ligands. The studies show that the phenomenological distribution of coordination complexes in a metal-ligand electrospray experiment is a function of both ligand nucleophilicity and interligand repulsion and that the latter factor begins exerting influence even in the case of relatively small ligands like the substituted methyl-formamide and methyl-acetamide ligands.
authors: Idaho National Laboratory, Idaho Falls, Idaho, United States.
authors: Garold L
authors: GL
authors: Gresham
authors: Adriana
authors: A
authors: Dinescu
authors: Michael T
authors: MT
authors: Benson
authors: Michael J
authors: MJ
authors: Van Stipdonk
authors: Gary S
authors: GS
authors: Groenewold
mesh_terms: D000577:Amides / Q000737:chemistry*; D007477:Ions / Q000737:chemistry; D008024:Ligands; D008958:Models, Molecular; D008968:Molecular Conformation; D011789:Quantum Theory; D021241:Spectrometry, Mass, Electrospray Ionization; D014502:Uranyl Nitrate / Q000737:chemistry*
chemical_list: D000577:Amides; D007477:Ions; D008024:Ligands; D014502:Uranyl Nitrate