No abstract

Three ternary oxides, SnWO4, PbWO4, and BiVO4, containing p-block cations with ns2np0 electron configurations, so-called lone pair cations, have been studied theoretically using density functional theory and UV-visible diffuse reflectance spectroscopy. The computations reveal significant differences in the underlying electronic structures that are responsible for the varied crystal chemistry of the lone pair cations. The filled 5s orbitals of the Sn2+ ion interact strongly with the 2p orbitals of oxygen, which leads to a significant destabilization of symmetric structures (scheelite and zircon) favored by electrostatic forces. The destabilizing effect of this interaction can be significantly reduced by lowering the symmetry of the Sn2+ site to enable the antibonding Sn 5s-O 2p states to mix with the unfilled Sn 5p orbitals. This interaction produces a localized, nonbonding state at the top of the valence band that corresponds closely with the classical notion of a stereoactive electron lone pair. In compounds containing Pb2+ and Bi3+ the relativistic contraction of the 6s orbital reduces its interaction with oxygen, effectively diminishing its role in shaping the crystal chemical preferences of these ions. In PbWO4 this leads to a stabilization of the symmetric scheelite structure. In the case of BiVO4 the energy of the Bi 6s orbital is further stabilized. Despite this stabilization, the driving force for a stereoactive lone pair distortion appears to be enhanced. The energies of structures exhibiting distorted Bi3+ environments are competitive with structures that possess symmetric Bi3+ environments. Nevertheless, the "lone pair" that results associated with a distorted Bi3+ environment in BiVO4 is more diffuse than the Sn2+ lone pair in beta-SnWO4. Furthermore, the distortion has a much smaller impact on the electronic structure near the Fermi level.

The CUO molecule, formed from the reaction of laser-ablated U atoms with CO in a noble gas, exhibits very different stretching frequencies in a solid argon matrix [804.3 and 852.5 wave numbers (cm(-1))] than in a solid neon matrix (872.2 and 1047.3 cm(-1)). Related experiments in a matrix consisting of 1% argon in neon suggest that the argon atoms are interacting directly with the CUO molecule. Relativistic density functional calculations predict that CUO can bind directly to one argon atom (U-Ar = 3.16 angstroms; binding energy = 3.2 kilocalories per mole), accompanied by a change in the ground state from a singlet to a triplet. Our experimental and theoretical results also suggest that multiple argon atoms can bind to a single CUO molecule.

Insights about the redox speciation of neptunium in an aqueous mineral acid electrolyte were obtained through a combination of in situ EXAFS (extended X-ray absorption fine structure) spectroelectrochemistry, density functional theory (DFT), and simple geometric modeling. A single solution of neptunium in 1 M perchloric acid was used to extract metrical information about the Np coordination environment, in terms of hydration numbers (n) and Np-O interatomic distances. Four aquo ions - Np

Laser-ablated uranium atoms have been reacted with CO molecules during condensation with neon at 4 K. Absorptions at 1047.3 and 872.2 cm -1 are assigned to the CUO molecule formed from the insertion reaction that requires activation energy. Isotopic substitution shows that the upper band is largely U-C and the lower band mostly U-O in vibrational character. Absorptions at 2051.5, 1361.8, and 841.0 cm -1 are assigned to the OUCCO molecule, which is formed by the CO addition reaction to CUO and ultravioletvisible photon-induced rearrangment of the U(CO) 2 molecule. The OUCCO molecule undergoes further photochemical rearrangment to the (C 2 )UO 2 molecule, which is characterized by symmetric and antisymmetric OUO stretching vibrations at 843.2 and 922.1 cm -1 . The uranium carbonyls U(CO) x (x ) 1-6) are produced on deposition or on annealing. Evidence is also presented for the CUO -anion and U(CO) x -(x ) 1-5) anions, which are formed by electron capture. Relativistic density functional theoretical calculations have been performed for the aforementioned species, which lend strong support to the experimental assignments of the infrared spectra. It is predicted that CUO is a linear singlet molecule with the shortest U-C bond yet characterized, and it has a U-C triple bond with substantial U 5f character. The theoretical analysis also finds that a distorted tetrahedral geometry of (C 2 )UO 2 lies much lower in energy than either the bent/linear OUCCO structures or the U(CO) 2 uranium dicarbonyl.

Laser-ablated U atoms react with CO in excess argon to produce CUO, which is trapped in a triplet state in solid argon at 7 K, based on agreement between observed and relativistic density functional theory (DFT) calculated isotopic frequencies ((12)C(16)O, (13)C(16)O, (12)C(18)O). This observation contrasts a recent neon matrix investigation, which trapped CUO in a linear singlet state calculated to be about 1 kcal/mol lower in energy. Experiments with krypton and xenon give results analogous to those with argon. Similar work with dilute Kr and Xe in argon finds small frequency shifts in new four-band progressions for CUO in the same triplet states trapped in solid argon and provides evidence for four distinct CUO(Ar)(4-n)(Ng)(n) (Ng = Kr, Xe, n = 1, 2, 3, 4) complexes for each Ng. DFT calculations show that successively higher Ng complexes are responsible for the observed frequency progressions. This work provides the first evidence for noble gas-actinide complexes, and the first example of neutral complexes with four noble gas atoms bonded to one metal center.

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The coordination and bonding of equatorial hydroxide, carbonyl, cyanide (CN-), and isocyanide (NC-) ligands with uranyl dication, [UO2]2+, has been studied using density functional theory with relativistic effective core potentials. Good agreement is seen between experimental and calculated geometries of [UO2(OH)4]2-. Newly predicted ground-state structures of [UO2(OH)5]3-, [UO2(CO)4]2+, [UO2(CO)5]2+, [UO2(CN)4]2-, [UO2(CN)5]3-, [UO2(NC)4]2-, and [UO2(NC)5]3- are reported. Four-coordinate uranyl isocyanide complexes are the predicted gas-phase species while five-coordinate uranyl cyanide complexes are energetically favorable in aqueous solution. Small energy differences between cyanide and isocyanide complexes indicate the energetic feasibility of mixed cyanide and isocyanide complexes. A D2d uranyl tetrahydroxide is the dominant gas-phase and aqueous species, but formation of uranyl carbonyl complexes is seen to be exothermic in the gas-phase and endothermic in aqueous solution.