Activation of Water by Pentavalent Actinide Dioxide Cations: Characteristic Curium Revealed by a Reactivity Turn after Americium.

Swapping of an oxygen atom of water with that of a pentavalent actinide dioxide cation, AnO2+ also called an "actinyl", requires activation of an An-O bond. It was previously found that such oxo exchange in the gas phase occurs for the first two actinyls, PaO2+ and UO2+, but not the next two, NpO2+ and PuO2+. The An-O bond dissociation energies (BDEs) decrease from PaO2+ to PuO2+, such that the observation of a parallel decrease in the An-O bond reactivity is intriguing. To elucidate oxo exchange, we here extend experimental studies to AmO2+, americyl(V), and CmO2+, curyl(V), which were produced in remarkable abundance by electrospray ionization of Am3+ and Cm3+ solutions. Like other AnO2+, americyl(V) and curyl(V) adsorb up to four H2O molecules to form tetrahydrates AnO2(H2O)4+ with the actinide hexacoordinated by oxygen atoms. It was found that AmO2+ does not oxo-exchange, whereas CmO2+ does, establishing a "turn" to increasing the reactivity from americyl to curyl, which validates computational predictions. Because oxo exchange occurs via conversion of an actinyl(V) hydrate, AnO2(H2O)+, to an actinide(V) hydroxide, AnO(OH)2+, it reflects the propensity for actinyl(V) hydrolysis: PaO2+ hydrolyzes and oxo-exchanges most easily, despite the fact that it has the highest BDE of all AnO2+. A reexamination of the computational results for actinyl(V) oxo exchange reveals distinctive properties and chemistry of curyl(V) species, particularly CmO(OH)2+.

High-resolution photoelectron imaging of cold C60⁻ anions and accurate determination of the electron affinity of C60.

High-resolution photoelectron imaging and spectroscopy of cold C60⁻ anions are reported using a newly built photoelectron imaging apparatus coupled with an electrospray ionization source and a temperature-controlled cryogenic ion trap. Vibrationally resolved photoelectron spectra are obtained for the detachment transition from the ground state of C60⁻ to that of C60 at various detachment wavelengths from 354.84 nm to 461.35 nm. The electron affinity of C60 is accurately measured to be 2.6835 ± 0.0006 eV. Numerous unexpected vibrational excitations are observed in the photoelectron spectra due to the Jahn-Teller effect in C60⁻ and Hertzberg-Teller vibronic coupling in both C60⁻ and C60. Both the relative intensities of vibrational peaks and their photoelectron angular distributions provide evidence for the vibronic couplings. The observed p-wave-like behavior in the angular distribution of the 00° transition suggests that the electron is detached from an s-type orbital.

Probing the electronic structure and chemical bonding in tricoordinate uranyl complexes UO2X3- (X = F, Cl, Br, I): competition between Coulomb repulsion and U-X bonding.

While uranyl halide complexes [UO2(halogen)n](2-n) (n = 1, 2, 4) are ubiquitous, the tricoordinate species have been relatively unknown until very recently. Here photoelectron spectroscopy and relativistic quantum chemistry are used to investigate the bonding and stability of a series of gaseous tricoordinate uranyl complexes, UO2X3(-) (X = F, Cl, Br, I). Isolated UO2X3(-) ions are produced by electrospray ionization and observed to be highly stable with very large adiabatic electron detachment energies: 6.25, 6.64, 6.27, and 5.60 eV for X = F, Cl, Br, and I, respectively. Theoretical calculations reveal that the frontier molecular orbitals are mainly of uranyl U-O bonding character in UO2F3(-), but they are from the ligand valence np lone pairs in the heavier halogen complexes. Extensive bonding analyses are carried out for UO2X3(-) as well as for the doubly charged tetracoordinate complexes (UO2X4(2-)), showing that the U-X bonds are dominated by ionic interactions with weak covalency. The U-X bond strength decreases down the periodic table from F to I. Coulomb barriers and dissociation energies of UO2X4(2-) → UO2X3(-) + X(-) are calculated, revealing that all gaseous dianions are in fact metastable. The dielectric constant of the environment is shown to be the key in controlling the thermodynamic and kinetic stabilities of the tetracoordinate uranyl complexes via modulation of the ligand-ligand Coulomb repulsions.

Photoelectron spectroscopy and the electronic structure of the uranyl tetrachloride dianion: UO2C(4(2-).

The uranyl tetrachloride dianion (UO(2)Cl(4)(2-)) is observed in the gas phase using electrospray ionization and investigated by photoelectron spectroscopy and relativistic quantum chemical calculations. Photoelectron spectra of UO(2)Cl(4)(2-) are obtained at various photon energies and congested spectral features are observed. The free UO(2)Cl(4)(2-) dianion is found to be highly stable with an adiabatic electron binding energy of 2.40 eV. Ab initio calculations are carried out and used to interpret the photoelectron spectra and elucidate the electronic structure of UO(2)Cl(4)(2-). The calculations show that the frontier molecular orbitals in UO(2)Cl(4)(2-) are dominated by the ligand Cl 3p orbitals, while the U-O bonding orbitals are much more stable. The electronic structure of UO(2)Cl(4)(2-) is compared with that of the recently reported UO(2)F(4)(2-) [P. D. Dau, J. Su, H. T. Liu, J. B. Liu, D. L. Huang, J. Li, and L. S. Wang, Chem. Sci. 3 1137 (2012)]. The electron binding energy of UO(2)Cl(4)(2-) is found to be 1.3 eV greater than that of UO(2)F(4)(2-). The differences in the electronic stability and electronic structure between UO(2)Cl(4)(2-) and UO(2)F(4)(2-) are discussed.

Photoelectron spectroscopy and theoretical studies of UF5(-) and UF6(-).

The UF(5)(-) and UF(6)(-) anions are produced using electrospray ionization and investigated by photoelectron spectroscopy and relativistic quantum chemistry. An extensive vibrational progression is observed in the spectra of UF(5)(-), indicating significant geometry changes between the anion and neutral ground state. Franck-Condon factor simulations of the observed vibrational progression yield an adiabatic electron detachment energy of 3.82 ± 0.05 eV for UF(5)(-). Relativistic quantum calculations using density functional and ab initio theories are performed on UF(5)(-) and UF(6)(-) and their neutrals. The ground states of UF(5)(-) and UF(5) are found to have C(4v) symmetry, but with a large U-F bond length change. The ground state of UF(5)(-) is a triplet state ((3)B(2)) with the two 5f electrons occupying a 5f(z3)-based 8a(1) highest occupied molecular orbital (HOMO) and the 5f(xyz)-based 2b(2) HOMO-1 orbital. The detachment cross section from the 5f(xyz) orbital is observed to be extremely small and the detachment transition from the 2b(2) orbital is more than ten times weaker than that from the 8a(1) orbital at the photon energies available. The UF(6)(-) anion is found to be octahedral, similar to neutral UF(6) with the extra electron occupying the 5f(xyz)-based a(2u) orbital. Surprisingly, no photoelectron spectrum could be observed for UF(6)(-) due to the extremely low detachment cross section from the 5f(xyz)-based HOMO of UF(6)(-).

Guiding electron emissions by excess negative charges in multiply charged molecular anions.

Using photoelectron imaging, we show the effects of excess negative charges on the directions of outgoing electrons in multiply charged anions. Photoemissions are observed to occur either in a perpendicular or parallel direction, depending on the molecular configurations and origins of the detached electrons. Detachment of the π electrons from biphenyl-disulfonate dianions is shown to occur in a perpendicular direction due to the Coulomb repulsion from the two terminal charges, whereas detachment from the sulfonate groups in linear aliphatic disulfonates occurs in parallel directions.

Vibrational State-Selective Resonant Two-Photon Photoelectron Spectroscopy of AuS(-) via a Spin-Forbidden Excited State.

Vibrational state-selective resonant two-photon photoelectron spectra have been obtained via a triplet intermediate state ((3)Σ(-)) of AuS(-) near its detachment threshold using high-resolution photoelectron imaging of cryogenically cooled AuS(-) anions. Four vibrational levels of the (3)Σ(-) excited state are observed to be below the detachment threshold. Resonant two-photon absorptions through these levels yield vibrational state-selective photoelectron spectra to the (2)Σ final state of neutral AuS with broad and drastically different Franck-Condon distributions, reflecting the symmetries of the vibrational wave functions of the (3)Σ(-) intermediate state. The (3)Σ(-) excited state is spin-forbidden from the (1)Σ(+) ground state of AuS(-) and is accessed due to strong relativistic effects. The nature of the (3)Σ(-) excited state is confirmed by angular distributions of the photoelectron images and quantum calculations.

Probing the nature of gold-carbon bonding in gold-alkynyl complexes.

Homogeneous catalysis by gold involves organogold complexes as precatalysts and reaction intermediates. Fundamental knowledge of the gold-carbon bonding is critical to understanding the catalytic mechanisms. However, limited spectroscopic information is available about organogolds that are relevant to gold catalysts. Here we report an investigation of the gold-carbon bonding in gold(I)-alkynyl complexes using photoelectron spectroscopy and theoretical calculations. We find that the gold-carbon bond in the ClAu-CCH(-) complex represents one of the strongest gold-ligand bonds-even stronger than the known gold-carbon multiple bonds, revealing an inverse correlation between bond strength and bond order. The gold-carbon bond in LAuCCH(-) is found to depend on the ancillary ligands and becomes stronger for more electronegative ligands. The strong gold-carbon bond underlies the catalytic aptness of gold complexes for the facile formation of terminal alkynyl-gold intermediates and activation of the carbon-carbon triple bond.

A joint photoelectron spectroscopy and theoretical study on the electronic structure of UCl5- and UCl5.

We report a combined photoelectron spectroscopic and relativistic quantum chemistry study on gaseous UCl5(-) and UCl5. The UCl5(-) anion is produced using electrospray ionization and found to be highly electronically stable with an adiabatic electron binding energy of 4.76±0.03 eV, which also represents the electron affinity of the neutral UCl5 molecule. Theoretical investigations reveal that the ground state of UCl5(-) has an open shell with two unpaired electrons occupying two primarily U 5f(z³) and 5f(xyz) based molecular orbitals. The structures of both UCl5(-) and UCl5 are theoretically optimized and confirmed to have C(4v) symmetry. The computational results are in good agreement with the photoelectron spectra, providing insights into the electronic structures and valence molecular orbitals of UCl5(-) and UCl5. We also performed systematic theoretical studies on all the uranium pentahalide complexes UX5(-) (X=F, Cl, Br, I). Chemical bonding analyses indicate that the U-X interactions in UX5(-) are dominated by ionic bonding, with increasing covalent contributions for the heavier halogen complexes.