Electronic Structure and Anion Photoelectron Spectroscopy of Uranium-Gold Clusters UAun-, n = 3-7.

A collaborative effort between experiment and theory toward elucidating the electronic and molecular structures of uranium-gold clusters is presented. Anion photoelectron spectra of UAun-(n = 3-7) were taken at the third (355 nm) and fourth (266 nm) harmonics of a Nd:YAG laser, as well as excimer (ArF 193 nm) photon energies, where the experimental adiabatic electron affinities and vertical detachment energies values were measured. Complementary first-principles calculations were subsequently carried out to corroborate experimentally determined electron detachment energies and to determine the geometry and electronic structure for each cluster. Except for the ring-like neutral isomer of UAu6 where one unpaired electron is spread over the Au atoms, all other neutral and anionic UAun clusters (n = 3-7) were calculated to possess open-shell electrons with the unpaired electrons localized on the central U atom. The smaller clusters closely resemble the analogous UFn species, but significant deviations are seen starting with UAu5 where a competition between U-Au and Au-Au bonding begins to become apparent. The UAu6 system appears to mark a transition where Au-Au interactions begin to dominate, where both a ring-like and two heavily distorted octahedral structures around the central U atom are calculated to be nearly isoenergetic. With UAu7, only ring-like structures are calculated. Overall, the calculated electron detachment energies are in good agreement with the experimental values.

Intramolecular Proton Transfer in the Radical Anion of Cytidine Monophosphate Sheds Light on the Sensitivities of Dry vs Wet DNA to Electron Attachment-Induced Damage.

Single-strand breaks (SSBs) induced via electron attachment were previously observed in dry DNA under ultrahigh vacuum (UHV), while hydrated electrons were found not able to induce this DNA damage in an aqueous solution. To explain these findings, crossed electron-molecular beam (CEMB) and anion photoelectron spectroscopy (aPES) experiments coupled to density functional theory (DFT) modeling were used to demonstrate the fundamental importance of proton transfer (PT) in radical anions formed via electron attachment. Three molecular systems were investigated: 5'-monophosphate of 2'-deoxycytidine (dCMPH), where PT in the electron adduct is feasible, and two ethylated derivatives, 5'-diethylphosphate and 3',5'-tetraethyldiphosphate of 2'-deoxycytidine, where PT is blocked due to substitution of labile protons with the ethyl residues. CEMB and aPES experiments confirmed the cleavage of the C3'/C5'-O bond as the main dissociation channel related to electron attachment in the ethylated derivatives. In the case of dCMPH, however, electron attachment (in the aPES experiments) yielded its parent (intact) radical anion, dCMPH-, suggesting that its dissociation was inhibited. The aPES-measured vertical detachment energy of the dCMPH- was found to be 3.27 eV, which agreed with its B3LYP/6-31++G(d,p)-calculated value and implied that electron-induced proton transfer (EIPT) had occurred during electron attachment to the dCMPH model nucleotide. In other words, EIPT, subduing dissociation, appeared to be somewhat protective against SSB. While EIPT is facilitated in solution compared to the dry environment, the above findings are consistent with the stability of DNA against hydrated electron-induced SSB in solution versus free electron-induced SSB formation in dry DNA.

Effect of a Single Platinum Atom within a Small Metal Oxide Cluster: Reaction of DMMP with Size-Selected Pt1Zr2O7 Supported on HOPG.

Chemical warfare agents (CWAs) are a persistent threat facing civilians and military personnel across the modern geopolitical landscape. The development of the next generation of protective and sensing materials stands to benefit from an improved fundamental understanding of the interaction of CWA molecules with the active components of such candidate materials. The use of model systems in well-controlled environments offers a route to glean such information and has been applied here to investigate the fundamental interaction of a nerve agent simulant molecule, dimethyl methylphosphonate (DMMP), with a small cluster model of a single atom catalyst (SAC) active site. The cluster models, Pt1Zr2O7, were prepared by depositing mass-selected cluster anions synthesized in the gas phase onto a 100 K highly oriented pyrolytic graphite (HOPG) substrate surface prepared in ultra-high vacuum (UHV) at sub-monolayer coverage. Upon deposition, the cluster anions lost their charge to the electrically conductive surface to yield free-standing neutral clusters. The HOPG-supported clusters were characterized by X-ray photoelectron spectroscopy (XPS) to determine the oxidation states and chemical environment of the metal atoms present within the clusters. The reactivity of the clusters with DMMP was investigated via temperature-programmed desorption/reaction (TPD/R) and XPS experiments in which the clusters were exposed to DMMP and incrementally heated to higher temperatures. In contrast to two other HOPG-supported clusters, (ZrO2)3 and Pt1Ti2O7, recently investigated in our laboratory, Pt1Zr2O7 decomposed DMMP to primarily evolve a methane species, which was completely absent for the other clusters.

Theoretical and Experimental Study of the Spectroscopy and Thermochemistry of UC+/0/.

A combination of high-level ab initio calculations and anion photoelectron detachment (PD) measurements is reported for the UC, UC-, and UC+ molecules. To better compare the theoretical values with the experimental photoelectron spectrum (PES), a value of 1.493 eV for the adiabatic electron affinity (AEA) of UC was calculated at the Feller-Peterson-Dixon (FPD) level. The lowest vertical detachment energy (VDE) is predicted to be 1.500 eV compared to the experimental value of 1.487 ± 0.035 eV. A shoulder to lower energy in the experimental PD spectrum with the 355 nm laser can be assigned to a combination of low-lying excited states of UC- and excited vibrational states. The VDEs calculated for the low-lying excited electronic states of UC at the SO-CASPT2 level are consistent with the observed additional electron binding energies at 1.990, 2.112, 2.316, and 3.760 eV. Potential energy curves for the Ω states and the associated spectroscopic properties are also reported. Compared to UN and UN+, the bond dissociation energy (BDE) of UC (411.3 kJ/mol) is predicted to be considerably lower. The natural bond orbitals (NBO) calculations show that the UC0/+/- molecules have a bond order of 2.5 with their ground-state configuration arising from changes in the oxidation state of the U atom in terms of the 7s orbital occupation: UC (5f27s1), UC- (5f27s2), and UC+ (5f27s0). The behavior of the UN and UC sequence of molecules and anions differs from the corresponding sequences for UO and UF.

Water activation and splitting by single anionic iridium atoms.

Mass spectrometric analysis of anionic products that result from interacting Ir- with H2O shows the efficient generation of [Ir(H2O)]- complexes and IrO- molecular anions. Anion photoelectron spectra of [Ir(H2O)]-, formed under various source conditions, exhibit spectral features that are due to three different forms of the complex: the solvated anion-molecule complex, Ir-(H2O), as well as the intermediates, [H-Ir-OH]- and [H2-Ir-O]-, where one and two O-H bonds have been broken, respectively. The measured and calculated vertical detachment energy values are in good agreement and, thus, support identification of all three types of isomers. The calculated reaction pathway shows that the overall reaction Ir- + H2O → IrO- + H2 is exothermic. Two minimum energy crossing points were found, which shuttle intermediates and products between singlet and triplet potential surfaces. This study presents the first example of water activation and splitting by single Ir- anions.

Electronic Properties of UN and UN- from Photoelectron Spectroscopy and Correlated Molecular Orbital Theory.

The results of calculations of the properties of the anion UN- including electron detachment are described, which further expand our knowledge of this diatomic molecule. High-level electronic structure calculations were conducted for the UN and UN- diatomic molecules and compared to photoelectron spectroscopy measurements. The low-lying Ω states were obtained using multireference CASPT2 including spin-orbit effects up to ∼20,000 cm-1. At the Feller-Peterson-Dixon (FPD) level, the adiabatic electron affinity (AEA) of UN is estimated to be 1.402 eV and the vertical detachment energy (VDE) is 1.423 eV. The assignment of the UN excited states shows good agreement with the experimental results with a VDE of 1.424 eV. An Ω = 4 ground state was obtained for UN- which is mainly associated with the 3H ΛS state. Thermochemical calculations estimate a bond dissociation energy (BDE) for UN- (U- + N) of 665.9 kJ/mol, ∼15% larger than that of UN and UN+. The NBO analysis reveals U-N triple bonds for the UN, UN-, and UN+ species.

Ultrasmall Cluster Model for Investigating Single Atom Catalysis: Dehydrogenation of 1-Propanamine by Size-Selected Pt1Zr2O7 Clusters Supported on HOPG.

The selective dehydrogenation of hydrocarbons and their functionalized derivatives is a promising pathway in the realization of endothermic fuel systems for powering important technologies such as hypersonic aircraft. The recent surge in interest in single atom catalysts (SACs) over the past decade offers the opportunity to achieve the ultimate levels of selectivity through the subnanoscale design tailoring of novel catalysts. Experimental techniques capable of investigating the fundamental nature of the active sites of novel SACs in well-controlled model studies offer the chance to reveal promising insights. We report here an approach to accomplish this through the soft landing of mass-selected, ultrasmall metal oxide cluster ions, in which a single noble metal atom bound to a metal oxide moiety serves as a model SAC active site. This method allows the preparation of model catalysts in which monodispersed neutral SAC model active sites are decorated across an inert electrically conductive support at submonolayer surface coverage, in this case, Pt1Zr2O7 clusters supported on highly oriented pyrolytic graphite (HOPG). The results contained herein show the characterization of the Pt1Zr2O7/HOPG model catalyst by X-ray photoelectron spectroscopy (XPS), along with an investigation of its reactivity toward the functionalized hydrocarbon molecule, 1-propanamine. Through temperature-programmed desorption/reaction (TPD/R) experiments it was shown that Pt1Zr2O7/HOPG decomposes 1-propanamine exclusively into propionitrile and H2, which desorb at 425 and 550 K, respectively. Conversely, clusters without the single platinum atom, that is, Zr2O7/HOPG, exhibited no reactivity toward 1-propanamine. Hence, the single platinum atom in Pt1Zr2O7/HOPG was found to play a critical role in the observed reactivity.

Au as a Surrogate for F: The Case of UAu6 vs UF6.

Here, anion photoelectron spectroscopy and first-principles quantum chemistry are used to demonstrate to what degree Au can act as a surrogate for F in UF6 and its anion. Unlike UF6, UAu6 exhibits strong ligand-ligand, i.e., Au-Au, interactions, resulting in three low-lying isomers, two of which are three-dimensional while the third isomer has a ring-like quasi two-dimensional structure. Additionally, all the UAu6 isomers have open-shell electrons, which in nearly all cases are localized on the central U atom. The adiabatic electron affinity and vertical detachment energy are measured to be 3.05 ± 0.05 and 3.28 ± 0.05 eV, respectively, and are in very good agreement with calculations.

Electron Binding Energy Spectra of AlnPt- Clusters─A Combined Experimental and Computational Study.

Results of size-selected electron photo-detachment experiments and density functional theory calculations on anionic AlnPt-, n = 1-7, clusters are presented and analyzed. The measured and calculated spectra of electron binding energies are, overall, in excellent accord with each other. The analysis reveals the general importance of accounting for the multiplicity of structural forms of a given-size cluster that can contribute to its measured spectrum, especially when the clusters are fluxional and/or the conditions of the experiment allow for structural transitions. We show that for the systems studied here, the size-specific peculiarities of the measured spectra can be understood in terms of the combined contributions of corresponding different accessible stable equilibrium conformations, bona-fide transition-state configurations, and electronic-crossing structures that may play the role of effective barriers in electronically nonadiabatic dynamics.

Experimental and Computational Description of the Interaction of H and H- with U.

The results of ab initio correlated molecular orbital theory electronic structure calculations for low-lying electronic states are presented for UH and UH- and compared to photoelectron spectroscopy measurements. The calculations were performed at the CCSD(T)/CBS and multireference CASPT2 including spin-orbit effects by the state interacting approach levels. The ground states of UH and UH- are predicted to be 4Ι9/2 and 5Λ6, respectively. The spectroscopic parameters Te, re, ωe, ωexe, and Be were obtained, and potential energy curves were calculated for the low energy Ω states of UH. The calculated adiabatic electron affinity is 0.468 eV in excellent agreement with an experimental value of 0.462 ± 0.013 eV. The lowest vertical detachment energy was predicted to be 0.506 eV for the ground state, and the adiabatic ionization energy (IE) is predicted to be 6.116 eV. The bond dissociation energy (BDE) and heat of formation values of UH were obtained using the IE calculated at the Feller-Peterson-Dixon level. For UH, UH-, and UH+, the BDEs were predicted to be 225.5, 197.9, and 235.5 kJ/mol, respectively. The BDE for UH is predicted to be ∼20% lower in energy than that for ThH. The analysis of the natural bond orbitals shows a significant U+H- ionic component in the bond of UH.

Molecular Properties of Thorium Hydrides: Electron Affinities and Thermochemistry.

High-level electronic structure calculations of the ground and low-lying energy electronic states for ThHx and ThHx- for x = 2-5 are reported and compared to available anion photoelectron detachment experiments. The adiabatic electron affinities (EAs) are predicted to be 0.82, 0.88, 0.51, and 2.36 eV for x = 2 to 5, respectively, at the Feller-Peterson-Dixon (FPD) level. The vertical detachment energies (VDEs) are predicted to be 0.84, 0.88, 0.81, and 4.38 eV for x = 2-5, respectively. The corresponding experimental VDEs are 0.871 eV for x = 2, 0.88 eV for x = 3, and 4.09 eV for x = 5. As for ThH, there is a significant spin-orbit (SO) correction for the EA of ThH2, and this correction decreases substantially for x > 2. The observed ThH2- photoelectron spectrum has many transitions as predicted at the CASPT2-SO level. The FPD bond dissociation energies (BDEs) increase from 67 to 75 kcal/mol for x = 2 to x = 4 at the FPD level. The BDE for ThH5 is much lower as it is a complex of H2 with ThH3. The hydride affinities for x = 2 to 4 are all comparable and near 70 kcal/mol. A natural bond orbital analysis is consistent with a significant Th+-H- ionic contribution to the Th-H bonds. There is very little participation of the 5f orbitals in the bonding and the valence electrons on the Th are dominated by 7s and 6d for the neutrals and anions except for ThH2- where there is a significant contribution from the 7p.

Photoelectron Spectroscopic and Computational Study of the Deprotonated Gallic Acid and Propyl Gallate Anions.

Antioxidants play important roles in eliminating reactive oxygen species (ROS), which have been associated with various degenerative diseases, such as cancer, aging, and inflammatory diseases. Gallic acid (GA) and propyl gallate (PG) are well-known antioxidants and have been widely studied in vitro and in vivo. The biological antioxidant abilities of GA and PG are related to the electronic structure of their dehydro-radicals. In this work, we report a combined photoelectron spectroscopic and computational study of the deprotonated gallic acid anion, [GA - H]-, and deprotonated propyl gallate anion, [PG - H]-. Adiabatic electron affinities of the dehydro-gallic acid radical, [GA - H]· and of the dehydro-propyl gallate radical, [PG - H]·, are measured to be 2.90 ± 0.05 eV and 2.85 ± 0.05 eV, respectively, and compared to computational results.

ThAu2 -, ThAu2O-, and ThAuOH- anions: Photoelectron spectroscopic and theoretical characterization.

The thorium-gold negative ions ThAu2 -, ThAu2O-, and ThAuOH- have been observed and experimentally characterized by anion photoelectron spectroscopy. These experiments are accompanied by extensive ab initio electronic structure calculations using a relativistic composite methodology based primarily on coupled cluster singles and doubles with perturbative triples calculations. The theoretical electron affinities (EAs) at 0 K agree with the experimental adiabatic EAs to within 0.02 eV for all species. Two separate isomers were located in the calculations for ThAuOH-, and detachment from both of these appears to be present in the photoelectron spectrum. Excited electronic states of the neutral molecules are reported at the equation of motion-coupled cluster singles and doubles level of theory. Atomization energies and heats of formation are also calculated for each neutral species and have expected uncertainties of 3 and 4 kcal/mol, respectively. The σ bonds between Th and Au are determined by natural bond orbital analysis to consist of predominately sd hybrids on Th bonding with the Au 6s orbital. In order to investigate the correspondence between the bonding in Th-Au and Th-F molecules, a limited number of calculations were also carried out on most of the F-analogs of this study. These results demonstrate that Au does behave like F in these cases, although the Th-F σ bonds are much more ionic compared to Th-Au. This results in an EA for ThF2 that is 10 kcal/mol smaller than that of ThAu2. The EA values for the Th(IV) species, i.e., ThX2O and ThXOH, only differed, however, by 3-4 kcal/mol.

Molecular-level electrocatalytic CO2 reduction reaction mediated by single platinum atoms.

The activation and transformation of H2O and CO2 mediated by electrons and single Pt atoms is demonstrated at the molecular level. The reaction mechanism is revealed by the synergy of mass spectrometry, photoelectron spectroscopy, and quantum chemical calculations. Specifically, a Pt atom captures an electron and activates H2O to form a H-Pt-OH- complex. This complex reacts with CO2via two different pathways to form formate, where CO2 is hydrogenated, or to form bicarbonate, where CO2 is carbonated. The overall formula of this reaction is identical to a typical electrochemical CO2 reduction reaction on a Pt electrode. Since the reactants are electrons and isolated, single atoms and molecules, we term this reaction a molecular-level electrochemical CO2 reduction reaction. Mechanistic analysis reveals that the negative charge distribution on the Pt-H and the -OH moieties in H-Pt-OH- is critical for the hydrogenation and carbonation of CO2. The realization of the molecular-level CO2 reduction reaction provides insights into the design of novel catalysts for the electrochemical conversion of CO2.

Interaction of Th with H0/-/+: Combined Experimental and Theoretical Thermodynamic Properties.

High-level electronic structure calculations of the low-lying energy electronic states for ThH, ThH-, and ThH+ are reported and compared to experimental measurements. The inclusion of spin-orbit coupling is critical to predict the ground-state ordering as inclusion of spin-orbit switches the coupled-cluster CCSD(T) ordering of the two lowest energy states for ThH and ThH+. At the multireference spin-orbit SO-CASPT2 level, the ground states of ThH, ThH-, and ThH+ are predicted to be the 2Δ3/2, 3Φ2, and 3Δ1 states, respectively. The adiabatic electron affinity is calculated to be 0.820 eV, and the vertical detachment energy is calculated to be 0.832 eV in comparison to an experimental value of 0.87 ± 0.02 eV. The observed ThH- photoelectron spectrum has many transitions, which approximately correlate with excitations of Th+ and/or Th. The adiabatic ionization energy of ThH including spin-orbit corrections is calculated to be 6.181 eV. The natural bond orbital results are consistent with a significant contribution of the Th+H- ionic configuration to the bonding in ThH. The bond dissociation energies for ThH, ThH-, and ThH+ using the Feller-Peterson-Dixon approach were calculated to be similar for all three molecules and lie between 259 and 280 kJ/mol.

Trapping of H2 - in aluminum hydride, Al4H14.

Ever since our first experimental and computational identification of Al4H6 as a boron analog [X. Li et al., Science 315, 356 (2007)], studies on aluminum hydrides unveiled a richer pattern of structural motifs. These include aluminum-rich hydrides, which follow shell closing electron counting models; stoichiometric clusters (called baby crystals), which structurally correspond to the bulk alane; and more. In this regard, a mass spectral identification of unusually high intense peak of Al4H14 -, which has two hydrogen atoms beyond stoichiometry, has remained mostly unresolved [X. Li et al., J. Chem. Phys. 132, 241103 (2010)]. In this Communication, with the help of global minima methods and density functional theory-based calculations, we identify the lowest energy bound structure with a unique Al-H-H-Al bonding. Our electronic structural analysis reveals that two Al2H6 units trap a transient, metastable H2 -. In other words, three stable molecules, two Al2H6 and an H2, are held together by a single electron. Our studies provide a pathway to stabilize transient species by making them part of a more extensive system.

Metal-Metal Bonding in Actinide Dimers: U2 and U2.

Understanding direct metal-metal bonding between actinide atoms has been an elusive goal in chemistry for years. We report for the first time the anion photoelectron spectrum of U2-. The threshold of the lowest electron binding energy (EBE) spectral band occurs at 1.0 eV, which corresponds to the electron affinity (EA) of U2, whereas the vertical detachment energy of U2- is found at EBE ∼ 1.2 eV. Electronic structure calculations on U2 and U2- were carried out with state-of-the-art theoretical methods. The computed values of EA(U2) and EA(U) and the difference between the computed dissociation energies of U2 and U2- are found to be internally consistent and consistent with experiment. Analysis of the bonds in U2 and U2- shows that while U2 has a formal quintuple bond, U2- has a quadruple bond, even if the effective bond orders differ only by 0.5 unit instead of one unit. The resulting experimental-computational synergy elucidates the nature of metal-metal bonding in U2 and U2-.

Photoelectron Spectroscopic Study of Ascorbate and Deprotonated Ascorbate Anions Using an Electrospray Ion Source and a Cryogenically Cooled Ion Trap.

Reactive oxygen species (ROS) in biological systems are formed through a variety of mechanisms. These species are very reactive and have been associated with many diseases, including cancer and cardiovascular disease. One way of removing ROS from the body is through the use of radical scavengers, which are compounds capable of giving up an electron to neutralize the ROS yet form a stable radical species themselves. A common radical scavenger is ascorbic acid, also known as vitamin C. At physiological pH, ascorbic acid is predominately present as the ascorbate anion, C6H7O6-. The ascorbate anion, as well as the dianion (C6H6O62-), is an effective antioxidant due to its ability to donate an electron from a lone pair generated by deprotonation. An electrospray ionization source was added to our pulsed anion photoelectron spectrometer to study ascorbate anions and deprotonated ascorbate dianions via photoelectron spectroscopy. The antioxidant behavior of the ascorbate anion and the deprotonated ascorbate dianion was confirmed based on the experimental vertical detachment energy (VDE), and, therefore, the ionization energy of the anions, 3.85 and 2.68 eV, respectively.

The electron affinity of the uranium atom.

The results of a combined experimental and computational study of the uranium atom are presented with the aim of determining its electron affinity. Experimentally, the electron affinity of uranium was measured via negative ion photoelectron spectroscopy of the uranium atomic anion, U-. Computationally, the electron affinities of both thorium and uranium were calculated by conducting relativistic coupled-cluster and multi-reference configuration interaction calculations. The experimentally determined value of the electron affinity of the uranium atom was determined to be 0.309 ± 0.025 eV. The computationally predicted electron affinity of uranium based on composite coupled cluster calculations and full four-component spin-orbit coupling was found to be 0.232 eV. Predominately due to a better convergence of the coupled cluster sequence for Th and Th-, the final calculated electron affinity of Th, 0.565 eV, was in much better agreement with the accurate experimental value of 0.608 eV. In both cases, the ground state of the anion corresponds to electron attachment to the 6d orbital.

Ligated aluminum cluster anions, LAln- (n = 1-14, L = N[Si(Me)3]2).

A wide range of low oxidation state aluminum-containing cluster anions, LAln- (n = 1-14, L = N[Si(Me)3]2), were produced via reactions between aluminum cluster anions and hexamethyldisilazane (HMDS). These clusters were identified by mass spectrometry, with a few of them (n = 4, 6, and 7) further characterized by a synergy of anion photoelectron spectroscopy and density functional theory (DFT) based calculations. As compared to a previously reported method which reacts anionic aluminum hydrides with ligands, the direct reactions between aluminum cluster anions and ligands promise a more general synthetic scheme for preparing low oxidation state, ligated aluminum clusters over a large size range. Computations revealed structures in which a methyl-group of the ligand migrated onto the surface of the metal cluster, thereby resulting in "two metal-atom" insertion between Si-CH3 bond.