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.

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-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.

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-.

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.

Study of the Reaction of Hydroxylamine with Iridium Atomic and Cluster Anions (n = 1-5).

Elucidating the multifaceted processes of molecular activation and subsequent reactions gives a fundamental view into the development of iridium catalysts as they apply to fuels and propellants, for example, for spacecraft thrusters. Hydroxylamine, a component of the well-known hydroxylammonium nitrate (HAN) ionic liquid, is a safer alternative and mimics the chemistry and performance standards of hydrazine. The activation of hydroxylamine by anionic iridium clusters, Irn- (n = 1-5), depicts a part of the mechanism, where two hydrogen atoms are removed, likely as H2, and Irn(NOH)- clusters remain. The significant photoelectron spectral differences between these products and the bare clusters illustrate the substantial electronic changes imposed by the hydroxylamine fragment on the iridium clusters. In combination with DFT calculations, a preliminary reaction mechanism is proposed, identifying the possible intermediate steps leading to the formation of Ir(NOH)-.

Simultaneous Functionalization of Methane and Carbon Dioxide Mediated by Single Platinum Atomic Anions.

Mass spectrometric analysis of the anionic products of interaction among Pt-, methane, and carbon dioxide shows that the methane activation complex, H3C-Pt-H-, reacts with CO2 to form [H3C-Pt-H(CO2)]-. Two hydrogenation and one C-C bond coupling products are identified as isomers of [H3C-Pt-H(CO2)]- by a synergy between anion photoelectron spectroscopy and quantum chemical calculations. Mechanistic study reveals that both CH4 and CO2 are activated by the anionic Pt atom and that the successive depletion of the negative charge on Pt drives the CO2 insertion into the Pt-H and Pt-C bonds of H3C-Pt-H-. This study represents the first example of the simultaneous functionalization of CH4 and CO2 mediated by single atomic anions.

Photoelectron spectroscopic study of dipole-bound and valence-bound nitromethane anions formed by Rydberg electron transfer.

Close-lying dipole-bound and valence-bound states in the nitromethane anion make this molecule an ideal system for studying the coupling between these two electronically different states. In this work, dipole-bound and valence-bound nitromethane anions were generated by Rydberg electron transfer and characterized by anion photoelectron spectroscopy. The presence of the dipole-bound state was demonstrated through its photoelectron spectral signature, i.e., a single narrow peak at very low electron binding energy, its strong Rydberg quantum number, n*, dependence, and its relatively large anisotropy parameter, ß. This work goes the furthest yet in supporting the doorway model of electron attachment to polar molecules.

Mapping the Electronic Structure of the Uranium(VI) Dinitride Molecule, UN2.

A combined anion photoelectron spectroscopic and relativistic coupled-cluster computational study of the electronic structure of the UN2 molecule is presented. Because the photoelectron spectrum of the uranium dinitride negative ion, UN2-, directly reflects the electronic structure of neutral UN2, we have measured and relied upon the photoelectron spectrum of the UN2- anion as a means of mapping the electronic structure of neutral UN2. In addition to the electron affinity of the UN2 ground state, energy levels of the UN2 excited states were well characterized by the close interplay between the experiment and high-level theory. We found that both electron attachment and electronic excitation significantly bend the UN2 molecule and elongate its U≡N bond. Implications for the activation of UN2 are discussed.

Reply to the Comment on "Realization of Lewis Basic Sodium Anion in the NaBH3 - Cluster".

We reply to the comment by S. Pan and G. Frenking who challenged our interpretation of the Na- :→BH3 dative bond in the recently synthesized NaBH3 - cluster. Our conclusion remains the same as that in our original paper (https://doi.org/10.1002/anie.201907089 and https://doi.org/10.1002/ange.201907089). This conclusion is additionally supported by the energetic pathways and NBO charges calculated at UCCSD and CASMP2(4,4) levels of theory. We also discussed the suitability of the Laplacian of electron density (QTAIM) and Adaptive Natural Density Partitioning (AdNDP) method for bond type assignment. It seems that AdNDP yields more sensible results. This discussion reveals that the complex realm of bonding is full of semantic inconsistencies, and we invite experimentalists and theoreticians to elaborate this topic and find solutions incorporating different views on the dative bond.

Excess electrons bound to H2S trimer and tetramer clusters.

We have prepared the hydrogen sulfide trimer and tetramer anions, (H2S)3- and (H2S)4-, measured their anion photoelectron spectra, and applied high-level quantum chemical calculations to interpret the results. The sharp peaks at low electron binding energies in their photoelectron spectra and their diffuse Dyson orbitals are evidence for them both being dipole-bound anions. While the dipole moments of the neutral (H2S)3 and (H2S)4 clusters are small, the excess electron induces structural distortions that enhance the charge-dipolar attraction and facilitate the binding of diffuse electrons.

"Outlaw" Dipole-Bound Anions of Intra-Molecular Complexes.

Using the example of silatranes XSi(OCH2CH2)3N (X = Me, H, F, Cl), XS, it was found that the effect of the dipole-bound (DB) electron on the cage intramolecular complexes does not fit into the standard views. Upon the transition from XS to the DB anions XS-, the unusual shortening of the internuclear Si···N distance is always observed. For X = Cl, it is equal to 0.15 Å, which is a record length for all DB anions known from the literature. The formation of DB anions with the cage structure has principal features, controlled not only by the "critical" value of the dipole moment (µ > 2.5 D), but also by a geometric factor, such as the degree of pyramidality of the N(CH2)3 moiety-the positive end of the molecular dipole of XS. It was a surprise that the effect of the substituent X on the extent of the structural rearrangement in the process XS → XS- cannot be explained using the values of the electron detachment energy of XS- or the initial strength of the coordination Si ← N bond in XS. The unique sensitivity of the silatrane geometry to the addition of an excess electron is governed by the rate of increase of their dipole moment with the shortening of the dative Si ← N contact. The conclusions drawn are supported by the high-accuracy CCSD and CCSD(T) calculations and the experimental (RET-PES) data. There is no real reason to doubt that the peculiarities of the formation of DB anions of XS- can also be characteristic of many hundreds of their structural analogues XM(YCH2CH2)3N (M = Si, Ge, Sn, Pb, Ti, Al, Cr, Fe, Ni...; Y = O, NR, CH2, S), i.e., substituted 5-azabicyclo[3.3.3]undecans.

The ground state, quadrupole-bound anion of succinonitrile revisited.

Using a combination of Rydberg electron transfer and negative ion photoelectron spectroscopy, we revisited an earlier study which, based on several separate pieces of evidence, had concluded that trans- and gauche-succinonitrile can form quadrupole bound anions (QBAs) and dipole bound anions (DBAs), respectively. In the present work, succinonitrile anions were formed by Rydberg electron transfer and interrogated by negative ion photoelectron spectroscopy. The resulting anion photoelectron spectra exhibited distinctive spectral features for both QBA and DBA species in the same spectrum, thereby providing direct spectroscopic confirmation of previous indirect conclusions. Just as importantly, this work also introduces the integrated combination of Rydberg electron transfer and anion photoelectron spectroscopy as a powerful, tandem technique for studying diffuse excess electron states.

Observation of the dipole- and quadrupole-bound anions of 1,4-dicyanocyclohexane.

Quadrupole-bound anions are negative ions in which their excess electrons are loosely bound by long-range electron-quadrupole attractions. Experimental evidence for quadrupole-bound anions has been scarce; until now, only trans-succinonitrile had been experimentally confirmed to form a quadrupole-bound anion. In this study, we present experimental evidence for a new quadrupole-bound anion. Our combined Rydberg electron transfer/anion photoelectron spectroscopy study demonstrates that the ee conformer of 1,4-dicyanocyclohexane (DCCH) supports a quadrupole-bound anion state, and that the cis-DCCH conformer forms a dipole-bound anion state. The electron binding energies of the quadrupole- and dipole-bound anions are measured as 18 and 115 meV, respectively, both of which are in excellent agreement with theoretical calculations by Sommerfeld.

Realization of Lewis Basic Sodium Anion in the NaBH3 - Cluster.

We report a Na:- →B dative bond in the NaBH3 - cluster, which was designed on the principle of minimum-energy rupture, prepared by laser vaporization, and characterized by a synergy of anion photoelectron spectroscopy and electronic structure calculations. The global minimum of NaBH3 - features a Na-B bond. Its preferred heterolytic dissociation conforms with the IUPAC definition of dative bond. The lone electron pair revealed on Na and the negative Laplacian of electron density at the bond critical point further confirm the dative nature of the Na-B bond. This study represents the first example of a Lewis adduct with an alkalide as the Lewis base.

Spectroscopic Measurement of a Halogen Bond Energy.

Halogen bonding (XB) has emerged as an important bonding motif in supramolecules and biological systems. Although regarded as a strong noncovalent interaction, benchmark measurements of the halogen bond energy are scarce. Here, a combined anion photoelectron spectroscopy and density functional theory (DFT) study of XB in solvated Br- anions is reported. The XB strength between the positively-charged σ-hole on the Br atom of the bromotrichloromethane (CCl3 Br) molecule and the Br- anion was found to be 0.63 eV (14.5 kcal mol-1 ). In the neutral complexes, Br(CCl3 Br)1,2 , the attraction between the free Br atom and the negatively charged equatorial belt on the Br atom of CCl3 Br, which is a second type of halogen bonding, was estimated to have interaction strengths of 0.15 eV (3.5 kcal mol-1 ) and 0.12 eV (2.8 kcal mol-1 ).