Vertical Electron Detachment Energies Research Articles

Strongly bound anions featuring bismuth fluoride building blocks

The stability of polynuclear superhalogen anions composed of BiF5 building blocks was investigated using ab initio electronic structure methods and flexible basis sets. A comprehensive exploration of the ground state potential energy surfaces of (Bi2F11)−, (Bi3F16)− and (Bi4F21)− anions, which can be viewed as comprising BiF5 fragments and an additional fluorine atom, led to the identification of their isomeric structures. It was found that the most stable isomers, predicted to dominate at room temperature, correspond to chain-like extended structures containing BiF6 subunits, with fluorine ligands arranged octahedrally around Bi atoms, sharing F atoms to form Bi–F–Bi bridging linkages. The vertical electron detachment energies of the (BinF5n+1)− anions (n = 1–4) were found to be very high (ranging from 10.91 to 13.36 eV) and increased with the number of bismuth atoms (n) and thus the BiF5 building blocks involved in the structure. Thermodynamic stability of the (BinF5n+1)− anions (i.e., their susceptibility to fragmentation) was also verified and discussed.

Observation of the Transition from Triple Bonds to Single Bonds between Ru-Ge Bonding in RuGeO(CO)n- (n = 3-5).

This work reports the observation and characterization of heterobinuclear transition-metal main-group metal oxide carbonyl complex anions, RuGeO(CO)n- (n = 3-5), by combining mass-selected photoelectron velocity map imaging spectroscopy and quantum chemistry calculations. The experimentally determined vertical electron detachment energy of RuGeO(CO)3- surpasses those of RuGeO(CO)4- and RuGeO(CO)5-, which is attributed to distinctive bonding features. RuGeO(CO)3- manifests one covalent σ and two Ru-to-Ge dative π bonds, contrasting with the sole covalent σ bond present in RuGeO(CO)4- and RuGeO(CO)5-. Unpaired spin density distribution analysis reveals a 17-electron configuration at the Ru center in RuGeO(CO)3- and an 18-electron configuration in RuGeO(CO)4- and RuGeO(CO)5-. This work closes a gap in the quantitative physicochemical characterization of heteronuclear oxide carbonyl complexes, enhancing our insights into catalytic processes of CO/GeO on the metal surface at the molecular level.

New-generation electron-propagator methods for vertical electron detachment energies of molecular anions: benchmarks and applications to model green-fluorescent-protein chromophores.

Ab initio electron-propagator calculations continue to be useful companions to experimental investigations of electronic structure in molecular anions. A new generation of electron-propagator methods recently has surpassed its antecedents' predictive accuracy and computational efficiency. Interpretive clarity has been conserved, for no adjustable parameters have been introduced in the preparation of molecular orbitals or in the formulation of approximate self-energies. These methods have employed the diagonal self-energy approximation wherein each Dyson orbital equals a canonical Hartree-Fock orbital times the square root of a probability factor. Numerical tests indicate that explicitly renormalized, diagonal self-energies are needed when Dyson orbitals have large valence nitrogen, oxygen or fluorine components. They also demonstrate that even greater accuracy can be realized with generalizations that do not employ the diagonal self-energy approximation in the canonical Hartree-Fock basis. Whereas the diagonal methods have fifth-power arithmetic scaling factors, the non-diagonal generalizations introduce only non-iterative sixth-power contractions. Composite models conserve the accuracy of the most demanding combinations of self-energy approximations and flexible basis sets with drastically reduced computational effort. Composite-model results on anions that resemble the chromophore of the green fluorescent protein illustrate the interpretive capabilities of explicitly renormalized self-energies. Accurate predictions on the lowest vertical electron detachment energy of each anion confirm experimental data and the utility of the diagonal self-energy approximation.

Strongly Bound Polynuclear Anions Comprising Scandium Fluoride Building Blocks.

The stability of polynuclear anions composed of ScF3 building blocks was studied by using ab initio and density functional theory electronic structure methods and flexible basis sets. Thorough exploration of ground state potential energy surfaces of (Sc2F7)-, (Sc3F10)-, and (Sc4F13)- anions which may be viewed as comprising ScF3 fragments and the additional fluorine atom led to determining the isomeric structures thereof. It was found that the most stable isomers which are predicted to dominate at room temperature correspond to the compact structures enabling the formation of a large number of Sc-F-Sc bridging linkages rather than to the chain-like structures. The vertical electron detachment energies of the (ScnF3n+1)- anions were found to be very large (spanning the 10.85-12.29 eV range) and increasing with the increasing number of scandium atoms (n) and thus the ScF3 building blocks involved in the structure. Thermodynamic stability of (ScnF3n+1)- anions (i.e., their susceptibility to fragmentation) was also verified and discussed.

Empirically Optimized One-Electron Pseudopotential for the Hydrated Electron: A Proof-of-Concept Study.

Mixed quantum-classical molecular dynamics simulations have been important tools for studying the hydrated electron. They generally use a one-electron pseudopotential to describe the interactions of an electron with the water molecules. This approximation shows both the strength and weakness of the approach. On the one hand, it enables extensive statistical sampling and large system sizes that are not possible with more accurate ab initio molecular dynamics methods. On the other hand, there has (justifiably) been much debate about the ability of pseudopotentials to accurately and quantitatively describe the hydrated electron properties. These pseudopotentials have largely been derived by fitting them to ab initio calculations of an electron interacting with a single water molecule. In this paper, we present a proof-of-concept demonstration of an alternative approach in which the pseudopotential parameters are determined by optimizing them to reproduce key experimental properties. Specifically, we develop a new pseudopotential, using the existing TBOpt model as a starting point, which correctly describes the hydrated electron vertical detachment energy and radius of gyration. In addition to these properties, this empirically optimized model displays a significantly modified solvation structure, which improves, for example, the prediction of the partial molar volume.

A Simple Strategy to Design Polycyclic Superhalogens.

Polycyclic hydrocarbon (PH) radicals and anions such as C9H7-, C11H7-, C13H9-, and C15H9- generally possess low electron affinity (EA) and vertical detachment energy (VDE), respectively, due to their aromaticity and, consequently, enhanced stability. In this work, we propose a simple strategy to design polycyclic superhalogens (PSs) by replacing all hydrogens by cyano (CN) groups. Superhalogens can be defined as the radicals having higher EA than halogens or anions having higher VDE than halides (3.64 eV). Our density functional calculations suggest that the EA (VDE) of PS radicals (anions) exceeds 5 eV. All these PS anions are aromatic, except C11(CN)7-, which is anti-aromatic. The superhalogen property of these PSs can be attributed to the EA of CN ligands, leading to the delocalization of extra electronic charge significantly as explained using prototype C5H5-x(CN)x systems. We also notice that the 'superhalogenity' (superhalogen behavior) of C5H5-x(CN)x- is directly related to their aromaticity. We have also shown that the substitution of CN is energetically favorable, which confirms their experimental viability. Our findings should motivate experimentalists to synthesize these superhalogens for further exploration and future applications.

Accurate Vertical Electron Detachment Energies and Multiphoton Resonant Photoelectron Spectra of Biochromophore Anions in Aqueous Solution.

We introduce a new methodology for calculating vertical electron detachment energies (VDEs) of biologically relevant chromophores in their deprotonated anionic forms in aqueous solution. It combines a large-scale mixed DFT/EFP/MD approach with the high-level multireference perturbation theory XMCQDPT2 and the Effective Fragment Potential (EFP) method. The methodology includes a multiscale flexible treatment of inner (∼1000 water molecules) and outer (∼18000 water molecules) water shells around a charged solute, capturing both the effects of specific solvation and the properties of bulk water. VDEs are calculated as a function of system size for getting a converged value at the DFT/EFP level of theory. The XMCQDPT2/EFP approach, adapted for calculating VDEs, supports the DFT/EFP results. When corrected for a solvent polarization contribution, the XMCQDPT2/EFP method yields the most accurate estimate to date of the first VDE for aqueous phenolate (7.3 ± 0.1 eV), which agrees well with liquid-jet X-ray photoelectron spectroscopy data (7.1 ± 0.1 eV). We show that the geometry of the water shell and its size are essential for accurate VDE calculations of aqueous phenolate and its biologically relevant derivatives. By simulating photoelectron spectra of aqueous phenolate upon two-photon excitation at wavelengths resonant with the S0 → S1 transition, we also provide interpretation of recent multiphoton UV liquid-microjet photoelectron spectroscopy experiments. We show that its first VDE is consistent with our estimate of 7.3 eV, when experimental two-photon binding energies are corrected for the resonant contribution.

Quasidegeneracy of (H2C = B = CH2)– borata-alkene synthon and (H3B–C≡CH)– boryl-substituted acetylene anion induced by an excess electron attachment

Using ab initio methods we examined neutral and anionic BC2H4 systems. The most stable neutral isomer was found to correspond to H2CBCH2 structure. Among other neutral isomers, H3B–CCH system was identified as the highest energy isomer. The energetic order of anionic isomers was found to be different as (H2CBCH2)– and (H3B–CCH)– species were identified as nearly isoenergetic and the most stable. The energetic effect caused by an excess electron attachment was found responsible for rendering (H3B–CCH)– isomer the most stable on the anionic potential energy surface. Vertical electron detachment energies (1.71–4.32 eV) characterizing (BC2H4)– anions were predicted and discussed.

Investigation of the Structures and Chemical Bonding of Mn2Ge6- and Mn2Ge7- Clusters via Anion Photoelectron Spectroscopy and Theoretical Calculations.

We present joint anion photoelectron spectroscopy and theoretical studies for Mn2Ge6- and Mn2Ge7-. Experimental results show that Mn2Ge6- and Mn2Ge7- have vertical electron detachment energies of 2.58 ± 0.08 and 2.88 ± 0.08 eV, respectively. Both Mn2Ge6- and Mn2Ge6 have Cs symmetric structures with a Mn atom attached to a pentagonal bipyramid MnGe6. Both Mn2Ge7- and Mn2Ge7 have C2v symmetric structures, which can be considered as two Mn2Ge4 tetragonal bipyramids sharing a MnGeMn face. According to chemical bonding analyses, Mn2Ge6 could be considered as a (MnII)(MnGe62-) complex. Theoretical calculations predict that the extension of Mn2Ge7 to Mn2nGe4n+3 may be able to produce a new type of Mn-doped germanium nanostructure.

Structural evolution of dicarbon-silver cluster anions: from flat to 3-dimensional and from attached to core–shell

Coinage-metal based nanocomposites and interfaces attract much attention due to a variety of technological applications in the areas of materials, catalysis, molecular electronics, etc. In particular, carbon-silver systems exhibit significant prospects for multiple practical uses. The reported study at a DFT level concentrates on such cluster anions with a diatomic carbon component and presents a variety of isomers evolving with size from planar to nonplanar geometries. Structures, stabilities, charge distributions, vertical electron-detachment energies and simulated IR spectra for C2Agn – (n = 4–17) are discussed, with dicarbon attached outside or inserted inside the silver entity. In particular, the spectra show two main bands associated with the relative motions of the carbon and silver parts as a whole and stretching vibration in C2. Their relative intensities are correlated with the structural features of the systems. A sequential mechanism is proposed for formation of the metastable core–shell C2Ag17 – species (found to release the carbon core upon electron detachment). The results are believed to be accessible for experimental verification.

Electron Propagator Theory of Vertical Electron Detachment Energies of Anions: Benchmarks and Applications to Nucleotides

A new generation of ab initio electron-propagator self-energy approximations that are free of adjustable parameters is tested on a benchmark set of 55 vertical electron detachment energies of closed-shell anions. Comparisons with older self-energy approximations indicate that several new methods that make the diagonal self-energy approximation in the canonical Hartree-Fock orbital basis provide superior accuracy and computational efficiency. These methods and their acronyms, mean absolute errors (in eV), and arithmetic bottlenecks expressed in terms of occupied (O) and virtual (V) orbitals are the opposite-spin, non-Dyson, diagonal second-order method (os-nD-D2, 0.2, OV2), the approximately renormalized quasiparticle third-order method (Q3+, 0.15, O2V3) and the approximately renormalized, non-Dyson, linear, third-order method (nD-L3+, 0.1, OV4). The Brueckner doubles with triple field operators (BD-T1) nondiagonal electron-propagator method provides such close agreement with coupled-cluster single, double, and perturbative triple replacement total energy differences that it may be used as an alternative means of obtaining standard data. The new methods with diagonal self-energy matrices are the foundation of a composite procedure for estimating basis-set effects. This model produces accurate predictions and clear interpretations based on Dyson orbitals for the photoelectron spectra of the nucleotides found in DNA.

An ab initio study on the stability of isolated phosphaalkene synthons.

Using ab initio methods and flexible basis sets, we examined the electronic, geometric, and thermodynamic stabilities of selected phosphaalkene synthons matching the (PCR2)- formula (R = H, CH3, C6H5, C6F5, and Mes). All isolated synthons considered were found to be electronically stable and susceptible to neither fragmentation nor isomerization processes. The structures corresponding to the most stable isomers of the studied phosphaalkene synthons contain a PC double-bond (whose presence was confirmed by natural bond orbital occupancies of σ(P-C) and π(P-C) approaching 2 electrons) and two R substituents connected to the carbon atom in either (PCR2)- (for R = H, CH3, C6H5, and Mes) or (PCF-R-R)- (for R = C6F5) manner. Vertical electron detachment energies (spanning the 0.924-3.118 eV range) characterizing the phosphaalkene synthons were predicted and discussed.

Excess-electron capture and energy transfer to bulk water for aqueous DNA nucleotide.

We performed QM/MM simulations to investigate excess-electron attachment to four aqueous DNA nucleotide anions (dRT-). The negative QM/MM vertical electron affinities (-0.86 to -0.59 eV) reveal that aqueous dRT- anions improbably capture the excess electron near 0 eV. Comparing with the calculations in the gas phase and without the background charges, it can be found that first-shell water molecules have a larger contribution to the promotion of the ability of the excess-electron capture and the bulk-water polarization has a small effect on vertical electron affinities. The phosphate group hampers the attachment of the very low-energy excess electron to aqueous dRT-. The large adiabatic electron affinities (1.45-1.96 eV) and vertical detachment energies (1.92-2.44 eV) reveal that stable dRT2- dianions could be formed after dRT- anions catch the higher-energy excess electron (>0.59 eV). We computed the energy changes in the dRT2- structural relaxations. The QM-region conformational changes cause small energy alterations (-0.28 to 0.35 eV). The QM/MM energy decreases are 2.31-2.73 eV which mainly come from QM computations (3.49-4.00 eV) embedded in the background charges. The analysis of excess-electron distributions indicates that the polarization of bulk water and structural relaxations of dianions induce the excess-electron redistributions in the QM region and produce large QM-energy decreases. The MM energy changes are -1.27 to -1.11 eV for four aqueous dianions. The negative values demonstrate that the energy of the MM region would increase in dRT2- structural relaxations. In contrast with the values of the polarized QM computations, about 30% of the energy released by the QM region is transferred to bulk water in the MM region. The large energy dissipation probably suppresses DNA damage by the low-energy electron.

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.

Investigation of the Failure of Marcus Theory for Hydrated Electron Reactions.

Reactions of the hydrated electron with a wide variety of substrates have been found to exhibit unusually similar activation energies in a manner incompatible with Marcus electron transfer theory. Given the fundamental linear response assumption of Marcus theory, one possible explanation for this apparent failure is that the underlying free energy surfaces governing the reactions are not harmonic; i.e., hydrated electron structural fluctuations exhibit non-Gaussian behavior. In this work, we test this hypothesis by using simulations to calculate the hydrated electron vertical detachment energy distribution. We consider both cavity and noncavity models for the hydrated electron, between which the actual hydrated electron behavior is expected to lie. Our results identify a possible origin for non-Gaussian behavior of the hydrated electron but show that it is not of sufficient magnitude to explain the failure of Marcus theory to describe its reactions. Thus, other explanations must be sought.

CF4-n (SO3) n (n = 1–4): a new series of organic superhalogens

Superhalogens are species having higher electron affinity (EA) or vertical detachment energy (VDE) than those of halogen atoms. CF4 is a molecule having no positive EA or VDE. In the quest for new organic superhalogens, we substitute SO3 in the place of F in CF4 successively and study the resulting CF4-n (SO3) n (n = 1–4) neutral molecules and their anions using density functional theory and quantum theory of atoms in molecule at the ωB97xD/6-311 + G(d) and B3LYP/6-311 + G(d) levels, respectively. The EA of CF4-n (SO3) n and VDE of their anions, being greater than 5 eV, suggest that all these molecules behave as superhalogens. There is significant structural relaxation in C(SO3)4 molecule, leading to the remarkably high VDE of corresponding anions. The superhalogen properties have been explained on the basis of the electronic charge delocalisation over SO3 moieties. We believe that these findings will add a new chapter in the exploration of organic superhalogens.

Identification of Ge═O Double Bonds in Planar MnGe3O and MnGe4O Clusters: Anion Photoelectron Spectroscopy and Ab Initio Calculations.

The structures, chemical bonding, and magnetic properties of MnGe3O-/0 and MnGe4O-/0 are investigated by photoelectron spectroscopy and ab initio calculations. The experimental vertical electron detachment energies of MnGe3O- and MnGe4O- are measured to be 2.06 ± 0.04 and 2.64 ± 0.04 eV, respectively. The structures of MnGe3O-/0 and MnGe4O-/0 can be viewed as evolved from quadrilateral MnGe3 or tetrahedral MnGe3. It is found that both MnGe3O- and MnGe3O have Cs symmetric planar structures with an O atom attached to quadrilateral MnGe3. The structure of MnGe4O- is nonplanar with an O atom and an additional Ge atom attached to tetrahedral MnGe3, while that of MnGe4O is planar with an O atom and an additional Ge atom attached to quadrilateral MnGe3. Chemical bonding analyses reveal the existence of Ge═O double bonds in MnGe3O and MnGe4O. Magnetic property analyses suggest that MnGe3O and MnGe4O are ferromagnetic with total spin magnetic moments of 5 μB.

Understanding the Temperature Dependence and Finite Size Effects in Ab Initio MD Simulations of the Hydrated Electron

The hydrated electron is of interest to both theorists and experimentalists as a paradigm solution-phase quantum system. Although the bulk of the theoretical work studying the hydrated electron is based on mixed quantum/classical (MQC) methods, recent advances in computer power have allowed several attempts to study this object using ab initio methods. The difficulty with employing ab initio methods for this system is that even with relatively inexpensive quantum chemistry methods such as density functional theory (DFT), such calculations are still limited to at most a few tens of water molecules and only a few picoseconds duration, leaving open the question as to whether the calculations are converged with respect to either system size or dynamical fluctuations. Moreover, the ab initio simulations of the hydrated electron that have been published to date have provided only limited analysis. Most works calculate the electron's vertical detachment energy, which can be compared to experiment, and occasionally the electronic absorption spectrum is also computed. Structural features, such as pair distribution functions, are rare in the literature, with the majority of the structural analysis being simple statements that the electron resides in a cavity, which are often based only on a small number of simulation snapshots. Importantly, there has been no ab initio work examining the temperature-dependent behavior of the hydrated electron, which has not been satisfactorily explained by MQC simulations. In this work, we attempt to remedy this situation by running DFT-based ab initio simulations of the hydrated electron as a function of both box size and temperature. We show that the calculated properties of the hydrated electron are not converged even with simulation sizes up to 128 water molecules and durations of several tens of picoseconds. The simulations show significant changes in the water coordination and solvation structure with box size. Our temperature-dependent simulations predict a red-shift of the absorption spectrum (computed using TD-DFT with an optimally tuned range-separated hybrid functional) with increasing temperature, but the magnitude of the predicted red-shift is larger than that observed experimentally, and the absolute position of the calculated spectra are off by over half an eV. The spectral red-shift at high temperatures is accompanied by both a partial loss of structure of the electron's central cavity and an increased radius of gyration that pushes electron density onto and beyond the first solvation shell. Overall, although ab initio simulations can provide some insights into the temperature-dependent behavior of the hydrated electron, the simulation sizes and level of quantum chemistry theory that are currently accessible are inadequate for correctly describing the experimental properties of this fascinating object.

An ab initio study on the stability of isolated borata-alkene synthons

• Isolated (H 2 C = BH 2 ) – synthon is a thermodynamically stable system. • (H 2 C = B R 2 ) – synthons ( R = CH 3 , C 6 H 5 , C 6 F 5 , Mes) are also stable as isolated species. • The presence of C = B double bond is confirmed in all borata-alkene systems studied. • Excess electron binding energies of (H 2 C = B R 2 ) – synthons span the 1.70–3.29 eV range. • Na + (H 2 C = B R 2 ) – salts preserve the structure and bonding pattern of embedded synthons. Using ab initio electronic structure methods with flexible basis sets, we examined the electronic, geometric, and thermodynamic stability of selected borata-alkene synthons (H 2 C = B R 2 ) – ( R = H, CH 3 , C 6 H 5 , C 6 F 5 , Mes). The isolated (H 2 C = BH 2 ) – synthon was found to be electronically stable and susceptible to neither fragmentation nor isomerization processes. Geometric and electronic stabilities of the remaining (H 2 C = B R 2 ) – anions were confirmed likewise. The structures of borata-alkene synthons studied contain planar H 2 C = B < fragment with the apparent double-bond character of the C–B connection confirmed by natural bond orbital occupancies of σ(C–B) and π(C–B) bonds approaching 2 e and by the shapes and spatial localizations of the corresponding NBOs. Vertical electron detachment energies (1.70–3.29 eV) of the (H 2 C = B R 2 ) – synthons were predicted and compared to those of the representative alkenyl carbanions. The stability and structures of sodium salts comprising the (H 2 C = B R 2 ) – unit and their ability to preserve the basic properties of the borata-alkene synthons were determined and discussed.

Design of a Novel Series of Hetero-Binuclear Superhalogen Anions MM'X4 - (M = Li, Na; M' = Be, Mg, Ca; X = Cl, Br).

A series of hetero-binuclear superatom motifs involving chloride/bromide ligands, that is, MM′X4 − (M = Li, Na; M′ = Be, Mg, Ca; X = Cl, Br) anions, have been characterized by using many-body perturbation theory calculations. Large vertical electron detachment energies (VDEs, 5.470–6.799 eV) confirm the superhalogen identity of these anions. A larger VDE value can be obtained by introducing small M or large M′ central atoms and small halogen ligand atoms. Thus, one isomer of LiCaCl4 − possesses the largest VDE value. Besides, when the extra electron is shared by all ligand atoms or three bridging ligand atoms, the isomers have relatively larger VDE values.