Vertical Electron Binding Energies Research Articles

New-Generation Electron-Propagator Methods for Molecular Electron-Binding Energies.

A new generation of electron-propagator methods for the calculation of electron binding energies has surpassed its antecedents with respect to accuracy, efficiency, and interpretability. No adjustable parameters are introduced in these fully ab initio procedures. Numerical tests versus several databases of valence, vertical electron binding energies of closed-shell molecules and atoms have been performed. Easily interpreted self-energy approximations with cubic arithmetic scaling produce mean absolute errors (MAEs) of 0.2 and 0.3 eV for electron detachments and attachments, respectively. The most accurate explicitly renormalized methods with fifth-power arithmetic scaling yield MAEs below 0.1 eV for detachments and attachments. Approximate renormalization leads to more efficient fifth-power alternatives for electron detachments that achieve similar accuracy with fewer bottleneck operations. Composite protocols generate excellent predictions versus highly accurate basis-extrapolated standards and experiments. The validity of the diagonal self-energy approximation and the accuracy of the approximate renormalizations are confirmed. The success of these perturbative methods based on canonical Hartree-Fock orbitals rests on a Hermitized, intermediately normalized superoperator metric. The results of all of the new-generation calculations may be analyzed in terms of final-state orbital relaxation and differential correlation effects.

Superhalogen Anions Supported by the Systems Comprising Alternately Aligned Boron and Nitrogen Central Atoms.

Using DFT/(B3LYP/wB97XD/B2PLYPD) and OVGF electronic structure methods with flexible atomic orbital basis sets, we examined the series of polynuclear superhalogen anions matching the (BF3(BN) n F4n+1)– formula (for n = 1-10,13,18-20) containing alternately aligned boron and nitrogen central atoms decorated with fluorine ligands. It was found that the equilibrium structures of these anions correspond to fully extended chains (with each B and N central atom surrounded by four substituents arranged in a tetrahedral manner) and thus mimic the globally stable fully extended (all-trans) conformations of higher n-alkanes. The vertical electron detachment energies of the (BF3(BN) n F4n+1)– anions were found to exceed 8 eV in all cases and gradually increase with the increasing number of n. The approximate limiting value of vertical electron binding energy that could be achieved for such polynuclear superhalogen anions was estimated as equal to ca. 10.7 eV.

Below Band Gap Formation of Solvated Electrons in Neutral Water Clusters?

Below band gap formation of solvated electrons in neutral water clusters using pump–probe photoelectron imaging is compared with recent data for liquid water and with above band gap excitation studies in liquid and clusters. Similar relaxation times on the order of 200 fs and 1–2 ps are retrieved for below and above band gap excitation, in both clusters and liquid. The independence of the relaxation times from the generation process indicates that these times are dominated by the solvent response, which is significantly slower than the various solvated electron formation processes. The analysis of the temporal evolution of the vertical electron binding energy and the electron binding energy at half-maximum suggests a dependence of the solvation time on the binding energy.

Ultrafast Internal Conversion and Solvation of Electrons in Water, Methanol, and Ethanol.

Ultrafast internal conversion from the first excited state of a solvated electron in water, methanol, and ethanol is investigated using time-resolved photoelectron spectroscopy of liquid microjets and a spectral retrieval method. Photoelectron spectra corrected for inelastic scattering clearly reveal well-separated signals from the excited and ground states, and the latter enables us to analyze the solvation dynamics in the ground state after internal conversion. Measurements with 25 fs time resolution identify a rapid increase in the vertical electron binding energy of the solvated electron owing to nuclear wave packet motions in the excited state and allow us to precisely determine the internal conversion time.

Temperature-dependent structure-property modeling of viscosity for ionic liquids

In this paper we present the methodology for assessing the ionic liquids’ viscosity at six temperature points (25, 35, 45, 50, 60 and 70 [°C]), which utilizes only the in silico approach. The main idea of such assessment is based on the “correction equation” describing the correlation between experimentally measured viscosity and theoretically derived density (calculated with use of molecular mechanics), given at 6 different temperature points. The equation allows for recalculating the viscosity of ILs at 25 °C to determine the viscosity of ILs at other, higher temperature. Since the equation needs the basic viscosity value (at 25 °C) we additionally developed QSPR model that allows predicting it. According to our model, the viscosity of ILs is dependent to the size and branching of the cation and size, shape, symmetry and the vertical electron binding energy of IL’s anion. With those novel tools, it is possible to predict the viscosity of new ionic liquids at different temperatures without the need of experimental measurements.

Towards the Application of Structure-Property Relationship Modeling in Materials Science: Predicting the Seebeck Coefficient for Ionic Liquid/Redox Couple Systems.

This work focuses on determining the influence of both ionic-liquid (IL) type and redox couple concentration on Seebeck coefficient values of such a system. The quantitative structure-property relationship (QSPR) and read-across techniques are proposed as methods to identify structural features of ILs (mixed with LiI/I2 redox couple), which have the most influence on the Seebeck coefficient (Se ) values of the system. ILs consisting of small, symmetric cations and anions with high values of vertical electron binding energy are recognized as those with the highest values of Se . In addition, the QSPR model enables the values of Se to be predicted for each IL that belongs to the applicability domain of the model. The influence of the redox-couple concentration on values of Se is also quantitatively described. Thus, it is possible to calculate how the value of Se will change with changing redox-couple concentration. The presence of the LiI/I2 redox couple in lower concentrations increases the values of Se , as expected.

Wavelength Dependence of UV Photoemission from Solvated Electrons in Bulk Water, Methanol, and Ethanol.

We have measured the wavelength dependence (340-215 nm) of one-photon photoemission from the ground electronic state of solvated electrons in bulk water, methanol, and ethanol. In every case, the vertical electron binding energy (VBE) gradually increased with photon energy, indicating that the photoelectron kinetic energy diminishes as a result of electron-vibration inelastic scattering prior to emission from the liquid surface. In contrast, the VBE of the Rydberg electron in DABCO (1,4-diazabicyclo[2,2,2]octane), which has a surface-excess density, revealed no clear wavelength dependence. These results suggest that the solvated electrons are created predominantly in the bulk and that VBEs measured using UV photoemission spectroscopy of liquids generally require energy corrections to account for inelastic scattering effects. From the wavelength dependence, we have re-estimated the VBEs of solvated electrons in bulk water, methanol, and ethanol to be 3.3, 3.1, and 3.1 eV, respectively. Hydrated electrons were also identified by photoemission spectroscopy using 90 nm radiation.

Chemisorption-induced two- to three-dimensions structural transformations in gold pentamer (CO)(n)Au5(-) (n =0-5).

Understanding the geometry structures of gold clusters, especially with adsorbates, is essential for designing highly active gold nanocatalysts. Here, CO chemisorption onto the Au5(-) cluster is investigated using the density functional calculations. It is found that chemisorption of CO molecules can induce previously unreported two- to three-dimensions (3D) structural changes. Even a single CO chemisorption induces a major structural change to explain the huge blue-shift in photoelectron spectroscopy (PES). The apex site in the parent Au5(-) cluster is not always the most preferred site for the chemisorption, and two bridged adsorption CO molecules are observed in the lowest-energy (CO)3Au5(-) cluster. A clear splitting is observed in the first PES of (CO)4Au5 (-), and calculated planar and 3D geometries are likely coexisting in the cluster beam. The fifth CO adsorption leads to the structural transformation of Au5 skeleton to create more apex sites to accommodate five CO molecules. The structural properties, together with the vertical electron detachment energy (VDE) and binding energies calculations indicate that the chemisorption-saturated number is 5.

Renormalized coupled cluster approaches in the cluster-in-molecule framework: predicting vertical electron binding energies of the anionic water clusters (H2O)(n)(-).

Anionic water clusters are generally considered to be extremely challenging to model using fragmentation approaches due to the diffuse nature of the excess electron distribution. The local correlation coupled cluster (CC) framework cluster-in-molecule (CIM) approach combined with the completely renormalized CR-CC(2,3) method [abbreviated CIM/CR-CC(2,3)] is shown to be a viable alternative for computing the vertical electron binding energies (VEBE). CIM/CR-CC(2,3) with the threshold parameter ζ set to 0.001, as a trade-off between accuracy and computational cost, demonstrates the reliability of predicting the VEBE, with an average percentage error of ∼15% compared to the full ab initio calculation at the same level of theory. The errors are predominantly from the electron correlation energy. The CIM/CR-CC(2,3) approach provides the ease of a black-box type calculation with few threshold parameters to manipulate. The cluster sizes that can be studied by high-level ab initio methods are significantly increased in comparison with full CC calculations. Therefore, the VEBE computed by the CIM/CR-CC(2,3) method can be used as benchmarks for testing model potential approaches in small-to-intermediate-sized water clusters.

Towards an understanding of the nature of superhalogen anions: an ab initio study of the system

The calculations performed at the OVGF/6-311+G//HF/6-311+G(d) level for the anion matching the superhalogen formula and utilizing very strong electron acceptors (C6F5 groups) as ligands revealed that the resulting vertical electron binding energy of (7.15 eV) is smaller than the analogous value predicted for (9.92 eV). This finding seems surprising taking into account that the electron affinity of the C6F5 radical (4.5 eV) significantly exceeds that of the fluorine atom (3.4 eV). As a consequence, it is postulated that the simple and useful concept of the ‘collective effects’ introduced in the past to explain the large electronic stability of superhalogen anions needs to be further re-examined.

Structure of the Aqueous Electron: Assessment of One-Electron Pseudopotential Models in Comparison to Experimental Data and Time-Dependent Density Functional Theory

The prevailing structural paradigm for the aqueous electron is that of an s-like ground-state wave function that inhabits a quasi-spherical solvent cavity, a viewpoint that is supported by numerous atomistic simulations using various one-electron pseudopotential models. This conceptual picture has recently been challenged, however, on the basis of results obtained from a new electron-water pseudopotential model that predicts a more delocalized wave function and no well-defined solvent cavity. Here, we examine this new model in comparison to two alternative, cavity-forming pseudopotential models. We find that the cavity-forming models are far more consistent with the experimental data for the electron's radius of gyration, optical absorption spectrum, and vertical electron binding energy. Calculations of the absorption spectrum using time-dependent density functional theory are in quantitative or semiquantitative agreement with experiment when the solvent geometries are obtained from the cavity-forming pseudopotential models, but differ markedly from experiment when geometries that do not form a cavity are used.

A one-electron model for the aqueous electron that includes many-body electron-water polarization: Bulk equilibrium structure, vertical electron binding energy, and optical absorption spectrum

Previously, we reported an electron-water pseudopotential designed to be used in conjunction with a polarizable water model, in order to describe the hydrated electron [L. D. Jacobson et al., J. Chem. Phys. 130, 124115 (2009)]. Subsequently, we found this model to be inadequate for the aqueous electron in bulk water, and here we report a reparametrization of the model. Unlike the previous model, the current version is not fit directly to any observables; rather, we use an ab initio exchange-correlation potential, along with a repulsive potential that is fit to reproduce the density maximum of the excess electron's wave function within the static-exchange approximation. The new parametrization performs at least as well as the previous model, as compared to ab initio benchmarks for (H(2)O)(n) (-) clusters, and also predicts reasonable values for the diffusion coefficient, radius of gyration, and absorption maximum of the bulk species. The new model predicts a vertical electron binding energy of 3.7 eV in bulk water, which is 1.4 eV smaller than the value obtained using nonpolarizable models; the difference represents the solvent's electronic reorganization energy following electron detachment. We find that the electron's first solvation shell is quite loose, which may be responsible for the electron's large, positive entropy of hydration. Many-body polarization alters the electronic absorption line shape in a qualitative way, giving rise to a high-energy tail that is observed experimentally but is absent in previous simulations. In our model, this feature arises from spatially diffuse excited states that are bound only by electronic reorganization (i.e., solvent polarization) following electronic excitation.

Accurate calculation of C1s core electron binding energies of some carbon hydrates and substituted benzenes

Approaches, using density functional theory (DFT), to calculate accurate adiabatic and vertical carbon 1s core electron binding energies (CEBE) of some alkanes, alkenes, alkynes and methyl- and fluorine-substituted benzenes are investigated. The approaches tested can be schematized as follows; Δ E KS ( PW 86 × - PW 91 c / TZP + C rel ) / / DFT ( PW 86 × - PW 91 c / TZP ) where Δ E KS is the difference between the Kohn–Sham total energy of the core–hole cation M +, E KS(M +), and the Kohn–Sham total energy of the neutral ground state molecule M, E KS(M). The geometry of M is optimized with DFT(PW86x-PW91c/TZP). For the adiabatic C1s CEBE calculation, the geometry of M + is optimized whereas, for the vertical C1s CEBE calculation, the geometry of M + is identical to the neutral ground state molecule M. C rel represents relativistic corrections. We tested two cases; C rel = 0 eV, and C rel = 0.05 eV. The relativistic correction turned out to be not necessary, because inclusion of the relativistic correction always increased deviation. The current results suggest a systematic error in the calculations that is fortuitously offset by the neglect of relativistic effects. The best approach resulted in average absolute deviations (maximum absolute deviations) from adiabatic experimental values of 0.045 eV (0.130 eV) for calculations of the corresponding C1s CEBE of the alkanes, alkenes, and substituted benzenes for 120 cases. The absolute uncertainty in the experimental measurements is estimated to be 0.03 eV. The average absolute deviation of 0.045 eV is close to the magnitude of the experimental uncertainty. Agreement between theory and experiment is better for adiabatic C1s CEBE than for vertical C1s CEBE.

Direct Measurement of Vertical Electron Binding Energies of Solvated Electrons in Methanol and Ethanol

Abstract Time-resolved photoelectron spectroscopy at ultralow kinetic energy was applied to liquid beams of NaI solutions in methanol and ethanol. Solvated electrons were formed from I− ions in these solutions by charge transfer to solvent reactions. The vertical electron binding energies of the solvated electrons were determined for the first time. Both of the binding energies were found to be 3.10 ± 0.1 eV. This is in reasonable agreement with the dielectric continuum model of solvated electrons using the previously measured vertical binding energy of a hydrated electron. This indicates that the cavity radii of solvated electrons in water, methanol, and ethanol are approximately the same (0.33–0.35 nm).

BX4− and AlX4− Superhalogen Anions (X = F, Cl, Br): An ab Initio Study

The vertical electron detachment energies (VDEs) of 30 MX 4 (-) (M = B, Al; X = F, Cl, Br) anions were calculated at the OVGF level with the 6-311+G(3df) basis sets. The largest vertical electron binding energy was found for the AlF 4 (-) system (9.789 eV). The strong VDE dependence on the symmetry of the species, ligand type, ligand-central atom distance, and bonding/nonbonding/antibonding character of the highest occupied molecular orbital was observed and discussed.

Coupled-cluster and explicitly correlated perturbation-theory calculations of the uracil anion

A valence-type anion of the canonical tautomer of uracil has been characterized using explicitly correlated second-order Moller-Plesset perturbation theory (RI-MP2-R12) in conjunction with conventional coupled-cluster theory with single, double, and perturbative triple excitations. At this level of electron-correlation treatment and after inclusion of a zero-point vibrational energy correction, determined in the harmonic approximation at the RI-MP2 level of theory, the valence anion is adiabatically stable with respect to the neutral molecule by 40 meV. The anion is characterized by a vertical detachment energy of 0.60 eV. To obtain accurate estimates of the vertical and adiabatic electron binding energies, a scheme was applied in which electronic energy contributions from various levels of theory were added, each of them extrapolated to the corresponding basis-set limit. The MP2 basis-set limits were also evaluated using an explicitly correlated approach, and the results of these calculations are in agreement with the extrapolated values. A remarkable feature of the valence anionic state is that the adiabatic electron binding energy is positive but smaller than the adiabatic electron binding energy of the dipole-bound state.

Electron photoemission from charged films: Absolute cross section for trapping –5eV electrons in condensed CO2

The electron trapping or attachment cross section of carbon dioxide (CO2) condensed as thin films on a spacer of Ar is obtained using a simple model for electron trapping in a molecular film and then charge releasing from the same film by photon absorption. The measurements are presented for different electron exposures and impact energies, film thicknesses, and probing photon energies. The cross section for trapping an electron of incident energy between 0 and 5 eV reveals three different attachment processes characterized by a maximum at about 0.75 eV, a structured feature around 2.25 eV, and a shoulder around 3.75 eV. From the measurement of their dependence with the probing photon energy, the two lowest processes produce traps having a vertical electron binding energy of approximately 3.5 eV, whereas the highest one yields a slightly higher value of approximately 3.7 eV. The 0.75 eV maximum corresponds to the formation of vibrational Feshbach resonances in (CO2)n- anion clusters. The 2.25 eV feature is attributed to the formation of a vibrationally excited 2Piu anion in (CO2)n- clusters, followed by fast decay into its vibrational ground state without undergoing autodetachment. Finally, 3.75 eV shoulder is assigned to the well-known dissociative electron attachment process from 2Piu anion state producing the O- anion in the gas phase and the (CO2)nO- anions in clusters.

Negatively Charged Xanthine. I. Anions Formed by Canonical Isomers

The possibility of an excess electron binding to canonical isomers of xanthine in the gas phase was studied at the coupled-cluster level with single and double excitations using the 6-31++G** basis sets supplemented with the 4(sp)3d set of diffuse functions. It was found that xanthine should exist in the gas phase as one canonical tautomer while all the other tautomers are not likely to be detected experimentally because of their significant thermodynamic instability. On the other hand, all canonical tautomers (except one) were found to be capable of forming electronically stable anionic states of dipole-bound nature. However, the only thermodynamically stable anion (with vertical electron binding energy estimated to be 0.041 eV) based on xanthine tautomers is the one supported by the most stable neutral species.

Electron Binding Energies of Aqueous Alkali and Halide Ions: EUV Photoelectron Spectroscopy of Liquid Solutions and Combined Ab Initio and Molecular Dynamics Calculations

Photoelectron spectroscopy combined with the liquid microjet technique enables the direct probing of the electronic structure of aqueous solutions. We report measured and calculated lowest vertical electron binding energies of aqueous alkali cations and halide anions. In some cases, ejection from deeper electronic levels of the solute could be observed. Electron binding energies of a given aqueous ion are found to be independent of the counterion and the salt concentration. The experimental results are complemented by ab initio calculations, at the MP2 and CCSD(T) level, of the ionization energies of these prototype ions in the aqueous phase. The solvent effect was accounted for in the electronic structure calculations in two ways. An explicit inclusion of discrete water molecules using a set of snapshots from an equilibrium classical molecular dynamics simulations and a fractional charge representation of solvent molecules give good results for halide ions. The electron binding energies of alkali cations computed with this approach tend to be overestimated. On the other hand, the polarizable continuum model, which strictly provides adiabatic binding energies, performs well for the alkali cations but fails for the halides. Photon energies in the experiment were in the EUV region (typically 100 eV) for which the technique is probing the top layers of the liquid sample. Hence, the reported energies of aqueous ions are closely connected with both structures and chemical reactivity at the liquid interface, for example, in atmospheric aerosol particles, as well as fundamental bulk solvation properties.

Photoelectron spectroscopy of the doubly-charged anions [MIVO(mnt)2]2- (M = Mo, W; mnt = S2C2(CN)2(2-): access to the ground and excited states of the [MVO(mnt)2]- anion.

Photodetachment photoelectron spectroscopy was used to investigate the electronic structure of the doubly charged complexes [MIVO(mnt)2]2- (M = Mo, W; mnt = 1,2-dicyanoethenedithiolato). These dianions are stable in the gas phase and are minimal models for the active sites of the dimethyl sulfoxide reductase family of molybdenum enzymes and of related tungsten enzymes. Adiabatic and vertical electron binding energies for both species were measured, providing detailed information about molecular orbital energy levels of the parent dianions as well as the ground and excited states of the product anions [MVO(mnt)2]-. Density functional theory calculations were used to assist assignment of the detachment features. Differences in energy between these features provided the energies of ligand-to-metal charge-transfer transitions from S(pi) and S(sigma) molecular orbitals to the singly occupied metal-based orbital of the products [MVO(mnt)2]-. These unique data for the M(V) species were obtained at the C(2)(v)() geometry of the parent M(IV) dianions. However, theoretical calculations and available condensed phase data suggested that a geometry featuring differentially folded dithiolene ligands (Cs point symmetry) was slightly lower in energy. The driving force for ligand folding is a favorable covalent interaction between the singly occupied metal-based molecular orbital (a1 in C2v) point symmetry; highest occupied molecular orbital (HOMO)) and the least stable of the occupied sulfur-based molecular orbitals (b1 in C2v point symmetry, HOMO-1) that is only possible upon reduction to the lower symmetry. This ligand folding induces a large increase in the intensity predicted for the a' S(pi) --> a' dx2 - y2 charge-transfer transition originating from the HOMO-2 of [MVO(mnt)2](-) under Cs point symmetry. Electronic absorption spectra are available for the related species [MoVO(bdt)2]- (bdt = 1,2-benzenedithiolato) and for the oxidized form of dimethyl sulfoxide reductase. The intense absorptions at approximately 1.7 eV have been assigned previously to S(sigma) --> Mo transitions, assuming C2v geometry. The present work indicates that the alternative a' S(pi) --> a' dx2 - y2 of Cs geometry must be considered. Overall, this study confirms that the electronic structure of the M-dithiolene units are exquisitely sensitive to dithiolene ligand folding, reinforcing the proposal that these units are tunable conduits for electron transfer in enzyme systems.