Electron localization functions and local measures of the covariance

The electron localization measure proposed by Becke and Edgecombe is shown to be related to the covariance of the electron pair distribution. Just as with the electron localization function, the local covariance does not seem to be, in and of itself, a useful quantity for elucidating shell structure. A function of the local covariance, however, is useful for this purpose. A different function, based on the hyperbolic tangent, is proposed to elucidate the shell structure encapsulated by the local covariance; this function also seems to work better for the electron localization measure of Becke and Edgecombe. In addition, we propose a different measure for the electron localization that incorporates both the electron localization measure of Becke and Edgecombe and the Laplacian of the electron density; preliminary indications are that this measure is especially good at elucidating the shell structure in valence regions. Methods for evaluating electron localization functions directly from the electron density, without recourse to the Kohn-Sham orbitals, are discussed.     

Electron and hole mobilities in polymorphs of benzene and naphthalene: role of intermolecular interactions

The hole and electron mobilities of the polymorphs of benzene and naphthalene crystals are estimated through quantum chemical calculations. The reorganization energy (lambda) and the charge-transfer matrix elements (Hmn) calculated for the two molecules reveal that these crystals can be used for dual applications, for both hole and electron conductance. The electron mobilities are five to eight times more than the hole mobilities for benzene while for naphthalene, the hole mobilities are almost an order magnitude more than the electron mobilities. The transfer matrices for both hole and electron conductance decrease monotonically with increase in the intermolecular distances. Calculations for various unique stacked dimers as determined from the radial distribution functions in both the crystals for the two molecules show strong dependence on the orientations of the rings and for similar intermolecular separations; Hmnhole is larger than Hmnelectron. The crystal mobilities are calculated from the weighted average over all the unique pair of molecules. The overall preference in a crystal for hole or electron mobility depends on the mutual competition of lambdahole/lambdaelectron and Hmnhole/Hmnelectron. From our microscopic understanding of essential parameters, specific dimers are identified from the crystalline solids of the two polymorphs and experimental strategies are suggested to enrich such pairs in aggregates for enhancing mobilities for these organic solids.     

A surface science approach to ultrafast electron transfer and solvation dynamics at interfaces

Excess electrons in polar media, such as water or ice, are screened by reorientation of the surrounding molecular dipoles. This process of electron solvation is of vital importance for various fields of physical chemistry and biology as, for instance, in electrochemistry or photosynthesis. Generation of such excess electrons in bulk water involves either photoionization of solvent molecules or doping with e.g. alkali atoms, involving possibly perturbing interactions of the system with the parent-cation. Such effects are avoided when using a surface science approach to electron solvation: in the case of polar adsorbate layers on metal surfaces, the substrate acts as an electron source from where photoexcited carriers are injected into the adlayer. Besides the investigation of electron solvation at such interfaces, this approach allows for the investigation of heterogeneous electron transfer, as the excited solvated electron population continuously decays back to the metal substrate. In this manner, electron transfer and solvation processes are intimately connected at any polar adsorbate-metal interface. In this tutorial review, we discuss recent experiments on the ultrafast dynamics of photoinduced electron transfer and solvation processes at amorphous ice-metal interfaces. Femtosecond time-resolved two-photon photoelectron spectroscopy is employed as a direct probe of the electron dynamics, which enables the analysis of all elementary processes: the charge injection across the interface, the subsequent electron localization and solvation, and the dynamics of electron transfer back to the substrate. Using surface science techniques to grow and characterize various well-defined ice structures, we gain detailed insight into the correlation between adsorbate structure and electron solvation dynamics, the location (bulk versus surface) of the solvation site, and the role of the electronic structure of the underlying metal substrate on the electron transfer rate.     

Photoinduced electron transfer and geminate recombination in liquids on short time scales: Experiments and theory

The coupled processes of intermolecular photoinduced forward electron transfer and geminate recombination between the (hole) donor (Rhodamine 3B) and (hole) acceptors (N,N-dimethylaniline) are studied in three molecular liquids: acetonitrile, butyronitrile, and benzonitrile. Two color pump-probe experiments on time scales from approximately 100 fs to hundreds of picoseconds give information about the depletion of the donor excited state due to forward electron transfer and the survival kinetics of the radicals produced by forward electron transfer. The data are analyzed with a model presented previously that includes distance dependent forward and back electron transfer rates, donor and acceptor diffusion, solvent structure, and the hydrodynamic effect in a mean-field theory of through solvent electron transfer. The forward electron transfer is in the normal regime, and the Marcus equation for the distance dependence of the transfer rate is used. The forward electron transfer data for several concentrations in the three solvents are fitted to the theory with a single adjustable parameter, the electronic coupling matrix element Jf at contact. Within experimental error all concentrations in all three solvents are fitted with the same value of Jf. The geminate recombination (back transfer) is in the inverted region, and semiclassical treatment developed by Jortner [J. Chem. Phys. 64, 4860 (1976)] is used to describe the distance dependence of the back electron transfer. The data are fitted with the single adjustable parameter Jb. It is found that the value of Jb decreases as the solvent viscosity increases. Possible explanations are discussed.     

Recent advances in photoinduced electron transfer processes of fullerene-based molecular assemblies and nanocomposites

Photosensitized electron-transfer processes of fullerenes hybridized with electron donating or other electron accepting molecules have been surveyed in this review on the basis of the recent results reported mainly from our laboratories. Fullerenes act as photo-sensitizing electron acceptors with respect to a wide variety of electron donors; in addition, fullerenes in the ground state also act as good electron acceptors in the presence of light-absorbing electron donors such as porphyrins. With single-wall carbon nanotubes (SWCNTs), the photoexcited fullerenes act as electron acceptor. In the case of triple fullerene/porphyrin/SWCNT architectures, the photoexcited porphyrins act as electron donors toward the fullerene and SWCNT. These mechanisms are rationalized with the molecular orbital considerations performed for these huge supramolecules. For the confirmation of the electron transfer processes, transient absorption methods have been used, in addition to time-resolved fluorescence spectral measurements. The kinetic data obtained in solution are found to be quite useful to predict the efficiencies of photovoltaic cells.     

Effective homogeneity of the exchange-correlation and non-interacting kinetic energy functionals under density scaling

Correlated electron densities, experimental ionisation potentials, and experimental electron affinities are used to investigate the homogeneity of the exchange-correlation and non-interacting kinetic energy functionals of Kohn-Sham density functional theory under density scaling. Results are presented for atoms and small molecules, paying attention to the influence of the integer discontinuity and the choice of the electron affinity. For the exchange-correlation functional, effective homogeneities are highly system-dependent on either side of the integer discontinuity. By contrast, the average homogeneity-associated with the potential that averages over the discontinuity-is generally close to 4/3 when the discontinuity is computed using positive affinities for systems that do bind an excess electron and negative affinities for those that do not. The proximity to 4/3 becomes increasingly pronounced with increasing atomic number. Evaluating the discontinuity using a zero affinity in systems that do not bind an excess electron instead leads to effective homogeneities on the electron abundant side that are close to 4/3. For the non-interacting kinetic energy functional, the effective homogeneities are less system-dependent and the effect of the integer discontinuity is less pronounced. Average values are uniformly below 5/3. The study provides information that may aid the development of improved exchange-correlation and non-interacting kinetic energy functionals.     

Quantum-chemical investigation of the structures and electronic spectra of the nucleic acid bases at the coupled cluster CC2 level

We study the ground-state structures and singlet- and triplet-excited states of the nucleic acid bases by applying the coupled cluster model CC2 in combination with a resolution-of-the-identity approximation for electron interaction integrals. Both basis set effects and the influence of dynamic electron correlation on the molecular structures are elucidated; the latter by comparing CC2 with Hartree-Fock and M?ller-Plesset perturbation theory to second order. Furthermore, we investigate basis set and electron correlation effects on the vertical excitation energies and compare our highest-level results with experiment and other theoretical approaches. It is shown that small basis sets are insufficient for obtaining accurate results for excited states of these molecules and that the CC2 approach to dynamic electron correlation is a reliable and efficient tool for electronic structure calculations on medium-sized molecules.     

A semi-empirical MO theory for ionization potentials and electron affinities

A method for calculating the vertical ionization potentials and electron affinities according to their fundamental definition as differences between energies of the singlet ground and doublet ionized states is developed for cyclic hydrocarbons. The method adopts a new approach based on the central idea of a recent ab initio IP and EA calculation in which orbital exponents are optimized for both ground and ionized states. Hence, all the semi-empirical parameters of the MO theory are written as functions of the effective nuclear charge which, in turn, is made self-consistent with the molecular electronic charge distribution of the species. Although the MO theory is developed in the π electron approximation, the changes in the σ electron density, resulting from the loss or gain of a π electron, are explicitly considered in the calculation. The theory is compared to the earlier work of Hoyland and Goodman and tested against the first five polyacenes and on the condensed ring aromatics phenanthrene, pyrene, and perylene. Except for perylene, the results are in close agreement with the latest photoelectron spectroscopic measurements.     

Optimization of electron beam crosslinking of wire and cable insulation

The computer simulations based on Monte Carlo (MC) method and the ModeCEB software were carried out in connection with electron beam (EB) radiation set-up for crosslinking of electric wire and cable insulation. The theoretical predictions for absorbed dose distribution in irradiated electric insulation induced by scanned EB were compared to the experimental results of irradiation that was carried out in the experimental set-up based on ILU 6 electron accelerator with electron energy 0.5–2.0 MeV.The computer simulation of the dose distributions in two-sided irradiation system by a scanned electron beam in multilayer circular objects was performed for various process parameters, namely electric wire and cable geometry (thickness of insulation layers and copper wire diameter), type of polymer insulation, electron energy, energy spread and geometry of electron beam, electric wire and cable layout in irradiation zone. The geometry of electron beam distribution in the irradiation zone was measured using CTA and PVC foil dosimeters for available electron energy range. The temperature rise of the irradiated electric wire and irradiation homogeneity were evaluated for different experimental conditions to optimize technological process parameters. The results of computer simulation are consistent with the experimental data of dose distribution evaluated by gel-fraction measurements. Such conformity indicates that ModeCEB computer simulation is reliable and sufficient for optimization absorbed dose distribution in the multi-layer circular objects irradiated with scanned electron beams.     

Materials and structures for the electron transport layer of efficient and stable perovskite solar cells

The electron transport layer plays a vital function in extracting and transporting photogenerated electrons, modifying the interface, aligning the interfacial energy level and minimizing the charge recombination in perovskite solar cells. This review summarizes the recent research progress on electron transport materials of metal oxides, organic molecules and multilayers. The doped metal oxides as electron transport materials in regular perovskite solar cells show improved device performance relative to their non-doped counterpart due to enhanced electron mobility and energy level alignment. The non-fullerene organic electron transport materials with better electron mobility and tunable energy level alignment need to be further designed and developed despite their advantages of mechanical flexibility and wide range tunability. The multilayer electron transport materials are suggested to be an important direction of research for efficient and stable perovskite solar cells because of their favorable synergistic interaction.     

Theoretical study on two types of weak interactions between methylenecyclopropane and XY (X,Y = H,F, Cl,and Br)

We present theoretical evidence that the two types of interactions exist in the complexes formed between methylenecyclopropane (MECP) and XY (X, Y = H, F, Cl, and Br). Two seats of XY interacted with MECP are located: (a) is via the pseudo‐π bonding electron pair associated with a C? C bond of the cyclopropane ring and (b) is via the typical‐π bonding of electron pair of the C?C bond of MECP. These two types of weak interactions are compared based on the calculated geometries, interaction energies, frequency changes, and topological properties of electron density. The integration of electron density over the interatomic surface is found to be a good measure for the strength of weak interaction. Furthermore, the total electron density and separated σ and π electron densities are also computed and discussed in this article. The separated electron density shows σ electron density determined the strength and π electron density influenced the direction of the hydrogen/halogen bond. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Ultrashort electron pulses for diffraction, crystallography and microscopy: theoretical and experimental resolutions

Pulsed electron beams allow for the direct atomic-scale observation of structures with femtosecond to picosecond temporal resolution in a variety of fields ranging from materials science to chemistry and biology, and from the condensed phase to the gas phase. Motivated by recent developments in ultrafast electron diffraction and imaging techniques, we present here a comprehensive account of the fundamental processes involved in electron pulse propagation, and make comparisons with experimental results. The electron pulse, as an ensemble of charged particles, travels under the influence of the space-charge effect and the spread of the momenta among its electrons. The shape and size, as well as the trajectories of the individual electrons, may be altered. The resulting implications on the spatiotemporal resolution capabilities are discussed both for the N-electron pulse and for single-electron coherent packets introduced for microscopy without space-charge.     

Energetics and bonding in aluminosilicate rings with alkali metal and alkaline-Earth metal charge-compensating cations

The stabilizing effect of alkali and alkaline-earth metal ions on the oxygen donors of four- and six-membered faujausite-like rings has been calculated in terms of Kohn-Sham core-level (O1s) energy shifts with respect to these same complexes without cations. The results confirm and complement earlier investigations by Vayssilov and co-workers where Na(+) and K(+) were the only complexing cations. The oxygen donor centers in six-membered rings are stabilized by -3.6 ± 0.4, -3.9 ± 0.5, -7.3 ± 0.1, and -7.6 ± 0.2 eV by K(+), Na(+), Ca(2+), and Mg(2+) adions, respectively. The energy shifts are even greater for four-membered rings where the stabilization effects attain -3.7 ± 0.1, -4.1 ± 0.1, -8.1 ± 0.1, and -9.0 ± 0.1 eV, respectively. These effects are also observed on the low-lying σ-bonding and antibonding molecular orbitals (MOs) of the oxygen framework, but in a less systematic fashion. Clear relationships with the core-level shifts are found when the effects of alkali metal complexation are evaluated through electron localization/delocalization indices, which are defined in terms of the whole wave function and not just of the individual orbitals. Complexation with cations not only involves a small but significant electron sharing of the cation with the oxygen atoms in the ring but also enhances electron exchange among oxygen atoms while reducing that between the O atoms and the Si or Al atoms bonded to them. Such changes slightly increase from Na to K and from Mg to Ca, whereas they are significantly enhanced for alkaline-earth metals relative to alkali metals. With respect to Al-free complexes, Si/Al substitution and cation charge compensation generally enhance electron delocalization among the O atoms, except between those that are linked through an Al atom, and cause either an increased or a decreased Si-O ionicity (smaller/higher electron exchange) depending on the position of O in the chain relative to the Al atom(s). The generally increased electron delocalization among O atoms in the ring is induced by significant electron transfer from the adsorbed metal to the atoms in the ring. This same transfer establishes an electric field that leads to a noticeable change in the ring-atom core-level energies. The observed shifts are larger for the oxygen atoms because, being negatively charged, they are more easily polarizable than Al and Si. The enhanced electron delocalization among O atoms upon cation complexation is also manifest in Pauling's double-bond nature of the bent σ-bonding MO between nonadjacent oxygen centers in O-based ring structures.     

Ionization potentials and electron affinities of Cu,Ag, and Au: Electron correlation and relativistic effects

The electron correlation and relativistic effects on ionization potentials and electron affinities of Cu, Ag, and Au are investigated in the framework of the coupled cluster method and different 1-component approximations to the relativistic Dirac-Coulomb Hamiltonian. The first-order perturbation approach based on the massvelocity and Darwin terms is found to be sufficiently accurate for Cu and Ag while it fails for Au. The spin-averaged Douglas-Kroll no-pair method gives excellent results for the studied atomic properties. The ionization potentials obtained within this method and the coupled cluster scheme for the electron correlation effects are 7.733(7.735) eV for Cu, 7.461(7.575) eV for Ag, and 9.123(9.225) eV for Au (experimental values given in parentheses). The calculated (experimental) electron affinity results for Cu, Ag, and Au are 1.236(1.226), 1.254(1.303), and 2.229(2.309) eV, respectively. There is a marked relativistic effect on both the ionization potential and electron affinity of Ag which sharply increases for Au while Cu exhibits only a little relativistic character. A similar pattern of relativistic effects is also observed for electric dipole polarizabilities of the coinage metal atoms and their ions. The coupled cluster dipole polarizabilities of the coinage metal atoms calculated in this article in the Douglas-Kroll no-pair formalism (Cu: 46.50 au; Ag: 52.46 au; Au: 36.06 au) are compared with our earlier data for their singly positive and singly negative ions. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 63: 557–565, 1997     

Electron and positron pair emission by low energy positron impact on surfaces

The emission of electron pairs from surfaces has the power to reveal details about the electron–electron interaction in condensed matter. This process, stimulated by a primary electron or photon beam, has been studied both in experiment and theory over the last two decades. An additional pathway, namely positron–electron pair emission, holds the promise to provide additional information. It is based on the notion that the Pauli exclusion principle does not need to be considered for this process.We have commissioned a laboratory based positron source and performed a systematic study on a variety of solid surfaces. In a symmetric emission geometry we can explore the fact that positron and electron are distinguishable particles. Following fundamental symmetry arguments we have to expect that the available energy is shared unequally among positron and electron. Experimentally we observe such a behavior for all materials studied. We find an universal feature for all materials in the sense that on average the positron carries a larger fraction of the available energy. This is qualitatively accounted for by a simplified scattering model. Numerical results, which we obtained by a microscopic theory of positron–electron emission from surfaces, reveal however that there are also cases in which the electron carries more energy. Whether the positron or the electron is more energetic depends on details of the bound electron state and of the emission geometry. The coincidence intensity is strongly material dependent and there exists an almost monotonic relation between the singles and coincidence intensity. These results resemble the findings obtained in electron and photon stimulated electron pair emission. An additional reaction channel is the emission of an electron pair upon positron impact. We will discuss the energy distributions and the material dependence of the coincidence signal which shows similar features as those for positron–electron pairs.     

Resonant vibrational excitation and de-excitation of CO(v) by low energy electrons

Integral cross sections and rate coefficients for vibrational excitation of the excited carbon-monoxide molecule, via the (2)Pi shape resonance in the energy region from 0 to 5 eV have been calculated. Cross sections are calculated by using our recently measured cross sections for the ground level CO excitation and the most recent cross sections for elastic electron scattering, applying the principle of detailed balance. Rate coefficients are calculated for Maxwellian electron energy distribution, with mean electron energies below 5 eV. By using extended Monte Carlo simulations, electron energy distribution functions (EEDF) and rate coefficients are determined in nonequilibrium conditions, in the presence of homogeneous external electric field. Nonequilibrium rates are calculated for typical, moderate values of the electric field over gas number density ratios, E/N, from 1 to 220 Td. Maxwellian and nonequilibrium rate coefficients are compared and the difference is attributed to a specific shape of the electron energy distribution functions under considered conditions.     

Pulse EPR Methods for Studying Chemical and Biological Samples Containing Transition Metals

This review discusses the application of pulse EPR to the characterization of disordered systems, with an emphasis on samples containing transition metals. Electron nuclear double‐resonance (ENDOR), electron‐spin‐echo envelope‐modulation (ESEEM), and double electron–electron resonance (DEER) methodologies are outlined. The theory of field modulation is outlined, and its application is illustrated with DEER experiments. The simulation of powder spectra in EPR is discussed, and strategies for optimization are given. The implementation of this armory of techniques is demonstrated on a rich variety of chemical systems: several porphyrin derivatives that are found in proteins and used as model systems, otherwise highly reactive aminyl radicals stabilized with electron‐rich transition metals, and nitroxide–copper–nitroxide clusters. These examples show that multi‐frequency continuous‐wave (CW) and pulse EPR provides detailed information about disordered systems.     

Free energies for biological electron transfer from QM/MM calculation: method, application and critical assessment

Computer simulations of biological electron transfer reactions are reviewed with a focus on the calculation of reaction free energy (driving force) and reorganization free energy. Then a mixed quantum mechanical/molecular mechanical (QM/MM) approach is described which is designed for computation of these quantities for pure electron transfer reactions with large donor-acceptor separation distances. The method is applied to intra-protein electron transfer in Ru(bpy)(2)(im)His33 cytochrome c and the results compared to experimental data. Several modeling aspects which are important for successful calculation of free energies with QM/MM are discussed in detail.     

Coupling Photocatalysis and Redox Biocatalysis Toward Biocatalyzed Artificial Photosynthesis

In green plants, solar‐energy utilization is accomplished through a cascade of photoinduced electron transfer, which remains a target model for realizing artificial photosynthesis. We introduce the concept of biocatalyzed artificial photosynthesis through coupling redox biocatalysis with photocatalysis to mimic natural photosynthesis based on visible‐light‐driven regeneration of enzyme cofactors. Key design principles for reaction components, such as electron donors, photosensitizers, and electron mediators, are described for artificial photosynthesis involving biocatalytic assemblies. Recent research outcomes that serve as a proof of the concept are summarized and current issues are discussed to provide a future perspective.     

Multiple scattering Xα calculations on the Σ and it electron shape resonances in chloroethylenes and fluoroethylenes

The continuum OS MS Xα method is used to interpret the multiple resonances observed in the electron transmission spectra of tetrachloroethylene and 1,1-dichloroethylene. Good agreement with experiment is found for these molecules and for their fluorine analogues which exhibit one shape resonance. Dissociative electron attachment studies on the halogenoethylenes are interpreted in detail.