Can electrons attract one another?

Electrons are believed to avoid one another in space (correlation) due to the Coulomb repulsion and/or the Pauli exclusion principle. It is shown, using examples of two-electron systems, that indeed the mean electron-electron distance increases in case of the ground electronic state as compared to the independent electron model. It is demonstrated however that there exist excited states, often of low energy, in which the electrons, while having a lot of free physical space (with nuclei being absent), choose to be close to each other in their motion (“anticorrelation”), as if they mutually attracted one another. The source of this effect, quantummechanical in nature, is the orthogonality of the eigenfunctions, that forces the electronic wave functions to differ widely, even at the price of short electron-electron distances. There are also excited states with a mixed behaviour, with complex and often intriguing correlation-anticorrelation patterns.     

Five-coordinate Fe(III)NO and Fe(II)CO porphyrinates: where are the electrons and why does it matter?

Recent years have seen dramatic growth in our understanding of the biological roles of nitric oxide (NO). Yet, the fundamental underpinnings of its reactivities with transition metal centers in proteins and enzymes, the stabilities of their structures, and the relationships between structure and reactivity remains, to a significant extent, elusive. This is especially true for the so-called ferric heme nitrosyls ([FeNO](6) in the Enemark-Feltham scheme). The Fe-CO and C-O bond strengths in the isoelectronic ferrous carbonyl complexes are widely recognized to be inversely correlated and sensitive to structural, environmental, and electronic factors. On the other hand, the Fe-NO and N-O bonds in [FeNO](6) heme complexes exhibit seemingly inconsistent behavior in response to varying structure and environment. This report contains resonance Raman and density functional theory results that suggest a new model for FeNO bonding in five-coordinate [FeNO](6) complexes. On the basis of resonance Raman and FTIR data, a direct correlation between the nu(Fe)(-)(NO) and nu(N)(-)(O) frequencies of [Fe(OEP)NO](ClO(4)) and [Fe(OEP)NO](ClO(4)).CHCl(3) (two crystal forms of the same complex) has been established. Density functional theory calculations show that the relationship between Fe-NO and N-O bond strengths is responsive to FeNO electron density in three molecular orbitals. The highest energy orbital of the three is sigma-antibonding with respect to the entire FeNO unit. The other two comprise a lower-energy, degenerate, or nearly degenerate pair that is pi-bonding with respect to Fe-NO and pi-antibonding with respect to N-O. The relative sensitivities of the electron density distributions in these orbitals are shown to be consistent with all published indicators of Fe-N-O bond strengths and angles, including the examples reported here.     

The role of π electrons in the formation of benzene-containing lithium-bonded complexes

The intermolecular interactions in C₆H₆???LiX (X=OH, NH₂, F, Cl, Br, NC, CN) complexes are investigated by using second‐order Møller–Plesset perturbation theory (MP2) calculations and quantum theory of “atoms in molecules” (QTAIM) studies, and the role of π electrons is studied in the formation of these benzene‐containing lithium‐bonded complexes. The molecular electrostatic potentials of benzene and LiX determine the geometries of the lithium‐bonded complexes. The electron densities at the lithium bond critical points in the πC₆H₆???LiX complexes are obviously stronger than those in the σC₆H₆???LiX complexes, which indicates that the intermolecular interactions in the C₆H₆???LiX complexes are mainly attributable to π‐type interaction. The topological and energy properties at the lithium bond critical points in both the C₆H₆???LiX and πC₆H₆???LiX complexes are linear with the interaction energies, thereby showing the crucial role of the π electrons in the formation of these complexes. Electron localization function (ELF) analysis indicates that the formation of the lithium bonds leads to the reduction of the ELF π‐electron density and volume, and the reduction of the π‐electron volume is linear with the interaction energies with the correction coefficient 0.9949.     

Restricted open-shell Kohn–Sham theory: N unpaired electrons

We present an energy expression for restricted open-shell Kohn–Sham theory for N unpaired electrons. It is shown that it is possible to derive an explicit energy expression for all low-spin multiplets of systems that exhibit neither radial nor cylindrical symmetry. The approach was implemented in the CPMD code and tested for iron complexes.     

An algorithm for depth–dose curves of electrons fitted to Monte Carlo data

An accurate algorithm has been developed for the depth profiles of energy deposition by plane-parallel electron beams normally incident on semi-infinite absorbers. The energies of electrons considered are from 0.1 to 20 MeV, and the absorbers considered are elemental materials of atomic numbers between 4 and 92. Adjustable coefficients in the algorithm have been determined so as to minimize its deviations from the Monte Carlo data published by Tabata et al. (1994). The algorithm is also applicable to light compounds and mixtures.     

Why are quasicrystals quasiperiodic?

In the past two decades significant progress has been made in the search for stable quasicrystals, the determination of their structures and the understanding of their physical properties. Now, quasiperiodic ordering states are not only known for intermetallic compounds, but also for mesoscopic systems such as ABC-star terpolymers, liquid crystals or different kinds of colloids. However, in spite of all these achievements fundamental questions concerning quasicrystal formation, growth and stability are still not fully answered. This tutorial review introduces the research in these fields and addresses some of the open questions concerning the origin of quasiperiodicity.     

Breaking bonds by mechanical stress: when do electrons decide for the other side?

Using first-principles molecular dynamics, we have simulated reactions that can be induced by mechanical stress in a polymer. We have stretched a small piece of poly(ethylene glycol) (PEG) in water at finite temperature. Both the molecule and the solvent were described quantum mechanically on an equal level. The formation of ions was observed, which corresponds to a heterolytic bond cleavage. We were able to monitor the motion of the electrons during the reactions. Our simulations show that the electron transfer and the breaking of the bond occur almost simultaneously and that both processes are initiated by the approach of a solvent molecule toward the destabilized bond.     

Metrology for metalloproteins—where are we now, where are we heading?

The use of the amount of certain proteins in biological samples as markers for distinguishing between a healthy and a diseased state has become increasingly important in clinical diagnosis. As about 30 % of all proteins contain metals in one form or another, either as a cofactor or covalently bound as part of the protein, some of these proteins are regularly analyzed in clinical laboratories. With the increasing number of measurements of those proteins performed all over the world, the necessity of obtaining reliable and comparable results is becoming a focal point for scientists and politicians. Directives such as the EC directive covering in vitro diagnostic medical devices (Directive 98/79/EC) and standards such as EN ISO 17511:2003 demand the traceability of the results obtained for analytes in samples of human origin. However, no reference measurement procedures with results traceable to the SI exist for many metalloproteins. In this article, the situation for a few important metalloproteins, such as hemoglobin, transferrin, superoxide dismutase, ceruloplasmin, and C-reactive protein, for which specific efforts have been made in recent years to achieve comparable and traceable results worldwide, is discussed. These proteins also serve as examples of the difficulties scientists face when they wish to quantify proteins and the pitfalls they should avoid to achieve reliable results.

How useful are direct alpha-measurements of fecal ash? What are their limits?

The Cogema Radiotoxicology Laboratory at La Hague carries out more than 40 000 analyses per year. This figure includes approximately 12 000 systematic detections of actinides in urine and more than 400 analyses of actinides in feces performed following suspicion of an internal contamination incident. The radiochemical analysis of feces, which involves fractionated separation, is a difficult and time-consuming technique which is poorly suited to emergency detections of artificial radioelements. With feces that may contain artificial activity, the laboratory, therefore, uses a feces screening method consisting in a global count of the fecal ash. This method requires the preliminary determination of the background of the natural environmental radioactivity. This prompted our laboratory to carry out a global -count of the fecal ash of 30 people that both work on the site and live in the region but who are not exposed to artificial radioelements. A threshold value of 0.2 Bq/g of natural uranium and its derivatives in fecal ash was established.     

How coordinating are non-coordinating anions?

Electrochemistry of (p-CH₃PhNC)₆Cr⁺³ and optical spectroscopy of [1,2-(9-fluorenyl)₂C₂H₄]ZrMe⁺ have been used to craft scales of coordinating ability of various anions. Coordinating ability also influences the barriers to intramolecular rearrangement in (RCp)₂ZrMe⁺ as well as activity of zirconocenium ions as polymerization catalysts.     

Why are water-hydrophobic interfaces charged?

We report ab initio molecular dynamics simulations of hydroxide and hydronium ions near a hydrophobic interface, indicating that both ions behave like amphiphilic surfactants that stick to a hydrophobic hydrocarbon surface with their hydrophobic side. We show that this behavior originates from the asymmetry of the molecular charge distribution which makes one end of the ions strongly hydrophobic while the other end is even more hydrophilic than the regular water (H2O) molecules. The effect is more pronounced for the hydroxide than for the hydronium. Our results are consistent with several experimental observations and explain why hydrophobic surfaces in contact with water acquire a net negative charge, a phenomenon that has important implications for biology and polymer science.     

Cars spectrum of O2− formed by the trapping of photo-generated electrons on a ZnO surface

Using the planar optical waveguide geometry, two-photon absorption in ZnO generates electrons which are trapped by physisorbed O₂. The CARS spectrum of transient O₂⁻ on a ZnO surface indicates the presence of at least three bound species with vibrational frequencies of 1121, 1133 and 1142 cm⁻¹. The required CARS sensitivity in a UHV environment was achieved by establishing an interference condition within a ZnO waveguide to eliminate the contribution from the non-resonant susceptibility of the ZnO.     

Time-dependent aspects of electron degradation—I. Subexcitation electrons in helium or neon admixed with nitrogen

We present an analysis of the kinetic competition between the moderation of subexcitation electrons in a background gas and the excitation of impurity molecules. The central element of our method is the energy distribution of subexcitation electrons as a function of energy and time. The distribution obeys a linear partial differential equation of the first order, which is in general analytically solvable. The analysis is applied to He and Ne gases admixed with N₂, as studied by Cooper, Denison, and Sauer in their pulse radiolysis experiments. The temporal behavior of N₂ fluorescence, calculated from our analysis, agrees with experiment. The present treatment is an example of a general theory, which represents an extension of the Spencer-Fano theory to time-dependent cases.     

How important is the back reaction of electrons via the substrate in dye-sensitized nanocrystalline solar cells?

The role of the conducting glass substrate (fluorine-doped tin oxide, FTO) in the back reaction of electrons with tri-iodide ions in dye-sensitized nanocrystalline solar cells (DSCs) has been investigated using thin-layer electrochemical cells that are analogues of the DSCs. The rate of back reaction is dependent on the type of FTO and the thermal treatment. The results show that this back-reaction route cannot be neglected in DSCs, particularly at lower light intensities, where it is the dominant route for the back transfer of electrons to tri-iodide. This conclusion is confirmed by measurements of the intensity dependence of the photovoltages of DSCs with and without blocking layers. It follows that blocking layers should be used to prevent the back reaction in mechanistic studies in which the light intensity is varied over a wide range. Conclusions based on studies of the intensity dependence of the parameters of DSCs such as photovoltage and electron lifetime in cells without blocking layers, must be critically re-examined.     

Thermalization lengths of “subexcitation electrons” in water determined by photoinjection from metals into electrolyte solutions

Ranges of 0–4 eV electrons in aqueous electrolyte solutions have been measured using the technique of photoinjection. Variations of the method based on investigation of the dependence of the photoinduced charges on scavenger concentration, the intensity of incident light and time (in nanosecond scale) are used. The values of electron ranges in H₂O are found to amount to 80 Å and in D₂O—110 Å. The mechanism of thermalization of slow electrons in water is discussed. It is caused, mainly, by non-local losses associated with the excitation of the vibration-rotation matter spectrum. During thermalization the slow electrons are subject to numerous elastic scatterings whose cross-section is close to a gas-phase one.     

Measurement of the hall mobility of injected electrons in liquid tetramethylsilane between 22 and 164 °C

We have measured the Hall mobility of excess electrons in liquid tetramethylsilane (TMS) between room temperature and 164°C. The novel result is that, within an experimental error of ≈10%, the Hall mobility μ_h is equal to the drift mobility μ_d; μ_h = 101 ± 7 cm²/V s at 22°C and 53 ± 5 cm²/Vs at 164°C. This indicates that, contrary to what is observed in neopentane, traps are unimportant and, in particular, are not responsible for the mobility drop observed close to the critical point in TMS.