Asymmetric Electron Distribution Research Articles

Diradical character dependences of the first and second hyperpolarizabilities of asymmetric open-shell singlet systems

The static first and second hyperpolarizabilities (referred to as β and γ, respectively) of asymmetric open-shell singlet systems have been investigated using the asymmetric two-site diradical model within the valence configuration interaction level of theory in order to reveal the effect of the asymmetric electron distribution on the diradical character and subsequently on β and γ. It is found that the increase of the asymmetric electron distribution causes remarkable changes in the amplitude and the sign of β and γ, and that their variations are intensified with the increase of the diradical character. These results demonstrate that the asymmetric open-shell singlet systems with intermediate diradical characters can exhibit further enhancements of β and γ as compared to conventional asymmetric closed-shell systems and also to symmetric open-shell singlet systems with intermediate diradical characters.

First Measurements of Oblique ECE with a Real-Time Movable Line of Sight on TCV

Electron cyclotron (EC) emission (ECE) radiometers viewing perpendicular to the magnetic field are common on nearly all tokamaks for measuring the electron temperature with good spatio-temporal resolution. Two such radiometers are installed on TCV, one looking from the low field side (LFS) and the other from the high field side (HFS). The HFS radiometer is especially sensitive to non-Maxwellian emission in the presence of the strong EC current drive (ECCD) provided by the 3-MW second-harmonic (X2) EC system as the nonthermal radiation is not reabsorbed by the bulk when passing to the receiver. Simultaneous HFS and LFS measurements allow higher-order modeling of the electron distribution function as more constraints are provided by the dual measurements; however, the asymmetric nature of the electron distribution function required for ECCD to occur is not directly put in evidence by these lines of sight. Oblique ECE measurements of an asymmetric nonthermal electron distribution, on the other hand, are expected to also be asymmetric and can provide important information on the current-carrying features of the nonthermal population. A dedicated receiving antenna has been installed allowing real-time swept oblique ECE on TCV in both the co- and counter-looking directions. Proof-of-principle experiments are described in which Doppler-shifted emission is measured.

Divinylphenylene-Bridged Diruthenium Complexes Bearing Ru(CO)Cl(PiPr3)2Entities

The divinylphenylene-bridged diruthenium complexes (E,E)-[{(PiPr3)2(CO)ClRu}2(μ-HCCHC6H4CHCH-1,3)] (m-2) and (E,E)-[{(PiPr3)2(CO)ClRu}2(μ-HCCHC6H4CHCH-1,4)] (p-2) have been prepared and compared to their PPh3-containing analogues m-1 and p-1. The higher electron density at the metal atoms increases the contribution of the metal end groups to the bridge-dominated occupied frontier orbitals and stabilizes the various oxidized forms with respect to those of m-1 and p-1. This has been confirmed and quantified electrochemically, because the two reversible oxidation waves were observed at considerably lower potentials than for the PPh3 complexes. Owing to their greater stability, the one- and two-electron-oxidized forms m-2n+ and p-2n+ of both complexes could be generated and spectroscopically characterized inside an optically transparent thin layer electrolysis cell. UV/vis/near-IR and ESR spectroelectrochemistry indicates that the oxidation processes are centered at the organic bridging ligand. σ-Bonded divinylphenylenes thus constitute an unusual class of “noninnocent” ligands for organometallic compounds. Electronic transitions observed for the mono- and dioxidized forms closely resemble those of donor-substituted phenylenevinylene compounds, including oligo(phenylenevinylenes) (OPVs) and poly(phenylenevinylene) (PPV) in the respective oxidation states. Strong ESR signals and nearly isotropic g tensors are observed for the monocations in fluid and frozen solutions. The metal contribution to the redox orbitals is illustrated by a shift of the CO stretching bands to notably higher energies upon stepwise oxidation. The shifts strongly exceed those observed for the PPh3 containing, six-coordinated species (E,E)-[{(PPh3)2(CO)Cl(L)Ru}2(μ-HCCHC6H4CHCH)]n+ (L = substituted pyridine). IR spectroelectrochemistry reveals the presence of two electronically different transition-metal moieties in m-2+, while they resemble each other more closely in p-2+. Differences in electronic coupling are illustrated by the charge distribution parameters calculated from the spectra. Bulk electrolysis experiments confirm the results from the in situ spectroelectrochemistry and the overall stoichiometry of the redox processes. Quantum-chemical calculations were performed in order to provide insight into the nature and composition of the frontier orbitals. The electronic transitions observed for the neutral forms were assigned by TD DFT. IR frequencies calculated for m-2 and p-2 in their various oxidation states retrace the experimental observations. They fail, however, in the case of m-2+, where a symmetrical structure is calculated, as opposed to the distinctly asymmetric electron distribution observed by IR spectroscopy. Geometry-optimized structures were calculated for all accessible oxidation states. The structural changes following stepwise oxidation agree well with the experimental findings: e.g., a successive low-energy shift of the CC stretching vibration of the bridge. The radical cation m-2+ displays a broad composite electronic absorption band at low energy that extends into the mid-IR region.

“Separated” versus “Contact” Ion-Pair Structures in Solution from Their Crystalline States: Dynamic Effects on Dinitrobenzenide as a Mixed-Valence Anion

Qualitative structural concepts about dynamic ion pairs, historically deduced in solution as labile solvent-separated and contact species, are now quantified by the low-temperature isolation of crystalline (reactive) salts suitable for direct X-ray analysis. Thus, dinitrobenzenide anion (DNB(-)) can be prepared in the two basic ion-paired forms by potassium-mirror reduction of p-dinitrobenzene in the presence of macrocyclic polyether ligands: L(C) (cryptand) and L(E) (crown-ethers). The crystalline "separated" ion-pair salt isolated as K(L(C))(+)//DNB(-) is crystallographically differentiated from the "contact" ion-pair salt isolated as K(L(E))(+)DNB(-) by their distinctive interionic separations. Spectral analysis reveals pronounced near-IR absorptions arising from intervalence transitions that characterize dinitrobenzenide to be a prototypical mixed-valence anion. Most importantly, the unique patterns of vibronic (fine-structure) progressions that also distinguish the "separated" from the "contact" ion pair in the crystalline solid state are the same as those dissolved into THF solvent and ensure that the same X-ray structures persist in solution. Moreover, these distinctive NIR patterns are assigned with the aid of Marcus-Hush (two-state) theory to the "separated"ion pair in which the unpaired electron is equally delocalized between both NO(2)-centers in the symmetric ground state of dinitrobenzenide, and by contrast, the asymmetric electron distribution inherent to "contact"ion pairs favors only that single NO(2)-center intimately paired to the counterion. The labilities of these dynamic ion pairs in solution are thoroughly elucidated by temperature-dependent ESR spectral changes that provide intimate details of facile isomerizations, ionic separations, and counterion-mediated exchanges.

Electronic structures of rocksalt, litharge, and herzenbergite SnO by density functional theory

Density functional theory calculations have been performed on SnO in the litharge, herzenbergite, and rocksalt crystal structures. An asymmetric electron distribution was found around the Sn atoms in litharge and herzenbergite SnO which could be ascribed to a $\mathrm{Sn}\phantom{\rule{0.3em}{0ex}}5{s}^{2}$ sterically active ``lone pair.'' Analysis of the electronic structure shows that the states responsible for the asymmetric Sn electron distribution are due to the coupling of unfilled $\mathrm{Sn}(5p)$ with the antibonding combination arising from interaction of $\mathrm{Sn}(5s)$ and $\mathrm{O}(2p)$. The coupling of $\mathrm{Sn}(5p)$ was found to be active in both the formation of the asymmetric density and the stabilization of the litharge and herzenbergite phases. Due to the symmetry of the interaction the coupling of $\mathrm{Sn}(5p)$ with the antibonding states can only take place on distorted Sn sites, explaining the absence of an asymmetry in the rocksalt structure. In contrast to the classical view that the Sn(II) ``lone pair'' forms directly through hybridization of $\mathrm{Sn}\phantom{\rule{0.3em}{0ex}}5s$ and $5p$, our calculations confirm for the first time, through COOP analysis, that it is only through the interaction of the oxygen $2p$ states that formation of the asymmetric density is achieved.

Electronic Structure of Perovskite‐Type YBRh3: X‐Ray Photoelectron Spectroscopy and ab initio Band Calculations.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Electronic structure of perovskite-type YBRh 3: X-ray photoelectron spectroscopy and ab initio band calculations

The perovskite structure type YBRh 3 was synthesized by the arc melting method. Its electronic structure was studied by XPS and ab initio band calculations. The chemical shifts of the Y and Rh 3d levels from the elements were negative. The experimental and calculated valence band XPS coincided well with each other. The band calculation indicated that the metallic behavior of the compound originates from the Rh 4d electrons. The atomic distance between Y and Rh is 93% of the sum of the radii of the elements. The electron density has a maximum between the Y and Rh atoms, and the distance between the maximum point and center of the Y atom is 77% of its atomic radius. This indicates that charge transfer occurs from Y to Rh. The nearly equal electronegativities of B and Rh result in a covalent bond between them, which is shown as an asymmetric electron distribution in the circumference of the B and Rh atoms. The covalent bond results from hybridization of the B 2p and Rh 4d levels.

Noise breaking the twofold symmetry of photosynthetic reaction centers: electron transfer.

In this work we present a stochastic model to elucidate the unidirectionality of the primary charge separation process in the bacterial reaction centers where two symmetric ways of electron transfer (ET), starting from the common electron donor, are possible. We have used a model of three sites/molecules with ET beginning at site 1 with the option to proceed to site 2 or site 3. If the direct ET between sites 2 and 3 is not allowed and electron cannot escape from the system then it is shown that the different stochastic fluctuations in the energy of sites and the interaction between sites on these two ways are sufficient to cause the transient asymmetric electron distribution at site 2 and 3 during relaxation to the steady state. This means that overall asymmetric ET can be caused by the transient asymmetric electron distribution if there is a possibility for an electron to escape from the three-site system. To explore this possibility we have introduced a sink into the model at the end of each of the sites 2 and 3. The dependence of the asymmetry in electron transfer on the value of the sink parameter, introduced through an additional imaginary diagonal matrix element of the Hamiltonian, was investigated. Results show indeed that the unidirectionality of the electron transfer generated in the system of three molecules depends strongly on the sink parameter value.

The origin of the electron distribution in SnO

Gradient corrected density functional theory calculations have been performed on SnO in the litharge and idealized CsCl structures with the litharge structure in good agreement with experiment. The CsCl structured SnO has a spherical electron density whereas the litharge structured SnO has a nonspherical electron distribution. Such asymmetry is often attributed to a sterically active lone pair formed by the 5s2 electrons which does not take part in chemical bonding. However, analysis of the density of states and band structures indicates that the situation is more complicated. In CsCl structured SnO mixing of the Sn 5s with the oxygen 2p electronic states results in filled bonding and antibonding combinations. The antibonding combinations, at the top of valence band, can interact with Sn 5p to stabilize the structure, only when in the distorted litharge structure resulting in the asymmetric electron density. This is in contrast to the classical theory of hybridization of the tin 5s and 5p orbitals to form a “lone pair” as the asymmetric electron distribution is the result of the tin–oxygen covalent interactions.

Influence of a piezoelectric field on the electron distribution in a double GaN/Al0.14Ga0.86N heterojunction

C–V profiling of Al0.14Ga0.86N/GaN heterojunctions was performed. It was found that a heterojunction with the Al0.14Ga0.86N layer on top increases the electron concentration at the Al0.14Ga0.86N/GaN interface, while the reversed structure with the GaN layer on top decreases it. In accordance with this result, an Al0.14Ga0.86N/GaN double heterojunction was found to experience a strongly asymmetric electron distribution with an enhancement of the electron concentration at the interface closest to the sample surface. This effect is attributed to the presence of a piezoelectric field redistributing the electrons in the heterostructure.

Electron Distribution in the Two-Electron Reduced Isopolytungstate [W10O32]6-

A simple model is proposed to calculate the charge distribution in the two-electron-reduced isopolytungstate [W10O32].6- It is shown that the experimentally observed asymmetric excess electron distribution is due to competition of the electronic delocalization between equatorial metal sites and Coulomb repulsion.

Kinetics and mechanism of reduction of gold(III) complexes by dimethyl sulfide

The reactions between trans-[Au(CN) 2 X 2 ] - (X = Cl or Br) and Me 2 S have been studied by conventional and high-pressure stopped-flow spectrophotometry in acidic aqueous solution containing 10% (w/w) methanol. The overall stoichiometry Au III ∶Me 2 S is 1∶(1.0 ± 0.1) in agreement with the reaction: trans-[Au(CN) 2 X 2 ] - + Me 2 S + H 2 O → [Au(CN) 2 ] - + Me 2 SO + 2H + + 2X - . Initial rapid substitution processes result in the formation of a pre-equilibrium between transient gold(III) complexes, which are reduced to [Au(CN) 2 ] - in a subsequent slower redox process. Complexes trans-[Au(CN) 2 X(Me 2 S)] with an asymmetric electron distribution along the X–Au–S axis are reduced rapidly via an intermolecular process, in which Me 2 S attacks a co-ordinated halide. The complex trans-[Au(CN) 2 (Me 2 S) 2 ] , on the other hand, undergoes slow reduction to gold(I) involving a water molecule. The rapid halide-mediated oxidation of thioethers implies that reduction of metal ions in biological systems by such moieties should be favoured in extracellular environments, where the chloride concentrations are high.

Asymmetric binding of the primary acceptor quinone in reaction centers of the photosynthetic bacterium Rhodobacter sphaeroides R26, probed with Q-band (35 GHz) EPR spectroscopy

The reaction center (RC)-bound primary acceptor quinone Q A of the photosynthetic bacterium Rhodobacter sphaeroides R26 functions as a one-electron gate. The radical anion Q •− A is proposed to have an asymmetric electron distribution, induced by the protein environment. We replace the native ubiquinone-10 (UQ10) with specifically 13C-labelled UQ10, and use Q-band (35 GHz) EPR spectroscopy to investigate this phenomenon in closer detail. The direct observation of the 13C-hyperfine splitting of the g z-component of UQ10 •− A in the RC and in frozen isopropanol shows that the electron spin distribution is symmetric in the isopropanol glass, and asymmetric in the RC. Our results allow qualitative assessment of the spin and charge distribution for Q •− A in the RC. The carbonyl oxygen of the semiquinone anion nearest to the S = 2 Fe 2+-ion and Q B is shown to acquire the highest (negative) charge density.

Ultrashort powerful laser matter interaction: Physical problems, models, and computations

The different physical models, computational methods, and results describing the properties of a plasma produced by an ultrashort laser pulse interacting with a solid target at different laser intensities are presented. The basic issues affecting the laser-plasma interaction are considered, including: the different absorption mechanisms; the formation of a transient, nonequilibrium, asymmetric electron distribution function; the energy losses; the role of instabilities and the ponderomotive force; ion expansion and acceleration; and surface wave propagation along the boundary of the plasma. The article concludes with a discussion of the range of applicability of the different approximations and future perspectives for the field.

Short Interactions between Heterocyclic Sulfer Atoms and Thiocarbonyl Sulfer or Carbonyl Oxygen Atoms

Short intramolecular S---S=C contacts in N-(2-thiazolylidene)-benzenecarbothioamides (11 and 12), 0.60angstrom less than the corresponding separation predicted by Van der Waals radii, are accounted for by Coulombic interactions and 3d orbital participation as a result of electron delocalisation within each molecule. Similar considerations apply to S---O=C contacts, however short S---S intermolecular interactions between sulfides are due to the asymmetric electron distribution about the bonded sulfide atoms.

Negative absolute conductivity in quantum wires

The effect of negative absolute conductivity in quasi-one-dimensional quantum wire structures is obtained by the Monte Carlo simulation. This negative conductivity is associated with inelastic optical phonon scattering leading to an asymmetric electron distribution function established under conditions of intensive electron photoinjection. Simulation results suggest that quantum wires are ideal for the experimental observation of negative absolute conductivity. The oscillations of photoconductivity as a function of injection energy can reveal the spectrum of optical phonons in quantum wires which differs considerably from that in bulk materials.

Oscillations of photoconductivity and negative absolute conductivity in quantum wires

The phenomenon of oscillating photoconductivity as a function of photoinjection energy is studied by Monte Carlo simulation in quasi-one-dimensional quantum wire structures. It is demonstrated that the amplitude of the oscillations of photoconductivity may be so large that this leads to a negative absolute conductivity at injection energies that are multiples of the optical phonon energy. These oscillations are associated with inelastic optical phonon scattering, leading to an asymmetric electron distribution function established under conditions of intensive electron photoinjection to the subband bottom or close to energies that are multiples of the optical phonon energy. Simulation results suggest that quasi-one-dimensional quantum wires are ideal for the experimental observation of negative absolute conductivity at low lattice temperatures.

Investigation of superthermal asymmetric electron distributions using electron cyclotron wave transmission in tokamaks

The asymmetric electron distribution generated during lower hybrid current drive has been computed using a 3-D Fokker-Planck code. The superthermal tail and the resulting current are generally a combination of two components streaming in opposite toroidal directions. An appropriate diagnostic method for experimental investigation of the two superthermal populations is wave transmission of two equivalent rays with equal and opposite values of the refractive index. These equivalent rays can be realized by launching the waves from symmetric positions with respect to the equatorial plane at equal and opposite angles in the toroidal direction. Using an appropriate ray tracing code, the damping of the two rays is computed and it is shown that it results from electrons with opposite parallel velocities. The differential transmission is then a measure of the overall asymmetry of the electron momentum distribution.