Production of excited electrons ine + e − interactions

We present a complete lowest order calculation of the processe ⁺ e ^?→(e ^*)→e ⁺ e ^? ρ based on a phenomenological Lagrangian where interference with “standard” amplitudes and spin correlation effects are considered. The calculation has been implemented in a Monte Carlo generator of unweighted events and can be used to study interactions with polarized beams as well as the effects of other excited fermions.     

Dinuclear complexes with a μ-cyano ligand. Part VIII(1). Synthesis and characterization of several derivatives ofcis-[CrX(H2O)(en)2]2+ (X=Cl or Br)

Summary The new complex double saltscis-[CrX(H₂O)(en)₂][M(CN)₄] (X=Cl or Br; M=Ni, Pd or Pt) have been synthesised and by anation either in the solid-state or in non-aqueous solvents the corresponding new dinuclear complexes with cyano bridgecis-[(en)₂XCr-NC-M(CN)₃] have been prepared. The complexes have been characterized by chemical analysis and by i.r. and electronic spectroscopy. Powder diffraction patterns indicate that the double salts are isostructural within each series. On the other hand, the dinuclear complexes show low crystallinity. The two dinuclear complexes derived from [Pt(CN)₄]^2– show strong Pt-Pt interactions both in the solid state and in solution.     

Evaluation of chiral recognition ability of a novel uranyl-salophen-based receptor: an easy and rapid testing protocol

A novel member of a new class of chiral uranyl-salophen complexes has been synthesised. The chiral recognition ability of this receptor toward the enantiomers of two primary amines, a sulfoxide, and a quaternary ammonium chloride has been evaluated for the first time. The enantioselectivities obtained are encouraging. The NMR method developed for this purpose allows a fast, quantitative determination of the enantioselectivity of the host directly from its racemic mixture and could find application as a preliminary screening tool in the search for new receptors using combinatorial methods. The experiments carried out in this context demonstrated also that the activation barrier for the racemisation of such chiral uranyl-salophen receptors is much higher than the lower limit of 21 kcal mol(-1) previously reported.     

Vibrational Stark effect and vibrational static electric properties of N2O

The vibrational Stark effect together with nuclear relaxation and vibrational contributions to the static molecular electric properties of the N₂O molecule are computed using ab initio molecular orbital thoery. Contributions to the molecular properties are computed by finite-difference techniques involving the energy vs. the uniform electric-field strength. © 1994 John Wiley & Sons, Inc.     

Determination of the temperature dependence of the H-D spin-spin coupling constant and the isotope effect on the proton chemical shift for the compressed dihydride complex [CpIr(P-P)H2]2+

Complex [CpIr(dmpm)H(2)](2+) (dmpm = bis(dimethylphosphino)methane) has been reported to display temperature-dependent spin-spin coupling constant ((1)J(HD)) and isotope effect on the (1)H NMR chemical shift (Deltadelta). A combined electronic structure density functional theory + quantum nuclear dynamics study is used to determine from first-principles the unusual temperature dependence of the spin-spin coupling constant. It is found that the potential energy surface describing the motion of the Ir-H(2) unit has a deeper minimum in the dihydride region and is characterized by important anharmonicities. These anomalies affect the nature of the vibrational states of the unit and are the main reason for the unusual temperature dependence of (1)J(HD) and Deltadelta. These results suggest experimental tests to identify compressed dihydride transition metal complexes.     

Intrinsic reaction coordinate of perturbed potential energy surfaces: Construction of perturbed energy profiles

This contribution analyzes the changes in the intrinsic reaction coordinate (IRC ) and in the potential energy profiles (PEP ) caused by application of a uniform electric field evaluated using standard ab initio MO methods. Two typical organic processes have been studied, namely, the Friedel–Crafts and the Walden inversion reactions, which are modeled by suitable simple systems. The results for the Walden inversion reaction show that the IRC for the field-free and that for perturbed processes are almost coincident; in this case, using the field-free IRC to compute the perturbed energy profile is a very good approximation. On the contrary, for the Friedel–Crafts model reaction, the two IRC s differ slightly, so the energy profiles using the perturbed and the unperturbed IRC s are somewhat different. © 1993 John Wiley & Sons, Inc.     

Chronoamperometric and Voltammetric Characterization of Gold Ultramicroelectrode Arrays

Some prototypes of photolithographic‐built arrays of gold ultramicroelectrodes (UMEAs) have been electrochemically characterized and compared with a normal‐size electrode. The achievement of steady‐state conditions, effects of supporting electrolyte and the role of conditioning and activation have been considered. The results obtained show that it is possible to monitor the effective area of those devices by chronoamperometry, which confirms the convenience of using the internal auxiliary and reference electrodes and this evidences that steady‐state currents are holding in a wide range of scan rates. Additionally, some preliminary tests show the promising features of gold UMEAs in flow systems like FIA or HPLC.     

The aromatic fluctuation index (FLU): a new aromaticity index based on electron delocalization

In this work, the aromatic fluctuation index (FLU) that describes the fluctuation of electronic charge between adjacent atoms in a given ring is introduced as a new aromaticity measure. This new electronic criterion of aromaticity is based on the fact that aromaticity is related to the cyclic delocalized circulation of pi electrons. It is defined not only considering the amount of electron sharing between contiguous atoms, which should be substantial in aromatic molecules, but also taking into account the similarity of electron sharing between adjacent atoms. For a series of rings in 15 planar polycyclic aromatic hydrocarbons, we have found that, in general, FLU is strongly correlated with other widely used indicators of local aromaticity, such as the harmonic-oscillator model of aromaticity, the nucleus independent chemical shift, and the para-delocalization index (PDI). In contrast to PDI, the FLU index can be applied to study the aromaticity of rings with any number of members and it can be used to analyze both the local and global aromatic character of rings and molecules.     

Electronic structure of Li2Ga and Li9Al4, two solids containing infinite and uniform zigzag chains

The electronic structure of inorganic solids such as Li(2)Ga and Li(9)Al(4) containing infinite zigzag homoatomic chains is discussed. It is shown that Li(2)Ga, a solid for which a Zintl-type electron-counting approach would suggest that a half-filled pi-type band occurs as in trans-polyacetylene, is really a three-dimensional solid with strong covalent interchain connections and small effective charge transfer. The zigzag chains do not play a dominant role as far as the electronic structure near the Fermi level is concerned, and there is no reason for the occurrence of a Peierls distortion despite the possible analogy with trans-polyacetylene. It is suggested that even assuming that a Zintl-type approach is appropriate for electron counting purposes, the infinite zigzag chains in this compound and those in trans-polyacetylene are not isolobal. The bonding in Li(9)Al(4) and Li(2)Ga is very similar, and both phases are predicted to be stable three-dimensional metals.     

Role of carboxylate bridges in modulating nonheme diiron(II)/O(2) reactivity

A series of diiron(II) complexes of the dinucleating ligand HPTP (N,N,N',N'-tetrakis(2-pyridylmethyl)-2-hydroxy-1,3-diaminopropane) with one or two supporting carboxylate bridges has been synthesized and characterized. The crystal structure of one member of each subset has been obtained to reveal for subset A a (micro-alkoxo)(micro-carboxylato)diiron(II) center with one five- and one six-coordinate metal ion and for subset B a coordinatively saturated (micro-alkoxo)bis(micro-carboxylato)diiron(II) center. These complexes react with O(2) in second-order processes to form adducts characterized as (micro-1,2-peroxo)diiron(III) complexes. Stopped-flow kinetic studies show that the oxygenation step is sensitive to the availability of an O(2) binding site on the diiron(II) center, as subset B reacts more slowly by an order of magnitude. The lifetimes of the O(2) adducts are also distinct and can be modulated by the addition of oxygen donor ligands. The O(2) adduct of a monocarboxylate complex decays by a fast second-order process that must be monitored by stopped-flow methods, but becomes stabilized in CH(2)Cl(2)/DMSO (9:1 v/v) and decomposes by a much slower first-order process. The O(2) adduct of a dicarboxylate complex is even more stable in pure CH(2)Cl(2) and decays by a first-order process. These differences in adduct stability are reflected in the observation that only the O(2) adducts of monocarboxylate complexes can oxidize substrates, and only those substrates that can bind to the diiron center. Thus, the much greater stability of the O(2) adducts of dicarboxylate complexes can be rationalized by the formation of a (micro-alkoxo)(micro-1,2-peroxo)diiron(III) complex wherein the carboxylate bridges in the diiron(II) complex become terminal ligands in the O(2) adduct, occupy the remaining coordination sites on the diiron center, and prevent binding of potential substrates. Implications for the oxidation mechanisms of nonheme diiron enzymes are discussed.