Extremal electron pairs

Extremal pair functions for an n-electron wave function of a closed-shell state are defined as linear combinations of spin-orbital-product pair functions that make some functionals (e.g., r²₁₂ or r⁻¹₁₂) extremal. They are related to the natural spin geminals in the uncorrelated limit and are useful both for an analysis of wave functions in view of an understanding of the chemical bond and for the treatment of electron correlation. Numerical examples are shown and discussed for He₂ as well as the 10-electron systems Ne, HF, H₂O, NH₃, and CH₄. © 1996 John Wiley & Sons, Inc.     

Confirmation of chlorotriazine pesticides,their degradation products and organophosphorus pesticides in soil samples using gas chromatography-mass spectrometry with electron impact and positive- and negative-ion chemical ionization

Gas chromatography-mass spectrometry (GC-MS) with electron impact (EI), positive-ion chemical ionization (PCI) and negative-ion chemical ionization (NCI) were applied as confirmatory techniques for residue analysis of chlorotriazine pesticides, their degradation products and organophosphorus pesticides in soil samples. Clean-up was effected using a Florisil column with subsequent analysis by GC with a nitrogen-phosphorus detector. GC-MS with the EI mode of operation is the common mode of confirmation for all the pesticides. Further confirmation by either GC-MS with PCI and NCI for chlorotriazines and organophosphorus pesticides, respectively, is recommended. The method was applied to the determination of residue levels of atrazine, deethylatrazine, deisopropylatrazine, simazine, fenitrothion and tetrachlorvinphos in several soil samples at levels from 5 ng g^?1 to 9 μg g^?1.     

Competition between reactivity and relaxation in free electron attachment to molecules

The reactivity of the C₆F₅X (X=F, Cl, Br, I) molecules following low energy (0–15 eV) electron attachment is studied in the gas phase under single collision conditions, free molecular clusters and condensed molecules by means of crossed beams and surface experiments. All four molecules exhibit a very prominent resonance for low energy electron attachment (<1 eV, attachment cross section >10⁻¹⁴ cm²). Under collision free conditions thermal electron capture generates long lived molecular parent anions C₆F₅X^−*. Along the line Cl, Br, I dissociation into X⁻+C₆F₅ and X+C₆F₅-increasingly competes until for X=1 only chemical fragmentation is observed on the mass spectrometric time scale. In free molecular clusters chemical fragmentation is quantitatively quenched at low energies in favour of associative attachment yielding undissociated, relaxed ions (C₆F₅X)⁻ _n,n≥1. A further dissociative resonance at 6.5 eV in C₆F₅Cl is considerably enhanched in clusters. If these molecules are finally condensed on a solid surface, one observes a prominent Cl⁻ desorption resonance at 6.5 eV. While the quantitative quenching of the chemical reactivity at low energies is due to the additional possibilities of energy dissipation under aggregation, the enhanched reactivity at 6.5 eV is interpreted by the conversion of a core excited open channel resonance in single molecules into a closed channel (Feshbach) resonance when it is coupled to environmental molecules.     

Comparison of electron impact and chemical ionization mass spectra of some arenetricarbonylchromium complexes

The mass spectrometric behavior of a) the tricarbonylchromium complexes of a series of aromatic hydrocarbons, b) the dimethyldiphenyl compounds of the Group IV elements (i.e., diphenylpropane, dimethyldiphenylsilane, etc.) and c) the mono- and bis-tricarbonylchromium complexes of these ligands under electron impact and chemical ionization conditions are reported. The MH⁺ ion is base peak for all of the simple arenetricarbonylchromium complexes using chemical ionization, whereas [M — 3 CO]⁺ or ⁵²Cr⁺ dominate the spectra with electron impact ionization. The chemical ionization spectra of the series of Group IV element ligands do not exhibit signals in the molecular ion region, the base peak being [M — Ph]⁺. [M — CH₃]⁺ is the electron impact base peak for each of the ligands except the lead-containing compound, for which the base peak is ²⁰⁸Pb⁺. The mono-tricarbonylchromium complexes yield chemical ionization molecular ion clusters, but their base peaks arise via fragmentation of the Group IV element—aromatic ring bonds. Electron impact ionization spectra of the mono complexes are characterized by losses of CO and the production of Cr⁺ ions, neither of which occurs with chemical ionization. For the series of bis-tricarbonylchromium complexes, an MH⁺ ion is prominent only in the chemical ionization spectrum of the diphenylpropane complex. The electron impact induced spectra of the bis-tricarbonylchromium complexes are similar to those of the mono-complexes in that loss of CO is a prominent feature.     

Positive electron impact and chemical ionization mass spectra of some nitramine nitrates

The positive electron impact (EI) and isobutane chemical ionization (CI) mass spectra of six nitramine nitrates were studied with the aid of some accurate mass measurements. In the EI spectra, β fission relative to both the nitramine and nitrate ester is important. In the CI spectra a major ion occurs at [MH – 45]⁺ and was found to be mainly due to [M + 2H ? NO₂]⁺. All of the compounds except N-(2 hydroxyethyl)-N-(2′,4′,6′-trinitrophenyl)nitramine nitrate gave an [MH]⁺ ion. The [MH – 45]⁺ ion in the isobutane CI mass spectra of tetryl is also due to [M + 2H ? NO₂]⁺.     

Intensity of molecular ion peak in electron ionization mass spectra

Compounds from NIST??08 and Wiley 8th databases were considered as a representative subset of the general population of organic compounds which can be analyzed using mass spectrometry with electron ionization. The percentage of organic compounds as a function of intensity of molecular ion (M^+?) peak normalized to the base peak was determined for the first time. It was shown that only 26% compounds have high-intensity M^+? peaks (greater than 50% of base peak). Intensity of M^+? peak is less than or equal to 1 or 5% of base peak for 24 or 37% compounds respectively. It means that in case of trace-level analysis M^+? peak may not be registered for quarter (or even more) of organic compounds. It is well-known that the absence of M^+? peaks in electron ionization mass spectra reduces reliability of unknown compound identification based on library search. Therefore determination of molecular mass by independent technique (e.g., mass spectrometry with chemical ionization) is necessary for increasing the identification reliability.     

Nanosilicon stabilized with ligands: Effect of high-energy electron beam on luminescent properties

Silicon nanopowders with nitrogen heterocyclic carbene (NHC) and butyl as stabilizing ligands were synthesized by bottom up chemical methods. Transmission electron microscopy (TEM) was used to obtain nanoparticle size distribution with 1.8–2.5 mm average diameter. Optical characteristics (photoluminescence and infrared (IR) absorption spectra) of samples were investigated as fabricated and on different steps of irradiation by high-energy 7-MeV electrons. The photoluminescence (PL) spectral changes are slightly different for two cases, but in general, we can see a decrease in luminescence amplitude with fluence growth up to 1.2·10¹⁶ cm⁻², mainly for NHC stabilized nanosilicon. Main mechanisms of radiation-induced changes in nanosilicon sample optical properties are discussed by the joint use of PL and IR spectra analysis.     

Surface analysis of high performance carbon/bismaleimide composites exposed to electron irradiation

The aim of this article was to investigate the effects of electron irradiation in ultrahigh vacuum environment on the surface properties of high‐performance carbon/bismaleimide (BMI) composites used in aerospace. The changes in surface chemical composition with increasing irradiation fluence were studied by XPS. The evolution of surface morphology and surface roughness were observed by atomic force microscopy (AFM). The mass loss behavior occurring in the surface layer of the composites was examined. The results indicated that the electron irradiation in high vacuum caused rupture of chemical bonds and cross‐linking process in the surface layer, thereby leading to the mass loss behavior and the formation of carbonification layer in the surface of the carbon/BMI composites. The changes in the surface chemical composition were determined by a competing effect existing between the rupture of chemical bonds and the cross‐linking process at lower irradiation fluence, and by a degradation process only at higher fluence of electron irradiation. The surface morphology was altered and the surface roughness was increased significantly after electron irradiation. The mass loss ratio first increased obviously at lower fluences, and then reached a plateau value of 0.45% beyond 5 × 10¹⁵ cm^?2 fluence of electron irradiation. Copyright © 2011 John Wiley & Sons, Ltd.     

13C NMR spectroscopy of heteroaromatic compounds: A quantitative measure of electron deficiency and excessiveness

A quantitative measure, ^πΔ, for expressing the π-deficiency and π-excessiveness of heteroaromatic compounds has been developed through ¹³C NMR chemical shift studies. An example which correlates electrophilic reactivity in π-excessive systems with ^πΔ of these compounds is given. The following decreasing order of π-excessiveness and increasing π-deficiency of a number of heteroaromatic systems has been established (numbers in parentheses correspond to ^πΔ): tellurophene (1.83), selenophene (1.43), thiophene (1.35), furan (1.31), benzene (1.00), pyrazine (1.00) pyridine (0.89), pyrimidine (0.77), pyridazine (0.71), 1,2,4-triazine (0.64). These correlations also allow the prediction of ¹³C chemical shifts in substituted heteroaromatic compounds, where the ratio of the difference between a given substituted heterocyclic compound and its parent, and the identically substituted benzene derivative, has been determined.     

Influence of electron beam irradiation on physicochemical properties of poly(trimethylene carbonate)

Electron beam (EB) irradiation of poly(trimethylene carbonate) (PTMC), an amorphous, biodegradable polymer used in the field of biomaterials, results in predominant cross-linking and finally in the formation of gel fraction, thus enabling modification of physicochemical properties of this material without significant changes in its chemical structure. PTMC films (M_w: 167-553 kg mol⁻¹) were irradiated with different doses using an electron accelerator. Irradiation with a standard sterilization dose of 25 kGy caused neither significant changes in the chemical composition of the polymer nor significant deterioration of its mechanical properties. Changes in viscosity-, number-, weight-, and z-average molecular weights of PTMC for doses lower than the gelation dose (D_g) as well as gel-sol analysis and swelling tests for doses above D_g indicate domination of cross-linking over degradation. EB irradiation can be considered as an effective tool for increasing the average molecular weight of PTMC and sterilization of PTMC-based biomaterials.     

Small-angle electron scattering by formaldehyde and ketene: Effects of electron correlation and chemical binding

The total (elastic plus inelastic) intensities of 51 keV electrons scattered by H₂CO and H₂CCO have been measured over a range of K = (4π/λ) sin(θ/2) = 1–9.5 Å^?1 and compared with the theoretical intensities calculated with SCF and CI wave functions. Significant discrepancies are found between the experimental intensities and the theoretical ones based on the SCF wave functions. Most of the chemical binding and electron correlation effects observed in the total scattered intensities are reproduced by the theoretical intensities based on the CI wave functions calculated with the basis set including polarization functions on all atoms. © 1992 John Wiley & Sons, Inc.     

Nanoparticles synthesis by electron beam radiolysis

Electron beam (EB) irradiation is a useful method to generate stable silver nanoparticles without the interference of inherent impurities generated from chemical reactions. Our experiments were carried out using linear electron beam accelerators with two different EB absorbed dose rates: 2 kGy min^?1 and 7–8 kGy s^?1, and with different absorbed dose levels. The optimum conditions for silver nanoparticles (AgNPs) generation by radiolysis, or by radiolysis combined with chemical reduction, were established. In order to obtain a good yield for AgNPs synthesized by radiolysis, a high dose rate is required, resulting in a rapid production process. At low absorbed dose rates, the utilization of a stabilization agent is advisable. By modifying the experimental conditions, the ratio between the chemical and radiolytic reduction process can be adjusted, thus it is possible to obtain nanoparticles with tailored characteristics, depending on the desired application.

Open image in new window

     

Investigation of copper patinas using ion beam analysis and scanning electron microscopy

Ion beam analysis (IBA) techniques were applied successfully to the investigation of non‐corroded and artificially corroded patina layers grown on copper substrates in order to explore their potential use in the study of degradation phenomena of copper and copper alloys subjected to chemical treatment and exposed to selected environmental conditions. Rutherford backscattering spectroscopy (RBS) with deuterons as projectiles and the nuclear reactions ¹⁶O(d,p)¹⁷O and ³²S(p,p′γ)³²S were applied to the investigation of the depth distribution of oxygen and sulphur in near‐surface layers of synthetic patina consisting of mineral phases corresponding to chalcanthite as well as to cuprite + chalcanthite and antlerite + brochantite + chalcanthite. Electrochemical techniques (potentiodynamic polarization and cyclic voltammetry in 0.5 M Na₂SO₄) were used for artificial acceleration and study of the corrosion processes, and scanning electron microscopy (SEM/EDS) was used for examination of the surface morphology of the samples. A patinated roof sample from the Vienna Hofburg also was investigated using the same techniques. The measurement showed that IBA can provide valuable information for the study of patina near‐surface layers of thickness up to a few micrometres and indicated that cuprite was the mineral phase primarily formed on the copper substrates and the main component of the interface between the patina layer and the metallic substrate. The investigated copper patinas looked rather heterogeneous and were characterized by high porosity. Mixed patinas exhibited considerable stability to further corrosive attack. Copyright © 2005 John Wiley & Sons, Ltd.     

DNA double-helix-mediated long-range electron transfer

A theoretical analysis based upon large-scale self-consistent Hartree-Fock calculations at a semiempirical quantum theory level (CNDO/S) is performed to investigate long-range electron transfer in a donor-DNA-acceptor molecule, where the donor and acceptor moieties are tethered to the DNA. The π-stacked base pairs are found to dominate the long-range electronic coupling. Despite the π-electron mediated coupling, the exponential distance decay constant of the electron transfer rate is ∼ 1.2–1.6 Å⁻¹, values typical of electron transfer proteins. The calculated long-range electron transfer rate of the order of 10⁶ s⁻¹ for a metal-to-metal distance of 21 Å is found to be in agreement with kinetic measurements by Meade and Kayyem. Based on the current analysis, the π-electrons dominate the long-range electronic coupling interactions in DNA, but they do not lead to one-dimensional molecular wire-like properties. © 1996 John Wiley & Sons, Inc.     

Analysis of Aluminum-Brass Corrosion Products in Sea Water and in Chloride Solutions

Abstract

A separation and chemical analysis method has been developed of corrosion products forming on aluminum brass surfaces immersed in sodium sulfate solutions at pH= 7.25 (simulating river water) and in sea water at pH= 8.2. This method is based upon a selective solubilization of various oxidation compounds with the aid of proper solvents (i.e. dissolving the metal matrix only to a negligible extent).

These analyses can be performed on oxidation films of more than 100 Å thickness. The following solvents were used:

1) Methanol for solubilization of Na⁺, Cu⁺⁺ and Zn⁺⁺ chlorides and sulfates;

2) Glycine to solubilize bivalent metal compounds:Cu and Zn oxydes oxysulfates, oxycarbonates, oxychlorides of Cu and Zn;

3) NH₄ OH to solubilize Cu⁺⁺ and aluminum compounds.

A fair agreement was observed between the formulae derived by stoichiometric calculation applied to chemical analyses and the values obtained by X-ray analysis.

Agreement was satisfactory for all compounds, except aluminum, determined by chemical analysis and identified by X-rays only in very thick oxidation films (≥ 10³ Å). The role of chemical analysis is therefore determinant to finalize the formation mechanism of aluminum brass passivation films.     

Synthesis and characterization of zirconium(IV) iodovanadate and its use as electron exchanger

Samples of zirconium(IV) iodovanadate have been synthesized under varying mixing ratios by adding a mixture of aqueous solution of 0.1 M potassium iodate and 0.1 M sodium metavanadate to aqueous solution of 0.1 M zirconium oxychloride at pH 1. The ion exchange capacity of the material for Na⁺ ion was found to be 2.20 meq g⁻¹ of dry exchanger. The material has been characterized on the basis of chemical composition, FTIR and TGA. The chemical stability of the product has been checked in neutral, acidic and basic media. The product has been used as electron exchanger. The oxidation of Fe(II), Sn(II), ascorbic acid and thioglycolic acid was achieved by batch-equilibrium technique successfully. The maximum redox capacity of the exchanger has been found to be 4.20 meq g⁻¹ of exchanger by column process.     

An ESR study of electron loss and electron capture by silver (I) ions in methyl cyanide

Exposure of solutions of silver (I) perchlorate in methyl cyanide to ⁶⁰Co γ-rays at 77 K gave Ag(II) and Ag(O) centres in addition to radicals formed from the solvent. Their ESR spectra showed hyperfine features from ¹⁰⁹Ag and ¹⁰⁷Ag together with features from four equivalent ¹⁴N nuclei. The results are contrasted with those previously reported for solutions of silver (I) nitrate in methyl cyanide, which gave Ag(II) and Ag(O) centres whose ESR spectra were devoid of any ¹⁴N hyperfine features.     

Degradation of prednisone in aqueous solutions by electron beam irradiation

A novel degradation study of prednisone (PRE) in aqueous systems (10 mg L^?1) was performed under electron beam irradiation (EBI) in various conditions. The data demonstrate that the highest degradation caused by EBI was obtained with the oxidation of hydroxyl radicals. The proper amount of hydrogen peroxide could increase the PRE total carbon content removal rate. Nine degradation products of PRE in aqueous solution were preliminarily analyzed using ultra-high performance liquid chromatography in combination with mass spectrometry. Furthermore, the chemical analysis and possible degradation pathway were proposed with the assistance of theoretical calculation results by density functional theory.     

Chemical ionization and electron impact mass spectra of thiosulfinates,thiosulfonates and sulfinyl sulfones

The chemical ionization and electron impact mass spectra of some thiosulfinates, thiosulfonates and sulfinyl sulfones have been studied. The electron impact mass spectra of four of the six thiosulfonates show molecular ions of less than 1%. Inconclusive evidence was obtained for sulfenyl sulfinate type intermediates in the electron impact spectra of thiosulfonates. The electron impact spectra of thiosulfonates were similar to those of thiosulfonates. The chemical ionization (isobutane) mass spectra of thiosulfinates and thiosulfonates generally show protonated molecular ions [MH]⁺ as base peaks and [MH+1]⁺ and [MH+2]⁺ peaks.     

The 13C chemical shift and the anisotropy effect of the carbene electron-deficient centre: Simple means to characterize the electron distribution of carbenes

Both the ¹³C chemical shift and the calculated anisotropy effect (spatial magnetic properties) of the electron-deficient centre of stable, crystalline, and structurally characterized carbenes have been employed to unequivocally characterize potential resonance contributors to the present mesomerism (carbene, ylide, betaine, and zwitter ion) and to determine quantitatively the electron deficiency of the corresponding carbene carbon atom. Prior to that, both structures and ¹³C chemical shifts were calculated and compared with the experimental δ(¹³C)/ppm values and geometry parameters (as a quality criterion for obtained structures).