Reactions in clusters of acetone and fluorinated acetones triggered by low energy electrons

Electron attachment to clusters of acetone (A), trifluoroacetone (TFA) and hexafluoroacetone (HFA) is studied in a crossed beam experiment with mass spectrometric detection of the anionic products. We find that the electron attachment properties in A change dramatically on going from isolated molecules to clusters. While single acetone is a very weak electron scavenger (via a dissociative electron attachment (DEA) resonance near 8.5 eV), clusters of A capture electrons at very low energy (close to 0 eV). The final ionic products consist of an ensemble of molecules (M) subjected to the loss of two neutral H₂ molecules ((M_n−2H₂)⁻, n ≥ 2). Their formation at low energies can only be explained by invoking new cyclic structures and polymers. In clusters of TFA, anionic complexes containing non-decomposed molecules (M_n⁻) including the monomer (M⁻) and ionic products formed by the loss of one and two HF molecules are observed. Loss of HF units is also interpreted by the formation of new cyclic structures in the anionic system. HFA is a comparatively stronger electron scavenger forming a non-decomposed anion via a narrow resonant feature near 0 eV in the gas phase. In HFA clusters, the non-decomposed parent anion is additionally observed at higher electron energies in the range 3–9 eV. The M⁻ signal carries signatures of self-scavenging processes, i.e., inelastic scattering by one molecule and capture of the completely slowed down electron by a second molecule within the same cluster. The scavenging spectrum is hence an image of the electronically excited states of the neutral molecule.     

On the structure of the LiH dimer and its negative ion

Ab initio calculations are presented on the structure of the lithium hydride dimer and its negative ion. The most interesting finding of this study is that the anion, resulting primarily from the binding of an electron in the large dipole field (≈ 14.8 debye) of linear LiHLiH, is stable with respect to the neutral dimer. Similar results are predicted for the alkali halide dimers [(MX)₂]. We suggest that the appearance potential for MX⁻ production via the dissociative attachment process, e⁻ + (MX)₂ → MX⁻ + M + X, may be due to the linear form of the dimers. In earlier work by Ebinghaus, it was assumed that the appearance potential for MX⁻ production from this reaction is due to the cyclic form of the dimers.     

Angular distributions of the slow and fast groups of excited hydrogen atom produced in e-H2 collisions

The angular dependence has been investigated for the slow and the fast groups of H*(n = 4) resulting from H₂ dissociative electron impact. The slow group is isotropic due to coexistence of several processes and/or effects of predissociation. The fast group shows a sin²θ dependence and is produced mainly through Rydberg states of II_u symmetry.     

Translational energy distributions and production mechanisms of the excited hydrogen atom (n = 3,4) produced in e-NH3 collisions

The Balmer α and β lines produced in e-NH₃ collisions have been measured precisely with the use of a Fabry-Perot interferometer. These lines are not polarized. The translational energy distributions of (H^*(n = 3,4) were determined from analysis of Doppler lineshapes and have five components; their peaks lie at 1, 3, 2, 4–5 and 8–12 eV. The excitation function [H^*(n = 4)] has five thresholds at 22.5, 29.0, 33.3, 38.9 and 41.7 eV, and indicates that five dissociation processes contribute to the formation of H^*. Excitation to the Rydberg states converging to the (2a₁)^?1 state of NH₃⁺ is a major process for the formation of the first and the second components. Doubly excited Rydberg states play important roles in the dissociative excitation of NH₃.     

Catalytic electrooxidation of hydrazine at the nickel ferricyanide modified electrode: can an array of surface bound one-electron redox centers act in concert?

The [NiFe(CN)₆]^2−/− derivatized nickel electrode represents an electrocatalytic surface for the mediated oxidation of hydrazine. The four-electron oxidation of hydrazine is employed as a probe of multiple electron charge transfer processes at the Ni[Fe(CN)₆]^2−/− interface since the observed product distribution is sensitive to the detailed mechanism and stoichiometry of electron transfer giving rise to production of NH₃ if only one-electron transfer processes occur, while yielding N₂H₂ or N₂ if multi-electron processes are present. A correlation between surface overlayer structure and charge transfer pathway is observed. A rather facile, pseudo-first order heterogeneous charge transfer rate (~3 × 10⁻² cm s⁻¹) was found for N₂ production. The kinetics of the processes involved in this electrocatalysis were analyzed according to the Savéant-Andrieux theory. The electrocatalytic behavior of this system can be varied from R type behavior to R + S or (SR) and then to S + E behavior by varying the amount of the surface attached catalyst.     

Laboratory investigations of negative ion molecule reactions of formic and acetic acids: implications for atmospheric measurements by ion-molecule reaction mass spectrometry

Laboratory measurements of gas-phase ion-molecule reactions of several negative ion species with formic and acetic acid have been carried out. A flow reactor operating at a temperature of 293 ± 3 K and total gas pressures of either 3 or 9 hPa was used. The negative reagent ion species investigated included OH⁻, O₂⁻, O₃⁻, CO₄⁻, CO₃⁻, CO₃⁻H₂O, HCO₃⁻H₂O, NO₃⁻, NO₃⁻H₂O, NO₂⁻, and NO₂⁻H₂O. The reactions were found to proceed either via proton transfer or clustering. Our measurements of ion-molecule reactions of negative ions with gaseous formic and acetic acids provide a firm base for quantitative detection of these acidic trace gases in the atmosphere by negative ion ion-molecule reaction mass spectrometry.     

Dissociative electron attachment to gas-phase glycine

By using a high-resolution electron energy monochromator low-energy electron attachment to gas-phase glycine (H₂NCH₂COOH, or G) has been studied by means of mass spectrometric detection of the product anions. In the same way as for several other biologically relevant molecules no stable parent anion was formed by free electron attachment. The largest dissociative electron attachment (DEA) cross-section, approximately 5×10^–20 m², was observed for (G–H)^–+H at an electron energy of 1.25 eV. Glycine and formic acid (HCOOH) have several common features, because a precursor ion can be characterized by electron attachment to the unoccupied * orbital of the –COOH group. At higher incident electron energies several smaller fragment anions are formed. Except for H^–, which could not be observed in this study, there was good agreement with an earlier investigation by Gohlke et al.     

Detection of CH in an expanding argon/acetylene plasma using cavity ring down absorption spectroscopy

Cavity ring down (CRD) absorption spectroscopy is used to measure the methylidyne (CH) radical in an Ar/C₂H₂ plasma. The rotational spectrum of the A ²Δ (v′=0) ← X ²Π (v′′=0) transition around 430 nm is recorded to determine the total CH ground state density, both as a function of the current through the arc producing the low-pressure Ar plasma and as a function of the injected acetylene flow. Total ground state densities between 5×10¹⁵ and 8×10¹⁶ m⁻³ are detected. The trends show that the methylidyne radical plays a minor role in the growing mechanism of hydrogenated amorphous carbon films and is predominantly formed in the charge exchange/dissociative recombination channel starting from the C₂H radical.     

Determination of Erbium by Two-Color Three-Photon Resonance Ionization Mass Spectrometry

Excited states and autoionization states of the erbium atom were investigated by the use of multicolor resonance ionization mass spectrometry. Among the observed first excited states, a level [4f¹²(³H₆)6s6p(³P₁)] located 17,348 cm⁻¹from the ground state is regarded as the most efficient state for excitation within the wavelength range investigated (560–600 nm), while a level located 17,080 cm⁻¹from this first excited state (E= 34,458 cm⁻¹) is identified as the best second excited state for the optimal photoionization scheme. Many ionization schemes adopting an autoionization state are also investigated, and the most efficient scheme is identified as 4f¹²6s²(³H₆) → 4f¹²(³H₆)6s6p(³P₁⁰), 17,348 cm⁻¹→ 34,458 cm⁻¹→ continuum state, which corresponds to the two-color (ω₁+ ω₂+ ω_1,2) scheme. Various concentrations of standard solutions for erbium are determined and the minimum amount detectable by two-color three-photon ionization was determined to be 20 pg.     

Electrochemical, spectroelectrochemical and theoretical studies on the reduction and deprotonation of the photovoltaic sensitizer [(H3-tctpy)Ru(NCS)3] (H3-tctpy=2,2′:6′,2′′-terpyridine-4,4′,4′′-tricarboxylic acid)

The electrochemical reduction of the black dye photosensitizer [(H₃-tctpy)Ru^II(NCS)₃]⁻ (H₃-tctpy=2,2′:6′,2′′-terpyridine-4,4′,4′′-tricarboxylic acid) used in photovoltaic cells has been found to be a complex process when studied in dimethylformamide. At low temperatures, fast scan rates and at a glassy carbon electrode, the chemically reversible ligand based one-electron reduction process [(H₃-tctpy)Ru(NCS)₃]⁻+e⁻[(H₃-tctpy√⁻)Ru(NCS)₃]²⁻ is detected. This process has a reversible half-wave potential (E^r_1/2) of −1585±20 mV versus Fc/Fc⁺ at 25°C. Under other conditions, a deprotonation reaction occurs upon reduction, which produces [(H_3−x-tctpy^x−)Ru(NCS)₃]^(1+x)− and hydrogen gas. Mechanistic pathways giving rise to the final products are discussed. The E^r_1/2-value for the ligand based reductions of the deprotonated complex is 0.70 V more negative than for [(H₃-tctpy)Ru(NCS)₃]⁻. Consequently, data obtained from molecular orbital calculations are consistent with the reaction [(H₃-tctpy)Ru(NCS)₃]⁻+e⁻→[(H₂-tctpy⁻)Ru(NCS)₃]²⁻+1/2H₂ yielding the monodeprotonated complex as the major product obtained after electrochemical reduction of [(H₃-tctpy)Ru(NCS)₃]⁻. The E^r_1/2-values for the metal based Ru^II/III process differ by 0.30 V when data obtained for the protonated and deprotonated forms of the black dye are compared. Electronic spectra obtained during the course of experiments in an optically transparent thin layer electrolysis configuration are consistent with the overall reaction scheme proposed on the basis of voltammetric measurements and molecular orbital calculations. Reduction studies on the free ligand, H₃-tcpy, are consistent with results obtained with [(H₃-tctpy)Ru(NCS)₃]⁻.     

UV–visible spectrum of the phenyl radical and kinetics of its reaction with NO in the gas phase

Pulse radiolysis transient UV–visible absorption spectroscopy was used to study the UV–visible absorption spectrum (225–575 nm) of the phenyl radical, C₆H₅(), and kinetics of its reaction with NO. Phenyl radicals have a strong broad featureless absorption in the region of 225–340 nm. In the presence of NO phenyl radicals are converted into nitrosobenzene. The phenyl radical spectrum was measured relative to that of nitrosobenzene. Based upon σ(C₆H₅NO)_{270 nm}=3.82×10⁻¹⁷ cm² molecule⁻¹ we derive an absorption cross-section for phenyl radicals at 250 nm, σ(C₆H₅())_{250 nm}=(2.75±0.58)×10⁻¹⁷ cm² molecule⁻¹. At 295 K in 200–1000 mbar of Ar diluent k(C₆H₅()+NO)=(2.09±0.15)×10⁻¹¹ cm³ molecule⁻¹ s⁻¹.     

Double electropolymer modified platinum electrode to follow the kinetic process H2O2 + ascorbic acid. Influence of the reaction on amperometric biosensor applications

A Pt electrode modified by a polypyrrole/poly(orthophenylenediamine) bilayer membrane able to entrap large molecules such as glucose oxidase was used to investigate (at 27°C and pH 7) the kinetics of ascorbic acid (AA) oxidation by hydrogen peroxide (H₂O₂ + AA → 2H₂O + DAA) by following the H₂O₂ concentration as a function of time. The largely unmatched rejection characteristics of this device towards AA permitted it to operate even in the presence of high AA/H₂O₂ ratios, e.g. 1000: 1. Under these conditions, pseudo-first-order kinetic constant values ranging from 3.26 × 10⁻³ to 4.10 × 10⁻³ s⁻¹ were obtained at [AA] = 2 mM and initial [H₂O₂] = 2 μM. The potential influence of the above reaction on sensitivity and reliability of H₂O₂-detecting biosensors in the presence of AA is discussed critically, taking into account also the recent, and sometimes conflicting, literature views on the problem.     

An extraction system for low-energy hydrogen ions formed by electron impact

A system has been developed for extracting near-zero kinetic energy H⁻ and D⁻ ions formed by dissociative electron attachment. It is the essential part of a new set-up for vibrational spectroscopy of hydrogen molecules. A magnetic field is used to collimate the probing electron beam. Ions produced by electron collision with the target molecules are collected by the combined action of this field and an electrostatic field penetrating into the interaction region. Highly effective extraction is achieved by taking into account the correct out-of plane displacement of ion trajectories which is usually neglected in similar arrangements. The extraction conditions are mass dependent so that by proper tuning, mass selection of detected ions is achieved. The new system is also used for detecting positive ions created by electron collisions with hydrogen atoms and molecules.     

MRD CI calculations of excited states of the SiH3 radical,including potential curves for the inversion mode and the dissociation SiH3 → SiH2 + H

Using the MRD CI method and large basis sets the vertical spectrum of silyl radical (SiH₃) has been calculated. The lowest excited state is the 4s Rydberg state, 41000 cm⁻¹ (5.2 eV) above the ground state. Only one excited valence state (2²E) was encountered, all other states are of Rydberg type. From potential curves for the inversion mode (symmetric bending motion) it was inferred that all Rydberg states are planar, whereas the valence excited state is highly pyramidalized. The investigation of the dissociation reaction SiH₃ → SiH₂ + H leads to the conclusion that the first excited state is dissociative.     

Conformations of triplet carbonyl compounds: Formaldehyde, acetaldehyde, propionaldehyde and acetone

A conformational study on the lowest triplet states of formaldehyde, acetaldehyde, propionaldehyde and acetone has been done using a minimal basis set, within the unrestricted Hartree—Fock framework.For the C₃H₆O species, the energy hypersurfaces (E θ₁, θ₂, θ₃) were generated, where energy is a function of the methyl rotations (θ₁, θ₂) and C---O out-of-plane bending for acetone, and a function of methyl rotation (θ₁), C₂H₅---C rotation (θ₂) and CHO out-of-plane deformation (θ₃) for propionaldehyde.The analysis of the hypersurface equations revealed the location and relative energies of the critical points (minima, first and second order saddle points as well as maxima): the barriers to inversion at the carbonyl group were 2.7 kcal mol⁻¹ for acetone and 4.2 kcal mol⁻¹ for propionaldehyde. Partial geometry optimization reduced these barriers to 2.5 and 2.4 kcal mol⁻¹ respectively.For comparison, both the pyramidal minimum and planar saddle point for the inversion of triplet formaldehyde and acetaldehyde were totally optimized; the resultant barriers were 2.0 kcal mol⁻¹ and 2.3 kcal mol⁻¹, respectively. The barrier to rotation about the bond to the α-carbon was 1.1 kcal mol⁻¹ for pyramidal acetone, 1.0 for acetaldehyde and ranged from 0.8 to 1.8 kcal mol⁻¹ for the various propionaldehyde conformers.     

Ion and solvent transfer discrimination at a nickel hydroxide film exposed to LiOH by combined electrochemical quartz crystal microbalance (EQCM) and probe beam deflection (PBD) techniques

A combined electrochemical quartz crystal microbalance (EQCM) and probe beam deflection (PBD) instrument was used to monitor the mobile species transfers associated with the redox processes of thin (Γ100–150 nmol cm⁻²) α- and β-nickel hydroxide films exposed to aqueous LiOH solution. A comparison of the measured PBD signal with the predicted PBD profiles, calculated by temporal convolution analysis of the current and mass responses, enabled the contributions to redox switching of anion (OH⁻) and solvent (H₂O) transfers to be discriminated quantitatively. The responses from the combined instrument are reconciled in terms of H⁺ deintercalation/intercalation within the nickel hydroxide structure as OH⁻ ions enter/exit the film. Hydroxide ion movement is associated with a counterflux of water. Thin nickel hydroxide films show a gradual α→β phase transformation with continuous voltammetric cycling, especially when the films are exposed to high concentrations of electrolyte. α-Films are characterised by OH⁻ transfers that dominate the H⁺ and H₂O movements; β-films are characterised by an increased participation of water and protons to the exchange dynamics.     

On the Mechanism of Formation of Intratrack Yields of Water Radiolysis Products upon Irradiation with Fast Electrons and Positrons: 2. Concentration and Time Dependences of Yields

A mathematical model for the formation of main transient and final radiolysis products generated in tracks of fast electrons and positrons in water and aqueous solutions was constructed and described in terms of equations of inhomogeneous chemical kinetics in part 1 of this study. The model takes into account the reactions of a solute with epithermal electrons, thermal, and hydrated electrons; the ambipolar character of diffusion of charged intratrack particles; and new pathways of the formation of hydrogen and positronium due to the appearance of weakly bound states of electrons. In the present paper, the model was quantitatively fitted to experimental data on both time variation in the yields of radiolytic products (H₃O⁺, e _aq ⁻ , H, OH, OH⁻, H₂O₂) in pure water and the yields of hydrogen (H₂, H), hydrated electron (e _aq ⁻ ) and positronium (Ps) in various dilute and concentrated aqueous solutions.__________Translated from Khimiya Vysokikh Energii, Vol. 39, No. 5, 2005, pp. 330–338.Original Russian Text Copyright © 2005 by Stepanov, Byakov.     

Adsorption of -histidine on copper surface as evidenced by surface-enhanced Raman scattering spectroscopy

The adsorption of -histidine on a copper electrode from H₂O- and D₂O-based solutions is studied by means of surface-enhanced Raman scattering (SERS) spectroscopy. Different adsorption states of histidine are observed depending upon pH, potential, and the presence of the SO²⁻₄ and Cl⁻ ions. In acidic solutions of pH 1.2 the imidazole ring of the adsorbed histidine remains protonated and is not involved in the chemical coordination with the surface. The SO²⁻₄ and Cl⁻ ions compete with histidine for the adsorption sites. In solutions of pH 3.1 three different adsorption states of histidine are observed depending on the potential. Histidine adsorbs with the protonated imidazole ring oriented mainly perpendicularly to the surface at potentials more positive than −0.2 V. Transformation of that adsorption state occurs at more negative potentials. As this takes place, histidine adsorbs through the α-NH₂ group and the neutral imidazole ring. The Cl⁻ ions cause the protonation and detachment of the α-NH₂ group from the surface and the formation of the ion pair NH⁺₃ … Cl⁻ can be observed. In the neutral solution of pH 7.0 histidine adsorbs through the deprotonated nitrogen atom of the imidazole ring and the α-COO⁻ group at E ≥ −0.2 V. However, this adsorption state is transformed into the adsorption state in which the α-NH₂ group and/or neutral imidazole ring participate in the anchoring of histidine to the surface, once the potential becomes more negative. In alkaline solutions of pH 11.9 histidine is adsorbed on the copper surface through the neutral imidazole ring.     

A Spectroscopic Study of the Dissolution of Cesium Phosphomolybdate and Zirconium Molybdate by Ammonium Carbamate

Through a combination of Raman spectroscopy, multi-element NMR spectroscopy and chemical analysis, the differences between the action of carbonate and carbamate as agents for dissolving Cs₃PMo₁₂O₄₀⋅xH₂O(s) (CPM) and ZrMO₂O₇(OH)₂(H₂O)₂(s) (ZM) have been elucidated. Alkaline H₂NCO⁻₂/HCO₃⁻/CO₃²⁻ solutions, derived from the dissolution of ammonium carbamate (NH₄H₂NCO₂; AC), dissolve CPM by base hydrolysis of the PMo₁₂O₄₀³⁻ Keggin anion, ultimately forming [MoO₄]²⁻ and PO₄³⁻ when excess base is present. If the initial concentration of H₂NCO₂⁻/HCO₃⁻/ CO₃²⁻ is lowered, base hydrolysis is incomplete and the dissolved species include [Mo₇O₂₄]⁶⁻ and [P₂Mo₅O₂₃]⁶⁻, and undissolved solid Cs₃PMo₁₂O₄₀, Cs_xNH_7−xPMo₁₁O₃₉, and Cs_xNH_6−xMo₇O₂₄ remain. Na₂CO₃ solutions dissolve Cs₃PMo₁₂O₄₀ through a similar mechanism, but the dissolution rate is much lower. We attribute this difference to the different buffering effects of H₂NCO₂⁻/HCO₃⁻/CO₃²⁻ and CO₃²⁻/HCO₃⁻ solutions, and the instability of carbamic acid, the protonated form of H₂NCO₂⁻ (which rapidly decomposes into NH₃ and CO₂). The ability of NH₃ to produce NH₄⁺ and OH⁻, together with the evolution of CO₂ gas, drive the reaction forward. Low temperature measurements under conditions where pure H₂NCO₂⁻ is kinetically stable, allowed the rates of dissolution of CPM by H₂NCO₂⁻ and CO₃²⁻ to be compared directly, confirming the faster dissolution by H₂NCO₂⁻. Compared to CPM, the dissolution of ZM by H₂NCO₂⁻/HCO₃⁻/CO₃²⁻ is a much slower process and is driven by the formation of soluble Zr^IV-carbonate complexes and MoO₄²⁻. The driving force for the dissolution of ZM is the superior complexing ability of carbonate over carbamate; consequently solutions containing a higher carbonate concentration dissolve ZM faster.