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.     

Bis‐Coenzyme Q0: Synthesis,Characteristics, and Application

A methylene‐bridged bis‐coenzyme Q₀, bis(2,3‐dimethoxy‐5‐methyl‐l,4‐benzoquinone)methane (Bis‐CoQ₀), that shows intramolecular electronic communications has been synthesized for the first time. By employing electrochemical, in situ UV/Vis, and electron paramagnetic resonance (EPR) spectroelectrochemical techniques, the unstable reduced intermediate species—monoradicals, diamagnetic dianions and tetraanions of Bis‐CoQ₀—have been observed. The electron‐transfer process can be defined as a three‐step reduction process with a total of four electrons in solution in CH₃CN. The chemical reaction in the third redox step process was confirmed by variable temperature cyclic voltammetry. In an aprotic CH₃CN solution, the peak potential separation between electron‐transfer steps diminished sequentially with increasing concentration of water. The hydrogen‐bonding interactions between water and the electrochemically reduced intermediates of Bis‐CoQ₀ can be estimated by peak potential shifts. The electronic communications of Bis‐CoQ₀ may have been blocked when one reduction peak was observed with proper quantities of water in CH₃CN solution. The antioxidant defense capacity of Bis‐CoQ₀‐protected cells has also been assessed.     

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.     

Negative Ion Formation by Thermal Surface Ionization of Sulfur Dioxide

The generation of negative ions from SO₂ in the gas phase was studied using the thermal surface ionization method. Six anion types were measured: O⁻, S⁻, SO⁻, and SO₂⁻ and anions with m/z=96 and m/z=128. The most abundant anion formed was S⁻ and the formation routes are discussed for each of the six anions. O⁻, S⁻, and SO⁻ are formed via dissociative electron attachment to the molecule, whereas the generation of SO₂⁻ and anions with m/z=96 and m/z=128 are probably associated with the formation of H₂SO₄ in the gas inlet system and the ion source. Using statistical thermodynamics the dissociation temperatures of SO₂ and SO in the gas phase are calculated and values of above 1800 °C are obtained for both molecules. We also estimated the optimal filament temperatures for the formation of all anions measured, indicating that for SO₂ the optimal temperature is related to the electron affinity of the molecules: the optimal temperature increases with decreasing value of the electron affinity for the molecule corresponding to the respective anion.     

Origin of Resonant Character in the Electron Impact Two-Body Neutral-Fragmentation of Methane

The observation of peaks in the threshold region of two-body neutral fragmentation of methane molecule, i. e., CH₄→CH₃+H, by low energy electron (LEE) impact has been an enigma. The prevailing explanation that this resonant behavior is due to excitation energy transfer is unsatisfactory since this process is not expected to show peaks in the cross-sections unless there is the involvement of electron-molecule resonances. Our first-principles calculations now reveal that the observed peaks could be explained as due to the formation of negative ion resonances, which dominantly dissociate into two neutral fragments and a free-electron. This case of methane is a pointer to the possibility that such reactions contribute significantly to neutral radical production from molecules by LEE impact in comparison to dissociative electron attachment, and in general could play a significant role in electron-based chemical control.     

Processes of dissociative electron capture by 20-hydroxyecdysone molecules

The peculiarities of dissociative electron capture by 20-hydroxyecdysone molecules with the formation of fragment negative ions were studied. In the region of high electron energies (5–10 eV), processes of skeleton bond rupture are accompanied by the elimination of H₂O and H₂ molecules. In the region of thermal energies of electrons (≈0 eV), the mass spectrum is formed mainly by the [M−nH₂O]^.− (n=1–3) and [M−H₂−nH₂O]^.− (n=0−3) ions that are generated exclusively by the rearrangement. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 709–712, April, 2000.     

Identification of Fragment Ions by Their Kinetic Energy Gained During Dissociative Ionization by Electron Impact

A technique is described, which supports the plasma mass spectrometry to distinguish possible sources of ion peaks found in the mass spectrum of the neutral gas. The proposed method is based on the measurement of the kinetic energy which the fragment ions gain during dissociative ionization by electron impact inside the ion source of the spectrometer. This approach is of special interest for applications in plasma processes such as plasma assisted deposition or etching techniques where complicated molecules are involved. The principle of the method is demonstrated and discussed for the examination of various fragment ions as CH₃ ⁺, C₂H₂ ⁺, C₂H₃ ⁺, C₂H₅ ⁺ and CH₃O⁺ in the neutral gas spectrum of an 13.56 MHz rf discharge in an Argon-Tetraethoxysilane (TEOS) mixture.     

Very Low Energy Electrons Transform the Cyclobutane‐Pyrimidine Dimer into a Highly Reactive Intermediate

Electrons with virtually no kinetic energy (close to 0 eV) trigger the decomposition of cytotoxic cyclobutane‐pyrimidine dimer (CPD) into a surprisingly large variety of fragment ions plus their neutral counterparts. The response of CPD to low energy electrons is thus comparable to that of explosives like trinitrotoluene (TNT). The dominant unimolecular reaction is the splitting into two thymine like units, which can be considered as the essential molecular step in the photolyase of CPD. We find that CPD is significantly more sensitive towards low energy electrons than its thymine building blocks. It is proposed that electron attachment at very low energy proceeds via dipole bound states, supported by the large dipole moment of the molecule (6.2 D). These states act as effective doorways to dissociative electron attachment (DEA).     

Fragmentation of deprotonated d-ribose and d-fructose in MALDI—Comparison with dissociative electron attachment

We present a detailed, collaborative study on the fragmentation of deprotonated native d-ribose and d-fructose and the isotopically labelled 1-¹³C-d-ribose, 5-¹³C-d-ribose and C-1-d-d-ribose. The fragmentation is studied in a matrix assisted laser desorption/ionization time of flight mass spectrometer (MALDI ToF MS), both in in-source decay (ISD) and post-source decay (PSD) mode and compared with fragmentation through dissociative electron attachment (DEA). Fragmentation of deprotonated monosaccharides formed in the MALDI process, as well as their transient molecular anions formed upon electron attachment are characterized by loss of different numbers of H₂O and CH₂O units. Two different fragmentation pathways leading to cross-ring cleavage are identified. Metastable decay of deprotonated d-ribose proceeds either via an X-type cleavage yielding fragment anions at m/z = 119, 100 and 89, or via an A-type cleavage resulting in m/z = 89, 77 and 71. A fast and early metastable cross-ring cleavage of deprotonated d-ribose observed in in-source decay is dominated by X-type cleavage leading mainly to m/z = 100 and 71. For dissociative electron attachment to d-ribose a sequential dissociation was identified that includes metastable decay of the dehydrogenated molecular anion leading to m/z = 89. All other fragmentation reactions in DEA to d-ribose are likely to proceed directly and on a faster timescale (below 400 ns).     

Dissociative Electron Attachment to the Nitroamine HMX (Octahydro-1,3,5,7-Tetranitro-1,3,5,7-Tetrazocine)

In the present study, dissociative electron attachment (DEA) measurements with gas phase HMX, octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine, C₄H₈N₈O₈, have been performed by means of a crossed electron-molecular beam experiment. The most intense signals are observed at 46 and 176 u and assigned to NO₂ ^? and C₃H₆N₅O₄ ^?, respectively. Anion efficiency curves for 15 negatively charged fragments have been measured in the electron energy region from about 0–20 eV with an energy resolution of ~0.7 eV. Product anions are observed mainly in the low energy region, near 0 eV, arising from surprisingly complex reactions associated with multiple bond cleavages and structural and electronic rearrangement. The remarkable instability of HMX towards electron attachment with virtually zero kinetic energy reflects the highly explosive nature of this compound. Substantially different intensity ratios of resonances for common fragment anions allow distinguishing the nitroamines HMX and royal demolition explosive molecule (RDX) in negative ion mass spectrometry based on free electron capture.      

Low energy electron and O- reactions in films of O2 coadsorbed with benzene or toluene

A detailed understanding of nascent reactive events leading to DNA damage is required to describe ionizing radiation effects on living cells. These early, sub-picosecond events involve mainly low energy (E < 20 eV) secondary electrons (SE), and low energy (E < 5 eV) secondary ion (and neutral) fragments; the latter are created either by the primary radiation, or by SE via dissociative electron attachment (DEA). While recent work has shown that SE initiate DNA strand break formation via DEA, the subsequent damage induced by the DEA ion fragments in DNA, or its basic components is unknown. Here, we report 0-20 eV electron impact measurements of anion desorption from condensed films containing O2 and either benzene (C6H6), or toluene (C6H5CH3); these molecules represent the most fundamental structural analogs of pyrimidine bases. Our experiments show that all of the observed OH- yields are the result of reactive scattering of 1-5 eV O- fragments produced initially by DEA to O2. These O- reactions involve hydrogen abstraction from benzene or toluene, and result in the formation of benzyl radicals, or toluene radicals centered on either the ring or exocyclic methyl group. O- scatters over nm distances comparable to DNA dimensions, and reactions involve a transient anion collision complex. Anion desorption is found to depend on both, the temperature of hydrocarbon film formation (morphology), and the order of overlayer adsorption, e.g. O2 on benzene, or benzene on O2. Our measurements support the notion that in irradiated DNA similar secondary-ion reactions can be initiated by the abundant secondary electrons, and may lead to clustered damage.     

Thermal electron capture by some halopropanes

Thermal electron attachment rate constants for CF₃CHClCH₃, CF₂ClCFClCF₃ and CBrF₂CH₂CH₂Br have been measured with electron swarm method. Corresponding rate constants are equal to 7.6×10⁻¹¹, 5.5×10⁻⁹ and 1.5×10⁻⁸ cm³ molecule⁻¹ s⁻¹, respectively. The dissociative electron attachment (DEA) spectra for nine haloalkanes have been determined using negative ion mass spectrometry. The correlation between rate constants, position of the DEA peaks and vertical attachment energy (VAE) available in literature has been demonstrated.     

Electron attachment to antipyretics: possible implications of their metabolic pathways

The empty-level structures and formation of negative ion states via resonance attachment of low-energy (0-15 eV) electrons into vacant molecular orbitals in a series of non-steroidal anti-inflammatory drugs (NSAIDs), namely aspirin, paracetamol, phenacetin, and ibuprofen, were investigated in vacuo by electron transmission and dissociative electron attachment (DEA) spectroscopies, with the aim to model the behavior of these antipyretic agents under reductive conditions in vivo. The experimental findings are interpreted with the support of density functional theory calculations. The negative and neutral fragments formed by DEA in the gas phase display similarities with the main metabolites of these commonly used NSAIDs generated in vivo by the action of cytochrome P450 enzymes, as well as with several known active agents. It is concluded that xenobiotic molecules which possess pronounced electron-accepting properties could in principle follow metabolic pathways which parallel the gas-phase dissociative decay channels observed in the DEA spectra at incident electron energies below 1 eV. Unwanted side effects as, e.g., hepatoxicity or carcinogenicity produced by the NSAIDs under study in human organism are discussed within the "free radical model" framework, reported earlier to describe the toxic action of the well-known model toxicant carbon tetrachloride.     

Degradation of gas phase decabromodiphenyl ether by resonant interaction with low-energy electrons

Resonance electron attachment in a series of brominated phenyl ethers, including decabromodiphenyl ether (DBDE), was investigated in the gas phase by means of electron transmission spectroscopy (ETS) and dissociative electron attachment spectroscopy (DEAS). Attachment of thermal electrons to DBDE leads to various dissociative decay channels of the temporary molecular anion. In contrast to other bromophenyl ethers, the bromide anion is not the most intense negative fragment. The neutral counterparts of the observed [Br(2)](-) and [C(6)Br(4)O](-) anion fragments are ascribed to the closed-shell species octabromodibenzofuran and hexabromobenzene, respectively, although their formation implies complex atomic rearrangements. Density functional theory calculations are employed to evaluate electron affinities, thermodynamic energy thresholds for production of the anion fragments observed in the DEA spectra and the proton affinities of the corresponding neutral radicals. Since DBDE is one of the most widespread organic pollutants, the present gas-phase DEA study can provide indications on the reaction mechanisms which occur in vivo and cause injuries to living cells.     

CHCl3电子吸附速率常数的离子迁移谱

The dissociative electron attachment process for CHCl₃ at different electric field have been studied with nitrogen as drift and carrier gas using corona discharge ionization source ion mobility spectrometry (CD-IMS). The corresponding electron attachment rate constants varied from 1.26×10^-8 cm³/(molecules s) to 8.24×10^-9 cm³/(molecules s) as the electric field changed from 200 V/cm to 500 V/cm. At a fixed electric field in the drift region,the attachment rate constants are also detected at different sample concentration. The ion-molecule reaction rate constants for the further reaction between Cl^- and CHCl₃ are also detected, which indicates that the technique maybe becomes a new method to research the rate constants between ions and neural molecules. And the reaction rate constants between Cl^- and CHCl₃ are the first time detected using CD-IMS.     

Specific features of resonance electron capture mass spectra of ecdysteroid molecules

Specificity of the dissociative attachment of low-energy electrons to ecdysteroid molecules (viz., 20-hydroxyecdysone 2,3:20,22-diacetonide, 20-hydroxyecdysone 20,22-acetonide, and poststerone 2-acetate) was found, which is manifested as the formation of long-lived pseudo-molecular negative ions that appeared due to the elimination of the H₂ and H₂O molecules. These rearrangements are resulted from the formation of the system of conjugated double bonds in the ecdysteroid skeleton, stabilizing the lowest vacant molecular orbital.     

Dissociative Electron Attachment to Hexachlorobenzene

Gas phase molecules of hexachlorobenzene (C₆Cl₆) were investigated by means of dissociative electron attachment spectroscopy (DEAS). Three channels of molecular negative ions decay have been identified: abstraction of Cl⁻ and Cl₂⁻ as well as electron detachment (τ_a∼250 μs at 343 K). All three channels exhibit temperature dependence. The adiabatic electron affinity estimated using a simple but typically accurate Arrhenius model (EA_a=1.6–1.9 eV) turns out to be much higher than the quantum-chemical predictions (EA_a=0.9–1.0 eV). We discuss the possible reasons behind the observed discrepancy.     

Dissociative Electron Attachment to β‐Alanine

A detailed study on dissociative electron attachment (DEA) to β‐alanine (βA) in the gas phase is presented. Ion yields as a function of the incident electron energy from about 0 to 15 eV have been measured for most of the fragments. As for all α‐amino acids, the main reaction corresponds to the loss of a hydrogen atom, although many other fragments have been observed that involved more complex bond cleavages. Threshold energies have been calculated by using the G4(MP2) method for various decomposition reactions. Fragmentation pathways were also investigated to measure metastable decays of the intermediate fragment anion (βA?H)^? by using the mass‐analyzed ion kinetic energy (MIKE) scan technique. Comparisons with α‐alanine and other amino acids are made when relevant.     

Nonequilibrium effects in thermal plasma chemistry

Numerical calculations have been performed to assess the potential significance of nonequilibrium effects on chemical reactivity in thermal plasmas The calculations consider situations in which the electron temperature and/or the electron density are elevated above their equilibrium values corresponding to the local gas temperature. Such nonequilibrium may occur in the plasma torch itself or could be purposefully imposed by a controlled hybrid discharge in a downstream reactor region so as to augment reactivity over a longer residence time. The calculations account for finite ionization/recombination rates of atomic and molecular species, electron-impact dissociation, dissociative recombination, dissociative attachment, and predissociation effects, as well as thermal reactions between neutral chemical species. As an example of the possible nonequilibrium enhancement of molecular decomposition, initial consideration has focused on the dissociation rates of diatomic species where heavy particle reaction rates and cross sections can be reasonably estimated. The results show that for O₂ or H₂ in argon at moderate temperatures, electron-temperature elevation can give rise to a notable enhancement of the dissociation rate, in comparison with the equilibrium case. Depending on the situation, it is found that either relatively energetic electron-impact dissociation or dissociative attachment (for O₂) can dominate the enhanced dissociation rate—which can be more than a factor of 2 greater than in the absence of a discharge. Similar effects would be expected for the decomposition of more complicated molecules.     

The role of dissociative attachment from Rydberg states in enhancing H concentration in moderate- and low-pressure H2 plasma sources

A numerical code, recently developed for describing the kinetics of H₂ microwave discharges obtained in diamond deposition plasma reactors, was used to estimate the importance of dissociative attachment from H₂ Rydberg states in enhancing the production of H⁻ in this kind of discharge. It was also used to investigate H⁻ production in multicusp low-pressure magnetic plasmas. Results show that the dissociative attachment from Rydberg states can be as important as the mechanism involving vibrationally excited molecules in both types of plasmas.