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).     

Strong fragmentation processes driven by low energy electron attachment to various small perfluoroether molecules

Negative ion formation in the three perfluoroethers (PFEs) diglyme (C(6)F(14)O(3)), triglyme (C(8)F(18)O(4)) and crownether (C(10)F(20)O(5)) is studied following electron attachment in the range from ～0 to 15?eV. All three compounds show intense low energy resonances at subexcitation energies (<3?eV) decomposing into a variety of negatively charged fragments. These fragment ions are generated via dissociative electron attachment (DEA), partly originating from sequential decompositions on the metastable (μs) time scale as observed from the MIKE (metastable induced kinetic energy) scans. Only in perfluorocrownether a signal due to the non-decomposed parent anion is observed. Additional and comparatively weaker resonances are located in the energy range between ～10 and 17?eV which preferentially decompose into lighter ions. It is suggested that specific features of perfluoropolyethers (PFPEs) relevant in applications, e.g., the strong bonding to surfaces induced by UV radiation of the substrate or degradation of PFPE films in computer hard disc drives can be explained by their pronounced sensitivity towards low energy electrons.     

Metastable anions of dinitrobenzene: resonances for electron attachment and kinetic energy release

Attachment of free, low-energy electrons to dinitrobenzene (DNB) in the gas phase leads to DNB(-) as well as several fragment anions. DNB(-), (DNB-H)(-), (DNB-NO)(-), (DNB-2NO)(-), and (DNB-NO(2))(-) are found to undergo metastable (unimolecular) dissociation. A rich pattern of resonances in the yield of these metastable reactions versus electron energy is observed; some resonances are highly isomer-specific. Most metastable reactions are accompanied by large average kinetic energy releases (KER) that range from 0.5 to 1.32 eV, typical of complex rearrangement reactions, but (1,3-DNB-H)(-) features a resonance with a KER of only 0.06 eV for loss of NO. (1,3-DNB-NO)(-) offers a rare example of a sequential metastable reaction, namely, loss of NO followed by loss of CO to yield C(5)H(4)O(-) with a large KER of 1.32 eV. The G4(MP2) method is applied to compute adiabatic electron affinities and reaction energies for several of the observed metastable channels.     

Computational study of TNT synthesis in solvated nitration reaction systems

Mononitrotoluene (MNT) was incorporated into solvated reaction systems and was subjected to subsequent nitration (electrophilic and free radical substitution) to obtain corresponding dinitrotoluene (DNT) and trinitrotoluene (TNT) products. In the electrophilic nitration system, the energy barrier of the reaction to produce o,p‐dinitrotoluene from p‐nitrotoluene was found to decrease from 62.7 to 14.7 kJ/mol to 9.2 kJ/mol in solventless, hydrated, and methanol‐solvated molecular reaction systems, respectively. Further nitration to produce TNT in related solventless and solvated systems also led to a stepwise decreasing trend in the required energy, from 297.6 to 118.6 kJ/mol to 42.8 kJ/mol. Comparative synthesis using ·NO₂ as the nitrating reagent to obtain o,p‐DNT or TNT in the hydrated system shows a lower reaction energy barrier than that of the same reaction in the solventless system. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Positive and negative ions of the amino acid histidine formed in low‐energy electron collisions

Histidine is an aromatic amino acid crucial for the biological functioning of proteins and enzymes. When biological matter is exposed to ionising radiation, highly energetic particles interact with the surrounding tissue which leads to efficient formation of low‐energy electrons. In the present study, the interaction of low‐energy electrons with gas‐phase histidine is studied at a molecular level in order to extend the knowledge of electron‐induced reactions with amino acids. We report both on the formation of positive ions formed by electron ionisation and negative ions induced by electron attachment. The experimental data were complemented by quantum chemical calculations. Specifically, the free energies for possible fragmentation reactions were derived for the τ and the π tautomer of histidine to get insight into the structures of the formed ions and the corresponding neutrals. We report the experimental ionisation energy of (8.48 ± 0.03) eV for histidine which is in good agreement with the calculated vertical ionisation energy. In the case of negative ions, the dehydrogenated parent anion is the anion with the highest mass observed upon dissociative electron attachment. The comparison of experimental and computational results was also performed in view of a possible thermal decomposition of histidine during the experiments, since the sample was sublimated in the experiment by resistive heating of an oven. Overall, the present study demonstrates the effects of electrons as secondary particles in the chemical degradation of histidine. The reactions induced by those electrons differ when comparing positive and negative ion formation. While for negative ions, simple bond cleav ages prevail, the observed fragment cations exhibit partly restructuring of the molecule during the dissociation process.     

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.     

An investigation into the role of metastable argon atoms in the afterglow plasma of a low pressure discharge

An investigation into the behaviour of metastable argon atoms in a low pressure (250 Pa) pulsed electrical discharge was undertaken in an effort to find the cause of the persisting emission from sputtered metal atoms in the afterglow of an atomic fluorimeter. Results obtained by time-resolved emission and absorption measurements of several argon and copper spectral lines indicate that low energy electrons in the afterglow are converted to high energy electrons via the recombination of electrons with argon ions and the subsequent collisions of pairs of metastable argon atoms. The high energy electrons excite the sputtered metal atoms to give rise to a slow decaying emission tail in the afterglow. A probable change in the electron energy distribution in the afterglow may also have an effect on the observed emission. This phenomenon may be reduced by the use of a suitable quenching gas.     

Highly Selective Room Temperature Monoreduction of Dinitro‐arenes by Hydrogen Sulfide under Liquid–Liquid Biphasic Catalysis

Selective reduction of one of the nitro group present in dinitro aromatic compounds by a novel Zinin reagent, H₂S‐laden N‐methyldiethanolamine (MDEA) solution, has been explored in the presence of tetra‐n‐butyl phosphonium bromide as a phase transfer catalyst under the liquid–liquid mode of reaction. Under the room temperature reaction condition, reduction of 2,4‐dinitrotoluene (2,4‐DNT) with H₂S‐laden MDEA leads to the selective reduction of one nitro group present either at the fourth position to obtain 4‐amino‐2‐nitrotoluene (4A2NT) or at the second position to get 2‐amino‐4‐nitrotoluene (2A4NT). The reaction was very fast to achieve 100% conversion, and the selectivity of 4A2NT is much higher than the 2A4NT. A detailed parametric study was performed to analyze the effect of parameters on 2,4‐DNT conversion and selectivity of both the isomers. The apparent activation energy was found to be as high as 46.25 kJ/mol, and the reaction was found to be kinetically controlled. An empirical kinetic model has been developed to correlate with the conversion version time data obtained experimentally. The present system dealt with an industrial problem in dealing with H₂S, present in by‐product gaseous streams of many petroleum and natural gas industries. Novelties in the selective monoreduction lie in that fact that the reaction was done at room temperature (303 K), with a novel reagent, H₂S‐laden MDEA solution. Therefore waste‐minimization was effected to yield value‐added fine chemicals, that is, amines.     

Fundamental properties characterizing low-pressure microwave-induced plasmas as excitation sources for spectroanalytical chemistry

Experimental measurements of the spectroscopic temperature and the electron temperature in low-pressure rare gas plasmas sustained by a microwave generator operating at 2450 MHz have revealed divergent values. These measurements have been interpreted on the basis of a radiative recombination model originally proposed by Schlüter. The importance of Penning ionization by metastable rare gas atoms in the excitation of foreign atoms has been discussed in terms of this model.On the basis of the radiative recombination model for these plasmas, the parameters of analytical importance are the concentration and energy of electrons in a high energy electron group, the concentration and energy of electrons in a low energy electron group, and the concentration of metastable rare gas atoms. Measurements of the spectroscopic temperature of an argon plasma have revealed that the energy of electrons in the low energy electron group is not greatly affected by applied microwave power and pressure over the range from 1–25 torr. The energy of electrons in the high energy electron group is not greatly affected by pressure and applied microwave power over the range studied, but has been shown to depend on the ionization potential of the plasma gas. The total electron concentration is not greatly affected by gas pressure for low applied powers, but varies with applied power, particularly at low pressures. The concentration of metastable argon atoms has been shown to depend on both the applied power and pressure. Studies of the excitation of mercury by these plasmas have led to results which are consistent with the radiative recombination model.     

Towards Solar Energy Storage in the Photochromic Dihydroazulene–Vinylheptafulvene System

One key challenge in the field of exploitation of solar energy is to store the energy and make it available on demand. One possibility is to use photochromic molecules that undergo light‐induced isomerization to metastable isomers. Here we present efforts to develop solar thermal energy storage systems based on the dihydroazulene (DHA)/vinylheptafulvene (VHF) photo/thermoswitch. New DHA derivatives with one electron‐withdrawing cyano group at position 1 and one or two phenyl substituents in the five‐membered ring were prepared by using different synthetic routes. In particular, a diastereoselective reductive removal of one cyano group from DHAs incorporating two cyano groups at position 1 turned out to be most effective. Quantum chemical calculations reveal that the structural modifications provide two benefits relative to DHAs with two cyano groups at position 1: 1) The DHA–VHF energy difference is increased (i.e., higher energy capacity of metastable VHF isomer); 2) the Gibbs free energy of activation is increased for the energy‐releasing VHF to DHA back‐reaction. In fact, experimentally, these new derivatives were so reluctant to undergo the back‐reaction at room temperature that they practically behaved as DHA to VHF one‐way switches. Although lifetimes of years are at first attractive, which offers the ultimate control of energy release, for a real device it must of course be possible to trigger the back‐reaction, which calls for further iterations in the future.     

石墨烯膜质子传输巨大光效应的微观机理

Graphene monolayers are permeable to thermal protons and impermeable to other atoms and molecules, exhibiting their potential applications in fuel cell technologies and hydrogen isotope separation. Furthermore, the giant photoeffect in proton transport through catalytically activated graphene membranes was reported by Geim et al. Their experiment showed that the synergy between illumination and the catalytically active metal plays a key role in this photoeffect. Geim et al. suggested that the local photovoltage created between metal nanoparticles and graphene could funnel protons and electrons toward the metal nanoparticles for the production of hydrogen, while repelling holes away from them, causing the giant photoeffect. However, based on static electric field theory, this explanation is not convincing and the work lacks an analysis on the microscopic mechanism of this effect. Herein, we provide the exact microscopic mechanism behind this phenomenon. In semi-metal pristine graphene, most photon excited hot electrons relax to lower energy states within a timescale of 10⁻¹² s, while the typical timescale of a chemical reaction is 10⁻⁶ s. Thus, hot electrons excited by incident photons relax to lower energy states before reacting with protons through the graphene. When graphene is decorated with metal, electron transfer between the graphene and the metal, induced by different work functions, would result in the formation of interface dipoles. When using metals such as Pt, Pd, Ni, etc., which can strongly interact with graphene, local dipoles form. Protons are trapped around the negative poles of the local dipoles, while electrons are around the positive poles. Upon illumination, the electrons are excited to metastable excited states with higher energy levels. Due to the energy barriers around them, the free electrons in the metastable excited states will have a relatively longer lifetime, which facilitates the production of hydrogen through their effective reaction with protons that permeated through the graphene. The concentration of high-energy electrons under illumination was estimated, and the results showed that more electrons are energized to the excited state with strong illumination. According to the analysis, the giant photoeffect in proton transport through the catalytically activated graphene membrane is attributed to long-lived hot electrons and a fast proton transport rate. Since there is no change in the activation energy of the reaction, the metal catalyst increases the rate of the reaction by increasing the number of successful collisions between the reactants to produce the significant photoeffect. This might lead to a new microscopic mechanism that clarifies the role of the catalyst in improving the efficiency of photo(electro)catalytic reactions.     

Inelastic electron interaction (attachment/ionization) with deoxyribose

We have investigated experimentally the formation of anions and cations of deoxyribose sugar (C(5)H(10)O(4)) via inelastic electron interaction (attachment/ionization) using a monochromatic electron beam in combination with a quadrupole mass spectrometer. The ion yields were measured as a function of the incident electron energy between about 0 and 20 eV. As in the case of other biomolecules (nucleobases and amino acids), low energy electron attachment leads to destruction of the molecule via dissociative electron attachment reactions. In contrast to the previously investigated biomolecules dehydrogenation is not the predominant reaction channel for deoxyribose; the anion with the highest dissociative electron attachment (DEA) cross section of deoxyribose is formed by the release of neutral particles equal to two water molecules. Moreover, several of the DEA reactions proceed already with "zero energy" incident electrons. In addition, the fragmentation pattern of positively charged ions of deoxyribose also indicates strong decomposition of the molecule by incident electrons. For sugar the relative amount of fragment ions compared to that of the parent cation is about an order of magnitude larger than in the case of nucleobases. We determined an ionization energy value for C(5)H(10)O(4) (+) of 10.51+/-0.11 eV, which is in good agreement with ab initio calculations. For the fragment ion C(5)H(6)O(2) (+) we obtained a threshold energy lower than the ionization energy of the parent molecular ion. All of these results have important bearing for the question of what happens in exposure of living tissue to ionizing radiation. Energy deposition into irradiated cells produces electrons as the dominant secondary species. At an early time after irradiation these electrons exist as ballistic electrons with an initial energy distribution up to several tens of electron volts. It is just this energy regime for which we find in the present study rather characteristic differences in the outcome of electron interaction with the deoxyribose molecule compared to other nucleobases (studied earlier). Therefore, damage induced by these electrons to the DNA or RNA strands may start preferentially at the ribose backbone. In turn, damaged deoxyribose is known as a key intermediate in producing strand breaks, which are the most severe form of lesion in radiation damage to DNA and lead subsequently to cell death.     

Differentiation of isomeric dinitrotoluenes and aminodinitrotoluenes using electrospray high resolution mass spectrometry

Explosive detection and identification play an important role in the environmental and forensic sciences. However, accurate identification of isomeric compounds remains a challenging task for current analytical methods. The combination of electrospray multistage mass spectrometry (ESI‐MSⁿ) and high resolution mass spectrometry (HRMS) is a powerful tool for the structure characterization of isomeric compounds. We show herein that resonant ion activation performed in a linear quadrupole ion trap allows the differentiation of dinitrotoluene isomers as well as aminodinitrotoluene isomers. The explosive‐related compounds: 2,4‐dinitrotoluene (2,4‐DNT), 2,6‐dinitrotoluene (2,6‐DNT), 2‐amino‐4,6‐dinitrotoluene (2A‐4,6‐DNT) and 4‐amino‐2,6‐dinitrotoluene (4A‐2,6‐DNT) were analyzed by ESI‐MS in the negative ion mode; they produced mainly deprotonated molecules [M ? H]^?. Subsequent low resolution MSⁿ experiments provided support for fragment ion assignments and determination of consecutive dissociation pathways. Resonant activation of deprotonated dinitrotoluene isomers gave different fragment ions according to the position of the nitro and amino groups on the toluene backbone. Fragment ion identification was bolstered by accurate mass measurements performed using Fourier transform ion cyclotron resonance mass spectrometry (FT‐ICR/MS). Notably, unexpected results were found from accurate mass measurements performed at high resolution for 2,6‐DNT where a 30‐Da loss was observed that corresponds to CH₂O departure instead of the expected isobaric NO^? loss. Moreover, 2,4‐DNT showed a diagnostic fragment ion at m/z 116, allowing the unambiguous distinction between 2,4‐ and 2,6‐DNT isomers. Here, CH₂O loss is hindered by the presence of an amino group in both 2A‐4,6‐DNT and 4A‐2,6‐DNT isomers, but nevertheless, these isomers showed significant differences in their fragmentation sequences, thus allowing their differentiation. DFT calculations were also performed to support experimental observations. Copyright © 2014 John Wiley & Sons, Ltd.     

Role of Emissive and Non‐Emissive Complex Formations in Photoinduced Electron Transfer Reaction of CdTe Quantum Dots

Bimolecular photoinduced electron transfer (PET) from excited state CdTe quantum dot (QD*) to an electron deficient molecule 2,4‐dinitrotoluene (DNT) is studied in toluene. We observed two types of QD‐DNT complex formations; (i) non‐emissive complex, in which DNT is embedded deep inside the surface polymer layer of QD and (ii) emissive complex, in which DNT molecules are attached to QDs but approach to the QD core is shielded by polymer layer. Because of its non‐emissive nature, the lifetime of QD is not affected by dark complex formation, though the steady‐state emission is greatly quenched. However, emissive complex formation causes both, lifetime and steady‐state emission quenching. In our fitting model, consideration of Poisson distribution of the attached quencher (DNT) molecules at QD surface enables a comprehensive fitting to our time resolved data. QD‐DNT complex formation was confirmed by an isothermal titration calorimetry (ITC) study. Fitting to the time resolved data using a stochastic kinetic model shows moderate increase (0.05 ns^?1 to 0.072 ns^?1) of intrinsic quenching rate with increasing the QD particle size (from ≈3.2 nm to ≈5.2 nm). Our fitting also reveals that the number of DNT molecules attached to a single QD increases from ≈0.1–0.2 to ≈1.2–1.7, as the DNT concentration is increased from ≈1 mm to 17.5 mm . Complex formation at higher quencher concentration assures that the observed PET kinetics is a thermodynamically controlled process where solvent diffusion has no role on it.     

利用太赫兹时域光谱鉴别爆炸物2,4-DNT和 2,6-DNT

利用太赫兹时域光谱测得了2,4-DNT和 2,6-DNT在0.3-2.0THz频谱范围的吸收谱和折射谱。借助高斯03程序对2,6-DNT进行结构优化和频率计算。2,6-DNT在1.09, 1.36 and 1.55 THz有三个明显吸收并被归因于分子间的相互作用。本文使用太赫兹光谱法对2,4-DNT和 2,6-DNT混合物做了定量分析。分析的结果与实际值基本一致,相对误差约为8%。     

Birch Reduction of Benzene in a Low‐Temperature Plasma

Selective and specific dihydrogenation of benzene and other arenes has been observed in a low‐temperature helium plasma. A surface Birch reduction mechanism has been proposed in which benzene molecules adsorbed on the discharge surface capture low‐energy surface‐adsorbed electrons and subsequently undergo protonation (see picture). Gas‐phase oxidation processes accompany the reduction reaction.

     

Spontaneous and infrared induced electron detachment from negatively charged helium nanodroplets

A beam of helium nanodroplets, with sizes ranging up to N = 10⁷ or more atoms, is produced by fragmentation of a low entropy supersonic expansion. It subsequently is excited by electron impact, producing various charged and metastable droplet states depending on the electron energy. We will describe experiments with negatively charged cluster ions, which are observed for low energy impacts when N >2×10⁵. In these experiments, after a flight time in high vacuum of several milliseconds the droplets pass through a weak transverse field above an electron multiplier. A signal from spontaneously detached electrons is observed, which suggests that the ion, while long lived, is inherently metastable. Furthermore, when the beam is crossed with an infrared light beam above the detector, the detachment rate is significantly increased. The wavelength dependence of this light induced signal has a broad peak near 1.5μm. By deflection measurements it is found that the spontaneous detachment signal comes preferentially from smaller clusters, while the light induced signal comes predominantly from larger ones. By stopping potential measurements one can conclude that both kinds of detached electrons have energies below 1eV, with photo detached electrons the more energetic.     

Low-energy electron capture by 6-Aza-2-thiothymine: investigations by electron attachment and electron transmission spectroscopies

The interaction of low-energy (0-10 eV) electrons with 6-aza-2-thiothymine is investigated in the gas phase by studies of sharp structure in the total electron scattering cross section and by mass analysis of the stable or long-lived negative ions produced by electron attachment. The most efficient fragmentation process, occurring at 0.15 eV, involves the ejection of a closed-shell neutral molecule (CH3CN). Ab initio calculations support our proposal that this process leads to ring closure to form a stable four-member heterocyclic anion. A long-lived parent anion with an approximate lifetime of 75 microseconds is observed near zero electron energy, and evidence is also seen for the slow decay of this anion by ejection of CH3CN. Near 3.3 eV, an anion of m/e 41 is produced that is likely to be a metastable valence anion of bent CH3CN, but the dipole-bound anion cannot be ruled out.     

Resonant Formation of Strand Breaks in Sensitized Oligonucleotides Induced by Low‐Energy Electrons (0.5–9 eV)

Halogenated nucleobases are used as radiosensitizers in cancer radiation therapy, enhancing the reactivity of DNA to secondary low‐energy electrons (LEEs). LEEs induce DNA strand breaks at specific energies (resonances) by dissociative electron attachment (DEA). Although halogenated nucleobases show intense DEA resonances at various electron energies in the gas phase, it is inherently difficult to investigate the influence of halogenated nucleobases on the actual DNA strand breakage over the broad range of electron energies at which DEA can take place (<12 eV). By using DNA origami nanostructures, we determined the energy dependence of the strand break cross‐section for oligonucleotides modified with 8‐bromoadenine (^8BrA). These results were evaluated against DEA measurements with isolated ^8BrA in the gas phase. Contrary to expectations, the major contribution to strand breaks is from resonances at around 7 eV while resonances at very low energy (<2 eV) have little influence on strand breaks.     

Sensitizing DNA Towards Low‐Energy Electrons with 2‐Fluoroadenine

2‐Fluoroadenine (^2FA) is a therapeutic agent, which is suggested for application in cancer radiotherapy. The molecular mechanism of DNA radiation damage can be ascribed to a significant extent to the action of low‐energy (<20 eV) electrons (LEEs), which damage DNA by dissociative electron attachment. LEE induced reactions in ^2FA are characterized both isolated in the gas phase and in the condensed phase when it is incorporated into DNA. Information about negative ion resonances and anion‐mediated fragmentation reactions is combined with an absolute quantification of DNA strand breaks in ^2FA‐containing oligonucleotides upon irradiation with LEEs. The incorporation of ^2FA into DNA results in an enhanced strand breakage. The strand‐break cross sections are clearly energy dependent, whereas the strand‐break enhancements by ^2FA at 5.5, 10, and 15 eV are very similar. Thus, ^2FA can be considered an effective radiosensitizer operative at a wide range of electron energies.