Bond and site selectivity in dissociative electron attachment to gas phase and condensed phase ethanol and trifluoroethanol

Low-lying Negative Ion States Probed in Potassium - Ethanol Collisions

Dissociative electron transfer in collisions between neutral potassium atoms and neutral ethanol molecules yields mainly OH-, followed by C2H5O-, O-, CH3 - and CH2 -. The dynamics of negative ions have been investigated by recording time-of-flight mass spectra in a wide range of collision energies from 17.5 to 350 eV in the lab frame, where the branching ratios show a relevant energy dependence for low/intermediate collision energies. The dominant fragmentation channel in the whole energy range investigated has been assigned to the hydroxyl anion in contrast to oxygen anion from dissociative electron attachment (DEA) experiments. This result shows the relevant role of the electron donor in the vicinity of the temporary negative ion formed allowing access to reactions which are not thermodynamically attained in DEA experiments. The electronic state spectroscopy of such negative ions, was obtained from potassium cation energy loss spectra in the forward scattering direction at 205 eV impact energy, showing a prevalent Feshbach resonance at 9.36±0.10 eV withσ O H * / σ C H * ${{\sigma }_{OH}^{^{\ast}}/{\sigma }_{CH}^{^{\ast}}}$ character, while a less pronouncedσ O H * ${{\sigma }_{OH}^{^{\ast}}}$ contribution assigned to a shape resonance has been obtained at 3.16±0.10 eV. Quantum chemical calculations for the lowest-lying unoccupied molecular orbitals in the presence of a potassium atom have been performed to support the experimental findings.

Interaction of low-energy electrons with radiosensitizers

We provide an experimentalist's perspective on the present state-of-the-art in the studies of low-energy electron interactions with common radiosensitizers, including compounds used in combined chemo-radiation therapy and their model systems. Low-energy electrons are important secondary species formed during the interaction of ionizing radiation with matter. Their role in the radiation chemistry of living organisms has become an important topic for more than 20 years. With the increasing number of works and reviews in the field, we would like to focus here on a very narrow area of compounds that have been shown to have radio-sensitizing properties on the one hand, and high reactivity towards low-energy electrons on the other hand. Gas phase experiments studying electron attachment to isolated molecules and environmental effects on reaction dynamics are reviewed for modified DNA components, nitroimidazoles, and organometallics. In the end, we provide a perspective on the future directions that may be important for transferring the fundamental knowledge about the processes induced by low-energy electrons into practice in the field of rational design of agents for concomitant chemo-radiation therapy.

Fragmentation dynamics and absolute dissociative electron attachment cross sections in the low energy electron collision with ethanol

Dissociative electron attachment (DEA) to ethanol has been probed to study fragmentation dynamics using Time-of-Flight (ToF) mass spectrometric technique. Several fragment ions, namely, H-, O-, OH-, C2H3O- and C2H5O- have been observed. Extra effort has been made to detect low mass ions (here, H-). Absolute DEA cross sections for the formation of O- and OH- have been measured for the first time using relative flow technique (RFT). The threshold energy of different dissociation channels has been calculated using density functional theory (DFT) method. By combining the experimental and theoretical data, we found evidence of hydrogen migration in the production of O and C2H3O- ions.

Wavelength dependent photoionisation of ethanol clusters: Generation of hydrogen like C5+ ions at terawatt laser intensity

Photoionisation of ethanol clusters has been investigated at different laser wavelengths over intensity range of ~1012- 1013 W/cm2. At 266 nm ionisation, singly charged fragments and protonated cluster ions (up...

Low-Energy Electron Damage to Condensed-Phase DNA and Its Constituents

The complex physical and chemical reactions between the large number of low-energy (0-30 eV) electrons (LEEs) released by high energy radiation interacting with genetic material can lead to the formation of various DNA lesions such as crosslinks, single strand breaks, base modifications, and cleavage, as well as double strand breaks and other cluster damages. When crosslinks and cluster damages cannot be repaired by the cell, they can cause genetic loss of information, mutations, apoptosis, and promote genomic instability. Through the efforts of many research groups in the past two decades, the study of the interaction between LEEs and DNA under different experimental conditions has unveiled some of the main mechanisms responsible for these damages. In the present review, we focus on experimental investigations in the condensed phase that range from fundamental DNA constituents to oligonucleotides, synthetic duplex DNA, and bacterial (i.e., plasmid) DNA. These targets were irradiated either with LEEs from a monoenergetic-electron or photoelectron source, as sub-monolayer, monolayer, or multilayer films and within clusters or water solutions. Each type of experiment is briefly described, and the observed DNA damages are reported, along with the proposed mechanisms. Defining the role of LEEs within the sequence of events leading to radiobiological lesions contributes to our understanding of the action of radiation on living organisms, over a wide range of initial radiation energies. Applications of the interaction of LEEs with DNA to radiotherapy are briefly summarized.

Electroless deposition of Pd/Pt nanoparticles on electrochemically grown TiO2 nanotubes for ppb level sensing of ethanol at room temperature

This work presents a comparative sensing study of three sensors based on pristine TiO₂ nanotubes, Pd loaded TiO₂ nanotubes, and Pt loaded TiO₂ nanotubes. Pristine TiO₂ nanotubes were synthesized using an electrochemical anodization method and an electroless plating method was used for the uniform deposition of noble metal nanoparticles of either Pd or Pt over the surface of TiO₂ nanotubes. The samples were thoroughly characterized by XRD, FESEM, EDS, TEM, and XPS techniques. The sensitivity of all three sensors was investigated at room temperature (300 K) for different volatile organic compounds like ethanol, methanol, 2-propanol, acetone, and benzene. The results revealed that loading of Pd and Pt nanoparticles improved the response magnitude of the sensor remarkably as these noble metals possess better oxygen dissociation capability than pristine TiO₂. The Pd-TiO₂ nanotube sensor exhibited a maximum response magnitude of 20-98% towards 100-1000 ppb of ethanol at room temperature. Notably, the formation of Pd/Pt-TiO₂ discrete heterojunctions on the surface of TiO₂ nanotubes was found to be responsible for enhanced sensitivity of the sensors.

Electronic excitation of ethanol by low-energy electron impact

We report computed differential cross sections (DCSs) for electron impact excitation of the lower-lying states of both trans and gauche tautomers of ethanol, as well as total cross sections for the 15 eV-50 eV energy range. The Schwinger multichannel (SMC) method with pseudopotentials has been employed, and in our most sophisticated calculation in terms of multichannel coupling, 431 open target states have been considered. We found an overall good agreement with the available experimental data at intermediate scattering angles and at higher impact energies. Although we have used a Born-closure scheme for the higher partial waves, we have found discrepancies in the forward direction that were assigned to a poor description of the long-range component of the lower partial waves. Meanwhile, the lack of more Rydberg states could be related to the overestimated DCSs at lower energies. Missing open channels are usually evoked to explain the remaining discrepancies to experiment, but here, we argue that other factors should also be involved. Aiming at an improved description of the target states, we have proposed a simple procedure for selecting the pairs of hole and particle orbitals while keeping the single excitation prescription of the current SMC implementation. A quantitative assessment of the collision process should further consider the individual contribution of each tautomer, which presented quite distinct DCSs in some cases. Our computed excitation energies also support that the second absorption band of ethanol is comprised of three singlet states of each tautomer, rather than the previously suggested two or four states.

Oxygen vacancy modulation of titania nanotubes by cathodic polarization and chemical reduction routes for efficient detection of volatile organic compounds

In this work, we have synthesized a highly ordered TiO₂ nanotube array by an electrochemical anodization method. Then the oxygen vacancy level of the TiO₂ nanotubes was tuned by two different methods: i.e. (i) cathodic polarization by the application of a reverse potential and (ii) chemical reduction using a reducing agent (e.g. hydrazine hydrate) treatment at elevated temperature. Field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) confirmed there was no morphological deformation of TiO₂ nanotubes after the modulation of oxygen vacancies. X-ray diffraction spectroscopy (XRD) and TEM both confirmed the formation of highly crystalline anatase (101). The oxygen vacancy level of all the TiO₂ nanotubes was tested progressively with photoluminescence (PL) spectra, Raman spectra, energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectra (XPS). All the spectroscopy results confirmed the successful reduction of TiO₂ nanotubes with different levels of defects. All the nanotube samples with various oxygen vacancy levels were transformed to Au/TiO₂ nanotube/Ti type sandwich structured sensor devices and tested under exposure to 100 ppm of ethanol in air. Highly doped cathodic reduced nanotubes offered very high response magnitudes of 99.64% and 60% under exposure to 100 ppm of ethanol at 200 °C and 50 °C, respectively. Chemically reduced TiO₂ nanotubes offered moderate response magnitudes of 75.4% and 80% at 150 °C and 200 °C under exposure to 100 ppm of ethanol, which was found to be the best among all the samples due to the appreciably fast response (155 s) and recovery time (779 s). The developed sensors showed adequate stability and selectivity towards ethanol with a moderate dynamic range (20 to 200 ppm of ethanol) of detection. A general relation was drawn based on the experimental findings of this work to estimate the response magnitude of nanoscale metal oxide gas sensors with various levels of oxygen vacancies.

Evidence For a Water-Stabilised Ion Radical Complex: Photoelectron Spectroscopy and Ab Initio Calculations

A photoelectron spectrum corresponding to an unknown 174m/z anion complex has been recorded. Initially believed to be I−…CH3CH2OH (173m/z), the spectrum has been assigned as belonging to that of an I−…H2O…CH3CH2 radical anion complex. The major peaks in the photoelectron spectrum occur at 3.54eV and 4.48eV as the 2P3/2 and 2P1/2 spin-orbit states of iodine respectively. Ab initio calculations were performed in order to rationalise the existence of the complex, with all structures converging to a ‘ring-like’ geometry, with the iodide anion bound to both the water molecule as well as a hydrogen of the ethyl radical, with the other hydrogen of water bound to the unpaired electron site of the ethyl. Simulated vertical detachment energies of 3.59eV and 4.53eV were found to be in agreement with the experimental results.

Can the Electron-Accepting Properties of Odorants Be Involved in Their Recognition by the Olfactory System?

The present study examines the possible importance of the electron-accepting properties of odorant molecules and, in particular, the formation and decay of temporary negative ions via low-energy electron attachment as a possible contribution toward understanding odorant recognition by olfactory receptors (ORs). Fragments formed by dissociative electron attachment (DEA) of mustard oil odorants represented by a series of isothiocyanates are studied experimentally using DEA spectroscopy and DFT calculations. Relative intensities for the most abundant fragment species, S^- and SCN^-, are found to be characteristic of structurally similar odorants under investigation. This novel approach for the investigation of odorants may contribute to understanding the initial stages of the olfactory process and may provide a means to distinguish between odorants and their interactions with the olfactory receptor system.

Inducing chemical reactivity on specific sites of a molecule using the Coulomb interaction exerted by a low energy electron

Remarkable site-specificity in the resonant attachment of low energy electrons (LEEs) to molecular targets is proposed as an efficient method for inducing chemical reactivity on specific sites of molecules. The Coulomb interaction between the attached electron and the most polarizable molecular electrons localized on the attached site is the reason for site-specific chemical reactivity. The Coulombically induced site-specific chemical reactivity is best illustrated by the LEE induced chemical transformation of a weakly bound molecular complex into a strong covalent adduct. The chemical transformation occurs due to the Coulombically induced nucleophilicity on a specific molecular moiety in the complex. A simple strategy for inducing site-specific chemical reactivity using a LEE may find new avenues in chemical synthesis.

Detection of Four Distinct Volatile Indicators of Colorectal Cancer using Functionalized Titania Nanotubular Arrays

Screening of colorectal cancer is crucial for early stage diagnosis and treatment. Detection of volatile organic compounds (VOCs) of the metabolome present in exhaled breath is a promising approach to screen colorectal cancer (CRC). Various forms of volatile organic compounds (VOCs) that show the definitive signature for the different diseases including cancers are present in exhale breathe. Among all the reported CRC VOCs, cyclohexane, methylcyclohexane, 1,3-dimethyl- benzene and decanal are identified as the prominent ones that can be used as the signature for CRC screening. In the present investigation, detection of the four prominent VOCs related to CRC is explored using functionalized titania nanotubular arrays (TNAs)-based sensor. These signature biomarkers are shown to be detected using nickel-functionalized TNA as an electrochemical sensor. The sensing mechanism is based on the electrochemical interaction of nickel-functionalized nanotubes with signature biomarkers. A detailed mechanism of the sensor response is also presented.

Hybrid 3D structures of ZnO nanoflowers and PdO nanoparticles as a highly selective methanol sensor

The present study concerns the enhancement of methanol selectivity of three dimensional (3D) nanoflowers (NFs) of ZnO by dispersing nickel oxide (NiO) and palladium oxide (PdO) nanoparticles on the surface of the nanoflowers to form localized hybrid nano-junctions.

Stoichiometry, Length, and Wall Thickness Optimization of TiO2 Nanotube Array for Efficient Alcohol Sensing

The present study concerns development of an efficient alcohol sensor by controlling the stoichiometry, length, and wall thickness of electrochemically grown TiO2 nanotube array for its use as the sensing layer. Judicious variation of H2O content (0, 2, 10 and 100% by volume) in the mixed electrolyte comprising ethylene glycol and NH4F resulted into the desired variation of stoichiometry. The sensor study was performed within the temperature range of 27 to 250 °C for detecting the alcohols in the concentration range of 10-1000 ppm. The nanotubes grown with the electrolyte containing 2 vol % H2O offered the maximum response magnitude. For this stoichiometry, variation of corresponding length (1.25-2.4 μm) and wall thickness (19.8-9 nm) of the nanotubes was achieved by varying the anodization time (4-16 h) and temperatures (42-87 °C), respectively. While the variation of length influenced the sensing parameters insignificantly, the best response magnitude was achieved for ∼13 nm wall thickness. The underlying sensing mechanism was correlated with the experimental findings on the basis of structural parameters of the nanotubes.

Biomolecular damage induced by ionizing radiation: the direct and indirect effects of low-energy electrons on DNA

Many experimental and theoretical advances have recently allowed the study of direct and indirect effects of low-energy electrons (LEEs) on DNA damage. In an effort to explain how LEEs damage the human genome, researchers have focused efforts on LEE interactions with bacterial plasmids, DNA bases, sugar analogs, phosphate groups, and longer DNA moieties. Here, we summarize the current understanding of the fundamental mechanisms involved in LEE-induced damage of DNA and complex biomolecule films. Results obtained by several laboratories on films prepared and analyzed by different methods and irradiated with different electron-beam current densities and fluencies are presented. Despite varied conditions (e.g., film thicknesses and morphologies, intrinsic water content, substrate interactions, and extrinsic atmospheric compositions), comparisons show a striking resemblance in the types of damage produced and their yield functions. The potential of controlling this damage using molecular and nanoparticle targets with high LEE yields in targeted radiation-based cancer therapies is also discussed.

DNA strand breaks and crosslinks induced by transient anions in the range 2-20 eV

The energy dependence of the yields of single and double strand breaks (SSB and DSB) and crosslinks induced by electron impact on plasmid DNA films is measured in the 2-20 eV range. The yield functions exhibit two strong maxima, which are interpreted to result from the formation of core-excited resonances (i.e., transient anions) of the bases, and their decay into the autoionization channel, resulting in π → π* electronic transitions of the bases followed by electron transfer to the C-O σ* bond in the phosphate group. Occupancy of the σ* orbital ruptures the C-O bond of the backbone via dissociative electron attachment, producing a SSB. From a comparison of our results with those of other works, including theoretical calculations and electron-energy-loss spectra of the bases, the 4.6 eV peak in the SSB yield function is attributed to the resonance decay into the lowest electronically excited states of the bases; in particular, those resulting from the transitions 13A'(π2 → π3*) and 13A″(n2 → π3*) of thymine and 13A'(π → π*) of cytosine. The strongest peak at 9.6 eV in the SSB yield function is also associated with electron captured by excited states of the bases, resulting mostly from a multitude of higher-energy π → π* transitions. The DSB yield function exhibits strong maxima at 6.1 and 9.6 eV. The peak at 9.6 eV is probably related to the same resonance manifold as that leading to SSB, but the other at 6.1 eV may be more restricted to decay into the electronic state 13A' (π → π*) of cytosine via autoionization. The yield function of crosslinks is dominated by a broad peak extending over the 3.6-11.6 eV range with a sharper one at 17.6 eV. The different line shape of the latter function, compared to that of SSB and DSB, appears to be due to the formation of reactive radical sites in the initial supercoiled configuration of the plasmid, which react with the circular form (i.e., DNA with a SSB) to produce a crosslink.

Reactions in Fluorinated Acetic Acid Esters Triggered by Slow Electrons: Bond Cleavages, Hydrogen Transfer Reactions and Loss of Halocarbons

Low energy electron induced reactions in the four fluorinated acetic acid esters CF3COOCH3, CF3COOC2H5, CF3COOCH2CF3 and CH3COOCH2CF3 are studied by means of a crossed electron-molecular beam experiment and mass spectrometric detection of the anions. All target molecules exhibit a prominent resonance located near 1 eV which is subjected to a variety of unimolecular reactions. The observation of the carboxylate anion CF3COO–/CH3COO– indicates rupture of the O−R ester bond, which in turn demonstrates that the prominent cleavage of the O−H bond recently observed in trifluoroacetic acid and acetic acid is not blocked when O−H is replaced by O−R (R=−CH3, −C2H5, −CH2CF3). In CF3COOCH3 and CF3COOC2H5 an ion of the form CF2COO– is generated which is identified to arise from a complex reaction involving multiple bond cleavages and substantial rearrangement in the transitory negative ion ultimately generating CH3F and C2H5F, respectively, as neutral counterpart. A fragment ion of the type (M-H)– (M: target molecule), i.e. loss of a neutral hydrogen atom, is only observed in CH3COOCH2CF3 which directly demonstrates that dehydrogenation selectively occurs at the CH3 site.

Dissociative electron attachment to gas phase glycine: exploring the decomposition pathways by mass separation of isobaric fragment anions

Dissociative electron attachment to gas phase glycine generates a number of fragment ions, among them ions observed at the mass numbers 15, 16 and 26 amu. From stoichiometry they can be assigned to the chemically rather different species NH(-)/CH(3)(-)(15 amu), O(-)/NH(2)(-)(16 amu) and CN(-)/C(2)H(2)(-)(26 amu). Here we use a high resolution double focusing two sector mass spectrometer to separate these isobaric ions. It is thereby possible to unravel the decomposition reactions of the different transient negative ions formed upon resonant electron attachment to neutral glycine in the energy range 0-15 eV. We find that within the isobaric ion pairs, the individual components generally arise from resonances located at substantial different energies. The corresponding unimolecular decompositions involve complex reaction sequences including multiple bond cleavages and substantial rearrangement in the precursor ion. To support the interpretation and assignments we also use (13)C labelling of glycine at the carboxylic group.