Search

Laboratory studies play a crucial role in understanding the chemical nature of the interstellar medium (ISM), but the disconnect between experimental timescales and the timescales of reactions in space can make a direct comparison between observations, laboratory, and model results difficult. Here we study the survival of reactive fragments of the polycyclic aromatic hydrocarbon (PAH) coronene, where individual C atoms have been knocked out of the molecules in hard collisions with He atoms at stellar wind and supernova shockwave velocities. Ionic fragments are stored in the DESIREE cryogenic ion-beam storage ring where we investigate their decay for up to one second. After 10 ms the initially hot stored ions have cooled enough so that spontaneous dissociation no longer takes place at a measurable rate; a majority of the fragments remain intact and will continue to do so indefinitely in isolation. Our findings show that defective PAHs formed in energetic collisions with heavy particles may survive at thermal equilibrium in the interstellar medium indefinitely, and could play an important role in the chemistry in there, due to their increased reactivity compared to intact or photo-fragmented PAHs., Ion storage rings allow reactions to be studied over orders of magnitude in time, bridging the gap between typical experimental and astronomical timescales. Here the authors observe that polycyclic aromatic hydrocarbon fragments produced upon collision with He atoms at velocities typical of stellar winds and supernova shockwaves remain intact up to second timescales, thus may play an important role in interstellar chemistry.

Tirapazamine (TPZ) has been tested in clinical trials on radio‐chemotherapy due to its potential highly selective toxicity towards hypoxic tumor cells. It was suggested that either the hydroxyl radical or benzotriazinyl radical may form as bioactive radical after the initial reduction of TPZ in solution. In the present work, we studied low‐energy electron attachment to TPZ in the gas phase and investigated the decomposition of the formed TPZ− anion by mass spectrometry. We observed the formation of the (TPZ–OH)− anion accompanied by the dissociation of the hydroxyl radical as by far the most abundant reaction pathway upon attachment of a low‐energy electron. Quantum chemical calculations suggest that NH2 pyramidalization is the key reaction coordinate for the reaction dynamics upon electron attachment. We propose an OH roaming mechanism for other reaction channels observed, in competition with the OH dissociation., Low‐energy electrons make a choice: Upon the attachment of a low‐energy electron to the potential radiosensitizer compound tirapazamine in the gas phase, dissociation of the OH radical is by far the most abundant reaction obtained. Other dissociation channels as well as the metastable parent anion formed upon roaming of OH are computationally predicted.

Funding Information: This work was supported by the FWF, Vienna, Austria (P30332). The computational results presented have been achieved using the HPC infrastructure LEO of the University of Innsbruck. Publisher Copyright: © The Royal Society of Chemistry. Nicotinamide (C6H6N2O) is a biologically relevant molecule. This compound has several important roles related to the anabolic and metabolic processes that take place in living organisms. It is also used as a radiosensitizer in tumor therapy. As a result of the interaction of high-energy radiation with matter, low-energy electrons are also released, which can also interact with other molecules, forming several types of ions. In the present investigation, dissociative electron attachment to C6H6N2O has been studied in a crossed electron-molecular beams experiment in the electron energy range of about 0-15 eV. In the experiment, six anionic species were detected: C6H5N2O-, C5H4N-, NCO-, O-/NH2-, and CN-, with NCO- being the most prominent anion. We also provide detailed computational results regarding the energetic thresholds and pathways of the respective dissociative electron attachment (DEA) channels. The experimental results are compared with the theoretical ones and on this basis, the possible DEA reactions for the formation of anions at a given resonance energy were assigned as well as the generation of neutrals fragments such as pyridine and its several derivatives and radicals are predicted. The pyridine ring seems to stay intact during the DEA process. This journal is publishersversion published

PTDC/FIS‐ AQM/31281/2017 PD/00193/2012 We report absolute photoabsorption cross sections for gas‐phase 2‐ and 5‐ bromopyrimidine in the 3.7–10.8 eV energy range, in a joint theoretical and experimental study. The measurements were carried out using high‐resolution vacuum ultraviolet synchrotron radiation, with quantum chemical calculations performed through the nuclear ensemble approach in combination with time‐dependent density functional theory, along with additional Franck–Condon Herzberg–Teller calculations for the first absorption band (3.7–4.6 eV). The cross sections of both bromopyrimidines are very similar below 7.3 eV, deviating more substantially from each other at higher energies. In the 7.3–9.0 eV range where the maximum cross‐section is found, a single and broad band is observed for 5‐bromopyrimidine, while more discernible features appear in the case of 2‐bromopyrimidine. Several π* ← π transitions account for the most intense bands, while weaker ones are assigned to transitions involving the nitrogen and bromine lone pairs, the antibonding σ*Br orbital, and the lower‐lying Rydberg states. A detailed comparison with the available photo‐absorption data of bromobenzene is also reported. We have found significant differences regarding the main absorption band, which is more peaked in bromobenzene, becoming broader and shifting to higher energies in both bromopyrimidines. In addition, there is a significant suppression of vibrational structures and of Rydberg states in the pair of isomers, most noticeably for 2‐bromopyrimidine. publishersversion published

In the search for alternatives to chlorine-containing gases, tetrafluoroethane, CF3CH2F (R134a), a widely used refrigerant gas, has been recognized as a promising substitute for dichlorodifluoromethane, CCl2F2 (R12). When R12 is replaced by R134a, the global warming potential drops from 8100 to 1430, the ozone depletion potential changes from 1 to 0, and the atmospheric lifetime decreases from 100 to 14 years. Electron interactions in the gas phase play a fundamental role in the atmospheric sciences. Here, we present a detailed study on electron-driven fragmentation pathways of CF3CH2F, in which we have investigated processes induced by both electron ionization and electron attachment. The measurements allow us to report the ion efficiency curves for ion formation in the energy range of 0 up to 25 eV. For positive ion formation, R134a dissociates into a wide assortment of ions, in which CF3+ is observed as the most abundant out of seven ions with a relative intensity above 2%. The results are supported by quantum chemical calculations based on bound state techniques, electron-impact ionization models, and electron-molecule scattering simulations, showing a good agreement. Moreover, the experimental first ionization potential was found at 13.10 ± 0.17 eV and the second at around 14.25 eV. For negative ion formation, C2F3- was detected as the only anion formed, above 8.3 eV. This study demonstrates the role of electrons in the dissociation of R134a, which is relevant for an improvement of the refrigeration processes as well as in atmospheric chemistry and plasma sciences.

Benzaldehyde is a simple aromatic aldehyde and has a wide range of applications in the food, pharmaceutical, and chemical industries. The positive electron affinity of this compound suggests that low-energy electrons can be easily trapped by neutral benzaldehyde. In the present study, we investigated the formation of negative ions following electron attachment to benzaldehyde in the gas-phase. Calculations on elastic electron scattering from benzaldehyde indicate a π* valence bound state of the anion at −0.48 eV and three π* shape resonances (0.78, 2.48 and 5.51 eV). The excited state spectrum of the neutral benzaldehyde is also reported to complement our findings. Using mass spectrometry, we observed the formation of the intact anionic benzaldehyde at ∼0 eV. We ascribe the detection of the benzaldehyde anion to stabilization of the π* valence bound state upon dissociative electron attachment to a benzaldehyde dimer. In addition, we report the cross sections for nine fragment anions formed through electron attachment to benzaldehyde. Investigations carried out with partially deuterated benzaldehyde show that the hydrogen loss is site-selective with respect to the incident electron energy. In addition, we propose several dissociation pathways, backed up by quantum chemical calculations on their thermodynamic thresholds. The threshold calculations also support that the resonances formed at higher energies lead to fragment anions observable by mass spectrometry, whereas the resonances at low electron energies decay only by electron autodetachment.

Funding Information: This work was supported by EFRE (K-Regio project FAENOMENAL, Grant No. EFRE 2016-4) and the Austrian Science Fund FWF (Project No. P31149, I4130). This work was also supported by Fundação para a Ciência e a Tecnologia (FCT-MCTES), Radiation Biology and Biophysics Doctoral Training Programme (RaBBiT, PD/00193/2012); Applied Molecular Biosciences Unit - UCIBIO (UIDB/04378/2020) and CEFITEC Unit (UIDB/00068/2020); and scholarship Grant No. PD/BD/114447/2016 to J.A., F. Zappa acknowledges support from the Brazilian agency CNPq. K.v.H. kindly acknowledges the award of a LFUI guest professorship. The demand for nanoscale materials of ultra-high purity and narrow size distribution is addressed. Clusters of Au, C60, H2O, and serine are produced inside helium nanodroplets using a combination of ionization, mass filtering, collisions with atomic or molecular vapor, and electrostatic extraction, in a specific and novel sequence. The helium droplets are produced in an expansion of cold helium gas through a nozzle into vacuum. The droplets are ionized by electron bombardment and subjected to a mass filter. The ionic and mass-selected helium droplets are then guided through a vacuum chamber filled with atomic or molecular vapor where they collide and "pick up" the vapor. The dopants then agglomerate inside the helium droplets around charge centers to singly charged clusters. Evaporation of the helium droplets is induced by collisions in a helium-filled radio frequency (RF)-hexapole, which liberates the cluster ions from the host droplets. The clusters are analyzed with a time-of-flight mass spectrometer. It is demonstrated that using this sequence, the size distribution of the dopant cluster ions is distinctly narrower compared to ionization after pickup. Likewise, the ion cluster beam is more intense. The mass spectra show, as well, that ion clusters of the dopants can be produced with only few helium atoms attached, which will be important for messenger spectroscopy. All these findings are important for the scientific research of clusters and nanoscale materials in general. publishersversion published

Interactions of low-energy electrons with the FEBID precursor Cr(CO)6 have been investigated in a crossed electron–molecular beam setup coupled with a double focusing mass spectrometer with reverse geometry. Dissociative electron attachment leads to the formation of a series of anions by the loss of CO ligand units. The bare chromium anion is formed by electron capture at an electron energy of about 9 eV. Metastable decays of Cr(CO)5− into Cr(CO)4−, Cr(CO)4− into Cr(CO)3− and Cr(CO)3− into Cr(CO)2− are discussed. Electron-induced dissociation at 70 eV impact energy was found to be in agreement with previous studies. A series of Cr(CO)nC+ (0 ≤ n ≤ 3) cations formed by C–O cleavage is described for the first time. The metastable decay of Cr(CO)6+ into Cr(CO)5+ and collision-induced dissociation leading to bare Cr+, are discussed. In addition, doubly charged cations were identified and the ration between doubly and singly charged fragments was determined and compared with previous studies, showing considerable differences.

International audience; Nitroimidazoles are important compounds in medicine, biology, and the food industry. The growing need for their structural assignment, as well as the need for the development of the detection and screening methods, provides the motivation to understand their fundamental properties and reactivity. Here, we investigated the decomposition of protonated ronidazole [Roni+H]+ in low-energy and high-energy collision-induced dissociation (CID) experiments. Quantum chemical calculations showed that the main fragmentation channels involve intramolecular proton transfer from nitroimidazole to its side chain followed by a release of NH2CO2H, which can proceed via two pathways involving transfer of H+ from (1) the N3 position via a barrier of TS2 of 0.97 eV, followed by the rupture of the C–O bond with a thermodynamic threshold of 2.40 eV; and (2) the –CH3 group via a higher barrier of 2.77 eV, but with a slightly lower thermodynamic threshold of 2.24 eV. Electrospray ionization of ronidazole using deuterated solvents showed that in low-energy CID, only pathway (1) proceeds, and in high-energy CID, both channels proceed with contributions of 81% and 19%. While both of the pathways are associated with small kinetic energy release of 10–23 meV, further release of the NO• radical has a KER value of 339 meV.

Grant Nos. PD/BD/114447/2016 and PD/BD/114452/2016. researcher position No. IF-FCT IF/00380/2014. Doctoral Training Programme (RaBBiT, PD/00193/2012). S.D. acknowledges support from FWF (P30332). Polish National Science Center (NCN) under the Grant No. UMO-2014/14/A/ST4/00405 (J. R.). 5-trifluoromethanesulfonyl-uracil (OTfU), a recently proposed radiosensitizer, is decomposed in the gas-phase by attachment of low-energy electrons. OTfU is a derivative of uracil with a triflate (OTf) group at the C5-position, which substantially increases its ability to undergo effective electron-induced dissociation. We report a rich assortment of fragments formed upon dissociative electron attachment (DEA), mostly by simple bond cleavages (e.g., dehydrogenation or formation of OTf-). The most favorable DEA channel corresponds to the formation of the triflate anion alongside with the reactive uracil-5-yl radical through the cleavage of the O-C5 bond, particularly at about 0 eV. Unlike for halouracils, the parent anion was not detected in our experiments. The experimental findings are accounted by a comprehensive theoretical study carried out at the M06-2X/aug-cc-pVTZ level. The latter comprises the thermodynamic thresholds for the formation of the observed anions calculated under the experimental conditions (383.15 K and 3 × 10-11 atm). The energy-resolved ion yield of the dehydrogenated parent anion, (OTfU-H)-, is discussed in terms of vibrational Feshbach resonances arising from the coupling between the dipole bound state and vibrational levels of the transient negative ion. We also report the mass spectrum of the cations obtained through ionization of OTfU by electrons with a kinetic energy of 70 eV. The current study endorses OTfU as a potential radiosensitizer agent with possible applications in radio-chemotherapy. publishersversion published

1 pag., 1 fig. -- International Conference on Photonic, Electronic, and Atomic Collisions (ICPEAC) (30th. 2017. Cairns, Australia). -- Open Access funded by Creative Commons Atribution Licence 3.0, In the present study joint experimental and theoretical elastic differential cross sections for electron scattering from dichloromethane in the incident electron energy region 7 to 50eV are discussed.

Synopsis We report on the formation of anions through electron attachment to the coenzyme Q0 in the gas-phase. It represents the functional group of the electron carrier coenzyme Q10, which plays a key role in the electron transport chain in the mitochondrial complex. The parent anion is most abundantly formed in a near-zero eV resonance, presumably correlating with the electronic ground state of the molecule. This observation accounts for the ability of coenzyme Q10 to capture and transport electrons in biological processes.

9 págs.; 6 figs.; 1 tab., In this work, we report theoretical and experimental cross sections for elastic scattering of electrons by chlorobenzene (ClB). The theoretical integral and differential cross sections (DCSs) were obtained with the Schwinger multichannel method implemented with pseudopotentials (SMCPP) and the independent atom method with screening corrected additivity rule (IAM-SCAR). The calculations with the SMCPP method were done in the static-exchange (SE) approximation, for energies above 12 eV, and in the static-exchange plus polarization approximation, for energies up to 12 eV. The calculations with the IAM-SCAR method covered energies up to 500 eV. The experimental differential cross sections were obtained in the high resolution electron energy loss spectrometer VG-SEELS 400, in Lisbon, for electron energies from 8.0 eV to 50 eV and angular range from 7 to 110. From the present theoretical integral cross section (ICS) we discuss the low-energy shape-resonances present in chlorobenzene and compare our computed resonance spectra with available electron transmission spectroscopy data present in the literature. Since there is no other work in the literature reporting differential cross sections for this molecule, we compare our theoretical and experimental DCSs with experimental data available for the parent molecule benzene. Published by AIP Publishing., A.S.B., M.T.N.V., S.d’A.S., and M.H.F.B. acknowledge the Brazilian Agency Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), under CAPES/FCT Programme (Process No. 23038.002465/2014-87). M.T.N.V., S.d’A.S., and M.H.F.B. acknowledge support from the Brazilian Agency Conselho Nacional de Desenvolvimento Científico e Tecnológico. M.H.F.B. acknowledges support from Finep (under project CT-Infra), and M.T.N.V. from São Paulo Research Foundation (FAPESP). A.S.B., S.d’A.S., and M.H.F.B. acknowledge computational support from Professor Carlos M. de Carvalho at LFTC-DFis-UFPR and at LCPADUFPR and from CENAPAD-SP. F.F.S. acknowledges the Portuguese National Funding Agency FCT through researcher Contract No. IF-FCT IF/00380/2014 and together with P.LV. the research Grant No. UID/FIS/00068/2013. F.B. and G.G. acknowledge partial financial support from the Spanish Ministry MINECO (Project No. FIS2012-31230).

7 págs.; 3 figs.; 1 tab., We report elastic differential cross sections (DCSs) for electron interactions with acetone, C3H6O. The incident electron energy range was 7.0-50 eV, and the scattered electron angular range for the differential measurements varied from 10° to 120°. The calculated cross sections were obtained with two different methodologies, the Schwinger multichannel method with pseudopotentials (SMCPP), and the independent-atom method with screening-corrected additivity rule (IAM-SCAR). The present elastic DCSs have been found to agree well with the results of IAM-SCAR calculations above 20 eV, and also with the SMC calculations below 30 eV. Although some discrepancies were found for lower energies, the agreement between the SMCPP data and the DCSs obtained with the IAM-SCAR method improves, as expected, as the impact energy increases. Comparison with previous DCSs shows good agreement albeit the present data is extended down to lower electron impact energies. We find a low-lying π∗ shape resonance located at 2.6 eV, in agreement with recent results on electron collisions with acetone [M. G. P. Homem et al., Phys. Rev. A 92, 032711 (2015)10.1103/PhysRevA.92.032711]. The presence of a σ∗ resonance is also discussed. ©2016 American Physical Society, Acknowledge the Brazilian Agency Coordenacao de Aperfeicoamento de Pessoal de Nıvel Superior (CAPES), under CAPES/FCT Programme (Process No. 23038.002465/2014-87).E.L. acknowledges the Brazilian Agency Conselho Nacional de Desenvolvimento Cientıfico e Tecnologico (CNPq) and the Science Without Borders Programme for opportunities to study abroad. M.H.F.B. also acknowledges support from CNPq. F.F.S. acknowledges the Portuguese National Funding Agency FCT through Researcher Contract No. IF-FCT IF/00380/2014, and, together with P.L.V., Research GrantsNo. PTDC/FIS-ATO/1832/2012 and No. UID/FIS/00068/2013. This work has been partially supported by the Spanish MINECO(Project No. FIS2012-31230).We also acknowledge financial support from European Union programs (COST CM1301 and FP7-ITN ARGENT). This work was also supported by FCT-MCTES, Radiation Biology and Biophysics Doctoral Training Programme (RaBBiT, PD/00193/2012); UID/Multi/04378/2013 (UCIBIO).

We demonstrate that low-energy electron (< 3 eV) interaction with 2,3-dimethoxy-5-methyl-1,4- benzoquinone (DMB) in the gas phase leads to a wide assortment of anionic fragments. DMB is the functional head-group of ubiquinone electron carrier involved in the electron transport chain in mithochondrial membrane. This study is a key contribution to a more comprehensive understanding of the electron transport in the mithochondrial membrane by ubiquinone. publishersversion published

Positive and negative ion formation upon low energy electron interaction with chromium hexacarbonyl precursor molecules used in FEBID applications was studied. The obtained results revealed a noteworthy different fragmentation pattern compared to other precursors. This work contributes to the development and the possible production concerning highly pure nanoscale structures.