Stephan Denifl

University of Innsbruck, Innsbruck, Tyrol, Austria

Are you Stephan Denifl?

Claim your profile

Publications (164)525.28 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: We present high-resolution measurements of the dissociative electron attachment (DEA) to isolated gas-phase hypoxanthine (C5H4N4O, Hyp), a tRNA purine base. The anion mass spectra and individual ion efficiency curves from Hyp were measured as a function of electron energy below 9 eV. The mass spectra at 1 and 6 eV exhibit the highest anion yields, indicating possible common precursor ions that decay into the detectable anionic fragments. The (Hyp − H) anion (C5H3N4O−) exhibits a sharp resonant peak at 1 eV, which we tentatively assign to a dipole-bound state of the keto-N1H,N9H tautomer in which dehydrogenation occurs at either the N1 or N9 position based upon our quantum chemical computations (B3LYP/6-311+G(d,p) and U(MP2-aug-cc-pVDZ+)) and prior studies with adenine. This closed-shell dehydrogenated anion is the dominant fragment formed upon electron attachment, as with other nucleobases. Seven other anions were also observed including (Hyp − NH)−, C4H3N4 −/C4HN3O−, C4H2N3 −, C3NO−/HC(HCN)CN−, OCN−, CN−, and O−. Most of these anions exhibit broad but weak resonances between 4 and 8 eV similar to many analogous anions from adenine. The DEA to Hyp involves significant fragmentation, which is relevant to understanding radiation damage of biomolecules.
    The Journal of Chemical Physics 06/2015; 142(21):215101. DOI:10.1063/1.4921388 · 3.12 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Low energy electrons (0 - 8 eV) effectively decompose 4-nitroimidazole (4NI) and the two methylated isomers 1-methyl-5-nitroimidazole and 1-methyl-4-nitroimidazole via dissociative electron attachment (DEA). The involved unimolecular decompositions range from simple bond cleavages (loss of H•, formation of NO2-) to complex reactions possibly leading to a complete degradation of the target molecule (formation of CN-, etc.). At energies below 2 eV the entire rich chemistry induced by DEA is completely quenched by methylation as already demonstrated in a previous Communication (K. Tanzer, L. Feketeová, B. Puschnigg, P. Scheier, E. Illenberger, and S. Denifl, Angew. Chem. Int. E. 53 (2014) 12240). The observation that in 4NI neutral radicals and radical anions are formed via DEA at high efficiency already at threshold (0 eV) may have significant implications for the development of nitroimidazole based radiosensitisers in tumour radiation therapy.
    The Journal of Physical Chemistry A 05/2015; 119(25). DOI:10.1021/acs.jpca.5b02721 · 2.78 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Adsorption of helium on free, negatively charged fullerenes is studied in this work. Helium nanodroplets have been doped with fullerenes and ionised by electron attachment. For suitable experimental conditions, C−60 and C−70 anions are found to be complexed with a large number of helium atoms. Prominent anomalies in the ion abundances indicate the high stability of the commensurate 1×1 phase in which all hollow adsorption sites are occupied by one atom each. The adsorption energy for an additional helium atom is about 40% less than for atoms in the commensurate layer, similar to our previous findings for fullerene cations and in agreement with theoretical dissociation energies. Similarly, an anomaly in the adsorption energy occurs when 60 helium atoms are attached to C−60 or 65 to C−70. For C60, the anomaly coincides with the one observed for cationic complexes but for C70 it does not. Implications of these features are discussed in light of several theoretical studies of neutral and positively charged helium-fullerene complexes.
    Molecular Physics 05/2015; DOI:10.1080/00268976.2015.1018357 · 1.64 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Nitroimidazoles are important compounds with chemotherapeutic applications as antibacterial drugs or as radiosensitizers in radiotherapy. Despite their use in biological applications, little is known about the fundamental properties of these compounds. Understanding the ionization reactions of these compounds is crucial in evaluating the radiosensitization potential and in developing new and more effective drugs. Thus, the present study investigates the decomposition of negative and positive ions of 2-nitroimidazole and 4(5)-nitroimidazole using low- and high-energy Collision-Induced Dissociation (CID) and Electron-Induced Dissociation (EID) by two different mass spectrometry techniques and is supported by quantum chemistry calculations. EID of [M+H]+ leads to more extensive fragmentation than CID and involves many radical cleavages including loss of H• leading to the formation of the radical cation, M•+. The stability (metastable decay) and the fragmentation (high-energy CID) of the radical cation M•+ have been probed in a crossed-beam experiment involving primary electron ionization of the neutral nitroimidazole. Thus, fragments in the EID spectra of [M+H]+ that come from further dissociation of radical cation M•+ has been highlighted. The loss of NO• radical from M•+ is associated with a high Kinetic Energy Release (KER) of 0.98 eV. EID of [M−H]− also leads to additional fragments compared to CID, however, with much lower cross section. Only EID of [M+H]+ leads to a slight difference in the decomposition of 2-nitroimidazole and 4(5)-nitroimidazole.
    Physical Chemistry Chemical Physics 04/2015; 17(19). DOI:10.1039/C5CP01014D · 4.20 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The effects of interactions between He(-) and clusters of fullerenes in helium nanodroplets are described. Electron transfer from He(-) to (C60)n and (C70)n clusters results in the formation of the corresponding fullerene cluster dianions. This unusual double electron transfer appears to be concerted and is most likely guided by electron correlation between the two very weakly bound outer electrons in He(-). We suggest a mechanism which involves long range electron transfer followed by the conversion of He(+)into He2 (+), where formation of the He-He bond in He2 (+) releases sufficient kinetic energy for the cation and the dianion to escape their Coulombic attraction. By analogy with the corresponding dications, the observation of a threshold size of n ≥ 5 for formation of both (C60)n (2-) and (C70)n (2-) is attributed to Coulomb explosion rather than an energetic constraint. We also find that smaller dianions can be observed if water is added as a co-dopant. Other aspects of He(-) chemistry that are explored include its role in the formation of multiply charged fullerene cluster cations and the sensitivity of cluster dianion formation on the incident electron energy.
    The Journal of Chemical Physics 03/2015; 142(10):104306. DOI:10.1063/1.4913956 · 3.12 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Negative ion formation through dissociative electron attachment to the gas-phase volatile anaesthetics enflurane and isoflurane (C3H2ClF5O) and to two chlorinated ethanes, pentachloroethane (C2HCl5) and hexachloroethane (C2Cl6), have been studied in a crossed electron-molecular beam two sector field mass spectrometer experiment. Anion efficiency curves for the negatively charged fragments have been measured over an electron energy range of approximately 0–17 eV, with an energy resolution of ∼1 eV. For the chlorinated ethanes, resonance features were generally found at ∼ 0 eV and at 7.5 eV. For the volatile anaesthetics, no zero energy resonances were observed. Instead, product anions were detected mainly in the 2–3 eV and 9 eV energy regions, with the exception of Cl-, whose dominant resonance occurs at approximately 0.6 eV and 0.9 eV for isoflurane and enflurane, respectively. To aid in the interpretation of the experimental results, quantum chemical calculations providing thermochemical thresholds of anion formation are also presented.
    International Journal of Mass Spectrometry 02/2015; 379. DOI:10.1016/j.ijms.2015.01.009 · 2.23 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Cyanamide (NH2CN) is a molecule relevant for interstellar chemistry and the chemical evolution of life. In the present investigation, dissociative electron attachment to NH2CN has been studied in a crossed electron-molecular beams experiment in the electron energy range from about 0 eV to 14 eV. The following anionic species were detected: NHCN(-), NCN(-), CN(-), NH2 (-), NH(-), and CH2 (-). The anion formation proceeds within two broad electron energy regions, one between about 0.5 and 4.5 eV and a second between 4.5 and 12 eV. A discussion of possible reaction channels for all measured negative ions is provided. The experimental results are compared with calculations of the thermochemical thresholds of the anions observed. For the dehydrogenated parent anion, we explain the deviation between the experimental appearance energy of the anion with the calculated corresponding reaction threshold by electron attachment to the isomeric form of NH2CN-carbodiimide.
    The Journal of Chemical Physics 01/2015; 142(3):034301. DOI:10.1063/1.4905500 · 3.12 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Clusters of sulfur are grown by passing superfluid helium nanodroplets through a pickup cell filled with sulfur vapor. In some experiments the droplets are co-doped with C60. The doped droplets are collided with energetic electrons and the abundance distributions of positively and negatively charged cluster ions are recorded. We report, specifically, distributions of Sm+, Sn-, and C60Sm- containing up to 41 sulfur atoms. We also observe complexes of sulfur cluster anions with helium; distributions are presented for HenSm- with n <= 31 and m <= 3. The similarity between anionic and cationic C60Sm- spectra is in striking contrast to the large differences between spectra of Sm+ and Sn-.
    The Journal of Physical Chemistry C 01/2015; 119(20):150109144621000. DOI:10.1021/jp510870x · 4.84 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The formation of dianions in helium nanodroplets is reported for the first time. The fullerene cluster dianions (C60)n2− and (C70)n2− were observed by mass spectrometry for n≥5 when helium droplets containing the appropriate fullerene were subjected to electron impact at approximately 22 eV. A new mechanism for dianion formation is described, which involves a two-electron transfer from the metastable He− ion. As well as the prospect of studying other dianions at low temperature using helium nanodroplets, this work opens up the possibility of a wider investigation of the chemistry of He−, a new electron-donating reagent.
    Angewandte Chemie 12/2014; 126(50). DOI:10.1002/ange.201408172
  • [Show abstract] [Hide abstract]
    ABSTRACT: The self-assembly of salt nanocrystals from chemical reactions inside liquid helium is reported for the first time. Reaction is initiated by an electron impacting a helium nanodroplet containing sodium atoms and SF6 molecules, leading to preferential production of energetically favorable structures based on the unit cell of crystalline NaF. These favorable structures are observed as magic number ions (anomalously intense peaks) in mass spectra and are seen in both cationic and anionic channels in mass spectra, for example, (NaF)nNa+ and (NaF)nF−. In the case of anions the self-assembly is not directly initiated by electrons: the dominant process involves resonant electron-induced production of metastable electronically excited He− anions, which then initiate anionic chemistry by electron transfer.
    Angewandte Chemie International Edition 12/2014; 53(49). DOI:10.1002/anie.201409465 · 11.34 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The self-assembly of salt nanocrystals from chemical reactions inside liquid helium is reported for the first time. Reaction is initiated by an electron impacting a helium nanodroplet containing sodium atoms and SF6 molecules, leading to preferential production of energetically favorable structures based on the unit cell of crystalline NaF. These favorable structures are observed as magic number ions (anomalously intense peaks) in mass spectra and are seen in both cationic and anionic channels in mass spectra, for example, (NaF)nNa+ and (NaF)nF−. In the case of anions the self-assembly is not directly initiated by electrons: the dominant process involves resonant electron-induced production of metastable electronically excited He− anions, which then initiate anionic chemistry by electron transfer.
    Angewandte Chemie 12/2014; 126(49). DOI:10.1002/ange.201409465
  • [Show abstract] [Hide abstract]
    ABSTRACT: Niederenergetische Elektronen (NEEs) mit Energien unter 2 eV zersetzen 4-Nitroimidazol (4NI) sehr effizient über dissoziative Elektronenanlagerung (DEA). Diese Reaktionen umfassen einfache Bindungsbrüche, aber auch komplexere Prozesse wie mehrfache Bindungsbrüche und die Bildung neuer Moleküle. Alle diese Reaktionen zeigen sich als scharfe Strukturen im detektierten Anionensignal und werden als Feshbach-Resonanzen gedeutet, die effiziente Wegbereiter für DEA sind. Die außergewöhnlich vielfältigen chemischen Reaktionen von 4NI werden bei Methylierung von 4NI an der N1-Position komplett blockiert. Diese bemerkenswerten Resultate haben auch Auswirkung auf die Entwicklung von Radiosensibilisatoren für die Strahlungstherapie von Tumoren auf Basis von Nitroimidazol.
    Angewandte Chemie 11/2014; 126(45). DOI:10.1002/ange.201407452
  • [Show abstract] [Hide abstract]
    ABSTRACT: Low-energy electrons (LEEs) at energies of less than 2 eV effectively decompose 4-nitroimidazole (4NI) by dissociative electron attachment (DEA). The reactions include simple bond cleavages but also complex reactions involving multiple bond cleavages and formation of new molecules. Both simple and complex reactions are associated with pronounced sharp features in the anionic yields, which are interpreted as vibrational Feshbach resonances acting as effective doorways for DEA. The remarkably rich chemistry of 4NI is completely blocked in 1-methyl-4-nitroimidazole (Me4NI), that is, upon methylation of 4NI at the N1 site. These remarkable results have also implications for the development of nitroimidazole based radiosensitizers in tumor radiation therapy.
    Angewandte Chemie International Edition 11/2014; 53(45). DOI:10.1002/anie.201407452 · 11.34 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Electron ionization of the DNA nucleobase, adenine, and the tRNA nucleobase, hypoxanthine, was investigated near the threshold region (~5-20 eV) using a high-resolution hemispherical electron monochromator and a quadrupole mass spectrometer. Ion efficiency curves of the threshold regions and the corresponding appearance energies (AEs) are presented for the parent cations and the five most abundant fragment cations of each molecule. The experimental ionization energies (IEs) for adenine and hypoxanthine were determined to be 8.70 ± 0.3 eV and 8.88 ± 0.5 eV, respectively. Quantum chemical calculations (B3LYP/6 311+G(2d,p)) yielded a vertical IE of 8.08 eV and an adiabatic IE of 8.07 eV for adenine and a vertical IE of 8.51 eV and an adiabatic IE of 8.36 eV for hypoxanthine, and the lowest energy optimized structures of the fragment cations and their respective neutrals were calculated. The enthalpies of the possible reactions from the adenine and hypoxanthine cations were also determined computationally, which assisted in determining the most likely electron ionization pathways leading to the major fragment cations. Our results suggest that the imidazole ring is more stable than the pyrimidine ring in several of the fragmentation reactions from both adenine and hypoxanthine. This electron ionization study contributes to understanding the biological effects of electrons on nucleobases and contributes to the database of electronic properties of biomolecules, which is necessary for modeling damage of DNA in living cells that is induced by ionizing radiation.
    Physical Chemistry Chemical Physics 10/2014; 16(45). DOI:10.1039/C4CP03452J · 4.20 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The mechanism of ionization of helium droplets has been investigated in numerous reports but one observation has not found a satisfactory explanation: How are He+ ions formed and ejected from undoped droplets at electron energies below the ionization threshold of the free atom? Does this path exist at all? A measurement of the ion yields of He+ and He2+ as a function of electron energy, electron emission current, and droplet size reveals that metastable He*- anions play a crucial role in the formation of free He+ at subthreshold energies. The proposed model is testable.
    Physical Chemistry Chemical Physics 09/2014; 16(41). DOI:10.1039/C4CP03236E · 4.20 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Helium droplets provide the possibility to study phenomena at the very low temperatures at which quantum mechanical effects are more pronounced and fewer quantum states have significant occupation probabilities. Understanding the migration of either positive or negative charges in liquid helium is essential to comprehend charge-induced processes in molecular systems embedded in helium droplets. Here, we report the resonant formation of excited metastable atomic and molecular helium anions in superfluid helium droplets upon electron impact. Although the molecular anion is heliophobic and migrates toward the surface of the helium droplet, the excited metastable atomic helium anion is bound within the helium droplet and exhibits high mobility. The atomic anion is shown to be responsible for the formation of molecular dopant anions upon charge transfer and thus, we clarify the nature of the previously unidentified fast exotic negative charge carrier found in bulk liquid helium.
    Journal of Physical Chemistry Letters 07/2014; 5(14):2444-2449. DOI:10.1021/jz500917z · 6.69 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Electron attachment experiments have been performed with dimethyl disulphide, C2H6S2, in the gas phase by means of a crossed electron-molecular beam experiment. Ion yields for 8 anions have been measured in the energy range from similar to 0 to 15 eV. Many of the dissociative electron attachment products observed at low energy arise from surprisingly complex reactions associated with multiple bond cleavages as well as structural and electronic rearrangement. Quantum chemical calculations on the electronic properties of C2H6S2 have been performed in order to complement the experimental investigations.
    Chemical Physics Letters 06/2014; 605. DOI:10.1016/j.cplett.2014.05.016 · 1.99 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Dissociative electron attachment to PtBr2 in the gas phase was studied in the low electron energy range from zero up to 10 eV with an energy resolution of 150 meV. The experiments were carried out using a hemispherical electron monochromator coupled with a quadrupole mass spectrometer and pulse counting acquisition system. The only anion observed was Br−. This ion is formed at three resonance electron energies: 0.4 eV, 1.2 eV and 7 eV. By the measurements of the Br− formation at different sample temperatures (in the 360–460 K range) the 0.4 eV resonance was associated with the electron capture by HBr generated in the apparatus at elevated temperatures. In addition, the thermodynamic thresholds for dissociative electron attachment reactions for platinum(II) bromide were calculated and compared with the experimental results.
    International Journal of Mass Spectrometry 05/2014; 365-366:152-156. DOI:10.1016/j.ijms.2013.11.016 · 2.23 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Electron attachment to CO2 embedded in superfluid He droplets leads to ionic complexes of the form (CO2)n(-) and (CO2)nO(-) and, at much lower intensities, He containing ions of the form Hem(CO2)nO(-). At low energies (< 5 eV), predominantly the non-decomposed complexes (CO2)n(-) are formed via two resonance contributions, similar to electron attachment to pristine CO2 clusters. The significantly different shapes and relative resonance positions, however, indicate particular quenching and mediation processes in CO2@He. A series of further resonances in the energy range up to 67 eV can be assigned to electronic excitation of He and capture of the inelastically scattered electron generating (CO2)n(-) and two additional processes where an intermediately formed He* leads to the non-stoichiometric anions (CO2)nO(-).
    The Journal of Physical Chemistry A 05/2014; 118(33). DOI:10.1021/jp503179d · 2.78 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We present a detailed experimental investigation of anion production in electron collisions with ethylene, C2H4. The investigated energy range is between 0 and 90 eV where anions are formed by two processes, in the low energy regime by dissociative electron attachment (DEA) and at higher energy by dipolar dissociation (DD). These electron induced processes are studied in two different experimental apparatus using two different mass spectrometry techniques. One is a time of flight spectrometer operating with velocity slice imaging technique and the other is a two sector field mass spectrometer. The former allows efficient collection of ions compared to standard mass spectrometers, while the latter provides high mass resolution. Eight fragment anions formed via DEA in the electron energy range between 5 and 17 eV have been detected; two fragments have not reported as DEA products in any previous studies. DD in C2H4 leads to the formation of the same anions as found in the case of DEA. Quantum chemical calculations have been carried out to determine the thermochemical thresholds of anion formation.
    International Journal of Mass Spectrometry 05/2014; 365. DOI:10.1016/j.ijms.2014.01.006 · 2.23 Impact Factor

Publication Stats

2k Citations
525.28 Total Impact Points

Institutions

  • 2002–2015
    • University of Innsbruck
      • Institute for Ion Physics and Applied Physics
      Innsbruck, Tyrol, Austria
  • 2013
    • University of Birmingham
      Birmingham, England, United Kingdom
  • 2012
    • Freie Universität Berlin
      • Institute of Theoretical Physics
      Berlín, Berlin, Germany
  • 2007
    • University of Iceland
      • Science Institute
      Reikiavik, Capital Region, Iceland