Electron attachment to chlorouracil: a comparison between 6-ClU and 5-ClU

Low energy electron impact to the isomers 6-chlorouracil (6-ClU) and 5-chlorouracil (5-ClU) yields a variety of negative ion fragments with surprisingly high cross sections. These ions are dominantly formed via sharply structured resonance features at energies below the threshold for electronic excitation and result from dissociative electron attachment (DEA). The most dominant DEA channel is formation of (M-HCl)-, i.e., ejection of a neutral HCl molecule with the negative charge remaining on the ring. The reaction cross section is 9 x 10(-18) m2 and 5 x 10(-18) m2 for 6-Cl and 5-ClU, respectively, and thus about two orders of magnitude higher than the geometrical cross section of the molecule. Further reactions also operative via low energy resonances (<2.5 eV) are Cl- abstraction, dehydrogenation [formation of (M-H)-, M=ClU], and DEA processes associated with a ring opening. Most of the ion yield curves exhibit remarkably sharp structures which have not been observed before in DEA to a polyatomic system. Although some possibilities on their origin are discussed, their interpretation remains a challenge for theory and further experiments. While electron attachment to both 6-ClU and 5-ClU generates fragments of the same stoichiometric composition, their ion yields and also their relative intensities show some very pronounced differences which can be explained by the different structure but also the different energetic situation in the two isomers.     

Dissociative electron attachment to Pt(PF3)4--a precursor for focused electron beam induced processing (FEBIP)

Experimental absolute cross sections for dissociative electron attachment (DEA) to Pt(PF(3))(4) are presented. Fragment anions resulting from the loss of one, two, three and four PF(3) ligands as well as the Pt(PF(3))F(-) and the F(-) ions were observed. The parent anion Pt(PF(3)) is too short-lived to be detected. The dominant process is loss of one ligand, with a very large cross section of 20?000 pm(2); the other processes are about 200× weaker, with cross sections around 100 pm(2), the naked Pt(-) anion is formed with a cross section of only 1.8 pm(2). The resonances responsible for the DEA bands were assigned based on comparison with electron energy-loss spectra and spectra of vibrational excitation by electron impact. Bands around 0.5 eV and 2 eV were assigned to shape resonances with single occupation of virtual orbitals. A DEA band at 5.9 eV was assigned to a core-excited resonance corresponding to an electron very weakly bound to the lowest excited state. An F(-) band at 12.1 eV is assigned to a core excited resonance with a vacancy in an orbital corresponding to the 2nd ionization energy of the PF(3) ligand. Implications of these findings for FEBIP are discussed.     

Vibrational Feshbach resonances in uracil and thymine

Sharp peaks in the dissociative electron attachment (DEA) cross sections of uracil and thymine at energies below 3 eV are assigned to vibrational Feshbach resonances (VFRs) arising from coupling between the dipole bound state and the temporary anion state associated with occupation of the lowest sigma* orbital. Three distinct vibrational modes are identified, and their presence as VFRs is consistent with the amplitudes and bonding characteristics of the sigma* orbital wave function. A deconvolution method is also employed to yield higher effective energy resolution in the DEA spectra. The site dependence of DEA cross sections is evaluated using methyl substituted uracil and thymine to block H atom loss selectively. Implications for the broader issue of DNA damage are briefly discussed.     

Photon-stimulated desorption of F(-) ions from CF(3)Cl adsorbed on Si(111)-7x7

We report the photon-stimulated desorption of negative ions induced by direct dipolar dissociation and dissociative electron attachment. The photon-stimulated desorption of F(-) ions from CF(3)Cl physisorbed on a Si(111)-7x7 surface at 30 K in the photon energy range 12-35 eV was studied. The F(-) ion yield exhibits four resonances, at 12.8, 16.2, 19.5, and 22.3 eV, quite unlike the gas phase photodissociation cross section. The intensities of these resonances depend strongly on the CF(3)Cl coverage in a manner which varies from peak to peak. The resonances at 19.5 and 22.3 eV, which have a significant enhancement in the monolayer regime, are due to electron mediated dipolar dissociation of adsorbed CF(3)Cl molecules. The enhancement is attributed to surface electron attachment following molecular excitation. A significant enhancement in the monolayer regime has also been observed for the resonances at 12.8 and 16.2 eV. These two resonances are ascribable to a combination of electron mediated dipolar dissociation and dissociative electron attachment driven by photoelectrons generated in the neighboring molecules.     

Electron attachment to antipyretics: possible implications of their metabolic pathways

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

Phenol, chlorobenzene and chlorophenol isomers: resonant states and dissociative electron attachment

This paper reports a study of resonant dissociative electron attachment (DEA) to the phenol, chlorobenzene, p-, m-, and o-chlorophenol molecules. On the basis of spectroscopic and thermochemical approaches the resonant states of the molecular negative ions (NIs) and the structures of some dissociative decay products are assigned. In the electron energy range up to 3 eV, DEA processes are determined by the two 2[pi*]-shape resonances resulting mainly in formation of [M-H]- and/or Cl- ions. At higher electron energies the energy correlation between peaks in the negative ion effective yield curves and bands of UV spectra allowed identification of the core-excited resonances. The peculiarities of Cl- ion formation and the vibrational fine structure on the effective yield curves of the [M-H]- ions are discussed. The mass spectrometric procedures for measurement of relative cross sections for NI formation are described.     

Resonant electron capture by orotic acid molecules

Resonant electron attachment by orotic acid molecules (6-COOH-uracil) are studied in the energy range of 0–14 eV via negative ion mass spectrometry. Molecular ions, whose lifetimes relative to electron autodetachment are found to be ~300 μs are recorded in the region of thermal electron energies; they form in the valence state through a vibration-excited resonance mechanism. Unlike unsubstituted uracil, most dissociative processes occur in the low-energy region of <4 eV and are due to carboxylic anions. An absolute cross section of 2.4 × 10^?17 cm² is found for the most intense fragment ions [M–H]^– at an output energy of 1.33 eV. The kinetics of decarboxylation is considered for these ions. This could be a model reaction for the last stage of uridine monophosphate biosynthesis.     

Dissociative electron attachment to gas phase valine: a combined experimental and theoretical study

Using a crossed electron/molecule beam technique the dissociative electron attachment (DEA) to gas phase L-valine, (CH(3))(2)CHCH(NH(2))COOH, is studied by means of mass spectrometric detection of the product anions. Additionally, ab initio calculations of the structures and energies of the anions and neutral fragments have been carried out at G2MP2 and B3LYP levels. Valine and the previously studied aliphatic amino acids glycine and alanine exhibit several common features due to the fact that at low electron energies the formation of the precursor ion can be characterized by occupation of the pi* orbital of the carboxyl group. The dominant negative ion (M-H)(-) (m/Z=116) is observed at electron energies of 1.12 eV. This ion is the dominant reaction product at electron energies below 5 eV. Additional fragment ions with m/Z=100, 72, 56, 45, 26, and 17 are observed both through the low lying pi* and through higher lying resonances at about 5.5 and 8.0-9.0 eV, which are characterized as core excited resonances. According to the threshold energies calculated here, rearrangements play a significant role in the formation of DEA fragments observed from valine at subexcitation energies.     

Cross sections for electron trapping by DNA and its component subunits I: Condensed tetrahydrofuran deposited on Kr

We report cross sections for electron capture processes occurring in condensed tetrahydrofuran (THF) for incident electron energies in the range of 0-9 eV. The charge trapping cross section for 6-9 eV electrons is very small, and an upper limit of 4 x 10(-19) cm2 is estimated from our results. This latter is thus also an upper bound for the cross section for dissociative electron attachment process that is known to occur at these energies for condensed THF. At energies close to zero eV electron trapping proceeds via intermolecular stabilization. The cross section for this process is strongly dependent on the quantity of deposited THF. Since THF may model the furyl ring found in deoxyribose, these measurements indicate that this ring likely plays little role in either initiating or enhancing strand break damage via the attachment of the low energy secondary electrons produced when DNA is exposed to ionizing radiation.     

Electron attachment to molecules in a cluster environment

Low-energy dissociative electron attachment (DEA) to the CF(2)Cl(2) and CF(3)Cl molecules in a water cluster environment is investigated theoretically. Calculations are performed for the water trimer and water hexamer. It is shown that the DEA cross section is strongly enhanced when the attaching molecule is embedded in a water cluster, and that this cross section grows as the number of water molecules in the cluster increases. This growth is explained by a trapping effect that is due to multiple scattering by water molecules while the electron is trapped in the cluster environment. The trapping increases the resonance lifetime and the negative ion survival probability. This confirms qualitatively existing experiments on electron attachment to the CF(2)Cl(2) molecule placed on the surface of H(2)O ice. The DEA cross sections are shown to be very sensitive to the position of the attaching molecule within the cluster and the orientation of the electron beam relative to the cluster.     

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

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

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.     

Energy selective excision of CN- following electron attachment to hexafluoroacetone azine ((CF3)2C=N-N=C(CF3)2)

Low energy electron attachment (DEA) to hexafluoroacetone azine (HFAA) leads to a remarkable energy selective excision of CN(-) within a pronounced resonance located at 1.35 eV. The underlying dissociative electron attachment (DEA) reaction involves multiple bond cleavages and rearrangement within the neutral products. A series of further fragment ions (F(-), CF(3)(-), (CF(3))(2)C(-) and (CF(3))(2)CN(-)) are observed from resonant features above 2 eV and only (CF(3))(2)CN(-) is additionally formed within a narrow resonance below 1 eV. In contrast to CN(-) all the remaining fragment ions can be formed by simple bond cleavages with (CF(3))(2)CN(-) being the result of a symmetric decomposition of the target molecule by cleavage of the (N-N) bond with the excess charge localised on either of the identical fragments. Our ab initio calculations predict an adiabatic electron affinity of HFAA close to 2 eV with the geometry of the relaxed anion considerably distorted with respect to that of the neutral molecule.     

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.     

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.     

High resolution dissociative electron attachment to gas phase adenine

The dissociative electron attachment to the gas phase nucleobase adenine is studied using two different experiments. A double focusing sector field mass spectrometer is utilized for measurements requiring high mass resolution, high sensitivity, and relative ion yields for all the fragment anions and a hemispherical electron monochromator instrument for high electron energy resolution. The negative ion mass spectra are discussed at two different electron energies of 2 and 6 eV. In contrast to previous gas phase studies a number of new negative ions are discovered in the mass spectra. The ion efficiency curves for the negative ions of adenine are measured for the electron energy range from about 0 to 15 eV with an electron energy resolution of about 100 meV. The total anion yield derived via the summation of all measured fragment anions is compared with the total cross section for negative ion formation measured recently without mass spectrometry. For adenine the shape of the two cross section curves agrees well, taking into account the different electron energy resolutions; however, for thymine some peculiar differences are observed.     

The dependence of low-energy electron attachment to CF3Br on electron and vibrational energy

In a joint experimental and theoretical effort, we have studied dissociative electron attachment (DEA) to the CF3Br molecule at electron energies below 2 eV. Using two variants of the laser photoelectron attachment method with a thermal gas target (T(G) = 300 K), we measured the energy dependent yield for Br- formation over the range E = 3-1200 meV with resolutions of about 3 meV (E < 200 meV) and 35 meV. At the onsets for excitation of one and two quanta for the C-Br stretching mode nu3, downward cusps are detected. With reference to the recommended thermal (300 K) attachment rate coefficient k(A)(CF3Br) = 1.4 x 10(-8) cm3 s(-1), absolute cross sections have been determined for Br- formation. In addition, we studied Br- and (CF3Br)Br- formations with a seeded supersonic target beam (10% CF3Br in helium carrier gas, with a stagnation pressure of 1-4 bars and nozzle temperatures of 300 and 600 K) and found prominent structure in the anion yields due to cluster formation. Using the microwave pulse radiolysis swarm technique, allowing for controlled variation of the electron temperature by microwave heating, we studied the dependence of the absolute DEA rate coefficient on the mean electron energy E over the range of 0.04-2 eV at gas temperatures T(G) ranging from 173 to 600 K. For comparison with the experimental results, semiempirical resonance R-matrix calculations have been carried out. The input for the theory includes the known energetic and structural parameters of the neutral molecule and its anion; the parameters of the resonant anion curves are chosen with reference to the known thermal rate coefficient for the DEA process. For the gas temperature T(G) = 300 K, good overall agreement of the theoretical DEA cross section with the experimental results is observed; moreover, rate coefficients for Br- formation due to Rydberg electron transfer, calculated with both the experimental and the theoretical DEA cross sections, are found to agree with the previously reported absolute experimental values. At T(G) = 300 K, satisfactory agreement is also found between the calculated and experimental attachment rate coefficients for mean electron energies E = 0.04-2 eV. The strong increase of the measured rate coefficients with rising gas temperature, however, could be only partially recovered by the R-matrix results. The differences may result from the influence of thermal excitations of other vibrational modes not included in the theory.     

Empty-level structure and reactive species produced by dissociative electron attachment to tert-butyl peroxybenzoate

The energy and nature of the gas-phase temporary anion states of tert-butylperoxybenzoate in the 0-6 eV energy range are determined for the first time by means of electron transmission spectroscopy (ETS) and appropriate theoretical calculations. The first anion state, associated with electron capture into a delocalized π* MO with mainly ring and carbonyl character, is found to lie close to zero energy, i.e., sizably more stable (about 2 eV) than the ground (σ*) anion state of saturated peroxides. Dissociative decay channels of the unstable parent molecular anions are detected with dissociative attachment spectroscopy (DEAS), as a function of the incident electron energy, in the 0-14 eV energy range. A large DEA cross-section, with maxima at zero energy, 0.7 and 1.3 eV, is found for production of the (m/e = 121) PhCOO(-) anion fragment, together with the corresponding tert-butoxy neutral radical, following cleavage of the O-O bond. Although with much smaller intensities, a variety of other negative currents are observed and assigned to the corresponding anion fragments with the support of density functional theory calculations.     

Temporary anion states of pyrimidine and halopyrimidines

The empty-level electronic structures of pyrimidine and its 2-chloro, 2-bromo, and 5-bromo derivatives have been studied with electron transmission spectroscopy (ETS) and dissociative electron attachment spectroscopy (DEAS) in the 0-5 eV energy range. The spectral features were assigned to the corresponding anion states with the support of theoretical calculations at the ab initio and density functional theory levels. The empty orbital energies obtained by simple Koopmans' theorem calculations, scaled with empirical equations, quantitatively reproduced the energies of vertical electron attachment to π* and σ* empty orbitals measured in the ET spectra and predicted vertical electron affinities close to zero for the three halo derivatives. The total anion currents of the halo derivatives, measured at the walls of the collision chamber as a function of the impact electron energy, presented intense maxima below 0.5 eV. The mass-selected spectra showed that, in this energy, range the total anion current is essentially due to halide fragment anions. The DEA cross sections of the bromo derivatives were found to be about six times larger than that of the chloro derivative. The absolute cross sections at incident electron energies close to zero were evaluated to be 10(-16)-10(-15) cm(2).     

Electron capture by pentafluoronitrobenzene and pentafluorobenzonitrile

Electron attachment to pentafluorobenzonitrile (C(6)F(5)CN) and pentafluoronitrobenzene (C(6)F(5)NO(2)) is studied in the energy range 0-16 eV by means of a crossed electron-molecular beam experiment with mass spectrometric detection of the anions. We find that pentafluoronitrobenzene exclusively generates fragment anions via dissociative electron attachment (DEA), while pentafluorobenzonitrile forms a long lived parent anion within a narrow energy range close to 0 eV and additionally undergoes DEA at higher energies. This is in contrast to the behaviour of the non-fluorinated analogues as in nitrobenzene the non-decomposed anion is formed while in benzonitrile only DEA is observed. The associated reactions involve simple bond cleavages but also complex unimolecular decompositions associated with structural and electronic rearrangement also resulting in the deterioration of the cyclic structure.