Dissociative electron attachment to gas-phase formamide

Dissociative electron attachment (DEA) to gaseous formamide, HCONH(2), has been investigated in the energy range between 0 eV and 18 eV using a crossed electron/molecule beam technique. The negative ion fragments have been comprehensively monitored and assigned to molecular structures by comparison with the results for two differently deuterated derivatives, namely 1D-formamide, DCONH(2), and N,N,D-formamide, HCOND(2). The following products were observed: HCONH(-), CONH(2)(-), HCON(-), OCN(-), HCNH(-), CN(-), NH(2)(-)/O(-), NH(-), and H(-). NH(2)(-) was also separated from O(-) by using high-resolution negative ion mass spectrometry. Four resonant dissociation channels can be resolved, the strongest ones being located between 2.0 and 2.7 eV and between 6.0 and 7.0 eV. CN(-) as the most abundant fragment and HCONH(-) are the dominant products of the first of these two resonances. The most important products of the latter resonance are NH(2)(-), CN(-), H(-), CONH(2)(-), and OCN(-). It is thus found that the loss of neutral H is a site-selective process, dissociation from the N site taking place between 2.0 and 2.7 eV while dissociation from the C site occurs between 6.0 and 7.0 eV. The suitability of these reactions and thus of formamide as an agent for electron-induced surface functionalisation is discussed.     

Dissociative electron attachment to triflates

Gas phase studies of dissociative electron attachment to simple alkyl (CF(3)SO(3)CH(3)) and aryl (C(6)H(5)SO(3)CF(3) and CF(3)SO(3)C(6)H(4)CH(3)) triflates, model molecules of nonionic photoacid generators for modern lithographic applications, were performed. The fragmentation pathways under electron impact below 10 eV were identified by means of crossed electron-molecular beam mass spectrometry. Major dissociation channels involved C-O, S-O, or C-S bond scissions in the triflate moiety leading to the formation of triflate (OTf(-)), triflyl (Tf(-)), or sulfonate (RSO(3)(-)) anions, respectively. A resonance leading to C-O bond breakage and OTf(-) formation in alkyl triflates occurred at electron energies about 0.5 eV lower than the corresponding resonance in aryl triflates. A resonance leading to S-O bond breakage and Tf(-) formation in aryl triflates occurred surprisingly at the same electron energies as C-O bond breakage. In case of alkyl triflates S-O bond breakage required 1.4 eV higher electron energies to occur and proceeded with substantially lower yields than in aryl triflates. C-S bond scission occurred for all presently studied triflates at energies close to 3 eV.     

Dissociative electron attachment to abasic DNA

Thin films of the short single DNA strand, GCAT, in which one of the bases has been removed were bombarded with 3 to 15 eV electrons. The yield functions of the H(-), O(-) and OH(-) ions desorbed from these films exhibit a broad peak near 9 eV, which is attributed to dissociative electron attachment to the basic molecules. Whereas removal of any one of the bases considerably decreases N-glycosidic and backbone C-O bond scission, the creation of basic sites does not appreciably modify bond rupture leading to anion electron stimulated desorption. These seemingly contradictory results make it possible to propose a detailed mechanism leading to the transfer of electrons in the range 5-13 eV within DNA.     

Dissociative electron attachment in selected haloalkanes

Three mono-substituted and six poly-substituted haloalkanes derivatives were investigated by means of negative ion mass spectrometry (NIMS). Available electron transmission spectroscopy (ETS) data were used for the treatment of NIMS results. Comparison of the ETS spectra of mono-substituted bromo- and chloroalkanes show that these families of compounds exhibit similar regularities in the process of NI formation. Capture of the p-partial wave dominates in the lowest anion state formation. In the series of mono-substituted bromo- and chloroalkanes the energy difference (Shift) between the vertical attachment energy (VAE) in ETS spectra and the maximum of the NI formation in dissociative electron attachment (DEA) spectra correlates approximately linearly with the VAE value for the given molecule. The slopes of these dependencies are different for the mono-substituted chloro and bromo derivatives. It is conjectured that, in the derivatives di-substituted in the 1,2-positions, there is a shallow minimum in the repulsive anion potential curve, which makes possible temporary trapping of the anion. This delay is long enough for the rearrangement process and formation of the Hal2- ions. This effect is not detected in 1,1-halogenated derivatives.     

Dissociative electron attachment to glycyl-glycine, glycyl-alanine and alanyl-alanine

The processes of negative ions formation of dipeptides glycyl-glycine, glycyl-alanine and alanyl-alanine in the conditions of resonant electron capture have been studied with a help of negative ions mass spectrometry. Using a thermochemical approach, the main channels of fragment negative ions formation were found and the structure of the ions were established. The isobaric ions have been identified by the experiments with high mass resolution. The cross sections of fragment ions formation were measured. The metastable fragmentation of [M-H](-) and [M-COOH](-) ions in the energy range 4.5-7.5 eV have been found.     

Dissociative electron attachment to furan, tetrahydrofuran, and fructose

We study dissociative electron attachment to furan (FN) (C(4)H(4)O), tetrahydrofuran (THF) (C(4)H(8)O), and fructose (FRU) (C(6)H(12)O(6)) using crossed electron/molecular beams experiments with mass spectrometric detection of the anions. We find that FN and THF are weak electron scavengers and subjected to dissociative electron attachment essentially in the energy range above 5.5 eV via core excited resonances. In striking contrast to that, FRU is very sensitive towards low energy electrons generating a variety of fragment ions via a pronounced low energy feature close to 0 eV. These reactions are associated with the degradation of the ring structure and demonstrate that THF cannot be used as surrogate to model deoxyribose in DNA with respect to the attack of electrons at subexcitation energies (<3 eV). The results support the picture that in DNA the sugar moiety itself is an active part in the initial molecular processes leading to single strand breaks.     

Dissociative electron attachment to di-tert-butylperoxide, artemisinin, and beta-artemether

The gas-phase dissociative electron attachment spectra of di-tert-butylperoxide (DBP) and the antimalarial polycyclic peroxides artemisinin and beta-artemether are presented for the first time. The total anion currents measured at the walls of the collision chamber and the mass selected anion currents are reported in the 0-6 eV energy range. Electron attachment to DBP produces an intense current, peaking at 1.3 eV, due to the C4H9O- negative fragment, in line with the strongly O-O antibonding character of the singly occupied orbital of the parent molecular anion and the small (if any) thermodynamic energy threshold predicted by B3LYP calculations for the formation of this anion fragment. A five times less intense signal, with m/e = 57 and a maximum at 0.7 eV, is also observed. The calculations exclude that this signal can be associated with the C4H9- negative fragment, whereas they support its assignment to the C3H5O- species, generated by simultaneous dissociation and loss of a methane molecule from the parent molecular anion. In DBP, artemisinin, and beta-artemether, currents corresponding to the parent molecular anion are not detected, indicating that its survival time is shorter than the time required (about 10-6 s) to pass through the mass filter. In the latter two compounds, where simple O-O bond breaking does not generate separate fragments, the anion currents are much weaker than in DBP and the maximum total anion current peaks at zero energy.     

Dissociative electron attachment to NO probed by velocity map imaging

An experimental and theoretical investigation of the dissociative electron attachment process in nitric oxide is presented. Measurements using the recently developed ion momentum imaging conclusively show the presence of two resonance features in the O(-) channel. These are found to dissociate to give N atoms in the (2)D and (2)P excited states respectively, thus settling the controversies regarding the possible dissociation limits of this process. Though the angular distribution of O(-) shows the resonances contributing to these dissociations are of Π symmetry and a mixture of Π and Σ or Δ symmetry respectively, our calculations using R-matrix theory show no direct electron attachment channel leading to O(-) through these resonances, as all the allowed resonances below 10 eV decay to either O + N(-) or O(-) + N((4)S) channels. We propose that indirect mechanisms through curve crossings lead to the experimentally observed results.     

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.     

Dissociative electron attachment to polyatomic molecules: Ion kinetic energy measurements

Dissociative electron attachment to SO₂, NO₂, NF₃ and H₂O₂ is studied in terms of the kinetic energies of the dominant fragment ions. The O^? data from SO₂ show that the two major resonances at 4.6 and 7.2 eV respectively have the same dissociation limit. Similarly, the resonances at 1.8 and 3.5 eV in the O^? channel in NO₂ appear to have same dissociation limit of NO (X ²Π) + O^?, while the resonance at 8.5 eV appears to dissociate to give NO (a ⁴Π_i) along with O^?. We find considerable internal excitation of the neutral fragments in all these cases along with that of NF₃, whereas the negative ion resonance in H₂O₂ appears to fragment almost like a diatomic system with very little internal excitation of the OH and OH^? fragments.     

Dissociative electron attachment to dinitrogen pentoxide, N2O5

Electron attachment was studied in gaseous dinitrogen pentoxide, N(2)O(5), for incident electron energies between a few meV and 10 eV. No stable parent anion N(2)O(5) (-) was observed but several anionic fragments (NO(3) (-), NO(2) (-), NO(-), O(-), and O(2) (-)) were detected using quadrupole mass spectrometry. Many of these dissociative pathways were found to be coupled and provide detailed information on the dynamics of N(2)O(5) fragmentation. Estimates of the cross sections for production of each of the anionic fragments were made and suggest that electron attachment to N(2)O(5) is amongst the most efficient attachment reactions recorded for nonhalogenated polyatomic systems.     

Specific features of the resonant electron attachment to the chlorodibenzo-p-dioxin molecules

The processes of resonant dissociative electron attachment to the molecules of dibenzo-p-dioxin and its chlorinated derivatives containing one to four chlorine atoms (totally eight compounds) were investigated. It was established that 2,3,7-trichlorodibenzo-p-dioxin; 1,2,3,4-tetrachlorodibenzo-p-dioxin; 1,3,7,8-tetrachlorodibenzo-p-dioxin, and 2,3,7,8-tetrachlorodibenzo-p-dioxin molecules are chatacterized by positive electron affinities. At electron energies below 2 eV, the electron attachment is caused by the shape resonances. Based on the energy correlation between the negative ion resonance peaks at 3—4 eV and the UV band maxima, it was suggested that electron attachment in this energy region occurs by the mechanism of inter-shell resonance with the molecular singlet-excited states as parents. The possibility for the rearrangement processes resulting in oxy-anionic structures to occur is substantiated.     

Dissociative electron attachment to H2S probed by ion momentum imaging

The dissociation dynamics of negative ion resonance states in H(2)S formed upon electron attachment are studied using momentum imaging of the fragment H(-) and S(-) ions and compared with similar resonances in water. The H(-) momentum images show that dissociation dynamics at the 5.2 eV resonance are very similar to those of the 6.5 eV (B(1)) resonance in water. Unlike the 8.5 eV resonance in water, which has A(1) symmetry but is found to display considerable deviation from the axial recoil approximation in the momentum distribution of H(-) ions, the distribution from the corresponding resonance in H(2)S at 7.5 eV is found to follow the axial recoil approximation fairly well. The resonance state with B(2) symmetry at 10 eV is found to decay via four dissociation channels viz.-H(-) + H + S, H(-) + SH(A(2)Σ), H(-) + SH(X(2)Π) and S(-) + H + H channels, similar to those that were seen in the B(2) resonance in water at 12 eV, including sequential fragmentation in the multiple fragmentation channels. However, the angular distributions for the fragment ions from this resonance are found to be distinctly different from those in water, even while displaying considerable deviation from the axial recoil approximation similar to that in water.     

Electron attachment to gas-phase uracil

We present results about dissociative electron attachment (DEA) to gas-phase uracil (U) for incident electron energies between 0 and 14 eV using a crossed electron/molecule beam apparatus. The most abundant negative ion formed via DEA is (U-H)-, where the resonance with the highest intensity appears at 1.01 eV. The anion yield of (U-H)- shows a number of peaks, which can be explained in part as being due to the formation of different (U-H)- isomers. Our results are compared with high level ab initio calculations using the G2MP2 method. There was no measurable amount of a parent ion U-. We also report the occurrence of 12 other fragments produced by dissociative electron attachment to uracil but with lower cross sections than (U-H)-. In addition we observed a parasitic contaminating process for conditions where uracil was introduced simultaneously with calibrant gases SF6 and CCl4 that leads to a sharp peak in the (U-H)- cross section close to 0 eV. For (U-H)- and all other fragments we determined rough measures for the absolute partial cross section yielding in the case of (U-H)- a peak value of sigma (at 1.01 eV)=3 x 10(-20) m2.     

Dissociative attachment to ClCN and BrCN

We present calculated dissociative attachment cross sections for ClCN and BrCN in the 0-20 eV energy range. In this energy region, both Cl(-)Br(-) and CN(-) fragments are possible and are produced via dissociation along repulsive resonance curves. Electron scattering calculations, using the complex Kohn variational method and molecular structure calculations, were used to determine the three-dimensional surfaces and resonance parameters. The nuclear dynamics was studied in one, two, and three dimensions using time-dependent wave packet methods, employing the multiconfiguration time-dependent Hartree method for multiple dimensions. The calculated cross sections are reported and compared to the available experiments. Couplings between resonance states will also be examined and discussed.     

Dissociative electron attachment to C(2)F(5) radicals

Dissociative electron attachment to the reactive C(2)F(5) molecular radical has been investigated with two complimentary experimental methods; a single collision beam experiment and a new flowing afterglow Langmuir probe technique. The beam results show that F(-) is formed close to zero electron energy in dissociative electron attachment to C(2)F(5). The afterglow measurements also show that F(-) is formed in collisions between electrons and C(2)F(5) molecules with rate constants of 3.7 × 10(-9) cm(3) s(-1) to 4.7 × 10(-9) cm(3) s(-1) at temperatures of 300-600 K. The rate constant increases slowly with increasing temperature, but the rise observed is smaller than the experimental uncertainty of 35%.     

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.     

Theoretical studies of damage to 3'-uridine monophosphate induced by electron attachment

Low-energy electrons (LEE) are well known to induce nucleic acid damage. However, the damage mechanisms related to charge state and structural features remain to be explored in detail. In the present work, we have investigated the N1-glycosidic and C3'-O(P) bond ruptures of 3'-UMP (UMP=uridine monophosphate) and the protonated form 3'-UMPH with -1 and zero charge, respectively, based on hybrid density functional theory (DFT) B3 LYP together with the 6-31+G(d,p) basis set. The glycosidic bond breakage reactions of the 3'UMP and 3'UMPH electron adducts are exothermic in both cases, with barrier heights of 19-20 kcal mol(-1) upon inclusion of bulk solvation. The effects of the charge state on the phosphate group are marginal, but the C2'-OH group destabilizes the transition structure of glycosidic bond rupture of 3'-UMPH in the gas phase by approximately 5.0 kcal mol(-1). This is in contrast with the C3'-O(P) bond ruptures induced by LEE in which the charge state on the phosphate influences the barrier heights and reaction energies considerably. The barrier towards C3'-O(P) bond dissociation in the 3'UMP electron adduct is higher in the gas phase than the one corresponding to glycosidic bond rupture and is dramatically influenced by the C2'-OH group and bulk salvation, which decreases the barrier to 14.7 kcal mol(-1). For the C3'-O(P) bond rupture of the 3'UMPH electron adduct, the reaction is exothermic and the barrier is even lower, 8.2 kcal mol(-1), which is in agreement with recent results for 3'-dTMPH and 5'-dTMPH (dTMPH=deoxythymidine monophosphate). Both the Mulliken atomic charges and unpaired-spin distribution play significant roles in the reactions.     

Dissociative Electron Attachment to Hexachlorobenzene

Gas phase molecules of hexachlorobenzene (C₆Cl₆) were investigated by means of dissociative electron attachment spectroscopy (DEAS). Three channels of molecular negative ions decay have been identified: abstraction of Cl⁻ and Cl₂⁻ as well as electron detachment (τ_a∼250 μs at 343 K). All three channels exhibit temperature dependence. The adiabatic electron affinity estimated using a simple but typically accurate Arrhenius model (EA_a=1.6–1.9 eV) turns out to be much higher than the quantum-chemical predictions (EA_a=0.9–1.0 eV). We discuss the possible reasons behind the observed discrepancy.