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 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 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 gas-phase glycine

By using a high-resolution electron energy monochromator low-energy electron attachment to gas-phase glycine (H₂NCH₂COOH, or G) has been studied by means of mass spectrometric detection of the product anions. In the same way as for several other biologically relevant molecules no stable parent anion was formed by free electron attachment. The largest dissociative electron attachment (DEA) cross-section, approximately 5×10^–20 m², was observed for (G–H)^–+H at an electron energy of 1.25 eV. Glycine and formic acid (HCOOH) have several common features, because a precursor ion can be characterized by electron attachment to the unoccupied * orbital of the –COOH group. At higher incident electron energies several smaller fragment anions are formed. Except for H^–, which could not be observed in this study, there was good agreement with an earlier investigation by Gohlke et al.     

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 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 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.     

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.     

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 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 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.     

Ring-breaking electron attachment to uracil: following bond dissociations via evolving resonances

Calculations are carried out at various distinct energies to obtain both elastic cross sections and S-matrix resonance indicators (poles) from a quantum treatment of the electron scattering from gas-phase uracil. The low-energy region confirms the presence of pi(*) resonances as revealed by earlier calculations and experiments which are compared with the present findings. They turn out to be little affected by bond deformation, while the transient negative ions (TNIs) associated with sigma(*) resonances in the higher energy region ( approximately 8 eV) indeed show that ring deformations which allow vibrational redistribution of the excess electron energy into the molecular target strongly affect these shape resonances: They therefore evolve along different dissociative pathways and stabilize different fragment anions. The calculations further show that the occurrence of conical intersections between sigma(*) and pi(*)-type potential energy surfaces (real parts) is a very likely mechanism responsible for energy transfers between different TNIs. The excess electron wavefunctions for such scattering states, once mapped over the molecular space, provide nanoscopic reasons for the selective breaking of different bonds in the ring region.     

Metastable anions of dinitrobenzene: resonances for electron attachment and kinetic energy release

Attachment of free, low-energy electrons to dinitrobenzene (DNB) in the gas phase leads to DNB(-) as well as several fragment anions. DNB(-), (DNB-H)(-), (DNB-NO)(-), (DNB-2NO)(-), and (DNB-NO(2))(-) are found to undergo metastable (unimolecular) dissociation. A rich pattern of resonances in the yield of these metastable reactions versus electron energy is observed; some resonances are highly isomer-specific. Most metastable reactions are accompanied by large average kinetic energy releases (KER) that range from 0.5 to 1.32 eV, typical of complex rearrangement reactions, but (1,3-DNB-H)(-) features a resonance with a KER of only 0.06 eV for loss of NO. (1,3-DNB-NO)(-) offers a rare example of a sequential metastable reaction, namely, loss of NO followed by loss of CO to yield C(5)H(4)O(-) with a large KER of 1.32 eV. The G4(MP2) method is applied to compute adiabatic electron affinities and reaction energies for several of the observed metastable channels.     

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 photoionization of methyl chloride studied with threshold photoelectron-photoion coincidence velocity imaging

Utilizing threshold photoelectron-photoion coincidence (TPEPICO) velocity imaging, dissociation of state-selected CH(3)Cl(+) ions was investigated in the excitation energy range of 11.0-18.5 eV. TPEPICO time-of-flight mass spectra and three-dimensional time-sliced velocity images of CH(3)(+) dissociated from CH(3)Cl(+)(A(2)A(1) and B(2)E) ions were recorded. CH(3)(+) was kept as the most dominant fragment ion in the present energy range, while the branching ratio of CH(2)Cl(+) fragment was very low. For dissociation of CH(3)Cl(+)(A(2)A(1)) ions, a series of homocentric rings was clearly observed in the CH(3)(+) image, which was assigned as the excitation of umbrella vibration of CH(3)(+) ions. Moreover, a dependence of anisotropic parameters on the vibrational states of CH(3)(+)(1(1)A') provided a direct experimental evidence of a shallow potential well along the C-Cl bond rupture. For CH(3)Cl(+)(B(2)E) ions, total kinetic energy released distribution for CH(3)(+) fragmentation showed a near Maxwell-Boltzmann profile, indicating that the Cl-loss pathway from the B(2)E state was statistical predissociation. With the aid of calculated Cl-loss potential energy curves of CH(3)Cl(+), CH(3)(+) formation from CH(3)Cl(+)(A(2)A(1)) ions was a rapid direct fragmentation, while CH(3)Cl(+)(B(2)E) ions statistically dissociated to CH(3)(+) + Cl via internal conversion to the high vibrational states of X(2)E.     

Dissociative electron attachment to the hydrogen-bound OH in water dimer through the lowest anionic Feshbach resonance

The lowest energy Feshbach resonance state of the water dimer anion is computationally studied as the hydrogen-bonded OH moiety is stretched from its equilibrium position toward the hydrogen bond acceptor. The purpose is to treat a simple model system to gain insight into how hydrogen bonding may affect dissociative electron attachment to water in condensed phases. In the case of a water monomer anion, the analogous potential surface is known to be repulsive, leading directly to dissociation of H(-). In contrast, in the dimer anion, a barrier is found to dissociation of the hydrogen-bonded OH moiety such that the migrating hydrogen can be held near the Franck-Condon region in a quasibound vibrational state for a time long compared to the OH vibrational period. This behavior is found both for the case of an icelike dimer structure and for a substantial majority of liquidlike dimer structures. These findings raise the possibility that due to effects of hydrogen bonding, a molecule-centered anionic entity that is metastable both to electron detachment and to bond dissociation may live long enough to be considered as a species in the radiolysis of condensed water phases.     

Near-threshold electron attachment as Regge resonances: cross sections for K, Rb, and Cs atoms

We investigate the near-threshold formation of negative ions as Regge resonances in electron-atom scattering, with specific results obtained for e--K, e--Rb, and e--Cs. The complex angular momentum method, implemented within the Mulholland formulation of the total elastic cross sections, is employed. We demonstrate that for e--K, e--Rb, and e--Cs scattering, the near-threshold electron attachment cross sections are characterized by the Wigner threshold behavior, Ramsauer-Townsend minima, and Regge resonances, all discernible only through Regge partial cross section scrutiny. Regge partial, differential, and total elastic cross sections are presented and contrasted, as well as the differential cross section critical minima.     

Dissociative electron transfer to and from pyrimidine cyclobutane dimers: an electrochemical study

Cyclic voltammetry was used to study the reduction and oxidation behaviour of several pyrimidine cyclobutane dimers mimicking UV induced lesion in DNA strands in polar solvents (N,N-dimethylformamide and acetonitrile). Both electron injection and removal to and from the dimers, respectively, lead to their cleavage and reformation of the monomeric base. The influence of stereochemistry and substitution pattern at the cyclobutane motif on the reactivity has been studied. It appears that the repair process always proceeds in a sequential fashion with initial formation of a dimer ion radical intermediate, which then undergoes ring opening by homolytic cleavage of the two C-C bonds. Standard redox potentials for the formation of both radical anion and radical cation state of the dimers were determined. Quantum calculations on simplified model compounds reveal the reason for the finding that the exergonic homolytic cleavages of the carbon-carbon bonds are endowed with sizeable activation barriers. The consequences of these mechanistic studies on the natural enzymatic repair by photolyase enzyme are discussed.