Temporary Negative Ion States Research Articles

Elementary processes triggered in curcumin molecule by gas-phase resonance electron attachment and by photoexcitation in solution.

Electron-driven processes in isolated curcumin (CUR) molecules are studied by means of dissociative electron attachment (DEA) spectroscopy under gas-phase conditions. Elementary photostimulated reactions initiated in CUR molecules under UV irradiation are studied using the chemically induced dynamic nuclear polarization method in an acetonitrile solvent. Density functional theory is applied to elucidate the energetics of fragmentation of CUR by low-energy (0-15eV) resonance electron attachment and to characterize various CUR radical forms. The adiabatic electron affinity of CUR molecule is experimentally estimated to be about 1eV. An extra electron attachment to the π1* LUMO and π2* molecular orbitals is responsible for the most intense DEA signals observed at thermal electron energy. The most abundant long-lived (hundreds of micro- to milliseconds) molecular negative ions CUR- are detected not only at the thermal energy of incident electrons but also at 0.6eV, which is due to the formation of the π3* and π4* temporary negative ion states predicted to lie around 1eV. Proton-assisted electron transfer between CUR molecules is registered under UV irradiation. The formation of both radical-anions and radical-cations of CUR is found to be more favorable in its enol form. The present findings shed some light on the elementary processes triggered in CUR by electrons and photons and, therefore, can be useful to understand the molecular mechanisms responsible for a variety of biological effects produced by CUR.

Formation of negative and positive ions in the radiosensitizer nimorazole upon low-energy electron collisions.

A comprehensive investigation of low-energy electron attachment and electron ionization of the nimorazole radiosensitizer used in cancer radiation therapy is reported by means of a gas-phase crossed beam experiment in an electron energy range from 0 eV to 70 eV. Regarding negative ion formation, we discuss the formation of fifteen fragment anions in the electron energy range of 0 eV-10 eV, where the most intense signal is assigned to the nitrogen dioxide anion NO2 -. The other fragment anions have been assigned to form predominantly from a common temporary negative ion state close to 3 eV of the nitroimidazole moiety, while the morpholine moiety seems to act only as a spectator in the dissociative electron attachment event to nimorazole. Quantum chemical calculations have been performed to help interpreting the experimental data with thermochemical thresholds, electron affinities, and geometries of some of the neutral molecules. As far as positive ion formation is concerned, the mass spectrum at the electron energy of 70 eV shows a weakly abundant parent ion and C5H10NO+ as the most abundant fragment cation. We report appearance energy (AE) measurements for six cations. For the intact nimorazole molecular cation, the AE of 8.16 ± 0.05 eV was obtained, which is near the presently calculated adiabatic ionization energy.

Formation of Smaller Anions from CnN− (n = 1–3, 5–7) in the Circumstellar Medium

Abstract We probe a new pathway for the formation of smaller anions from the temporary negative ion states (anion resonances) of C n N− (n = 1–3, 5–7) in the circumstellar envelope of IRC+10216. C n N− (n = 1–3, 5–7) anions were collisionally excited to their resonance states and were observed to decay into a variety of smaller anions. The measured kinetic-energy-release distributions for the anionic fragments arising from each of the parent anions indicate a concerted manner of occurrence of these fragments, implying rich dissociation dynamics. and C2N− were found to be dominant fragments of these anions, suggesting their presence in the external layers of IRC+10216 where UV photons penetrate. C n N− (n = 1–3) were also observed to undergo dissociative and nondissociative double-electron detachments, with the former being dominant. The significance of this new pathway in determining the stability and abundance of anions in IRC+10216 is discussed.

Study of S− and SO− ion formation from dissociative electron attachment to SO2

A detailed experimental investigation of the dissociative electron attachment (DEA) process to sulfur dioxide molecules in the gas phase has been performed. The sulfur dioxide molecule is known to produce O−, S− and SO− ions, mainly forming two prominent resonant peaks at around 4.2 and 7.5 eV incident electron energies. The S− and SO− ions are studied over the second resonance around 7.5 eV incident electron energy in detail using the well-established velocity slice imaging (VSI) spectrometer. The time-sliced images, kinetic energy and angular distribution have been reported for the two ions over the second resonance for the first time. Both the ions are seen to be formed with very low kinetic energies with their respective neutral fragments being produced in internally excited states. The angular distribution data also signifies the presence of a linear combination of A1 and B2 temporary negative ion states responsible for the resonant peak at 7.5 eV incident electron energy.

Kinematic study of O− -ion formation from dissociative electron attachment to SO2

We report a complete kinematic study of ${\mathrm{O}}^{\ensuremath{-}}$-ion formation due to dissociative electron attachment to ${\mathrm{SO}}_{2}$ using the velocity slice imaging technique in the incident electron energy range over the resonances. Two resonances are observed at 5.2 and 7.5 eV, respectively. From the kinetic energy distribution, the two resonances are observed to have the same threshold energy, pointing to the fact that the two processes, giving rise to the two resonant peaks, have the same dissociation limit. From the angular distribution results we identified the involvement of an ${A}_{1}$ and a combination of ${A}_{1}+{B}_{2}$ temporary negative-ion state(s) for the first and second resonances, respectively.

Identification of overlapping resonances in dissociative electron attachment to chlorine molecules

A combined highly differential momentum imaging experiment and ab initio potential-energy curve calculation using density-functional theory (DFT) have been performed to understand the broad resonant peak around 5.7 eV due to dissociative electron attachment (DEA) to the chlorine molecule. Both the kinetic energy and angular distribution over the entire $2\ensuremath{\pi}$ angle of the fragment negative ions have been measured. Two heavily overlapping resonances are identified for the observed broad resonant peak that could settle the long-standing debate. The symmetry of the involved two temporary negative ion (TNI) states are determined from the angular distribution data. Experimental observations are strongly supported with the computed potential-energy curve using DFT.

On a simple way to calculate electronic resonances for polyatomic molecules.

We propose a simple method for calculation of low-lying shape electronic resonances of polyatomic molecules. The method introduces a perturbation potential and requires only routine bound-state type calculations in the real domain of energies. Such a calculation is accessible by most of the free or commercial quantum chemistry software. The presented method is based on the analytical continuation in a coupling constant model, but unlike its previous variants, we experience a very stable and robust behavior for higher-order extrapolation functions. Moreover, the present approach is independent of the correlation treatment used in quantum many-electron computations and therefore we are able to apply Coupled Clusters (CCSD-T) level of the correlation model. We demonstrate these properties on determination of the resonance position and width of the (2)Πu temporary negative ion state of diacetylene using CCSD-T level of theory.

Low-Energy Electron Interaction with the Phosphate Group in DNA Molecule and the Characteristics of Single-Strand Break Pathways.

We modeled the low-energy electron attachment to the sugar-phosphate-sugar (SPS) molecule for investigating the single-strand break (3' C-O and 5' C-O) at the DNA backbone. In particular, we predicted the electron capture at the phosphate center. We found that 0.6 eV electron can attach to the phosphate group, and the lifetime (∼40-55 fs) of the resulting temporary negative ion state is more than what we found for the lifetime of the metastable species (∼18-20 fs) formed at the cytosine base center. We treated the two competing dissociation channels in SPS molecule, that is, both the 3' C-O and 5' C-O lesions, separately. The activation energy barrier calculated for 5' C-O bond rupture is found to be less than that for 3' C-O bond dissociation. The overall low-energy electron transfer process is found to be mediated through a "shape resonance state" formed at the phosphate center.

Resonance electron attachment to plant hormones and its likely connection with biochemical processes.

Gas-phase formation of temporary negative ion states via resonance attachment of low-energy (0-6 eV) electrons into vacant molecular orbitals of salicylic acid (I) and its derivatives 3-hydroxy- (II) and 4-hydroxybenzoic acid (III), 5-cloro salicylic acid (IV) and methyl salicylate (V) was investigated for the first time by electron transmission spectroscopy. The description of their empty-level structures was supported by density functional theory and Hartree-Fock calculations, using empirically calibrated linear equations to scale the calculated virtual orbital energies. Dissociative electron attachment spectroscopy (DEAS) was used to measure the fragment anion yields generated through dissociative decay channels of the parent molecular anions of compounds I-V, detected with a mass filter as a function of the incident electron energy in the 0-14 eV energy range. The most intense negative fragment produced by DEA to isomers I-III is the dehydrogenated molecular anion [M-H](-), mainly formed at incident electron energies around 1 eV. The vertical and adiabatic electron affinities were evaluated at the B3LYP/6-31+G(d) level as the anion/neutral total energy difference. The same theoretical method was also used for evaluation of the thermodynamic energy thresholds for production of the negative fragments observed in the DEA spectra. The gas-phase DEAS data can provide support for biochemical reaction mechanisms in vivo.

Temporary Anion States of Three Herbicide Families

Electron scattering studies are used to locate the energies of temporary negative ion states of three chloro-substituted molecular families of herbicidal importance: salicylic and phenoxyacetic acids and acetamides. The correlation between these energies and the computed virtual orbital energies of the compounds is examined and used to put the latter on an absolute energy scale. Such scaling of orbital energies permits the anion states of other members of these families, for which experimental data may not be available, to be estimated from the calculated orbital energies. Studies of electron reduction rates often rely on calculated LUMO energies as molecular descriptors. The use of measured anion energies as well as appropriately scaled orbital energies should serve to improve such studies in these and in related herbicides.

Electron attachment to indole and related molecules

Gas-phase formation of temporary negative ion states via resonance attachment of low-energy (0-6 eV) electrons into vacant molecular orbitals of indoline (I), indene (II), indole (III), 2-methylen-1,3,3-trimethylindoline (IV), and 2,3,3-trimethyl-indolenine (V) was investigated for the first time by electron transmission spectroscopy (ETS). The description of their empty-level structures was supported by density functional theory and Hartree-Fock calculations, using empirically calibrated linear equations to scale the calculated virtual orbital energies. Dissociative electron attachment spectroscopy (DEAS) was used to measure the fragment anion yields generated through dissociative decay channels of the parent molecular anions of compounds I-V, detected with a mass filter as a function of the incident electron energy in the 0-14 eV energy range. The vertical and adiabatic electron affinities were evaluated at the B3LYP∕6-31+G(d) level as the anion∕neutral total energy difference. The same theoretical method is also used for evaluation of the thermodynamic energy thresholds for production of the negative fragments observed in the DEA spectra. The loss of a hydrogen atom from the parent molecular anion ([M-H](-)) provides the most intense signal in compounds I-IV. The gas-phase DEAS data can provide support for biochemical reaction mechanisms in vivo involving initial hydrogen abstraction from the nitrogen atom of the indole moiety, present in a variety of biologically important molecules.

Reply to “Comment on ‘Electron-induced bond breaking at low energies in HCOOH and glycine: The role of very short-livedσ∗anion states’ ”

Rescigno et al. [Phys. Rev. A80, 046701 (2009)] criticized our theoretical treatment of dissociative electron attachment in formic acid in which we show that this process can be explained by electron attachment into the temporary negative ion state formed by occupation of the ${\ensuremath{\sigma}}^{\ensuremath{\ast}}(\text{OH})$ orbital. We argue that their objections do not hold up to scrutiny.

Assignments of Normally Unoccupied Orbitals to the Temporary Negative Ion States of Several Lanthanide NMR Shift Reagents and Comments on Resonance Involvement in Electron Circular Dichroism

We have measured the total electron scattering cross sections of several NMR shift reagent molecules X(hfc)3, where X = Yb, Er, Eu and Pr, by means of electron transmission spectroscopy (ETS) to determine their vertical attachment energies. A strong low-energy resonance (<1 eV) is observed in all of the compounds except for Yb(hfc)3. We explain this anomaly in terms of the ground-state electron configuration of each molecule. Also, with the aid of restricted open-shell Hartree-Fock (ROHF) calculations on analogous molecules with truncated fluorocarbon chains, we have assigned specific normally unoccupied orbitals to the resonances observed in ETS. To our knowledge, these molecules are the largest for which this procedure has been successfully completed. Nolting et al. (J. Phys. B 1997, 30, 5491) have demonstrated that the above NMR shift reagents exhibit electron circular dichroism (ECD) between 1 and 10 eV. Using our new total cross section data, we comment on the possibility of resonance involvement in the generation of ECD.

Electron ionization time‐of‐flight mass spectrometry: Historical review and current applications

This review presents an overview of electron ionization time-of-flight mass spectroscopy (EITOFMS), beginning with its early development to the employment of modern high-resolution electron ionization sources. The EITOFMS is demonstrated to be ideally suited for analytical and basic chemical physics studies. Studies of the formation of positive ions by electron ionization time-of-flight mass spectroscopy have been responsible for many of the known ionization potentials of molecules and radicals, as well as accepted bond dissociation energies for ions and neutral molecules. The application of TOFMS has been particularly important in the area of negative ion physics and chemistry. A wide variety of negative ion properties have been discovered and studied by using these methods including: autodetachment lifetimes, metastable dissociation, Rydberg electron transfer reactions and field detachment, SF(6) Scavenger method for detecting temporary negative ion states, and many others.

Remote bond breaking by interacting temporary anion states

The cross section for bond breaking at the site of a dissociative temporary negative ion state through the dissociative electron attachment process can be considerably enhanced by the presence of a second longer-lived temporary negative ion state elsewhere in the molecule, even one quite remote from the first. In a series of chloroalkenes possessing both C-Cl and C==C bonds separated by various distances, we show that the cross sections are determined by the lifetime of the lower anion state created by the mixing of the anion states of these two moieties, with the wave function's coefficients giving the probability that the electron is located at the dissociative site. Furthermore, the lifetime of the composite anion state can be expressed in terms of these same coefficients and the lifetimes of the unmixed resonances. We also discuss how these results may give insight into the means by which strand breaks are induced in DNA by the attachment of slow electrons.

An investigation of electron helicity density in bromocamphor and dibromocamphor as a source of electron circular dichroism

We investigate the causes of electron-circular dichroism (ECD) in bromocamphor and dibromocamphor, focusing specifically on the electron helicity density of the target. Using electron transmission spectroscopy (ETS) and quantum chemical calculations, we have observed and assigned temporary negative ion states of bromocamphor and dibromocamphor. Further calculations were conducted to determine the helicity densities of these compounds. Large helicity densities are found in the regions of large wavefunction amplitude of the normally unoccupied molecular orbitals responsible for resonances in the scattering cross sections. We relate our ETS assignments and helicity density results to the chiral asymmetry data observed in electron-circular dichroism experiments by the Münster group (Nolting et al 1997 J. Phys. B: At. Mol. Opt. Phys. 30 5491). Our results support helicity density as a possible source of chiral asymmetry at certain resonance positions in bromocamphor and dibromocamphor.

Electronic transitions and resonance electron scattering measured by electron energy loss spectroscopy of lead phthalocyanine thin film

We have measured the electronic transitions of a lead phthalocyanine thin film by means of optical absorption and reflection electron energy loss spectroscopy in the range from 1 to 20 eV. We compare the optical absorption spectrum with the loss spectra and discuss their similarity. We identify the loss structures and we report a new absorption band at 10 eV. In addition we report that the inelastic electron scattering from the lower energy π→ π* transition and from the C–H stretching vibrational mode show energy-dependent structure, characteristic of resonance scattering via temporary negative ion states.

Very low energy electron scattering in nitromethane, nitroethane, and nitrobenzene

Absolute total integral and total backward scattering cross sections are reported for CH 3 NO 2 , C 2 H 5 NO 2 , and C 6 H 5 NO 2 at electron-impact energies from 30 meV to several eV, with some additional data at higher energies. These experimental data extend the range of species studied in low-energy electron scattering to include target molecules with dipole moments large enough to support dipole-bound states. Data provide a further test of the validity of the Born point-dipole approximation for the calculation of rotationally inelastic scattering cross sections for polar molecules, which are of importance in modeling the chemical and physical characteristics of industrial and natural plasmas. For CH 3 NO 2 and C 2 H 5 NO 2 , large cross sections are found, monotonically increasing with decreasing electron energy in qualitative and to some degree quantitative agreement with theory based on the Born model. As in earlier studies other polar species, the Born model is found to underestimate significantly backward-scattering cross sections. C 6 H 5 NO 2 shows a powerful dip in the total integral scattering cross sections <200 meV, a feature not found in the backward-scattering cross section and totally at variance with any simple theory. The suggestion is made that this behavior, in common with ClO 2 and Cl 2 O, may arise through interference between direct Born-type rotational excitation and an indirect channel involving temporary negative ion states in the continuum.

Low energy electron scattering from CH3Cl

Differential cross section measurements for the elastic scattering of electrons from CH3Cl at energies from 0.5 to 9.5 eV are reported for scattering angles of 30° and 100°. The angular scattering dependence is determined at selected energies over this range. At energies below 1.0 eV, the cross sections are in excellent agreement with calculations using the Born dipole approximation. At large angles and higher energies, the scattering is dominated by a 2A1 temporary negative ion state near 3.5 eV. Energy loss data at 3.5, 5.0, and 8.5 eV are reported and the relative contributions of various vibrational modes determined. Differential cross sections for vibrational excitation of the υ3(a1)C–Cl and υ4(e)CH stretching modes have been measured. The latter reveals a broad shape resonance of 2E symmetry peaking near 5.5 eV. Angular distributions for excitation of these same modes are also reported. Using fittings to the vibrational excitation functions, the resonance parameters have been extracted and used in a mixed semiempirical ab initio calculation to compute the relative strengths of the vibrational energy loss peaks. These compare favorably with the results of the experiment. Elastic cross sections integrated over angle are reported for low energies. They are substantially larger than the results from recent measurements of the total cross section.

Temporary negative-ion states of Na, K, Rb, and Cs.

Electron-transmission spectroscopy is used to observe structure in the electron-scattering cross section associated with the formation of temporary negative ions and channel-opening effects. A strong ``family'' resemblance is apparent among the resonances of the alkali metals. From published calculations and angular scattering studies, the lower-lying features are known with some confidence. Based on these, plausible assignments for some of the higher-lying resonances are put forth. We propose that the absence of resonances near the Cs 5 $^{2}$D and possibly the Na 4 $^{2}$P and 5 $^{2}$P thresholds is associated with the negative polarizabilities of these excited states.