Resonant Electron Capture Research Articles

Resonance capture of electrons by electroactive organic molecules

Bound states of electrons with electroactive aromatic molecules, promising for use in molecular electronics, are studied employing electron attachment spectroscopy Anions are produced in the gas phase through the capture of electrons with energies of up to 15 eV by molecules via the resonance mechanism. The possible pathways of fragmentation of molecular anions and the times of electron retention by the test molecules at various incident electron energies are measured. The resonance states identified are interpreted with the help of quantum-chemical calculations. The possibility of applying the results obtained to molecular electronics is discussed.

O‐(Pentafluorobenzyloxycarbonyl)benzoyl chloride: a novel electrophoric derivatisation reagent for amino compounds designed for negative ion chemical ionisation mass spectrometry

The synthesis of a novel electrophoric derivatisation reagent, o-(pentafluorobenzyloxycarbonyl)benzoyl chloride, is described. The reagent was tested against selected primary and secondary amino compounds as analytical targets. The derivatives exhibit excellent mass spectral properties under negative ion chemical ionisation (NICI), i.e. reduced fragmentation and thus high ion current for the targeted m/z during analysis. Since the reagent bears a pentafluorobenzyl ester group, resulting mass NICI mass spectra were expectedly dominated by dissociative resonance electron capture typically observed with these compounds. The reagent is suitable for detecting volatile primary and secondary amines with high sensitivity. Background is reduced by a shift in detected m/z and retention time, as demonstrated for the analysis of the drug methylphenidate from human plasma.

Fast side-chain losses in keV ion-induced dissociation of protonated peptides

In this paper, we compare ionization and dissociation of a series of singly and doubly protonated peptides, namely leucine enkephalin, bradykinin, LHRH and substance P as induced by collisions with keV H +, He + and He 2+. For all peptides under study, the fragmentation pattern depends strongly on the electronic structure of the projectile ions. Immonium ions, side-chains and their fragments dominate the spectrum whereas fragments due to peptide backbone cleavage are weak or even almost absent for He +. Here, resonant electron capture from the peptide is ruled out and only interaction channels accompanied by much higher excitation contribute. Cleavage of the side-chain linkage appears to be a process alternative to backbone fragmentation occurring after internal vibrational redistribution of excitation energy. Depending on the peptide, this process can lead to the loss of a side-chain cation (leucine enkephalin, LHRH) or a neutral side-chain (substance P).

On the delay mechanism of Cl 2 − diatomic anion dissociation up to the microsecond timescale

The dissociative capture of slow electrons by tetrachlorethylene (C2Cl4) has been investigated by resonant electron capture negative ion mass spectrometry. Metastable ions with fractional mass numbers 7.5, 17.5, and 19 corresponding to the C2Cl4− → Cl− + C2Cl3 and Cl2− → Cl− + Cl decays occurring at the microsecond timescale have been detected. It has been revealed that Cl2− anions, which are fragment ions, can dissociate at the microsecond timescale, which is very surprising for a system with one internal degree of freedom. This process is assumingly attributed to the rotational excitation of Cl2− anions. Thus, the experimental estimate of the time of rovibronic relaxation in the Cl2− anion has been obtained.

High resolution mass analysis of N- and C-terminal negative ions resulting from resonance electron capture by aliphatic amino acids

High mass resolving power was applied to study resonance electron capture by glycine, alanine, and valine, and accurate mass measurements helped to distinguish between some negative ions having the same nominal masses. It was established that the C- and N-terminal negative ions of the same nominal masses were formed at different electron energies from different resonance states. The typical fragmentation pathways in deprotonated amino acids via loss of water initiated by collisional activation were not observed upon resonant electron capture by aliphatic amino acids. Instead, [M-18](-) negative ions in the vicinity of 5 eV were found to be associated with simultaneous loss of either ammonia and a hydrogen atom or an amino group and a hydrogen molecule.

Complex fragmentation pathways of rhodanine and rhodanine-3-acetic acid upon resonant capture of low-energy electrons

A detailed gas-phase study of dissociative electron attachment (DEA) to rhodanine ( Rd) and its 3-acetic acid derivative ( Rdaa) in the 0–14 eV energy range has been carried out with a magnetic mass spectrometer under conditions of medium energy resolution (0.4 eV) of the incident electron beam, and high sensitivity. The DEA spectra reveal the occurrence of numerous and complex dissociative decay channels of the molecular anions formed by resonances, involving multiple bond cleavage and structural rearrangements. Along with a variety of anion fragments normally formed in the collision cell at incident electron energies <1 eV, dissociation of a series of metastable anion species (occurring in the time scale of microseconds) is also detected. The observation of these slow processes is consistent with the complexity of many dissociative channels, and allows to propose schemes for the fragmentation pathways. The DEA spectra of both Rd and Rdaa also display small yields of molecular anions at zero energy. Their lifetimes with respect to electron detachment are experimentally evaluated to be about 30 μs and 200 μs, respectively. These (vibrationally excited) ground anion states formed by capture of thermal electrons mainly into the ring π* (C S) LUMO survive long enough to redistribute their excess energy before undergoing different dissociation channels.

Resonant dissociative electron capture by the simplest amino acids and dipeptides

The formation of ions from amino acids (glycine and alanine) and dipeptides (glycylglycine, alanylalanine, and glycylalanine) under the resonant electron capture conditions was studied by negative ion resonant electron capture mass spectrometry. The isobaric ions were found, their effective yield curves were experimentally separated, and the elemental composition was determined. The thermochemical aspect of ion formation was considered, and probable dissociative channels of fragmentation ion formation and their structures were established on the basis of this aspect. Bond cleavage reactions only and H-shift processes were revealed. The rearrangements occur presumably through the stage of formation of intramolecular hydrogen bonds. The cross-sections of formation of ions [M − H]− were measured in the energy range 1.1–1.3 eV. The metastable decay channels of ions [M − H]− and [M − COOH]− were found in the energy range 4.5–7.5 eV for dipeptides, which enabled establishing the genetic relationship between the parental and daughter ions and revealing hidden fragmentation pathways.

Resonant electron capture byC60ions at a metal surface with projected band gap

${\text{C}}_{60}^{+}$ and ${\text{C}}_{60}^{2+}$ ions are scattered under grazing incidence from an atomically clean and flat Be(0001) surface at kilo-electron-volt energies. Distances for electron transfer are deduced from shifts of angular distributions for incident ${\text{C}}_{60}^{+}$ and ${\text{C}}_{60}^{2+}$ projectiles, which reflect changes in the interaction potentials at the instants of electron transfers. These distances are consistent with classical over-the-barrier model results indicating that the suppression of charge transfer, observed for atomic projectiles in front of metal surfaces with a projected band gap, is absent for fullerenes.

Resonance Electron Capture by Serine

Formation of negative ions via dissociative electron attachment (DEA) to the amino acid serine in the gas phase was studied using two different crossed electron/molecular beam techniques and quantum chemical calculations. Resonance electron capture mass spectrum and effective ion yield curves of 16 negative ions were measured over the electron energy range from close to 0 to 11 eV. The negative ions from serine were detected from resonance states in the vicinity of 0, 1.3, 5, and 8 eV. The dominant reaction channel at low electron energies was (M-H)(-). The relative cross section for this ion exceeds more than 20 times that of any other fragment negative ions. A high-resolution experiment was applied to study fine structures in (M-H)(-) cross section. We have found that the second OH group influences some dissociative channels. Quantum chemical calculations were applied to interpret products of the DEA reaction channels.

Dissociative Charge Exchange Dynamics of HOCO+ and DOCO+

Translational spectroscopy has been used to study the dissociation of HOCO and DOCO to both the OH/OD(X(2)Pi) + CO(X(1)Sigma(+)) and H/D((2)S) + CO(2)(X(1)Sigma(+)) product channels upon charge exchange between a fast (keV) beam of HOCO(+)/DOCO(+) and cesium. Analysis of the measured kinetic energy release distributions for the two product channels suggests that HOCO is initially produced in the 1(2)A'' electronic state by near resonant electron capture from cesium (approximately 4.31 eV above the ground state trans-HOCO conformer). This initial state undergoes rapid predissociation leading to OH + CO and H + CO(2) products, with respective branching fractions of 0.75 +/- 0.03 and 0.25 +/- 0.03. Possible mechanisms leading to dissociation and the observed dissociation dynamics are presented in light of theoretical studies of the low-lying electronic states of HOCO.

Plasma oscillations in fullerene molecules within the random phase approximation

A method is presented for calculating the density of plasma excitation modes in fullerene molecules, which makes it possible to describe more accurately the specific features of the spectra as compared to the models proposed previously. The method is based on the inclusion of the total contribution of π and σ electrons to the spectra of collective oscillations in the molecules. This allows one to consider fullerene molecules with an arbitrary symmetry and arbitrary substituents. A comparative analysis of the experimental curves obtained for resonance electron capture and the results of numerical simulations for C60, C70, and C76 molecules has demonstrated that not only the main maxima but also the ranges of the formation of negative ions coincide.

Slow decay of negative molecular fluorofullerene ions in the electron autodetachment process

The mean lifetimes of negative molecular ions of C60 fullerene and its fluoroderivatives C60F18 and C60F36 with respect to the autodetachment of electrons have been measured as functions of the ionizing electron energy by the method of mass spectrometry of electron resonance capture. The lifetimes of negative ions in the compounds under investigation are within the one-second range, and an increase in the number of addends leads to an increase in the lifetimes of negative ions by 1.5–2.5 orders of magnitude.

Rearrangement and predissociation processes in negative molecular ions of nitrobenzenes

The reactions of resonant electron capture by the molecules of benzene nitroderivatives has been studied in the gas phase. Some fragment negative ions were found to be unstable with respect to electron autodetachment. This circumstance has been used for the determination of their structure. In particular, it has been established that the low measured appearance energy of neutral component of [M-H](-) ion beam is a result of isomerization of nitrobenzenes' molecular ion, leading to the 2-nitrosophenol structure with the subsequent formation of the phenoxide anion in the autodetaching state. The effective yield curves of some types of fragment ions demonstrate fine vibrational structures, testifying the predissociation mechanism of ion formation. For all detected ions, the absolute cross sections of formation have been measured.

The fragmentation of negative ions of fullerene C60 trifluoromethyl derivatives

The formation and decay of gas-phase negative ions of trifluoromethylated fullerenes C60(CF3)n(n = 2–10) were studied. The resonance electron capture mass spectra were measured to find that the main fragmentation channel of negative ions was the detachment of trifluoromethyl groups. The degree of fragmentation directly depended on the energy of electrons and reached the complete splitting off of all the CF3 addends with the formation of C60− ions. The observed metastable ion signals were analyzed to determine the scheme of sequential fragmentation of negative ions.

Dominance of the Breit Interaction in the X-Ray Emission of Highly Charged Ions Following Dielectronic Recombination

The Breit interaction typically appears as a-more or less small-correction to the Coulomb repulsion acting among the electrons. We here propose two x-ray measurements on the angular distribution and linear polarization of the 1s2s(2)2p(1/2) J=1-->1s(2)2s(2) J=0 electric-dipole radiation of high-Z, beryllium-like ions, following the resonant electron capture into initially lithium-like ions, for which the Breit interaction strongly dominates the Coulomb repulsion and leads to a qualitative change in the expected x-ray emission pattern. The proposed measurements are feasible with present-day x-ray detectors and may serve a stringent test on relativistic corrections to the electron-electron interaction in the presence of strong fields.

Fragmentation of peptide negative molecular ions induced by resonance electron capture.

A simple robust method to study resonance gas-phase reactions between neutral peptides of low volatility and free electrons has been designed and implemented. Resonance electron capture (REC) experiments were performed by several neutral model peptides and two naturally occurring peptides. The assignment of negative ions (NIs) formed in these gas-phase reactions was based on high mass-resolving power experiments. From these accurate mass measurements, it was concluded that fragment NIs formed by low (1-2 eV) energy REC are of the same types as those observed in electron capture/transfer dissociation, where the positive charge is a factor. The main feature resulting from these REC experiments by peptides is the occurrence of z(n)-1 ions, which are invariably of the highest abundances in the negative ion mass spectra of larger peptides. [M-H](-) NIs presumably the carboxylate anion structure dominate the REC spectra of smaller peptides. There was no evidence for the occurrence of the complementary reaction, i.e., the formations of c(n)+1 ions. Instead, c(n) ions arose without hydrogen/proton transfer albeit with lower abundances than that observed for z(n)-1 ions. Only the amide forms of small peptides showed more abundant ion peaks for the c(n) ions than for the z(n)-1 ions. The mechanisms for the N-C(alpha) bond cleavage are discussed.

Photodissociation dynamics of Ar2+ and Ar3+ excited by 527 nm photons

The photofragmentation dynamics of Ar(2)(+) and Ar(3)(+) clusters has been investigated at a 527 nm wavelength (2.35 eV) using a setup that allows simultaneous detection of the ionic and neutral fragments in a coincidence experiment. Measurement of positions and times of flight enables in principle a complete description of the fragmentation dynamics. The photofragmentation dynamics of Ar(3)(+) clusters is similar to that of Ar(2)(+) with, in addition, the ejection of a third fragment that can be neutral or ionized via a resonant electron capture. This is attributed to the triangular geometry of the Ar(3)(+) ion.

Dissociation processes of pyrazoline and related molecules upon resonance electron capture and electronic excitation

The formation of negative ions from pyrazoline and its derivatives was studied by the negative-ion and resonance-electron-capture mass spectrometry techniques. The results were compared with those of the dissociation processes of the excited-state pyrazoline molecule, and resemblance to the fragmentation processes of the molecular negative ion was revealed.

Scattering of F atoms and anions on a TiO 2(1 1 0) surface

Results of a study of energy losses and electron transfer processes for grazing scattering of fluorine atoms and anions scattering along different azimuthal orientations of the TiO 2 crystal are presented. We observe strong variations in the overall intensity of scattered particles which are due to channelling effects. The energy losses do not show strong variations as a function of crystal azimuth except for the case of scattering along the (0 0 1) direction between the bridging oxygen atom rows, where we also observe differences in the energy losses of scattered ions and neutrals. We attribute this to the fact that larger F − survival occurs for trajectories staying farther from the surface, when also the energy losses remain small. The overall characteristics of energy losses are attributed mainly to trajectory effects due to scattering in regions of different electron density. Measurements of the ratio of scattered ions to the total scattered flux, i.e. the ion fractions which reflect electron capture and loss processes, show that these are not the same for incident anions and atoms. A strong difference for scattering along the (0 0 1) direction is observed, where at low incident energies a strong survival of incident ions occurs. These results are tentatively discussed in terms of non resonant electron capture at lattice O − sites and electron loss into the conduction band or by collisional detachment with bridging O atoms.

Photoelectron and UV absorption spectroscopy for determination of electronic configurations of negative molecular ions: Chlorophenols

The photoelectron and UV absorption spectra of p-, m-, and o-chlorophenols in the gas phase have been obtained. On the basis of DFT B3LYP/6-311++G(d, p) calculations, the photoelectron bands have been assigned to occupied molecular orbitals. From the TDDFT B3LYP/6-311++G(d, p) calculation results, the UV absorption bands have been assigned to excited singlet states of the molecules under investigation. For each excited state a dominant transition was found. It has been shown that the energies of these singlet transitions correlate with the energy differences between the ground-state molecular orbitals participating in them. Using the UV spectra interpretation, the electronic states of molecular anions detected earlier for the same compounds by means of the resonant electron capture mass-spectrometry have been determined.