Resonance electron interaction with five-membered heterocyclic compounds: Vibrational Feshbach resonances and hydrogen-atom stripping

Low-energy (0–15 eV) resonance electron attachment to a series of five-membered heterocyclic rings (isoxazole, imidazole, pyrazole, pyrrole, 1-methyl-, and 2-methylimidazole) is studied under gas-phase conditions by means of electron transmission spectroscopy and dissociative electron attachment spectroscopy (DEAS). Experimental spectral features are assigned on the basis of Hartree-Fock and density functional theory calculations. Sharp features, with a width of less than 0.1 eV, observed in the electron transmission spectra of imidazole, pyrazole, and pyrrole close to 0.45 eV, i.e., well below the energy of their lowest-lying π∗ shape resonances detected at 1.90, 1.87, and 2.33 eV, respectively, are associated with formation of negative ion states bound by long-range electron-molecule interactions. Effective range theory calculations which include both dipolar and polarization interactions support this interpretation. In addition to the general observation of cleavage of the N–H bond at incident electron energies close to 2 eV, elimination of as many as three hydrogen atoms from the molecular negative ions is detected at higher energies by DEAS with the only exception of methylated imidazoles. This complex process is associated with ring opening and formation of diatomic hydrogen as one of the neutral fragments, as indicated by the calculations to satisfy the energetic requirements. The present results are of importance for understanding the basic mechanisms of damages caused in living tissues by high-energy radiations

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 re-arrangements. 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 microseconds and 200 microseconds, respectively. These (vibrationally excited) ground anion states formed by capture of thermal electrons mainly into the ring pi*(C=S) LUMO survive long enough to re-distribute their excess energy before undergoing different dissociation channels

A relation between energies of the short-lived negative ion states and energies of unfilled molecular orbitals for a series of bromoalkanes

A series of bromoalkanes was investigated by means of electron transmission spectroscopy in the gas phase. Experimental values of vertical electron affinities associated with occupation of the LUMO by an incoming electron were assigned using ab initio quantum chemical calculations. The predicted vertical electron affinity values differ from measured ones by at most ±0.2 eV

Gas-phase dissociative electron attachment to flavonoids and possible similarities to their metabolic pathways.

The gas-phase empty-level structures and formation of anion states via resonance attachment of low-energy electrons to the flavonoids naringenin (III), quercetin (IV) and myricetin (V) and the smaller reference molecules chromone (I) and flavone (II) are investigated experimentally for the first time. Dissociative electron attachment spectroscopy (DEAS) is used to measure the fragment anion currents generated through dissociative decay channels of the 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. Due to the insufficient volatility of flavonoids III-V, the energies of vertical electron attachment associated with temporary occupation of the lower-lying virtual orbitals are measured with electron transmission spectroscopy (ETS) only in the smaller reference molecules I and II. The experimental findings are interpreted with the support of appropriate density functional theory calculations with the B3LYP functional. The experimental vertical electron attachment energies measured in the ET spectra of I and II are compared with the orbital energies of the neutral molecules scaled using an empirically calibrated linear equation. The vertical and adiabatic electron affinities are evaluated at the B3LYP/6-31+G(d) level as the anion/neutral total energy difference. The latter theoretical method is also used for evaluation of the most stable conformers of the neutral molecules, O-H bond dissociation energies and thermodynamic energy thresholds for production of the anion fragments observed in the DEA spectra. A possible role played by loss of an H2 molecule from the parent molecular anion in vivo in the mitochondrial respiratory chain is briefly discussed

Interconnections between dissociative electron attachment and electron-driven biological processes.

Small molecular species present in mitochondria as, e.g., quinones and oxygen, can capture cellular electrons thus behaving as electron carriers or reactive species, supporting the fundamental process of respiration, and providing protection from pathogens. When xenobiotics penetrate living cells, their delicate redox balance can be altered by capture of cellular electrons to form temporary negative ions. The latter can give rise to the formation of reactive species via dissociative electron attachment (DEA), as observed under gas-phase or electrochemical conditions. DEA to isolated biorelevant molecules studied in vacuo with the support of in silico methods can serve as a model to predict the behavior of these species in vivo under conditions of electron \u201cleakage\u201d in the lipid-protein-cytosol media or in enzymatic active centers. The present review summarizes the results of studies on the correlation between the biological activity of various classes of compounds and fragment species formed by DEA. The following classes of compounds are included into the present review: chlorinated organic pollutants, brominated ethers, captafol and 2,6-dichloroisonicotinic acid, atrazine and bromoxynil, non-steroidal anti-inflammatory drugs, natural polyphenolic compounds, anthralin, salicylic acid and related compounds, ascorbic acid, melatonin, tryptophan, indole and related compounds and some organic peroxides. Formation of temporary molecular anions and their decay are characterized using electron transmission spectroscopy (ETS) and DEA spectroscopy. Quantum-chemical calculations support the identification of the dissociation products. Cellular electron attachment to unnatural electron acceptors is likely to be the first step of cascade processes which constitute the molecular mechanisms of electron-driven biological processes. The fragment species detected with DEA are of importance for understanding the metabolism of xenobiotics, including side effects produced by drugs

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

Molecular anion formation in 9,10-anthraquinone: dependence of the electron detachment rate on temperature and incident electron energy.

Attachment of low-energy electrons to gas phase 9,10-anthraquinone (AQ) was observed with Electron Transmission Spectroscopy (ETS), and interpreted with the support of quantum chemical calculations. The ET spectrum displays three shape resonances at 0.45, 0.7 and 2.2 eV, associated with temporary electron capture into empty * molecular orbitals of AQ, the first two anion states being stable. According to TD-B3LYP calculations, the first -* core-excited resonance lies at about 1.8 eV, although no experimental evidence for this anion state was found. The long-lived parent molecular anion [AQ]\u2013 was observed by means of Electron Attachment Spectroscopy (EAS) using two different mass spectrometers and also by measuring the total anion current at the collision chamber walls. The molecular anion current shows maxima at zero energy, around 0.6 eV and at 1.8 eV. Association of these maxima with the corresponding resonant anion states is discussed. The experimentally measured electron detachment times from [AQ]\u2013 as a function of the incident electron energy and the temperature of the target molecule show a pronounced change of slope around 1.5 eV, regardless of the temperature. This unexpected behavior can be qualitatively reproduced within the framework of a multi-exponential approach which describes the electron detachment event in terms of a redistribution of the anion excess energy, regardless of the initial mechanism of temporary anion formation