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 (0-15 eV) resonance electron attachment and to characterize various CUR radical forms. The adiabatic electron affinity of CUR molecule is experimentally estimated to be about 1 eV. 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 0.6 eV, which is due to the formation of the π3* and π4* temporary negative ion states predicted to lie around 1 eV. 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.

On delicate balance between formation and decay of tetracyanoethylene molecular anion triggered by resonance electron attachment.

Low-energy (0-15 eV) resonance electron interaction with isolated tetracyanoethylene (TCNE) molecules is studied in vacuo by means of dissociative electron attachment (DEA) spectroscopy. Despite this molecule being relatively small, the long-lived molecular anions TCNE- are formed not only at thermal electron energy via a vibrational Feshbach resonance mechanism but also via shape resonances with the occupation of the π4* and π5* molecular orbitals by an incident electron. Dissociative decays of TCNE- are mostly observed at incident electron energy above the π7* temporary anion state predicted to lie at 1.69 eV by means of B3LYP/6-31G(d) calculations combined with the empirical scaling procedure. Electron attachment to the π6* orbital (predicted at 0.85 eV) leads to the generation of long-lived TCNE- species, which can decay via two competing processes: extra electron detachment, which appears in hundreds of microseconds, or elimination of two cyano groups to form the [TCNE - 2(CN)]- negative fragment on a tens of microsecond timescale. The latter is accompanied by the generation of a highly toxic cyanogen molecule as a neutral counterpart. Since the electron transfer to the acceptor molecule TCNE plays a key role in the formation of single-molecule magnets, the present data are of importance to understand the long-term behavior and likely harmful effects produced by cyanide-based prospective materials.

Dissociative Electron Attachment to Hexachlorobenzene.

Gas phase molecules of hexachlorobenzene (C6 Cl6 ) 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 (τa ∼250 µs at 343 K). All three channels exhibit temperature dependence. The adiabatic electron affinity estimated using a simple but typically accurate Arrhenius model (EAa =1.6-1.9 eV) turns out to be much higher than the quantum-chemical predictions (EAa =0.9-1.0 eV). We discuss the possible reasons behind the observed discrepancy.

Microsecond dynamics of molecular negative ions formed by low-energy electron attachment to fluorinated tetracyanoquinodimethane.

Low-energy (0-15 eV) electron interactions with gas-phase 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) molecules are studied under single collision conditions using dissociative electron attachment spectroscopy. The experimental findings are supported by density functional theory calculations of the virtual orbital energies and energetics of the dissociative decays. Long-lived molecular negative ions F4-TCNQ- are detected in a wide electron energy range (0-3 eV) with electron detachment times in the range of milliseconds. Although plenty of decay channels are observed, their intensities are found to be very small (two to four orders of magnitude relative to the F4-TCNQ- signal). These findings prove that the structure of this strong electron-accepting molecule bearing an excess electron is robust in its electronic ground state, even when highly (up to 6 eV) vibrationally excited. As many as nine metastable fragment anions formed slowly (in the 16-23 µs range) are found in the negative ion mass spectrum of F4-TCNQ, as never observed before in compounds possessing high electron-accepting ability. The present results shed some light on microsecond dynamics of isolated F4-TCNQ molecules under conditions of excess negative charge, which are important for understanding the functionality of nanoscale devices containing this molecule as a structural element.

5-Nitro-2,4-Dichloropyrimidine as an Universal Model for Low-Energy Electron Processes Relevant for Radiosensitization.

We report experimental results of low-energy electron interactions with.

Ionizing radiation and natural constituents of living cells: Low-energy electron interaction with coenzyme Q analogs.

Resonance electron attachment to short-tail analogs of coenzyme Q10 is investigated in the electron energy range 0 eV-14 eV under gas-phase conditions by means of dissociative electron attachment spectroscopy. Formation of long-lived (milliseconds) molecular negative ions is detected at 1.2 eV, but not at thermal energy. A huge increase in the electron detachment time as compared with the reference para-benzoquinone (40 µs) is ascribed to the presence of the isoprene side chains. Elimination of a neutral CH3 radical is found to be the most intense decay detected on the microsecond time scale. The results give some insight into the timescale of electron-driven processes stimulated in living tissues by high-energy radiation and are of importance in prospective fields of radiobiology and medicine.

Structural rearrangements as relaxation pathway for molecular negative ions formed via vibrational Feshbach resonance.

The low-energy (0-15 eV) resonance electron interaction with two organic acids, oxaloacetic and α-ketoglutaric, is studied under gas-phase conditions using dissociative electron attachment spectroscopy. The most unexpected observation is the long-lived (microseconds) molecular negative ions formed by thermal electron attachment via the vibrational Feshbach resonance mechanism in both compounds. Unlike oxaloacetic acid, for which only one slow (microseconds) dissociative decay is detected, as many as five metastable negative ions are observed for α-ketoglutaric acid. These results are analyzed using density functional theory calculations and estimations of electron affinity using the experimental electron detachment times. The results are of considerable interest for understanding the fundamental mechanisms responsible for the dynamics of highly excited negative ions and the transformation pathways of biologically relevant molecules stimulated by excess electron attachment.

Dissociative Electron Attachment to 2,3,6,7,10,11-Hexabromotriphenylene.

2,3,6,7,10,11-Hexabromotriphenylene (HBTP) and 2,3,6,7,10-pentabromotriphenylene (PBTP) were investigated by means of dissociative electron attachment spectroscopy (DEAS). The dominant decay channel of the transient molecular negative ions consists of elimination of Br- with resonances in the low electron energy region. Formation of long-lived parent anions with autodetachment lifetime τa = 310 µs is observed at thermal electron energies. The adiabatic electron affinities, EAa = 1.12 ± 0.1 eV in HBTP and 1.09 ± 0.1 eV in PBTP, evaluated using a simple Arrhenius approach are in good agreement with those predicted by DFT (XYG3/Def2-TZVPP//PBE0/Def2-TZVPP) calculations.

Electron stimulated ring opening in diphenylphthalide dicarboxylic acid: Its likely role in the unique properties of phthalide-based materials.

The electronic properties of diphenylphthalide dicarboxylic acid (DPDA) are studied under gas-phase conditions using dissociative electron attachment spectroscopy and in the condensed environment by means of total current spectroscopy. The experimental features are assigned with the support of density functional theory calculations of the energies of the lowest-lying anion states to describe both resonances responsible for low-energy (0-15 eV) electron attachment to the isolated molecule and the maxima in the density of unoccupied electronic states in the condensed ultrathin (up to 10 nm) films. Resonance electron attachment to DPDA is found to be followed by the opening of the γ-lactone ring in the molecular negative ions, an unusual mechanism leading to their stabilization. A similar mechanism is expected to be responsible for the unique properties of phthalide-based materials in the condensed state.

Fragmentation of chlorpyrifos by thermal electron attachment: a likely relation to its metabolism and toxicity.

The energies of formation and dissociative decays of temporary negative ions of the organophosphorus insecticide chlorpyrifos (CPF) are studied using electron transmission spectroscopy (ETS), dissociative electron attachment spectroscopy (DEAS) and quantum-chemical calculations. Three features are displayed by ETS at 2.4, 3.1 and 4.30 eV, which are ascribed to empty σ* MOs, a higher-lying π* MO and a core-excited state, respectively. Two stable π* anion states are predicted by the calculations. Most of the negative fragments are detected by DEAS at thermal energies of the incident electrons, being thus associated with the dissociation of stable (vibrationally excited) negative ion states formed by electron attachment into the π* LUMO and LUMO+1. The CPF- molecular anions (not observed in the present study) are expected to decay by fast dissociation to give the most abundant ([CPF - HCl]-) species, which in turn dissociates on the microsecond timescale, producing as much as six metastable peaks in the mass spectrum. The m/z = 196 and 169 negative fragments, structurally similar to the main metabolites of CPF, 3,5,6-trichloro-2-pyridinol and O,O-diethyl thiophosphate, respectively, are formed by the direct decomposition of CPF-. Active radicals able to abstract hydrogen atoms from lipid membranes are generated as neutral counterparts of the observed anion fragments. A likely involvement of DEA in the biotransformation of CPF by cytochrome P450 enzymes in a reductive environment producing toxic species and precursors of the main metabolites is briefly discussed.

Can the Electron-Accepting Properties of Odorants Be Involved in Their Recognition by the Olfactory System?

The present study examines the possible importance of the electron-accepting properties of odorant molecules and, in particular, the formation and decay of temporary negative ions via low-energy electron attachment as a possible contribution toward understanding odorant recognition by olfactory receptors (ORs). Fragments formed by dissociative electron attachment (DEA) of mustard oil odorants represented by a series of isothiocyanates are studied experimentally using DEA spectroscopy and DFT calculations. Relative intensities for the most abundant fragment species, S- and SCN-, are found to be characteristic of structurally similar odorants under investigation. This novel approach for the investigation of odorants may contribute to understanding the initial stages of the olfactory process and may provide a means to distinguish between odorants and their interactions with the olfactory receptor system.

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 (ET) spectroscopy, 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 pi( *) molecular orbitals of AQ, the first two anion states being stable. According to TD-B3LYP calculations, the first pi-pi( *) 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](-) 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](-) 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 multiexponential 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.