Long-lived molecular anions of brominated diphenyl ethers.

Resonance electron attachment in a series of brominated diphenyl ethers, namely 4-bromodiphenyl ether (BDPE), 4-bromophenyl ether (BPE), and decabromodiphenyl ether (DBDE), was investigated in the gas phase by means of dissociative electron attachment spectroscopy. In addition to channels of dissociation into stable fragments, long-lived molecular negative ions with an average lifetime relative to autodetachment of the order of 60 µs were found for the last two molecules. In the case of BDPE and BPE, the most intense dissociation channel is the bromine anion, and for DBDE-the [C6Br5O]- anion. The [C6Br5O]- anion sequentially decomposes with the elimination of the bromide anion on a microsecond time scale, which is confirmed by the registration of metastable ions with an apparent mass of 12.8 a.m.u. The electron affinity of the studied molecules and the appearance energy of fragment ions were estimated with CAM-B3LYP/6-311+G(d,p).

Dissociative electron attachment to p-fluoranil and p-chloranil.

The processes of formation and decay of negative molecular ions (NMI) of p-fluoranil [2,3,5,6-tetrafluoro-1,4-benzoquinone (FA)] and p-chloranil [2,3,5,6-tetrachloro-1,4-benzoquinone (CA)] in the gas phase were investigated. In both cases, long-lived NMIs were found in two resonances, at thermal electron energies and in the region of 0.8-0.9 eV, with lifetimes relative to electron ejection of τa ∼ 600 µs. The dissociation of molecular NIs with the formation of fragment ions [M-COF2]-, [M-CO]-, and Cl- proceeds at microsecond times, which is confirmed by the registration of the corresponding metastable ions. It is shown that the dependence of the lifetime of MNIs on the electron energy can be explained by the presence of a transition state at an energy of ∼0.5 eV.

Non-covalent anion structures in dissociative electron attachment to some brominated biphenyls.

The present work combines experiment and theory to reveal the behavior of bromo-substituted-biphenyls after an electron attachment. We experimentally determine anion lifetimes using an electron attachment-magnetic sector mass spectrometer instrument. Branching ratios of dissociative electron attachment fragments on longer timescales are determined using the electron attachment-quadrupole mass spectrometer instrument. In all cases, fragmentation is low: Only the Br- and [M-Br]- ions are detected, and [M-H]- is observed only in the case of 4-Br-biphenyl and parent anion lifetimes as long as 165 µs are observed. Such lifetimes are contradictory to the dissociation rates of 2- and 4-bromobiphenyl, as measured by the pulse radiolysis method to be 3.2 × 1010 and >5 × 1010 s-1, respectively. The discrepancy is plausibly explained by our calculation of the potential energy surface of the dissociating anion. Isolated in vacuum, the bromide anion can orbit the polarized aromatic radical at a long distance. A series of local minima on the potential energy surface allows for a roaming mechanism prolonging the detection time of such weakly bound complex anions. The present results illuminate the behavior recently observed in a series of bromo-substituted compounds of biological as well as technological relevance.

Dissociative electron attachment to 3-benzelidenephthalide and phenolphthalein molecules.

Electron attachment to the 3-benzelidenephthalide and phenolphthalein molecules and decay channels of their molecular anions were investigated by means of dissociative electron attachment spectroscopy and electron transmission spectroscopy. Interpretations of these experimental data were supported with UV-spectroscopy and density functional theory calculations. The average electron detachment times for the long-lived molecular anions of 3-benzelidenephthalide and phenolphthalein were measured to be 150 µs at 120 °C and 560 µs at 190 °C, respectively. The long-lived molecular anions of phenolphthalein are ascribed to an isomer formed by ring opening. The present results suggest that, opposite to phenolphthalein, polymeric materials based on 3-benzylidenephthalide cannot be switched to a high conductive state due to different mechanisms of stabilization of their long-lived molecular anions.

4-Bromobiphenyl: Long-lived molecular anion formation and competition between electron detachment and dissociation.

Electron attachment to the 4-bromobiphenyl molecule and the decay channels of its molecular anion were investigated by means of Dissociative Electron Attachment (DEA) spectroscopy with two different spectrometers. The first apparatus is equipped with a static magnet mass analyzer (Ufa group) and the second one with a quadrupole mass filter (Prague group). The dominant DEA channel at low electron energy leads to formation of Br- negative fragments. Long-lived (τa = 40 µs at the temperature of 80 °C) molecular negative ions were detected only in the Ufa experiment. We explored the involved potential energy surfaces and found that the molecular anion has two distinct structures with the C-Br distances of 1.92 Å and 2.8 Å. The statistical model based on the Arrhenius approximation fully explains the experimental observations and sheds light on the earlier anion dissociation kinetic studies in solution.

Dissociative electron attachment to 2,4,6-trichloroanisole and 2,4,6-tribromoanisole molecules.

2,4,6-trichloroanisole and 2,4,6-tribromoanisole were investigated by means of electron transmission spectroscopy and two different types of dissociative electron attachment spectrometers. The results obtained were interpreted with the support of density functional theory calculations. The dominant dissociative decay channels of the temporary molecular negative ions lead to the formation of Cl- and Br- in the low electron energy region. Formation of long-lived parent anions is observed at thermal electron energies. Their relative intensity depends on the experimental time window, ∼36 µs in the case of the static magnet mass analyzer and ∼200 µs for the quadrupole mass analyzer employed. The results obtained may be useful for rapid detection of these compounds in wine and pharmaceutical industries, as well as other branches connected to the food industry, e.g., packaging.

Electron attachment to the phthalide molecule.

Phthalide, the simplest chain of conductive polymer thin film, was investigated by means of Electron Transmission Spectroscopy, Negative Ion Mass Spectrometry, and density functional theory quantum chemistry. It has been found that formation of gas-phase long-lived molecular anions of phthalide around 0.7 eV takes place through cleavage of a C-O bond of the pentacyclic ring of the parent molecular anion to give a vibrationally excited (electronically more stable) open-ring molecular anion. The energy of the transition state for ring opening of the parent negative ion is calculated to be 0.65 eV above the neutral ground state of the molecule. The energy (2.64 eV) evaluated for the corresponding transition state in the neutral molecule is much higher, so that the process of electron detachment from the anion must lead to a neutral molecule with its initial pentacyclic structure. The average lifetime of the molecular negative ions formed at an electron energy of 0.75 eV and 80 °C is measured to be about 100 µs. The known switching effect of thin phthalide films could stem from the presence of a similar open/closed transition state also in the polymer.

Electron attachment to some naphthoquinone derivatives: long-lived molecular anion formation.

RATIONALE: Electron Affinity (EA) is one of the fundamental properties of a molecule. EA values can be measured with various experimental methods, although their availability is still relatively limited. We make an attempt to use Dissociative Electron Attachment Spectroscopy (DEAS) data for evaluation of the EAs of twelve naphthoquinone (NQ) derivatives. METHODS: Naphthoquinone (NQ) and eleven of its hydroxyl derivatives were investigated by means of DEAS. A combined investigation of NQ and juglone by means of the Electron Transmission Spectroscopy (ETS) and DEAS techniques, with the support of density functional theory (DFT) calculations, allowed us to elucidate the empty-level structures of NQ and its hydroxyl derivatives. RESULTS: All molecules under investigation form extremely long-lived molecular anions associated with three resonant states (except for NQ, where only two long-lived resonances were observed). The hydroxyl substituents of NQ cause an increase in EA and number of internal degrees of freedom (N), and, as a result, an increase in the mean electron autodetachment lifetimes of the molecular negative ions (NIs). Evaluation of the EAs from the measured lifetimes of the molecular NIs through a simple Arrhenius approximation gives results in reasonable agreement with those obtained with DFT calculations. CONCLUSIONS: NI lifetime measurements by means of a modified DEAS instrumentation can provide quantitative data of EA. A simple Arrhenius approximation seems to be adequate to describe the process of electron detachment from molecular anions.

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.

Temperature dependence of the mean autodetachment lifetime of the p-benzoquinone molecular radical anion.

Experimental data previously published from this laboratory (Asfandiarov NL, Pshenichnyuk SA, Fokin AI, Nafikova EP. Chem. Phys. 2004; 298: 263), on the temperature dependence of the mean autodetachment lifetime of the p-benzoquinone molecular radical anion, were analyzed in the framework of a simple statistical model. This model is a simplification of the well-known semiempirical approach elaborated by Christophorou et al. (Christophorou LG, Hadjiantoniou A, Carter JG. J. Chem. Soc., Faraday Trans. II 1973; 69: 1722). Evaluation of the vibrational energy storage of the target molecule and anion was made in a quantum approximation following a published procedure (Matejcik S, Märk TD, Spanel P, Smith D, Jaffke T, Illenberger E. J. Chem. Phys. 1994; 102: 2516). The results obtained are in reasonable agreement with the experimental data.

The role of free electrons in MALDI: electron capture by molecules of alpha-cyano-4-hydroxycinnamic acid.

Low-energy (0-12 eV) electron attachment to molecules of a typical matrix substance used for matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS), namely alpha-cyano-4-hydroxicynnamic acid, has been investigated in the gas phase at different temperatures ranging from 140 degrees C to 260 degrees C by means of electron capture negative-ion mass spectrometry (ECNI MS). The yield of negative ions, formed by electron capture, was measured as a function of incident electron energy for four different temperatures. The long-lived parent molecular anion, [M]- (m/z 189), was observed in the negative-ion mass spectra of the substance under investigation. Its autodetachment lifetime was estimated to be approximately 600 micros. It was found that at 140 degrees C the main decay channel of the long-lived temporary molecular anion of alpha-cyano-4-hydroxicynnamic acid is a formation of the [M-COOH]-; fragment negative ion (m/z 144) with an intensity of 37.2% in percentage terms in respect of the total anion current. There are also [M-H]-, [M-CO2]- and [CN]- fragments in the spectra with intensities of about 7.7%, 21.6% and 3.1% at 140 degrees C. It was shown that the escape of the CO2 molecule from the parent molecular anion is a slow process. It takes [M]- about 10 micros to decay on carbon dioxide molecules and [M-CO2]- fragment anions. Increasing the temperature of the target molecule alters the negative-ion mass spectra of alpha-cyano-4-hydroxicynnamic acid significantly. A possible role for the findings in typical MALDI MS experiments is discussed.

Electron capture negative ion mass spectra of some typical matrix-assisted laser desorption/ionization matrices.

A series of seven typical matrix-assisted laser desorption/ionization (MALDI) matrices has been investigated by means of electron capture negative ion mass spectrometry (ECNI-MS). It has been shown that the most effective matrices form deprotonated negative ions predominantly in the low-energy region. Relative dissociative cross sections have been measured for all molecules under investigation. The relative integrated abundance of [M - H](-) ion formation in the series changes by four orders of magnitude. It has been shown that 2,5-DHB (gentisic acid), one of the most effective MALDI matrices, has maximal relative intensity of [M - H](-) formation at the energy approximately equal 0.8 eV. This result is in accordance with a finding of Frankevich and Zenobi [Book of Abstracts, Workshop-school "Mass spectrometry in chemical physics, bio-physics and environmental sciences", Zvenigorod, Russia, April, 25-26, 2002, p. 40] that a probable origin of negative ions in MALDI is the process of low-energy (0.5-1 eV) dissociative electron capture by matrix molecules.

Violation of frozen shell approximation in dissociative electron capture by halogenated anthraquinones.

A series of halogenated anthraquinone (AQ) derivatives has been studied by means of electron capture negative ion (NI) mass spectrometry (ECNI-MS). 1Cl-AQ and 2Br-AQ display dramatically steep positive temperature dependencies of Hal(-) ion abundance in the low electron energy region. Molecular NI intensity decreases rapidly with increasing temperature in the case of 1I-AQ. In the case of 2Br-AQ, a metastable NI peak (m/z 22.9) corresponding to the process BrAQ(-) --> Br(-) + AQ(0) was recorded. This means that the characteristic dissociation lifetime of the molecular NI Br-AQ(-) is at least approximately 25 micros at the energy approximately 0.67 eV in the low-temperature spectrum (T approximately 80 degrees C), and at the energy approximately 0.13 eV in the hot spectrum (T approximately 290 degrees C). Together with the observed temperature dependence of the 2Br-AQ curves of effective yield (CEY), this proves that this anion dissociates according to Coulson's model. The same halogen anion behavior is observed in the case of 1Cl-AQ. There are three consecutive stages in the process of molecular NI dissociation of Cl- and Br-substituted AQ, namely, electron capture into the empty pi-orbital by means of the shape resonance mechanism, followed by a radiationless transition into the ground electronic pi-state of the anion, as predicted by Compton in the case of the parabenzoquinone molecule, and, finally, a fluctuative dissociation of the molecular NI accompanied by the transition from the pi-term into the sigma-term, so-called predissociation. Calculations show reasonable agreement with the experimental data. In the case of 1I-AQ, an effect of inversion of empty levels in the process of electron capture by the molecule takes place, a violation of the so-called frozen shell approximation. The phenomenon found may be of significance not only in the case of ECNI-MS, but also in other experimental investigations using low-energy electron-molecule and ion-molecule collisions.