N. L. Asfandiarov

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.

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.

Multiexponential decay of negative molecular ions as a result of target molecule distribution over vibrational states

The lifetimes of long-lived negative molecular ions SF6−-, C6H5NO2−, and C6F6− are measured with a static mass spectrometer. A great spread in published data for the lifetimes of these ions is explained using a concept of multiexponential decay of molecular ions. The influence of the Boltzmann distribution of neutral target molecules over vibrational states on the lifetime of negative molecular ions is studied in terms of the Illenberger-Smirnov-Kompaneits simple statistical model. It is shown that this distribution has a profound effect on the multiexponential decay of molecular anions and, as a consequence, on the lifetime of negative molecular ions measured on different mass spectrometers.

Shape resonances in slow electron scattering by aromatic molecules. I. Anthraquinone derivatives

A series of anthraquinone (C(14)O(2)H(8)) derivatives has been studied by means of electron capture negative ion mass spectrometry (ECNI-MS), photoelectron spectroscopy (PES), and AM1 quantum chemical calculations. Mean lifetimes of molecular negative ions M(-.) (MNI) have been measured. The mechanism of long-lived MNI formation in the epithermal energy region of incident electrons has been investigated. A simple model of a molecule (a spherical potential well with the repulsive centrifugal term) has been applied for the analysis of the energy dependence of cross sections at the first stage of the electron capture process. It has been shown that a temporary resonance of MNI at the energy approximately 0.5 eV corresponds to a shape resonance with lifetime 1-2.10(-13) s in the f-partial wave (l = 3) of the incident electron. The next resonant state of MNI at the energy approximately 1.7 eV has been associated with the electron excited Feshbach resonance (whose parent state is a triplet npi* transition). In all cases the initial electron state of the MNI relaxes into the ground state by means of a radiationless transition, and the final state of the MNI is a nuclear excited resonance with a lifetime measurable on the mass spectrometry timescale. Copyright 1999 John Wiley & Sons, Ltd.

Internal conversion as the main stabilization mechanism for long-lived negative molecular ions

The temperature dependence of the mean lifetime of Ph-N=N-Ph− azobenzene negative molecular ions on the captured electron energy is studied with a static mass spectrometer by the method of resonance electron capture. A family of respective experimental dependences is calculated accurate to 2–10%. It is shown that the molecular anions in the epithermal electron energy range can be stabilized through internal conversion, namely, a series of fast radiationless transitions without change in the multiplicity.

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.

Low Energy Electron Attachment by Haloalkanes

Electron attachment process in three halopropanes, CH3CH2CH2Br, CH3CHBrCH3 and CH3CHBrCF3, have been investigated using an electron swarm and electron capture negation mass spectrometry techniques. All compounds attach electrons in dissociative process, and the main product is bromine ion. The rate constants for thermal electron attachment are equal to 1.1·10−1, 1.4·10−12 and 4.1·10−10 cm3molec.−1 s−1 for CH3CH2CH2Br, CH3CHBrCH3 and CH3CHBrCF3, respectively.

Application of the united atom model for estimating the lifetime of negative molecular ions relative to electron autodetachment

A model proposed for describing the scattering of low-energy electrons (whose energy ranges between thermal energy and several electronvolts) from polyatomic molecules makes it possible to estimate the lifetime of shape resonances. The parameters of the model are determined by specific structural and experimental characteristics of molecules. The results of approximate computations of the lifetimes for negative ions of molecules with different symmetries (diatomic halogens, parabenzoquinone, fullerene C60, benzothiadiazoles, anthraquinone derivatives, and substituted benzene forms) are presented. The obtained data show that the lifetimes are sufficient for the formation of fragment ions observed in the mass spectra of negative ions.

Frozen shell approximation violation in negative ion formation from halogenated benzenes via dissociative attachment

A series of benzene derivatives (R(1)C(6)H(4)R(2)) has been studied by means of electron capture negative ion mass spectrometry (ECNI-MS), and PM3 quantum chemical calculations. The dissociation channel M(-.) --> Hal(-) + (M - Hal). is analysed from the point of view of symmetry conservation. Generally, a symmetry ban on dissociation may be avoided in at least two ways: (i) out-of-plane vibrations of the halogen atom in the molecular negative ion (MNI), mixing pi- and sigma-states of the anion; (ii) symmetrical in-plane vibration of the C-Hal bond, changing the order of the empty levels in the MNI with subsequent radiationless conversion into a sigma-state. Our analysis shows that neither of them provides a satisfactory explanation of the ECNI mass spectra for chlorobenzene, if one retains the usual assumption that an additional electron goes into the LUMO of the neutral molecule. Thus, it may be concluded that in this case electron capture causes a significant perturbation of the energy ordering of vacant orbitals, thus making the frozen shell approximation inapplicable. Copyright 2000 John Wiley & Sons, Ltd.

Interpreting electron transmission spectroscopy and negative ion mass spectrometry data using a spherical potential well model

Experimental data obtained using electron transmission spectroscopy and negative ion mass spectrometry based on resonance electron capture are interpreted within the framework of a spherical potential well model in application to a series of chloro-and bromoalkane molecules. Allowance for the scattering of a single partial p-wave of the incoming electron makes possible (i) reproduction of the ratio of a resonance peak width to the electron energy observed in the electron transmission spectra and (ii) establishment of a relation between the total cross section of electron scattering on a molecule and the dissociative electron attachment cross section. The proposed model offers a radical simplification of the approach developed previously based on the Fashbach-Fano resonance theory.