Dissociative electron capture by sulfide,sulfoxide, and sulfone molecules

Conclusions The low energy resonance in the negative ion mass spectra of alkyl vinyl, 2,3-epoxypropyl alkyl, and 2, 3-epithiopropyl alkyl sulfies and their sulfoxide and sulfone analogs is related to electron capture in the ^* orbital of the S-C bond. The oxidation state of the sulfur atom has no qualitative effect on the major processes involved in the formation of the negative ions.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 7, 1653–1657, July, 1984.     

Radiative relaxation and fragmentation dynamics of S 2p-excited hydrogen sulfide

Radiative relaxation of S 2p-excited hydrogen sulfide (H(2)S) is investigated by dispersed ultraviolet and visible fluorescence spectroscopies. We observe distinct changes in the fluorescence spectra as a function of excitation energy. Excitation to Rydberg states below the S 2p ionization threshold yields intense fluorescence from neutral and ionic atomic fragments (H, S(+), and S(2+)). In addition to the atomic emission, fluorescence of the molecular fragment ion HS(+) is preferably found after excitation of the S 2p electron into the unoccupied 6a(1) and 3b(2) orbitals with sigma(*) character. This is interpreted as evidence for ultrafast dissociation of the core-excited molecule prior to electronic relaxation. The rotationally resolved fluorescence spectra of the A (3)Pi-->X (3)Sigma(-) transition are analyzed in terms of the fragmentation dynamics leading to the formation of the excited molecular fragment ion, where changes in bond angle are discussed in terms of the rotational population.     

Dissociative electron capture of halocarbon caused by the internal electron transfer from water trimer anion

Dissociative electron capture dynamics of halocarbon absorbed on water cluster anion, caused by internal electron transfer from the water trimer anion to the halocarbon, have been investigated by means of the direct density functional theory (DFT)-molecular dynamics (MD) method. The CF(2)Cl(2) molecule and a water trimer anion e(-)(H(2)O)(3) were used as a halocarbon and a trapped electron, respectively. First, the structure of trapped electron state, expressed by e(-)(H(2)O)(3)-CF(2)Cl(2), was fully optimized. The excess electron was trapped by a dipole moment of water trimer. Next, initial geometries were randomly generated around the equilibrium point of the trapped electron state, and then trajectories were run. The direct DFT-MD calculations showed that the spin density distribution of excess electron is gradually changed from the water cluster (trapped electron state) to CF(2)Cl(2) as a function of time. Immediately, the Cl(-) ion was dissociated from CF(2)Cl(2)(-) adsorbed on the water cluster. The reaction was schematically expressed by e(-)(H(2)O)(3)-CF(2)Cl(2)-->[(H(2)O)(3)-->-CF(2)CL(2)](-) --> (H(2O)(3) + CF(2)CL + CI(-) (I) where [(H(2)O)(3)-CF(2)Cl(2)](-) indicates a transient intermediate state in which the excess electron is widely distributed on both the water cluster and CF(2)Cl(2). The mechanism of the electron capture of halocarbon from the trapped electron in water ice was discussed on the basis of the theoretical results. Also, the dynamics feature was compared with those of the direct electron capture reactions of CF(2)Cl(2) and CF(2)Cl(2)-(H(2)O)(3), i.e. e(-) + CF(2)Cl(2), and e(-) + CF(2)Cl(2)-(H(2)O)(3), investigated in our previous paper [Tachikawa and Abe, J. Chem. Phys., 2007, 126, 194310].     

Formation of negative hydrogen ion: Polarization electron capture and nonthermal shielding

The influence of the nonthermal shielding on the formation of the negative hydrogen ion (H(-)) by the polarization electron capture are investigated in partially ionized generalized Lorentzian plasmas. The Bohr-Lindhard method has been applied to obtain the negative hydrogen formation radius and cross section as functions of the collision energy, de Broglie wave length, Debye length, impact parameter, and spectral index of the plasma. The result shows that the nonthermal character of the plasma enhances the formation radius of the negative hydrogen, especially, for small Debye radii. It is found that the nonthermal effect increases the formation cross section of the negative hydrogen. It is also found that the maximum position of the formation cross section approaches to the collision center with an increase of the spectral index. In addition, it is found that the formation cross section significantly decreases with an increase of the Debye length, especially, for small spectral indices.     

Recombination of silicon ions by electron capture from atomic hydrogen and helium

 Ab initio potential-energy curves and coupling matrix elements of the Σ and Π molecular states involved in the collision of the Si²⁺, Si³⁺ and Si⁴⁺ multicharged ions on atomic hydrogen and helium have been determined by means of configuration interaction methods. The total and partial electron capture cross sections have been determined using a semiclassical or a quantal approach in the 0.002–0.1 au velocity range. A detailed comparison with very recent theoretical and experimental rate coefficient results is made. Received: 14 September 1999 / Accepted: 3 February 2000 / Published online: 2 May 2000     

Dissociative electron capture by 1,3-dithiolanes and 1,3-dioxalanes

A study was carried out on dissociative electron capture by 1,3-dithiolanes and 1,3-dioxalanes. The mechanisms for the fragmentation pathways of the negative ions differ significantly between the oxygen- and sulfur-containing compounds. The (M-H)^– ions are formed with excess internal excitation energy.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 1, pp. 208–210, January, 1990.     

Dissociative excitation and fragmentation of S8 by electron impact

The vacuum-ultraviolet emission spectrum from 136 nm to 168 nm following the dissociative excitation of a predominantly S(8) target by electron impact at 100 eV incident energy was measured. The relative cross sections for the dominant multiplets at 138.9, 142.9, 147.9, and 166.7 nm are presented. Excitation functions are shown for electron-impact energies from below threshold to 360 eV for the two most prominent emissions at 142.5 nm and 147.4 nm. Five thresholds are clearly apparent in both excitation functions. For the four highest energy channels, the energy separation between the adjacent thresholds is approximately constant and the cross sections reduce regularly as the threshold energies increase. We suggest possible fragmentation pathways of the dissociating S(8) molecule that reproduce the energies of our observed thresholds.     

Dissociative recombination of electrons with molecular hydrogen ions

To a first approximation, the perturbation theory yields an explicit analytical expression for the cross section of the dissociative recombination of electrons with molecular hydrogen ions. The possible nonadiabatic transitions during the separation of the nuclei which lead to the appearance of H⁺+H^–, H(1s)+H(n=2), H(1s)+H(n=3) in finite reaction channels were considered. Numerical results are presented for the cross sections of direct and reverse reactions. The expression =4. 2 · 10^–8 T^–1/2 cm³/sec² was obtained for the recombination rate at low temperatures; this expression is in agreement with known results. Several general details of the calculation and their possible implications for the case of heavy molecular ions are discussed.     

Dissociative electron capture and level width of quasistationary molecular states

Perturbation theory is applied to give the cross section for dissociative capture e+A₂A+A^–. From the requirement of quantitative agreement with experiment, the level widths for the quasistationary states are estimated as ₁1.2 eV; ₂0.2 eV; ₃0.05 eV for the first three terms for H ₂ ^– .Visiting worker from the Boris Kidri Institute of Nuclear Sciences, Belgrade.     

Long-lived electron capture dissociation product ions experience radical migration via hydrogen abstraction

To explore the mechanism of electron capture dissociation (ECD) of linear peptides, a set of 16-mer peptides were synthesized with deuterium labeled on the alpha-carbon position of four glycines. The ECD spectra of these peptides showed that such peptides exhibit a preference for the radical to migrate to the alpha-carbon position on glycine via hydrogen (or deuterium) abstraction before the final cleavage and generation of the detected product ions. The data show c-type fragment ions, ions corresponding to the radical cation of the c-type fragments, c*, and they also show c*-1 peaks in the deuterated peptides only. The presence of the c*-1 peaks is best explained by radical-mediated scrambling of the deuterium atoms in the long-lived, metastable, radical intermediate complex formed by initial electron capture, followed by dissociation of the complex. These data suggest the presence of at least two mechanisms, one slow, one fast. The abundance of H* and -CO losses from the precursor ion changed upon deuterium labeling indicating the presence of a kinetic isotope effect, which suggests that the values reported here represent an underestimation of radical migration and H/D scrambling in the observed fragments.     

Charge remote fragmentation in electron capture and electron transfer dissociation

Secondary fragmentations of three synthetic peptides (human αA crystallin peptide 1-11, the deamidated form of human βB2 crystallin peptide 4-14, and amyloid β peptide 25-35) were studied in both electron capture dissociation (ECD) and electron-transfer dissociation (ETD) mode. In ECD, in addition to c and z· ion formations, charge remote fragmentations (CRF) of z· ions were abundant, resulting in internal fragment formation or partial/entire side-chain losses from amino acids, sometimes several residues away from the backbone cleavage site, and to some extent multiple side-chain losses. The internal fragments were observed in peptides with basic residues located in the middle of the sequences, which was different from most tryptic peptides with basic residues located at the C-terminus. These secondary cleavages were initiated by hydrogen abstraction at the α-, β-, or γ-position of the amino acid side chain. In comparison, ETD generates fewer CRF fragments than ECD. This secondary cleavage study will facilitate ECD/ETD spectra interpretation, and help de novo sequencing and database searching.     

Dissociative excitation of water by electron impact

Absolute emission cross sections and threshold energies have been measured for radiation (1850–9000Å) from excited fragments (OH, O and H) produced by electron impact (0–1000 eV) on water vapour. The results are compared with previous experiments and the discrepancies are discussed. The measurements indicate that hydroxyl radicals excited in the A² ∑⁺ state originate from excitation of both singlet and triplet states of the water molecule. Excited atomic fragments arise partly from predissociation of Rydberg states of the water molecule converging to the third ionization potential.     

Dissociative double ionization of CO2: dynamics, energy levels, and lifetime

In a kinematically complete experiment on the dissociative double ionization of CO2 by electron impact, spontaneous and metastable decay have been observed via the channel CO2(2+) --> CO+ + O+. The metastable decay shows a lifetime of 5.8 +/- 1.5 micros. The measured kinetic energy release spectrum of the dissociation shows one broad peak. To understand the observed features, ab initio potential energy surface (PES) for the ground electronic state of CO2(2+) was computed using a multireference configuration interaction method and a correlation-consistent polarized-valence quadruple-zeta basis set, for a range of internuclear distances and O-C-O bond angles, and an analytic fit of the PES was obtained. The computed PES clearly indicates the metastability of the dication and yields a barrier height and an asymptotic limit in fair agreement with the reported data. A time-dependent quantum mechanical approach was used to compute the ground vibrational state wave function of CO2 in its ground electronic state. Assuming a Franck-Condon transition, the same function was taken to be the initial wave function at time t = 0 for the time evolution on the fitted PES for the ground electronic state of CO2(2+). The autocorrelation function was computed and Fourier transformed to obtain the excitation spectrum. Upon convolution with the instrument resolution function, the kinetic energy release spectrum was obtained, in good agreement with the experimental results, particularly at lower energies. The discrepancies at higher energies are attributed to the noninclusion of the excited states of CO2(2+) in the dynamical study.     

Correlation between electron capture negative chemical ionization mass spectrometric fragmentation and calculated internal energies for polychlorinated biphenyls

Correlationbse tween molecular structure and fragmentation observed in electron capture negative chemical ionization mass spectra (moderator gas = methane) of 49 selected tetrachlorinated, pentachlorinated, and hexachlorinated biphenyls have been investigated by using molecular modeling. The semiempirical general molecular orbital program MOPAC was used to calculate molecular properties for biphenyl and the 209 polychlorinated biphenyls. The mass spectrometric ionization and fragmentation processes were found to be linked to the number of chlorine atoms present on the biphenyl, and to the number of those chlorine atoms in the ortho (2, 2′, 6, and 6′) positions. The intensity of molecular ions increased with the number of chlorine atoms present, but this was counteracted by enhanced fragmentation as the number of ortho position chlorine atoms increased. The molecular parameters that were most closely linked with the number of ortho chlorine atoms were the twist angle between the phenyl rings and the energy of the lowest unoccupied molecular orbital (LUMO). It is suggested that fragmentation occurs when the energy of the ionizing electron exceeds the energy difference between the LUMO and LlJMO + 1 orbitals.     

The reactions between negative hydrogen ions and silane

Contracted CI-calculations have been performed in order to find out the mechanisms of the reactions involved when negative hydrogen ions react with silane. There were initially severe problems to find a balanced basis set to describe the reactions including correlation, particularly for the choice of diffuse functions. Finally, in agreement with earlier calculations, SiH ₅ ^– was found to be more stable than SiH₄+H^– by 21 kcal/mol but less stable than SiH ₃ ^– and H₂ by 6 kcal/mol. A barrier in the S _N2 reaction SiH₄+H^– SiH₅ has previously been predicted by calculations, which was not confirmed by the present CI calculations. The lack of a barrier is in agreement with experimental evidence. Contrary to what is expected from the orbital symmetry rules, which predict two allowed pathways, SiH ₅ ^– does not dissociate easily to the lower lying SiH ₃ ^– + H₂. A barrier of 57 kcal/mol, which was very difficult to locate, was finally found. In order to explain the experimental observation of SiH ₃ ^– and the lack of observation of SiH ₅ ^– a different mechanism for the reaction SiH₄+H^– SiH ₃ ^– + H₂ is suggested. For a direct proton transfer a barrier of less than 10 kcal/mol is predicted.     

Hydrogen sulfide conversion: How to capture hydrogen and sulfur by photocatalysis

H₂S is a notorious gas widely generated in the petrochemical industry. How to handle H₂S effectively and convert it into highly-valued products is vital. Photocatalysis is promising in this field, as it could directly utilize solar light and convert H₂S into H₂ and S. In this review, the properties of hydrogen sulfide (H₂S) is overviewed first, and conventional techniques (Claus process, thermolysis, non-thermal plasma, electrochemistry and other methods) for H₂S conversion are simply introduced. Basic knowledge of photocatalysis and general strategies for enhancing the activities of photocatalysts are presented as well. Then typical work for photocatalytic conversion of H₂S in gas phase and liquid phase are introduced case by case, with the generated H₂ as the main product in these systems. Furthermore, methods for extraction of elemental sulfur from H₂S by photocatalysis-related methods were discussed, with specific attention on photoelectrochemical cells and photovoltaic-electrochemical cells. In the end, current status of the research on photocatalytic conversion of H₂S is summarized, and challenges in this field is put forward. In addition, some other possible strategies for photocatalytic conversion of H₂S into highly-valued chemicals instead of hydrogen and elemental sulfur will be discussed, which is aimed to inspire researchers interested in this field.     

Investigating the breakup dynamics of dihydrogen sulfide ions recombining with electrons

This paper presents results concerning measurements of the dissociative recombination (DR) of dihydrogen sulfide ions. In combination with the ion storage ring CRYRING an imaging technique was used to investigate the breakup dynamics of the three-body channel in the DR of 32SD2(+). The two energetically available product channels S(3P) + 2D(2S) and S(1D) + 2D(2S) were both populated, with a branching fraction of the ground-state channel of 0.6(0.1). Information about the angle between the two deuterium atoms upon dissociation was obtained together with information about how the available kinetic energy was distributed between the two light fragments. The recombination cross sections as functions of energy in the interval of 1 meV to 0.3 eV in the center-of-mass frame are presented for 34SH2(+). The thermal rate coefficient for the DR of 34SH2(+) was determined to be (4.8+/-1.0) x 10(-7)(T/300)(-0.72+/-0.1) cm3 s(-1) over this interval.     

Towards A qualitative rationalisation of the fragmentation reactions of organic negative molecular ions

A set of axioms is formulated which provides a means for the qualitative rationalization and prediction of the fragmentation modes of organic molecular anions.     

Charge stripping reactions of ions formed from methane,ammonia, water and hydrogen sulphide by protonation and by electron impact

The minimum energy, Q_min, necessary to convert an ion m₁⁺ to a doubly charged ion m₁²⁺ is obtained for 19 different ions from methane, ammonia, water and hydrogen sulphide by charge stripping using nitrogen collision gas. The ions studied include the [MH]⁺ ions formed by chemical ionization in a high pressure source. Stable m₁²⁺ ions could not be formed in the case of [NH₄]⁺, [NH]^+˙, [H₂O]^+˙ and [OH]⁺. Even in these cases the value of Q_min could be estimated by studying the fragments formed from the unstable m₂₊ ions. In several cases, the energy required to form m₁²⁺ ions is less than the literature value for the ionization energy of m₁⁺. This is discussed in terms of the possibility of the presence of excited states of m₁⁺ in the present experiments.