Dissociative Electron Ionization Research Articles

Theoretical Investigation of Electron Impact Scattering on Imidazole.

This study presents the results of electron scattering calculations on a biologically important molecule, imidazole, using the UK molecular R-matrix method. The R-matrix calculations are performed using SE, SEP, and CC models, and the resonance detected in the present SEP model is found to be in better agreement with available experimental data than previous theoretical data. The study also reports an inelastic scattering cross section, which comprises dissociative electron attachment (DEA), excitation, and ionization cross section, for the first time. The total scattering cross sections are also reported for the first time. We confirm the presence of the two well-known π* shape resonances predicted earlier experimentally. Due to the scarcity of total scattering cross section (TCS) data for imidazole, we have compared the TCS of imidazole with its isoelectronic target isoxazole and drawn important conclusions. A comparison among the resonances of imidazole with those of isoxazole helps us to conclude that electron attachment to π* molecular orbitals is a general feature displayed by these five-membered heterocyclic compounds. The comprehensive electron scattering studies presented in this work are expected to provide a deeper understanding of electron-induced biochemical processes and fill gaps in the available data. Furthermore, this study is anticipated to inspire further investigations on imidazole and other five-membered heterocyclic ring molecules, which have significant applications in medicine.

C-I and C-F bond-breaking dynamics in the dissociative electron ionization of CF3I.

We present a comprehensive experimental study into the dissociative electron ionization dynamics of CF3I at energies ranging from 20 to 100 eV. A beam-gas instrument has been used to measure the absolute total ionization cross-section for the molecule over the energy range from 0 to 300 eV. Coupled with data from an electron-molecule crossed beam velocity-map imaging instrument, this allows absolute partial ionization cross-sections to be determined for formation of each ionic fragment. These reveal a number of fragmentation channels involving both C-I and C-F bond cleavage, in some cases followed by further fragmentation of the resulting molecular ion. Velocity-map images have been recorded for the I+ and CF3+ products of C-I bond cleavage and the CF2I+ products of C-F bond cleavage. Analysis of fragment kinetic energy distributions extracted from the images reveals that CF3+ product of C-I bond cleavage appears to be formed via a statistical mechanism occurring over long timescales, while the CF2I+ products of C-F cleavage are formed via a much faster, more direct dissociation mechanism involving one or more repulsive states of the parent molecular ion. The I+ fragments arising from C-I bond cleavage display behaviour intermediate between the two extremes. For all fragments, the images show little or no dependence on the energy of the incident electron, implying that the initially excited ion state or states undergo rapid relaxation to the dissociative state(s) in all cases. Only a very small fraction of the incident electron's kinetic energy is released into kinetic energy of the recoiling atomic and molecular fragments, implying that most of the available energy remains with the two departing electrons. The kinetic energy distributions obtained for the various fragments of dissociative electron ionization are compared with the corresponding distributions reported from photoionization studies in order to gain insight into the electronic states involved.

Spectroscopic Characterization of the Product Ions Formed by Electron Ionization of Adamantane.

A structural characterization of the products formed in the dissociative electron ionization of adamantane (C10H16) is presented. Molecular structures of product ions are suggested based on multiple‐photon dissociation spectroscopy using the Free Electron Laser for Infrared eXperiments (FELIX) in combination with quantum‐chemical calculations. Product ions are individually isolated in an ion trap tandem mass spectrometer and their action IR spectra are recorded. Atomic hydrogen loss from adamantane yields the 1‐adamantyl isomer. The IR spectrum of the C8H11 + product ion is best reproduced by computed spectra of 2‐ and 4‐protonated meta‐xylene and ortho‐ and para‐protonated ethylbenzenes. The spectrum of the product ion at m/z 93 suggests that it is composed of a mixture of ortho‐protonated toluene, para‐protonated toluene and 1,2‐dihydrotropylium, while the spectrum of the m/z 79 ion is consistent with the benzenium ion. This study thus suggests that adamantane is efficiently converted into aromatic species and astrophysical implications for the interstellar medium are highlighted.

Electron-impact-ionization dynamics of SF6

A detailed understanding of the dissociative electron ionization dynamics of $\mathrm{S}{\mathrm{F}}_{6}$ is important in the modeling and tuning of dry-etching plasmas used in the semiconductor manufacture industry. This paper reports a crossed-beam electron ionization velocity-map imaging study on the dissociative ionization of cold $\mathrm{S}{\mathrm{F}}_{6}$ molecules, providing complete, unbiased kinetic energy distributions for all significant product ions. Analysis of these distributions suggests that fragmentation following single ionization proceeds via formation of $\mathrm{S}{{\mathrm{F}}_{5}}^{+}$ or $\mathrm{S}{{\mathrm{F}}_{3}}^{+}$ ions that then dissociate in a statistical manner through loss of F atoms or ${\mathrm{F}}_{2}$, until most internal energy has been liberated. Similarly, formation of stable dications is consistent with initial formation of $\mathrm{S}{{\mathrm{F}}_{4}}^{2+}$ ions, which then dissociate on a longer time scale. These data allow a comparison between electron ionization and photoionization dynamics, revealing similar dynamical behavior. In parallel with the ion kinetic energy distributions, the velocity-map imaging approach provides a set of partial ionization cross sections for all detected ionic fragments over an electron energy range of 50--100 eV, providing partial cross sections for ${\mathrm{S}}^{2+}$, and enables the cross sections for $\mathrm{S}{{\mathrm{F}}_{4}}^{2+}$ from $\mathrm{S}{\mathrm{F}}^{+}$ to be resolved.

Electron ionization cross sections for the PH3 molecule

The partial single, double differential cross-sections with their total (sum of partial cross sections) of the phosphine (PH3) by direct and dissociative electron ionization have been evaluated by using modified Jain–Khare semi-empirical approach. To the best of my knowledge, no other data (experimental and/or theoretical) of differential cross sections is available till now for comparison. Partial and total integral ionization cross sections were also evaluated for PH3. Integral ionization cross-sections show good agreement with available experimental/or theoretical data. Ionization rate coefficients corresponding to partial ionization cross-sections have also been calculated by the Maxwell–Boltzmann distribution of energy.

The effect of N-heteroatoms and CH 3 substituents on dissociative electron–ion recombination of protonated single six membered ring compounds at room temperature

Electron–ion dissociative recombination rate constants have been determined for protonated single six-membered hydrocarbon rings with varying N-atom substitution in the rings and varying degrees of methyl substitutions to the rings. The species studied have been protonated forms of the xylenes (C 8H 10; with configurations o, m, and p), the picolines (C 6H 7N; with methyl substitutions located at 2, 3, and 4), mesitylene (C 9H 12) and 2, 5-lutidine (C 7H 9N). By operating at high reactant vapor pressures, ternary association has been made to dominate over recombination to create proton bound dimers and the rate constants of these species have been determined. The studies were made at room temperature in a flowing afterglow with a Langmuir probe to determine the reduction in electron density as a function of distance along the flow tube. All of the data except for mesitylene showed a consistent trend with the numbers of N-atom substitutions and CH 3 attached to the rings. For the protonated species, the rate constants increase with the number of nitrogens and decreased with number of CH 3 substituents attached to the ring. For proton bound dimers, the rate constants increase with both, the number of N-atoms and number of CH 3 substituents. For the xylene and picoline isomer's the measurements showed that the rate constants were independent of isomeric form, and thus, it is not necessary to study all isomeric forms. The relevance of these studies to the ionosphere of Titan is discussed.

Effect of isotopic content on the rate constants for the dissociative electron–ion recombination of N 2H +

The rate constants, α e, for dissociative electron–ion recombination of various isotopic combinations of N 2H + have been determined at 300 and 500 K in a variable temperature flowing afterglow using a Langmuir probe to determine the electron density. The values of α e at 300 K are 2.77 ( 14N 2H +), 2.12 ( 14N 2D +), 2.31 ( 15N 2H +) and 1.98 ( 15N 2D +) × 10 −7 cm 3 s −1. The equivalent values at 500 K are 2.84, 2.33, 2.93, and 2.33 respectively. This has shown that the greatest change occurs between H and D substitution α e( 14/15N 2D +)/ α e( 14/15N 2H +) ∼ 0.765/0.857 at 300 K and 0.820/0.795 at 500 K respectively. Values at 500 K are consistently larger than at 300 K. At both temperatures, the rate constants with H substitution are larger than with D substitution for both 14N and 15N. Values with 14N substitution are larger than with 15N for both H and D. At 500 K, the values are independent of whether the ion contains 14N or 15N. Gas-phase 15N fractionation enhancement is predicted in many regions of the interstellar medium including cold and pre-stellar cores. The effect of this fractionation on the recombination process is investigated. The relevance to storage ring measurements of recombination rate constants is considered and applications to the chemistry of the interstellar medium are discussed.

Influence of the structure of medium-sized aromatic precursors on the reactivity of their dications towards rare gases

The gas-phase reactivity of dications generated by dissociative electron ionization of several aromatic C m H n N o precursors with 4 ≤ m ≤ 13, 4 ≤ n ≤ 21, and 0 ≤ o ≤ 2 with rare gases is investigated. Whereas most of these reactions lead to monocations via simple electron transfer, proton transfer, or Coulomb explosion, the formation of organo rare-gas dications is observed in a few cases. Specifically, dications generated from 2,4,6-trimethylpyridine react with krypton and xenon to form organo rare-gas species as major products and under maintenance of the two-fold positive charge. Such a reactivity is not observed in the presence of lighter rare gases. The formation of organo rare-gas dications are also observed for dications generated from 3-vinylpyridine, N, N-dimethylaniline, isopropylbenzene, and 4-ethyltoluene as neutral precursors. In some cases, isomeric dications are characterized by very different reactivity toward rare gases, suggesting that the structure of the precursors is crucial and that electron ionization does not lead to a total scrambling of the structures of the doubly charged ions obtained.

Electron ionization of H2O

Relative partial ionization cross-sections and precursor-specific relative partial ionization cross-sections for fragment ions formed by electron ionization of H2O have been measured using time-of-flight mass spectrometry coupled with a 2D ion coincidence technique. We report data for the formation of H+, H2+, O2+, O+ and OH+ relative to the formation of H2O+, as a function of ionizing electron energy from 30 to 200eV. This data includes, for the first time, measurements on the formation all positive ion pairs and ion triples by dissociative multiple electron ionization of H2O. Through determinations of the kinetic energy release involved in ion pair formation we provide further evidence that indirect processes contribute significantly to the yield of H++OH+ ion pairs below the vertical double ionization threshold.

Differential and partial ionization cross sections for electron impact ionization of plasma processing molecules: CF4 and PF5

The semiempirical approach that requires the oscillator strength data as input has been employed to evaluate the single and double differential cross sections corresponding to the production of various cations, CFx+ (x=0–3), F+, CF2++ and CF3++ from CF4 and PFx+ (x=0–4), F+ and PFx++ (x=0–4) from the PF5 molecule, through dissociative electron ionization processes. The calculations are made as a function of secondary electron energy and angle at fixed incident electron energies of 100 and 200 eV. The corresponding partial and total ionization cross sections have also been derived in the energy range varying from ionization thresholds to 1000 eV. Comparison of the evaluated partial and total ionization cross sections is made with the experimental and theoretical data wherever available. The formula has been extended to the calculation of ionization rate coefficients as a function of electron temperature/energy using the Boltzmann energy distribution function.

Ionic and neutral mercaptothiocarbonyl: A tandem mass spectrometry and computational study

Dissociative electron ionization (70 eV) of ethyl carbomylmethanedithioate generates m/ z 77 ions of composition [H, C, S 2] +. The collision-induced dissociation (CID) mass spectrum of these ions is consistent with the mercaptothiocarbonyl connectivity HSCS + ( a +). Calculations predict that some of the a + ions may equilibrate with HC(S)S + ( b) + ions prior to dissociation. Neutralization-reionization mass spectrum (NRMS) of these ions confirmed that neutral HSCS is stable in the rarefied gas phase. The relative, dissociation, and isomerization energies for the ions and neutrals studied at B3LYP/6-311G(d,p) and G2/G2(MP2) levels, are used to support and interpret the experimental results.

C 2H 2S radical cations: Application of tandem mass spectrometry methodologies

Attempts have been made to characterize ionic and neutral C 2H 2S +/o isomers, viz., thioketene ( 1 +), ethynylthiol ( 2 +), and thiirene ( 3 +) using various tandem mass spectrometric methodologies. Dissociative electron ionization of thiophene, 1-propane thiol and thiirane were used to produce the C 2H 2S + ( m/ z 58) ions. The collisional activation (CA) mass spectrum of m/ z 58 ions from thiophene is highly compatible with thioketene ions which has been supported by the CA mass spectrum of metastably generated m/ z 58 ions and by comparison with the previously reported CA mass spectrum of thioketene ions. Theoretical calculations at the B3LYP/6-311G(d,p), and G2/G2(MP2) levels predict that low energy thioketene ions equilibrate with other isomeric ions viz., ethynylthiol and thiirene prior to dissociation by loss of H ( m/ z 57) and CH ( m/ z 45). The CA spectra of C 2H 2S + ions from 1-propane thiol and thiirane are nearly identical indicating a facile isomerization of the initial structures at energies lower than the dissociative threshold. The low energy CID spectra of the m/ z 58 ions from three precursors are found to be similar. However, significant differences are noticed in the relative abundances of product ions in the NR spectra of m/ z 58 ions from the three precursors. While the NR mass spectra of m/ z 58 ions from thiophene and thiirane confirms the stability of thioketene neutrals, the NR spectra of m/ z 58 ions from 1-propane thiol suggests formation of a mixture of ethynylthiol and thioketene ions. Calculations predict a different thermo-chemistry for 1 +– 3 +, but, due to a mixture of reacting isomeric ions whatever the precursor molecule is, similar results were obtained for the associative ion/molecule reactions between the C 2H 2S + ions with methylisocyanide.

The afterglow mystery of pulsed glow discharges and the role of dissociative electron–ion recombination

It is generally recognized that excited level populations and, hence, optical emission intensities in pulsed glow discharges exhibit a peak upon pulse termination, i.e., in the so-called afterglow. This afterpeak formation is attributed in many papers to electron–ion recombination, but up to now this hypothesis could not be confirmed quantitatively by numerical modelling, because of too low electron and ion number densities and recombination rate coefficients. In the present paper, we show the calculation results of a model, which includes also Ar2+ ions, beside the Ar+ ions, and which takes into account the thermalization of the electrons upon pulse termination, yielding higher recombination rate coefficients. The role of electron–Ar2+ dissociative recombination and electron–Ar+ three-body recombination as afterpeak formation mechanisms is investigated. Our study clearly shows the important role of Ar2+ ions and dissociative recombination in the afterpeak formation in the afterglow of pulsed discharges.

Generation and characterization of ionic and neutral chloro(hydroxy) phosphanyl [Cl–P–OH] +/[rad] and chloro(thiohydroxy) phosphanyl [Cl–P–SH] +/[rad] in the gas phase by tandem mass spectrometry and computational chemistry

Dissociative electron ionization (70 eV) of diethyl chlorothiophosphate ( I) and ethylphosphonodichloridate ( II) generates moderately abundant m/ z 83 ions of composition [P, Cl, O, H] +. From tandem mass spectrometry experiments and theoretical calculations at the B3LYP/6-31G(d, p), G2, and G2(MP2) levels it is concluded that the majority of the ions have the structure of ClPOH + ( 1a +) and it is separated by high-energy barriers from its isomers HP(O)Cl + ( 1b +), HPOCl + ( 1c +), and OP–ClH ( 1d +). Neutralization–reionization experiments confirm the theoretical prediction that radical 1a is a stable species in the gas phase. The 70 eV electron ionization of ethylphosphonothioic dichloride ( III) yields m/ z 99 ions, predominantly of the structure ClPSH + ( 2a +). This conclusion follows from tandem mass spectrometry experiments and theoretical calculations. The calculations predict that ( 2a +) is separated by high-energy barriers from its isomers HP(S)Cl + ( 2b +), HPSCl + ( 2c +), and SP–ClH ( 2d +). Neutralization–reionization experiments confirmed that 2a radical is a stable species in the rarefied gas phase, which is in agreement with theoretical calculations.

Reactivity of ferrocenylalkylazoles under conditions of dissociative ionization

Electron ionization mass spectra of biologically active ferrocenylalkylazoles CpFeC5H4CH(R)Az (R = H, Me, Et, Pr, or Ph; AzH is pyrazole, imidazole, indazole, benzoimidazole, benzotriazole, adenine, or their derivatives) were studied. The nature of the heterocyclic fragment (its ionization energy) is the main factor determining the reactivity of these compounds under conditions of dissociative electron ionization.

Selenoketene (H2CCSe)+• and selenoketyl cumulene (HCCSe)+ ions and their neutral counterparts: a tandem mass spectrometric and computational study

AbstractDissociative electron ionization (70eV) of selenophene (C4H4Se) generates m/z 106 ions of composition [H2, C2, 80Se]+• and m/z 105 ions of [H, C2, 80Se]+. From tandem mass spectrometric experiments, Density Functional Theory (DFT) and ab initio calculations, it is concluded that these ions have the structure of selenoketene H2CCSe+• (1a+• )and selenoketyl HCCSe+ (2a+) ions respectively. The calculations predict that selenoketene ion 1a+• is separated by high energy barriers from its isomers selenirene (H$\catcode`@=11 \def\overlsqmatrix#1{\null\vbox{\normalbaselines\m@th \ialign{\hfil$##$\hfil&&\quad\hfil$##$\hfil\crcr \mathstrut\crcr\noalign{\kern-\baselineskip} #1\crcr\mathstrut\crcr\noalign{\kern-\baselineskip}}}} \newdimen\rulwidth \def\downsqbrace#1{\setbox1=\hbox{#1} \rulwidth=\wd1\advance \rulwidth by -6pt \raise3pt\hbox{$\overlsqmatrix{\vrule height5pt\kern-0.5pt\raise5pt\hbox to \rulwidth{\hrulefill}\kern-0.5pt\vrule height5pt\cr\noalign{\vskip-3pt}\hbox{#1}\cr\noalign{\vskip-5pt}}$}} \catcode`@=12 \downsqbrace{{\rm CCHS}}$e)+• 1b+•, ethyne selenol (HCCSeH)+• 1c+•, (CCHSeH)+• 1d+• and (CCSeH2)+• 1e+•. The selenoketyl ion 2a+ is separated by high barriers from its isomers (CCHSe)+ 2b+, and (CCSeH)+ 2c+. Neutralization‐reionization mass spectra (NRMS) of these structurally characterized ions confirmed that the corresponding neutral analogues, selenoketene H2CCSe 1a and selenoketyl radical HCCSe 2a• are stable in the rarefied gas phase. The relative, dissociation, and isomerization energies for selenoketene and selenoketyl ions and neutrals studied at B3LYP/6–31G(d,p) and G2/G2(MP2) levels are used to support and interpret the experimental results. Copyright © 2005 John Wiley & Sons, Ltd.

Consecutive fragmentations of the cubane-like zinc cluster [CH3Zn(O-i-C3H7)]4upon electron ionization

The sequential dissociations of the tetranuclear zinc cluster-ion [(CH3)3Zn4(O-i-C3H7)4]+ obtained by dissociative electron ionization of neutral [(CH3)Zn(O-i-C3H7)]4 are investigated by tandem mass spectrometry. After initial loss of a neutral (CH3)Zn(O-i-C3H7) unit to afford [(CH3)2Zn3(O-i-C3H7)3]+, hydrogen migration leads to the expulsion of neutral acetone concomitant with [(CH3)2Zn3(O-i-C3H7)2(H)]+ as ionic fragment. Unimolecular dissociation of the latter gives rise to [(CH3)2Zn2(O-i-C3H7)]+ and neutral HZn(O-i-C3H7). As demonstrated by collisional ionization of the neutral product, in the fragmentation of the dinuclear cluster [(CH3)2Zn2(O-i-C3H7)]+ neutral CH3ZnH is formed, rather than the more stable fragments Zn + CH4. From the resulting mononuclear species CH3Zn(OC(CH3)2)+ acetone is eliminated to afford CH3Zn+ as a quasi-terminal fragment ion.

Generation and characterization of ionic and neutral methylene isothiocyanate by a combined tandem mass spectrometry and computational study

AbstractThe heterocumulene, methyleneisothiocyanate ion, CH2NCS+ (1a+), is generated by the dissociative electron ionization of 2‐mercaptoimidazole. This conclusion follows from tandem mass spectrometry experiments and theoretical calculations at the B3LYP/6‐311G** and G2/G2(MP2) levels. The calculations predict that 1a+ is separated by high energy barriers from its isomers CHNCHS (1b+), CHNCSH (1d+), CNCHSH (1e+) and CHNHCS (1f+). The low energy metastable ions 1a+ dissociate by loss of HCN via the pathway 1a+ → 1b+ → HCS+ + HCN. Neutralization‐reionization experiments confirm the theoretical prediction that the hitherto unknown heterocumulene CH2NCS. is a stable species in the rarefied gas phase. Copyright © 2004 John Wiley & Sons, Ltd.

Generation and characterization of ionic and neutral selenocumulene HC 3Se +/[rad] by tandem mass spectrometry and computational study

Dissociative electron ionization (70 eV) of diphenyl diselenide [(C 6H 5Se) 2] generates m/ z 117 ions of composition [H, C 3, 80 Se ] +. The collision-induced dissociation (CID) mass spectrum of these ions are consistent with the selenocumulene connectivity HCCCSe +. Neutralization–reionization mass spectrum (NRMS) of these structurally characterized ions confirmed that the corresponding neutral analog, hitherto unknown hetero cumulene, HCCCSe, is stable in the rarefied gas phase. The relative, dissociation, and isomerization energies for the ions and neutrals studied at B3LYP/6-31G(d,p) and B3LYP/6-311+G (2d,p) levels are used to support and interpret the experimental results.

Protonated silanoic acid HSi(OH)2+ and its neutral counterpart: a tandem mass spectrometric and CBS-QB3 computational study

Protonated silanoic acid, HSi(OH)2+, 1a+, is cleanly generated by the dissociative electron ionization of triethoxysilane, HSi(OC2H5)3, and tetraethoxysilane, Si(OC2H5)4. This follows from tandem mass spectrometric experiments and CBS-QB3 model chemistry calculations. The calculations predict that 1a+ (ΔHf(298 K) = 205 kJ mol−1) is separated by high barriers from its isomers HOSiOH2+, 1b+ and HSi(O)OH2+, 1c+. Low-energy (metastable) ions 1a+ dissociate by loss of H2O via the pathway 1a+ → 1b+ → SiOH+ + H2O. Analysis of the metastable peak for this process confirms that the isomerization step 1a+ → 1b+ is rate determining. The calculations further predict that the incipient ions 1b+ communicate via a low barrier with the proton-bound dimer SiO···H···OH2+, 1d+. This dimer ion is much lower in energy than its counterpart OSi···H···OH2+, 1e+, which is calculated to be only marginally stable. A comparison of the potential energy diagram for the silicon-containing ions 1a+–1e+ with that of their carbon analogues reveals that the dissociation chemistries of HSi(OH)2+ and HC(OH)2+ are only superficially similar. Neutralization–reionization experiments confirm the theoretical prediction that the HSi(OH)2• radical (ΔHf(298 K) = −455 kJ mol−1) is a stable species in the rarefied gas phase. However, owing to a mismatch of Franck–Condon factors a large fraction of the neutralized ions dissociates by loss of H• yielding Si(OH)2. Copyright © 2004 John Wiley & Sons, Ltd.