Metastable transitions in n-butane and n-butane-d7

Metastable ions have been investigated for n-butane d₇ molecular ions using a tandem mass spectrometer which samples unimolecular decay processes occuring during the time interval of c.2 μs to 4 ms after ion formation. Some 37% of ions formed by 70 eV electron impact decay on this time scale. The competing unimolecular processes observed, in order of relative importance, are methance, methyl radical and hydrogen atom elimination. The slow metastables sample the threshold energy regime of unimolecular reactions responsible for forming the ordinary mass spectrum of butance and very large isotope effect are noted for the deuterated molecule.     

Resonance capture of electrons by molecules of substituted pyrans

Negative-ion mass spectrometry in the mode of resonance capture of electrons and photoelectron spectroscopy in combination with quantum-chemical calculations showed that the formation of the resonance states of negative molecular ions in the reaction of electrons with molecules of the mechanism of intershell Feshbach resonance with the consecutive excitation of an electron from several higher occupied MO to one vacant MO. In a low-energy region, the resonance at 1.4 eV is a resonance of form and the resonance at 3–4 eV is the usual electron exciting Feshbach resonance with a parent triplet state (π.π*)³. The one and the same vacant π*_CC MO is “active” in all the resonances mentioned. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1892–1894, October, 1997.     

Positive and negative ions of the amino acid histidine formed in low‐energy electron collisions

Histidine is an aromatic amino acid crucial for the biological functioning of proteins and enzymes. When biological matter is exposed to ionising radiation, highly energetic particles interact with the surrounding tissue which leads to efficient formation of low‐energy electrons. In the present study, the interaction of low‐energy electrons with gas‐phase histidine is studied at a molecular level in order to extend the knowledge of electron‐induced reactions with amino acids. We report both on the formation of positive ions formed by electron ionisation and negative ions induced by electron attachment. The experimental data were complemented by quantum chemical calculations. Specifically, the free energies for possible fragmentation reactions were derived for the τ and the π tautomer of histidine to get insight into the structures of the formed ions and the corresponding neutrals. We report the experimental ionisation energy of (8.48 ± 0.03) eV for histidine which is in good agreement with the calculated vertical ionisation energy. In the case of negative ions, the dehydrogenated parent anion is the anion with the highest mass observed upon dissociative electron attachment. The comparison of experimental and computational results was also performed in view of a possible thermal decomposition of histidine during the experiments, since the sample was sublimated in the experiment by resistive heating of an oven. Overall, the present study demonstrates the effects of electrons as secondary particles in the chemical degradation of histidine. The reactions induced by those electrons differ when comparing positive and negative ion formation. While for negative ions, simple bond cleav ages prevail, the observed fragment cations exhibit partly restructuring of the molecule during the dissociation process.     

Low-energy tandem mass spectrometry of the molecular ion derived from fatty acid methyl esters: A novel method for analysis of branched-chain fatty acids

Low-energy collision-induced dissociation (CID) of the molecular ions of fatty acid methyl esters obtained by electron ionization (70 eV) decompose in the tandem quadrupole mass spectrometer to yield a regular homologous series of carbomethoxy ions. Even at energies up to 200 eV (E _lab), primarily carbomethoxy ions are present, with the most abundant found at m/z 101 at hi her energies. The lack of any other CID ions, including m/z 74 (McLafferty rearrangement) or m/z 87, suggest a rearranged molecular ion structure on leaving the first quadrupole mass analyzer. Analyses of various stable isotope variants support the hypothesis of alkyl radical migration to the carboxy carbonyl oxygen atom, with subsequent radical site directed cleavage either with or without a cyclization event. Decomposition of the molecular ions (70 eV) of several methyl branched fatty acid methyl esters, including phytanic acid, iso-methyl and anteiso-methyl branched acids, and tuberculostearic acid, reveals enhanced radical site cleavage at the alkyl branching positions. This method can be used to readily determine methyl (or alkyl) branching positions in a saturated fatty acid methyl ester.     

Electron‐induced dissociation of doubly protonated betaine clusters: controlling fragmentation chemistry through electron energy

The [M₂₁+2H]²⁺ cluster of the zwitterion betaine, M = (CH₃)₃NCH₂CO₂, formed via electrospray ionisation (ESI), has been allowed to interact with electrons with energies ranging from >0 to 50 eV in a Fourier transform ion cyclotron resonance (FT‐ICR) mass spectrometer. The types of gas‐phase electron‐induced dissociation (EID) reactions observed are dependent on the energy of the electrons. In the low‐energy region up to 10 eV, electrons are mainly captured, forming the charge‐reduced species, {[M₂₁+2H]^{+ .} }*, in an excited state, which stabilises via the ejection of an H atom and one or more neutral betaines. In the higher energy region, above 12 eV, a Coulomb explosion of the multiply charged clusters is observed in highly asymmetric fission with singly charged fragments carrying away more than 70% of the parent mass. Neutral betaine evaporation is also observed in this energy region. In addition, a series of singly charged fragments appears which arise from C? X bond cleavage reactions, including decarboxylation and CH₃ group transfer. These latter reactions may arise from access of electronic excited states of the precursor ions. Copyright © 2009 John Wiley & Sons, Ltd.     

Resonant electron capture by some amino acids and their methyl esters

Resonant electron capture mass spectra of aliphatic and aromatic amino acids and their methyl esters show intense [M-H](-) negative ions in the low-energy range. Ion formation results from a predissociation mechanism mediated by the low-energy pi*oo resonant state. Methylation in general has little influence on the electronic structure according to quantum chemical calculations, but the corresponding ions from the methyl esters, [M-Me](-), could be ascertained to arise only at higher resonance energies. Aromatic amino acids are characterized by an additional low-energy fragmentation channel associated with the generation of negative ions with loss of the side chain. The complementary negative ions of the side chains are more efficiently produced at higher energies. The results have significant implications in biological systems as they suggest that amino acids can serve as radiation protectors since they have been found to efficiently thermalize electrons.     

Low-energy resonance states upon electron capture by five-membered heterocycles and cyclopentadiene

New resonance states were discovered for the negative molecular ions of thiophene and selenophene and a series of resonances was found for various heterocyclic compounds in the region 3.0–3.6 eV. The low-energy resonances at 1.65–2.3 eV are formed by a resonance mechanism of a form of the molecular ground state, while an electronically excited Feschbach resonance is responsible for the series of resonance states at 3.0–3.6 eV. The mother state for the latter resonance states is the first triplet state of these molecules. The first triplet state of selenophene is at 3.6±0.15 eV.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 4, pp. 925–927, April, 1990.     

Unimolecular Reaction Mechanism of an Imidazolin‐2‐ylidene: An iPEPICO Study on the Complex Dissociation of an Arduengo‐Type Carbene

The photoionization and dissociative photoionization of Im(iPr)₂, 1,3‐diisopropylimidazolin‐2‐ylidene, was investigated by imaging photoelectron photoion coincidence (iPEPICO) with vacuum ultraviolet (VUV) synchrotron radiation. A lone‐pair electron of the carbene carbon atom is removed upon ionization and the molecular geometry changes significantly. Only 0.5 eV above the adiabatic ionization energy, IE_ad=7.52±0.1 eV, the carbene cation fragments, yielding propene or a methyl radical in parallel dissociation reactions with appearance energies of 8.22 and 8.17 eV, respectively. Both reaction channels appear at almost the same photon energy, suggesting a shared transition state. This is confirmed by calculations, which reveal the rate‐determining step as hydrogen‐atom migration from the isopropyl group to the carbene carbon center forming a resonance‐stabilized imidazolium ion. Above 10.5 eV, analogous sequential dissociation channels open up. The first propene‐loss fragment ion dissociates further and another methyl or propene is abstracted. Again, a resonance‐stabilized imidazolium ion acts as intermediate. The aromaticity of the system is enhanced even in vertical ionization. Indeed, the coincidence technique confirms that a real imidazolium ion is produced by hydrogen transfer over a small barrier. The simple analysis of the breakdown diagram yields all the clues to disentangle the complex dissociative photoionization mechanism of this intermediate‐sized molecule. Photoelectron photoion coincidence is a promising tool to unveil the fragmentation mechanism of larger molecules in mass spectrometry.     

Degradation of vitamin C by low-energy electrons

We report on the degradation of gas phase vitamin C (ascorbic acid, AA) induced by low-energy electrons. In the energy range of (0–12) eV, different negatively charged fragments, attributed to the dehydro-ascorbic acid anion ((AA–H)⁻), OH⁻, O⁻ and H⁻, are observed. The yield functions indicate that these ions are formed via dissociative electron attachment, DEA. While the formation of (AA–H)⁻ is exclusively observed at sub-excitation energies (<1.5 eV), the other fragments arise from resonance features at higher energies. Possible implications of these observations for radiation damage and food treatment by high energy radiation are considered.     

Comparison of collision-induced dissociation and electron-induced dissociation of <Emphasis Type="Italic">singly protonated</Emphasis> aromatic amino acids,cystine and related simple peptides using a hybrid linear ion trap–FT-ICR mass spectrometer

The gas-phase fragmentation reactions of singly protonated aromatic amino acids, their simple peptides as well as simple models for intermolecular disulfide bonds have been examined using a commercially available hybrid linear ion trap-Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. Low-energy collision-induced dissociation (CID) reactions within the linear ion trap are compared with electron-induced dissociation (EID) reactions within the FT-ICR cell. Dramatic differences are observed between low-energy CID (which occurs via vibrational excitation) and EID. For example, the aromatic amino acids mainly fragment via competitive losses of NH(3) and (H(2)O+CO) under CID conditions, while side-chain benzyl cations are major fragment ions under EID conditions. EID also appears to be superior in cleaving the S-S and S-C bonds of models of peptides containing an intermolecular disulfide bond. Systematic studies involving fragmentation as a function of electron energy reveal that the fragmentation efficiency for EID occurs at high electron energy (more than 10 eV) compared with the low-electron energy (less than 0.2 eV) typically observed for electron capture dissociation fragmentation. Finally, owing to similarities between the types of fragment ions observed under EID conditions and those previously reported in ultraviolet photodissociation experiments and the electron-ionization mass spectra, we propose that EID results in fragmentation via electronic excitation and vibrational excitation. EID may find applications in analyzing singly charged molecular ions formed by matrix-assisted laser desorption ionization.     

Electron capture dissociation Fourier transform ion cyclotron resonance mass spectrometry in the electron energy range 0-50 eV

Electron capture dissociation (ECD) of polypeptide cations was obtained with pencil and hollow electron beams for both sidekick and gas-assisted dynamic ion trapping (GADT) using Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) with an electrostatic ion transfer line. Increasing the number of trapped ions by multiple ICR trap loads using GADT improved the ECD sensitivity in comparison with sidekick ion trapping and ECD efficiency in comparison with single ion trap load by GADT. Furthermore, enhanced sensitivity made it possible to observe ECD in a wide range of electron energies (0-50 eV). The degree, rate and fragmentation characteristics of ECD FTICR-MS were investigated as functions of electron energy, electron irradiation time, electron flux and ion trapping parameters for this broad energy range. The results obtained show that the rate of ECD is higher for more energetic (>1 eV) electrons. Long electron irradiation time with energetic electrons reduces average fragment ion mass and decreases efficiency of formation of c- and z-type ions. The obtained dependencies suggest that the average fragment ion mass and the ECD efficiency are functions of the total fluence of the electron beam (electron energy multiplied by irradiation time). The measured electron energy distributions in low-energy ECD and hot ECD regimes are about 1 eV at full width half maximum in employed experimental configurations.     

Electron-attachment resonances of glycine zwitterions from quantum scattering calculations: modeling macrosolvation effects

A computational study of the quantum dynamics for low-energy electrons scattered by the isolated zwitterionic species of the glycine molecule is carried out using a model interaction potential described in the main text. The macroscopic effects of water solvation on the target molecule in the electron scattering problem are described through a continuum polarizable model (CPCM) which modifies the target molecular structure. In such a way, realistic molecular orbitals depicting the glycine zwitterion in solution are used to model the electron-molecule interaction. The results of the calculations indicate the presence of five different transient negative ions (TNIs) formed at energies from the threshold and up to about 6 eV. Although no nuclear motion was explicitly considered in the ensuing decay processes, the analysis of the nodal structures and density distributions for the resonant excess electron wavefunctions over the molecular space suggests possible anionic fragmentations that produce (Gly-H)-, H-, -CO2-, and -NH3. The likely consequences of such releases into the medium are briefly discussed.     

Resonance electron capture by uracil derivatives

The enol forms of uracil and its derivatives were detected in the gas phase by mass spectrometry. The [M - H]⁻ ion is produced by resonance electron capture to the lowest unoccupied molecular orbitals, the process being accompanied by the detachment of the hydrogen atom from the nitrogen atom of the diketo form (low-energy peak at 0.8 eV) and from the oxygen atom of the enol form (in the energy region of 1.4 eV). The gas phase contains ∼10⁻³% of the enol form. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1360–1362, June, 2005.     

Molecular size effect in NCO and NCS dianion resonances

Cross sections for electron-impact detachment and electron-impact dissociation of NCO- and NCS- were measured from about 3 to about 40 eV. The former are found to follow a classical prediction with a threshold energy of 9.1 +/- 0.1 eV for NCO- and 8.9 +/- 0.2 eV for NCS-. When the incoming electron binds to the monoanion, a short-lived dianion complex is formed, which is revealed as a resonance in the cross section. For NCO- a resonance is evident at 9.3 +/- 0.2 eV, which implies that the dianion lies above the monoanion by this amount of energy. In the case of NCS- two resonances are evident at 8.4 +/- 0.2 and 19.0 +/- 0.5 eV, respectively. The low-energy NCS dianion is less unstable than the dianion of NCO, which in turn is less unstable than the CN dianion (10-eV resonance). Thus the resonance shifts down in energy with the increasing size of the anion, a fact which is attributed to a decrease in Coulomb energy between the spatially separated electrons.     

Dissociative electron attachment to furan, tetrahydrofuran, and fructose

We study dissociative electron attachment to furan (FN) (C(4)H(4)O), tetrahydrofuran (THF) (C(4)H(8)O), and fructose (FRU) (C(6)H(12)O(6)) using crossed electron/molecular beams experiments with mass spectrometric detection of the anions. We find that FN and THF are weak electron scavengers and subjected to dissociative electron attachment essentially in the energy range above 5.5 eV via core excited resonances. In striking contrast to that, FRU is very sensitive towards low energy electrons generating a variety of fragment ions via a pronounced low energy feature close to 0 eV. These reactions are associated with the degradation of the ring structure and demonstrate that THF cannot be used as surrogate to model deoxyribose in DNA with respect to the attack of electrons at subexcitation energies (<3 eV). The results support the picture that in DNA the sugar moiety itself is an active part in the initial molecular processes leading to single strand breaks.     

Neutralization of methyl cation via chemical reactions in low-energy ion-surface collisions with fluorocarbon and hydrocarbon self-assembled monolayer films

Low-energy ion-surface collisions of methyl cation at hydrocarbon and fluorocarbon self-assembled monolayer (SAM) surfaces produce extensive neutralization of CH3+. These experimental observations are reported together with the results obtained for ion-surface collisions with the molecular ions of benzene, styrene, 3-fluorobenzonitrile, 1,3,5-triazine, and ammonia on the same surfaces. For comparison, low-energy gas-phase collisions of CD3+ and 3-fluorobenzonitrile molecular ions with neutral n-butane reagent gas were conducted in a triple quadrupole (QQQ) instrument. Relevant MP2 6-31G*//MP2 6-31G* ab initio and thermochemical calculations provide further insight in the neutralization mechanisms of methyl cation. The data suggest that neutralization of methyl cation with hydrocarbon and fluorocarbon SAMs occurs by concerted chemical reactions, i.e., that neutralization of the projectile occurs not only by a direct electron transfer from the surface but also by formation of a neutral molecule. The calculations indicate that the following products can be formed by exothermic processes and without appreciable activation energy: CH4 (formal hydride ion addition) and C2H6 (formal methyl anion addition) from a hydrocarbon surface and CH3F (formal fluoride addition) from a fluorocarbon surface. The results also demonstrate that, in some cases, simple thermochemical calculations cannot be used to predict the energy profiles because relatively large activation energies can be associated with exothermic reactions, as was found for the formation of CH3CF3 (formal addition of trifluoromethyl anion).     

Mechanism of negative ion formation from phenol and para-chlorophenol by interaction with free electrons

Dissociative electron attachment (DEA) to phenol and para-chlorophenol in the energy range 0-12 eV is studied. Analogies in formation of the resonance states in an ionic benzene and its derivatives are found to arise from the similarity of the aromatic base of the molecules. Differences in DEA processes are defined mainly by the influence of the functional OH-group and, to a lesser degree, by the presence of a chlorine atom. A correlation between the energies of the resonance states and ionization energies of p-chlorophenol and phenol, analogous to that found previously for phenol, is proved. On this basis it is established that the dominating mechanism for formation of molecular negative ions at energies above 5 eV is Feshbach resonance.Copyright 2000 John Wiley & Sons, Ltd.     

Structure of X-ray photoelectron, emission, and conversion spectra and molecular orbitals of uranium compounds

The fine structure of X-ray photoelectron spectra of uranium compounds in the range of electron binding energies from 0 to ∼50 eV is largely determined by the electrons of the outer and inner valence molecular orbitals arising from the valence atomic shells, including the U6p and Lns low-energy occupied atomic shells. This result is in agreement with the data of the electronic structure calculations of these compounds and confirmed by the nuclear electron (conversion) and X-ray emission spectroscopic investigations. It is shown that the fine structure of X-ray photoelectron spectra associated with the electrons of inner valence molecular orbitals makes it possible to judge the participation of the electrons of the occupied atomic shells in chemical bonding, the structure of the nearest environments of the atom, and the bond lengths in the compounds. The overall contribution of the electrons of these molecular orbitals to the absolute value of binding energy may prove to be comparable to the contribution of the electrons of the outer valence molecular orbitals to atomic bonding. This is a new and important fact in chemistry. Translated fromZhurnal Strukturnoi Khimii, Vol. 39, No. 6, pp. 1037–1046, November–December, 1998.     

Resonance electron capture mass spectra of some diterpenoid alkaloids

Formation of negative ions from molecules of some diterpenoid alkaloids under conditions of resonance electron capture was studied. Ions formed by capturing low-energy electrons in the lowest unoccupied molecular orbital similar to the *-C=O or *-Ph—CO₂ orbitals contribute predominantly to the mass spectra.     

Electron-induced reactions in condensed films of acetonitrile and ethane

Reactions in pure and mixed films of C(2)H(6) and CD(3)CN deposited on a Au surface at 35 K have been induced by low-energy electrons and investigated by Thermal Desorption Spectrometry (TDS). The incident electron energy (E(0)) was varied between 5 and 16 eV and a number of different products were identified. Beside the main products, CD(4), CD(3)H, and C(2)D(6), molecules resulting from atom scrambling during radical chain reactions (C(2)H(5)D) and recombination products (CD(3)CD(2)CN and C(2)H(5)CD(3)) were identified while others were characteristically absent. The quantity of the different products varied with E(0). The observed electron-driven processes are in accord with previous findings from gas phase experiments on dissociative electron attachment and electron impact ionization. On this basis, reaction mechanisms leading to the formation of the observed products are suggested for different ranges of E(0).