Subsequent Ion-molecule Reactions Research Articles

Ultraviolet-Laser-Induced Electron Transfer from Peptides to an Oxidizing Matrix: Study of the First Step of MALDI In-Source Decay Mass Spectrometry.

Although the N-H bond in peptide backbones is stronger than the C-H bond, hydrogen abstraction from the amide nitrogen is considered to be the initial step in the Cα-C bond cleavage of peptide backbones by matrix-assisted laser desorption/ionization in-source decay (MALDI-ISD) when using an oxidizing matrix. MALDI-ISD induces Cα-C bond cleavage in most amino acid residues, whereas the N-terminal sides of proline (Pro) residues preferentially undergo peptide bond cleavage, which cannot be explained by the previously proposed mechanism involving hydrogen abstraction from peptides. To explain the whole MALDI-ISD process, electron abstraction from peptides by the oxidizing matrix is proposed as the initial step in the MALDI-ISD process. The electron abstraction occurs from either nitrogen or oxygen in the peptide backbone and induces the cleavage of both Cα-C and N-H bonds in most amino acid residues, except for those on the N-terminal sides of Pro residues. Electron abstraction from the Pro residues induces the cleavage of both peptide and Cα-C bonds, which is consistent with MALDI-ISD experimental results. The electron transfer from the peptide to the oxidizing matrix occurs simultaneously with the formation of matrix ions, which is considered to be the initial ion formation process in MALDI. The resultant peptide radical cation produces protonated and neutral molecules/radicals, which undergo subsequent ion-molecule reactions in the MALDI plume, finally yielding the ions that are observed in MALDI-ISD spectrum. As a result, the fragment ions formed by MALDI-ISD are observed as both positive and negative ions.

Radical Generation from the Gas-Phase Activation of Ionized Lipid Ozonides

Reaction products from the ozonolysis of unsaturated lipids at gas–liquid interfaces have the potential to significantly influence the chemical and physical properties of organic aerosols in the atmosphere. In this study, the gas-phase dissociation behavior of lipid secondary ozonides is investigated using ion-trap mass spectrometry. Secondary ozonides were formed by reaction between a thin film of unsaturated lipids (fatty acid methyl esters or phospholipids) with ozone before being transferred to the gas phase as [M + Na]+ ions by electrospray ionization. Activation of the ionized ozonides was performed by either energetic collisions with helium buffer-gas or laser photolysis, with both processes yielding similar product distributions. Products arising from the decomposition of the ozonides were characterized by their mass-to-charge ratio and subsequent ion-molecule reactions. Product assignments were rationalized as arising from initial homolysis of the ozonide oxygen–oxygen bond with subsequent decomposition of the nascent biradical intermediate. In addition to classic aldehyde and carbonyl oxide-type fragments, carbon-centered radicals were identified with a number of decomposition pathways that indicated facile unimolecular radical migration. These findings reveal that photoactivation of secondary ozonides formed by the reaction of aerosol-bound lipids with tropospheric ozone may initiate radical-mediated chemistry within the particle resulting in surface modification.Graphical ᅟ

Electron attachment to propargyl chloride, 305–540 K

Electron attachment to propargyl chloride (HC≡C-CH(2)Cl) was studied in a flowing-afterglow Langmuir-probe apparatus from 305 to 540 K. The sole ion product in this temperature range is Cl(-). Electron attachment is very inefficient, requiring correction for a competing process of electron recombination with molecular cations produced in reaction between Ar(+) and propargyl chloride and subsequent ion-molecule reactions. The electron attachment rate coefficient was measured to be 1.6×10(-10)cm(3) s(-1) at 305 K and increased to 1.1×10(-9)cm(3) s(-1) at 540 K.

Characterization of high-pressure capacitively coupled hydrogen plasmas

Capacitively coupled very-high-frequency hydrogen plasmas have been systematically diagnosed in a wide range of a gas pressure from 5 mTorr to 10 Torr. The plasma parameters, ion species, and ion energy distributions (IEDs) are measured using a Langmuir probe, optical emission spectroscopy, and energy filtered mass spectrometer. The measurement results show that the ion species in a hydrogen plasma is determined from ionization channels and subsequent ion-molecule reactions. The ions are dominated by H2+ at a less-collisional condition of ≲20 mTorr, whereas those are dominated by H3+ at a collisional condition of ≳20 mTorr. The IED is determined by both the sheath potential drop and ion-neutral collisions in the plasma sheath. The IED is broadened for a collisional sheath at ≳0.3 Torr and the ion bombardment energy is lowered. For high-pressure discharge operated at ≈10 Torr, plasmas are characterized by a low electron temperature of ≈0.8 eV and a low ion bombardment energy of ≲15 eV.

Cyclization and rearrangement of monofluoroallylic cations from halonium metathesis in the gas phase

Products of the F+-for-O metathesis between a variety of fluorinated cations and α,β-unsaturated ketones have been examined using FT-ICR and sector mass spectrometry. The reactant ions CF3+, C3F7+, and CFO+ all undergo ion–molecule reactions with carbonyl compounds to replace oxygen with F+. In principle, the simple transposition of atoms corresponds to the transformation CO→CF+ and ought to produce monofluorinated allylic cations, which DFT calculations predict to be highly stable. The metathesis, however, is so exothermic that cationic rearrangements take place, as attested by several experimental data, including: (1) unimolecular loss of HF from the ion created by methacrolein; (2) the Brønsted acidity of the ion created by methacrolein in its subsequent ion–molecule reactions; and (3) unimolecular loss of ethylene from the ion created by senecialdehyde. DFT calculations suggest that cyclization of unsaturated cations takes place, even though that mechanism removes the positive charge from conjugation with double bonds. Evidence for cyclization is to be found in CF3+-adduct ions as well as the metathesis ions (for instance, the reaction of sorbital with CF3+, which forms CH2OCF3+, as well as the metathesis ion). Electrocyclic ring closure of fluoroallylic ions creates cyclopropyl cations, which represent transition states rather than stable structures on the DFT potential energy surface. Calculated energy barriers to forming monofluorinated cyclopropyl cations range from ΔH=160 to 266kJmol−1. The exothermicities of metatheses with CF3+ are calculated to be >50kJmol−1 higher than the respective barrier heights.

A mass spectrometry study of n-octane: Electron impact ionization and ion-molecule reactions

Electron impact ionization of n-octane over an energy range of 10–70 eV and the subsequent ion-molecule reactions with the parent molecule have been studied using Fourier-transform mass spectrometry. Molecular ion and fragment ions C1+–C6+ are produced from the electron impact with a total ionization cross section of 1.4±0.2×10−15 cm2 between 60 and 70 eV. C3H7+ is the most abundant ion at most of the ionizing energies with the exception for E⩽16 eV where C6H13+ and C6H12+ are the most abundant. Among the fragment ions only C4H7+ and smaller ions react readily with the parent molecule, primarily producing C5H11+ and C4H9+, with rate coefficients of 0.32–2.4×10−9 cm3 s−1. Essentially all of the ions, including the molecular ion and the large fragment ions, undergo decomposition upon collision with neutral molecules after they are kinetically excited to an energy range of 1–5 eV, forming a variety of small hydrocarbon ions. Many of the decomposition product ions in turn are capable of further reacting with n-octane. Isotope reagents have been utilized in experiments to probe the type of the ion-molecule reactions studied.

Electron impact dissociative ionization and the subsequent ion-molecule reactions in a methane beam

The cross-sections of the electron impact dissociative ionization of methane have been measured for electron energies from threshold to 100 eV. For CH 4 + the results agree with the existing measurements. For the fragments of CH 3 +, CH 2 +, CH +, and C +, our results overlap with the previous results within their reported error limits. These earlier measurements exceed their given error limits and hence are inconsistent. At higher backing pressures the subsequent ion-molecule reactions and their product ions have been measured.

Plasma polymerization anda-C:H film ablation in microwave discharges in methane diluted with argon and hydrogen

Neutral hydrocarbons are observed from a microwave discharge in a fast flow of (A) 0.5–6% methane in argon, (B) 0.5–6% methane in hydrogen, and (C) hydrogen over a previously depositeda-C:H film. System (A) produces polyacetylenic and other hydrocarbons through C8 by predominantly gas-phase reactions and deposits ana-C: H film. Reactions under conditions (B) and (C) produce hydrocarbon radicals and molecules with masses through 300 that in case (B) arise from both gas-phase reactions and film ablation, and in case (C) from film ablation alone. Proposals are made for the mechanisms of gas-phase polymerization, film deposition, and ablation. Hydrocarbon ions observed downstream from these discharges appear to arise from ionization of neutral species with a distribution determined by subsequent ion-molecule reactions and selective diffusion losses.

Multiphoton ionization and fragmentation mechanism of CS 2

Mechanisms underlying the multiphoton ionization and fragmentation of CS 2 at 266 nm and 193 nm are determined from studies of ion signal intensity as functions of laser intensity and reagent pressure. At 266 nm fragments are from highly excited states prepared by absorption of at least four photons. Atl93 nm the mechanism has changed.The one-photon state, CS 2 (Ã 1 B 2), predissociates into CS and S instead of absorbing one additional photon to form the molecular ion. The neutral CS and S fragments undergo multiphoton ionization and subsequent ion-molecule reactions produce CS+ 2 + and S 2 +.

Photoionization and ion cyclotron resonance studies of the ion chemistry of ethylene oxide

Time-resolved photoionozation mass spectrometry (PIMS), ion cyclotron resonance spectroscopy (ICR), and photoelectron spectroscopy have been employed to study the formation of the ethylene oxide molecular ion and its subsequent ion–molecule reactions which lead to the products C2H5O+ and C3H5O+. Earlier observations that a structurally and energetically modified species (C2H4O+) * is an intermediate in the production of C3H5O+ are confirmed. The PIMS data detail the effects of internal energy on reactivity, with the ratio of C3H5O+ to C2H5O+ increasing by an order of magnitude with a single quantum of vibrational energy. Evidence is presented for the formation of (C2H4O+) * in a collision-induced isomerization which yields a ring-opened structure by C–C bond cleavage. This species contains considerable internal excitation which is relaxed in collisions with ethylene oxide or bath gases such as SF6 prior to reaction. The relaxed ring-opened C2H4O+ ion reacts with neutral ethylene oxide by CH2+ transfer to yield an intermediate product ion C3H6O+ which gives C3H5O+ by loss of H. Isotopic product distributions observed in a mixture of ethylene oxide and ethylene oxide-d4 are consistent with this mechanism. The effects of ion kinetic energy on reactivity are explored using ICR techniques. Increased reactant ion kinetic energy leads to collision-induced dissociation of C2H4O+ rather than isomerization to the open form.

Ionization, attachment and negative ion reactions in carbon dioxide

From measurements of the relative abundances of O- and CO3- ions produced in carbon dioxide by electron attachment and subsequent ion-molecule reactions, and from current-growth data, values of the primary ionization coefficient, attachment coefficient and rate coefficient for the reaction O-+2CO2 to CO3-+CO2 have been obtained for 60*10-17<E/N<152*10-17 V cm2 at gas number densities N between 0.2 and 5.0*1016 cm-3.

Electron attachment and detachment in nitric oxide

The attachment coefficient and drift velocity of electrons in nitric oxide have been measured in a pulsed drift tube, over a reduced field range between 6 × 10–18 and 2 × 10–16 V cm2 molecule–1, at gas densities of ca. 3 × 10+18 molecule cm–3 and ambient temperatures between 293 and 493 K. The resultant attachment frequency, described as second order in nitric oxide pressure at room temperature, reached a peak value of 12 × 10–31 cm6 molecule–2 s–1 at 0.2 eV electron energy, falling off at higher and lower energies. The thermal rate found by extrapolation is 8 ± 2 × 10–31 cm6 molecule–2 s–1. At higher temperatures the measured coefficient fell, indicating the occurrence of detachment reactions. The detachment rate was measured in separate experiments using a d.c. drift tube and mass filter and found to be 5 ± 1 × 10–12 cm3 molecule–1 s–1, for near thermal ions. Comparison of the measured attachment and detachment rates with the statistical mechanical formula for the equilibrium constant yields a value of 0.026 ± 0.02 eV for the electron affinity of nitric oxide.The only stable ion formed in NO was mass 46, NO–2, formed by subsequent ion molecule reactions from the initial product NO–. In NO + O2 mixtures the rate of the following reaction was measured: NO–3+ NO → NO–2+ NO2, k= 3 × 10–15 cm3 molecule s–1. From equilibrium considerations, it is deduced using the value of this rate constant that the electron affinity of NO3 exceeds that of NO2 by at least 1.13 eV.

Electron attachment and negative ion-molecule reactions in pure oxygen

The formation of negative ions in oxygen by electron attachment and by subsequent ion-molecule reactions has been studied in the gas phase, using a drift-tube and mass filter. Pressures ranged between 0.65 and 13 mbar (0.5–10 Torr) and reduced fields (field/density) between 3 × 10–18 and 6 × 10–16 V cm2 part.–1. At reduced fields below 10–16 V cm2 part.–1 masses 32 (O–2), 64 (O–4) and 50 (O2˙H2O–) predominate. The O–2 formed initially is converted to O–4 in a three-body reaction, O–2+ 2O2⇌O–4+ O2, with a thermal rate constant of 3 ± 0.5 × 10–31 part.–2 ml2 s–1 at 298 K. The back reaction has a rate constant of 7 ± 1 × 10–15 part.–1 ml s–1. The hydrate concentration is related to that of the dimer via the reactions O–4+ H2O⇌O2˙ H2O–+ O2. The forward rate constant is estimated to be of the order of 10–9 part.–1 ml s–1, and the back measured as 2.5 ± 1 × 10–15 part.–1ml s–1. At higher fields the initially formed mass 16 (O–) is converted irreversibly to mass 48 (O–3): O–+ 2O2→O–3+ O2. The thermal rate constant is measured as 1.05 ± 0.1 × 10–30 part. –2 ml s–1 up to pressures of at least 13 mbar, without any significant change in order. The change in the rate constants with ion collisional energy has also been followed.