Threshold photoelectron photoion coincidence spectroscopy and selected ion flow tube cation-molecule reaction studies of cyclic-C4F8

Using tunable vacuum-UV radiation from a synchrotron, the threshold photoelectron and threshold photoelectron photoion coincidence (TPEPICO) spectra of cyclic-C4F8 in the range 11-25 eV have been recorded. The parent ion is observed very weakly at threshold, 11.60 eV, and is most likely to have cyclic geometry. Ion yield curves and branching ratios have been determined for five fragments. Above threshold, the first ion observed is C3F5+, at slightly higher energy C2F4+, then successively CF+, CF2+ and CF3+ are formed. The dominant ions are C3F5+ and C2F4+, with the data suggesting the presence of a barrier in the exit channel to production of C3F5+ whilst no barrier to production of C2F4+. In complementary experiments, the product branching ratios and rate coefficients have been measured in a selected ion flow tube (SIFT) at 298 K for the bimolecular reactions of cyclic-C4F8 with a large number of atomic and small molecular cations. Below the energy where charge transfer becomes energetically allowed, only one of the ions, CF2+, reacts. Above this energy, all but one of the remaining ions react. Experimental rate coefficients are consistently greater than the collisional values calculated from modified average dipole orientation theory. The inclusion of an additional ion-quadrupole interaction has allowed better agreement to be achieved. With the exception of N+, a comparison of the fragment ion branching ratios from the TPEPICO and SIFT data suggest that long-range charge transfer is the dominate mechanism for reactions of ions with recombination energy between 12.9 and 15.8 eV. For all other ions, either short-range charge transfer or a chemical reaction, involving cleavage and making of new bond(s), is the dominant mechanism.     

Isomeric effects in the gas-phase reactions of dichloroethene, C2H2CL2, with a series of cations

A study of the reactions of a series of gas-phase cations (NH(4)(+), H(3)O(+), SF(3)(+), CF(3)(+), CF(+), SF(5)(+), SF(2)(+), SF(+), CF(2)(+), SF(4)(+), O(2)(+), Xe(+), N(2)O(+), CO(2)(+), Kr(+), CO(+), N(+), N(2)(+), Ar(+), F(+), and Ne(+)) with the three structural isomers of dichloroethene, i.e., 1,1-C(2)H(2)Cl(2), cis-1,2-C(2)H(2)Cl(2), and trans-1,2-C(2)H(2)Cl(2) is reported. The recombination energy (RE) of these ions spans the range of 4.7-21.6 eV. Reaction rate coefficients and product branching ratios have been measured at 298 K in a selected ion flow tube (SIFT). Collisional rate coefficients are calculated by modified average dipole orientation (MADO) theory and compared with experimental data. Thermochemistry and mass balance have been used to predict the most feasible neutral products. Threshold photoelectron-photoion coincidence spectra have also been obtained for the three isomers of C(2)H(2)Cl(2) with photon energies in the range of 10-23 eV. The fragment ion branching ratios have been compared with those of the flow tube study to determine the importance of long-range charge transfer. A strong influence of the isomeric structure of dichloroethene on the products of ion-molecule reactions has been observed for H(3)O(+), CF(3)(+), and CF(+). For 1,1-C(2)H(2)Cl(2) the reaction with H(3)O(+) proceeds at the collisional rate with the only ionic product being 1,1-C(2)H(2)Cl(2)H(+). However, the same reaction yields two more ionic products in the case of cis-1,2- and trans-1,2-C(2)H(2)Cl(2), but only proceeds with 14% and 18% efficiency, respectively. The CF(3)(+) reaction proceeds with 56-80% efficiency, the only ionic product for 1,1-C(2)H(2)Cl(2) being C(2)H(2)Cl(+) formed via Cl(-) abstraction, whereas the only ionic product for both 1,2-isomers is CHCl(2)(+) corresponding to a breaking of the C=C double bond. Less profound isomeric effects, but still resulting in different products for 1,1- and 1,2-C(2)H(2)Cl(2) isomers, have been found in the reactions of SF(+), CO(2)(+), CO(+), N(2)(+), and Ar(+). Although these five ions have REs above the ionization energy (IE) of any of the C(2)H(2)Cl(2) isomers, and hence the threshold for long-range charge transfer, the results suggest that the formation of a collision complex at short range between these ions and C(2)H(2)Cl(2) is responsible for the observed effects.     

Selected ion flow tube study of the reactions between gas phase cations and CHCl2F, CHClF2, and CH2ClF

The branching ratios and rate coefficients have been measured at 298 K for the reactions between CHCl2F, CHClF2, and CH2ClF and the following cations (with recombination energies in the range 6.3-21.6 eV); H3O+, SFx+ (x = 1-5), CFy+ (y = 1-3), NO+, NO2+, O2+, Xe+, N2O+, O+, CO2+, Kr+, CO+, N+, N2+, Ar+, F+, and Ne+. The majority of the reactions proceed at the calculated collisional rate, but the reagent ions SF3+, NO+, NO2+, and SF2+ do not react. Surprisingly, although all of the observed product channels are calculated to be endothermic, H3O+ does react with CHCl2F. On thermochemical grounds, Xe+ appears to react with these molecules only when it is in its higher-energy 2P1/2 spin-orbit state. In general, most of the reactions form products by dissociative charge transfer, but some of the reactions of CH2ClF with the lower-energy cations produce the parent cation in significant abundance. The branching ratios produced in this study and by threshold photoelectron-photoion coincidence spectroscopy agree reasonably well over the energy range 11-22 eV. In about one-fifth of the large number of reactions studied, the branching ratios are in excellent agreement and appreciable energy resonance between an excited state and the ground state of the ionized neutral exists, suggesting that these reactions proceed exclusively by a long-range charge-transfer mechanism. Upper limits for the enthalpy of formation at 298 K of SF4Cl (-637 kJ mol-1), SClF (-28 kJ mol-1), and SHF (-7 kJ mol-1) are determined.     

Selected Ion Flow Tube Study of the Gas-Phase Reactions of CF(+), CF(2)(+), CF(3)(+), and C(2)F(4)(+) with C(2)H(4), C(2)H(3)F, CH(2)CF(2), and C(2)HF(3)

We study how the degree of fluorine substitution for hydrogen atoms in ethene affects its reactivity in the gas phase. The reactions of a series of small fluorocarbon cations (CF(+), CF(2)(+), CF(3)(+), and C(2)F(4)(+)) with ethene (C(2)H(4)), monofluoroethene (C(2)H(3)F), 1,1-difluoroethene (CH(2)CF(2)), and trifluoroethene (C(2)HF(3)) have been studied in a selected ion flow tube. Rate coefficients and product cations with their branching ratios were determined at 298 K. Because the recombination energy of CF(2)(+) exceeds the ionization energy of all four substituted ethenes, the reactions of this ion produce predominantly the products of nondissociative charge transfer. With their lower recombination energies, charge transfer in the reactions of CF(+), CF(3)(+), and C(2)F(4)(+) is always endothermic, so products can only be produced by reactions in which bonds form and break within a complex. The trends observed in the results of the reactions of CF(+) and CF(3)(+) may partially be explained by the changing value of the dipole moment of the three fluoroethenes, where the cation preferentially attacks the more nucleophilic part of the molecule. Reactions of CF(3)(+) and C(2)F(4)(+) are significantly slower than those of CF(+) and CF(2)(+), with adducts being formed with the former cations. The reactions of C(2)F(4)(+) with the four neutral titled molecules are complex, giving a range of products. All can be characterized by a common first step in the mechanism in which a four-carbon chain intermediate is formed. Thereafter, arrow-pushing mechanisms as used by organic chemists can explain a number of the different products. Using the stationary electron convention, an upper limit for Δ(f)H°(298)(C(3)F(2)H(3)(+), with structure CF(2)═CH-CH(2)(+)) of 628 kJ mol(-1) and a lower limit for Δ(f)H°(298)(C(2)F(2)H(+), with structure CF(2)═CH(+)) of 845 kJ mol(-1) are determined.     

Evidence of CH2O (a3A2) and C2H4 (a3B1u) produced from photodissociation of 1,3-trimethylene oxide at 193 nm

We investigated the dissociative ionization of formaldehyde (CH(2)O) and ethene (C(2)H(4)) produced from photolysis of 1,3-trimethylene oxide at 193 nm using a molecular-beam apparatus and vacuum-ultraviolet radiation from an undulator for direct ionization. The CH(2)O (C(2)H(4)) product suffers from severe dissociative ionization to HCO(+) (C(2)H(3) (+) and C(2)H(2) (+)) even though photoionization energy is as small as 9.8 eV. Branching ratios of fragmentation of CH(2)O and C(2)H(4) following ionization are revealed as a function of kinetic energy of products using ionizing photons from 9.8 to 14.8 eV. Except several exceptions, branching ratios of daughter ions increase with increasing photon energy but decrease with increasing kinetic energy. The title reaction produces CH(2)O and C(2)H(4) mostly on electronic ground states but a few likely on triplet states; C(2)H(4) (a(3)B(1u)) seems to have a yield greater than CH(2)O (a(3)A(2)). The distinct features observed at small kinetic energies of daughter ions are attributed to dissociative ionization of photoproducts CH(2)O (a(3)A(2)) and C(2)H(4) (a(3)B(1u)). The observation of triplet products indicates that intersystem crossing occurs prior to fragmentation of 1,3-trimethylene oxide.     

Electron ionization of acetylene

Relative partial ionization cross sections and precursor specific relative partial ionization cross sections for fragment ions formed by electron ionization of C2H2 have been measured using time-of-flight mass spectrometry coupled with a 2D ion-ion coincidence technique. We report data for the formation of H+, H+2, C2+, C+/C2+ 2, CH+/C2H+2, CH+2, C+2, and C2H+ relative to the formation of C2H+2, as a function of ionizing electron energy from 30-200 eV. While excellent agreement is found between our data and one set of previously published absolute partial ionization cross sections, some discrepancies exist between the results presented here and two other recent determinations of these absolute partial ionization cross sections. We attribute these differences to the loss of some translationally energetic fragment ions in these earlier studies. Our relative precursor-specific partial ionization cross sections enable us, for the first time, to quantify the contribution to the yield of each fragment ion from single, double, and triple ionization. Analysis shows that at 50 eV double ionization contributes 2% to the total ion yield, increasing to over 10% at an ionizing energy of 100 eV. From our ion-ion coincidence data, we have derived branching ratios for charge separating dissociations of the acetylene dication. Comparison of our data to recent ab initio/RRKM calculations suggest that close to the double ionization potential C2H2+2 dissociates predominantly on the ground triplet potential energy surface (3Sigma*g) with a much smaller contribution from dissociation via the lowest singlet potential energy surface (1Delta g). Measurements of the kinetic energy released in the fragmentation reactions of C2H2+2 have been used to obtain precursor state energies for the formation of product ion pairs, and are shown to be in good agreement with available experimental data and with theory.     

Ion-molecule reactions of [C,H(3), S](+) ions and evidence for long-lived triplet CH(3)S(+)

Gas-phase [C, H(3), S](+) ions obtained by electron impact from (CH(3))(2)S at 14 eV undergo two distinct low-pressure ion-molecule reactions with the parent neutral: proton transfer and charge exchange. The kinetics of these reactions studied by Fourier transform ion cyclotron resonance (FT-ICR) techniques clearly suggests the [C, H(3), S](+) species to be a mixture of isomeric ions. While proton transfer is consistent with reagent ions displaying the CH(2)SH(+) connectivity, the observed charge exchange strongly argues for the presence of thiomethoxy cations, CH(3)S(+), predicted to be stable only in the triplet state. Charge exchange reactions are also observed in the reaction of these same [C, H(3), S](+) ions with benzene, toluene and phenetole. For these substrates, the CH(2)SH(+) ions can promote proton transfer and electrophilic methylene insertion in the aromatic ring with elimination of H(2)S. The results obtained for the different substrates suggest that the fraction of long-lived fraction of thiomethoxy cations obtained at 14 eV by electron ionization of dimethyl sulfide amounts to ~(22 -/+ 4)% of the [C, H(3), S](+) fragments.     

Charge exchange in tandem mass spectrometry: dissociative single electron capture by doubly-charged toluene cations

Single electron capture by doubly-charged toluene cations upon collision with various target gases has been investigated by sector tandem mass spectrometry. Both non-dissociative and dissociative charge transfer reactions leading to C(7)H(7)(+) + H and to C(5)H(5)(+) + [C(2),H(3)] are detected. Seven atomic or molecular target gases have been used with ionisation energies ranging from 8.8 eV to 14 eV. The branching ratios between the different non-dissociative and dissociative exit channels have been determined as well as the translational energy release on the dissociation products. The experimental data are compared to the predictions of a two-state semi-classical theoretical model that takes into account the non-adiabatic transition responsible for the charge transfer reaction. A wide reaction window shows up but the internal energies of the C(7)H(8)(+) cations produced by single electron capture are observed to be larger than expected. We assign this effect partly to the influence of the large density of vibrational states and to the multichannel nature of the process. Excited states of the dication are also most probably involved in the charge exchange reaction.     

Sensitivity of charge transfer reactions to hydrocarbon ion structures

Charge transfer reactivities of hydrocarbon ions have been measured with time-of-flight techniques, and results correlated with theoretical structures computed by self-consistent field molecular orbital methods. Recombination energies, ion structures, heats of formation, reaction energetics and relative charge transfer cross-sections are presented for molecular and fragment ions produced by electron bombardment ionization of CH₄, C₂H₄, C₂H₆, C₃H₈ and C₄H₁₀ molecules. Even-electron bridged cations have low ion recombination energies and relatively low charge transfer cross-sections as compared with odd-electron hydrocarbon cations.     

Investigations of silicon-nitrogen hydrides from reaction of nitrogen atoms with silane: experiments and calculations

Using a quadrupole mass filter and vacuum-ultraviolet ionization, we measured the time-of-flight spectra of species at mass-to-charge ratios of m/ z = 45-42 from the reaction of N + SiH 4 in crossed molecular beams. Species with m/ z = 44 and 43 correspond to reaction products HSiNH/SiNH 2 and HSiN/HNSi, respectively; species with m/ z = 45 and 42 are assigned to isotopic variants and daughter ions, respectively, of those two reaction products. We measured the photoionization yields and branching ratios for dissociative ionization of reaction products as a function of photoionization energy. The ionization thresholds of products HSiNH/SiNH 2 and HSiN/HNSi were determined to be 6.7 and 9.2 eV, respectively. Furthermore, we calculated the equilibrium structures, electronic energies, and vibrational wavenumbers of various silicon-nitrogen hydrides H x SiNH y ( x + y = 0-3) using quantum-chemical methods. SiNH 2 (X (2)B 2) and HNSi (X (1)Sigma (+)) are more stable than HSiNH (X (2)A') and HSiN (X (1)Sigma (+)) by 0.82 and 2.81 eV, respectively. SiNH 2 (X (2)B 2), HSiNH (X (2)A'), HNSi (X (1)Sigma (+)), and HSiN (X (1)Sigma (+)) have adiabatic ionization energies of 6.81, 8.19, 10.21, and 10.23 eV, respectively. These experimental and calculated results indicate that SiNH 2 (X (2)B 2) and HNSi (X (1)Sigma (+)) are dominant among isomeric products in the reaction of N + SiH 4. This work presents the first observation of products from the reaction of N + SiH 4 in crossed beams and extensive calculations on pertinent silicon-nitrogen hydrides.     

Transition metals as electron traps. II. Structures,energetics and electron transfer dissociations of ternary Co,Ni and Zn–peptide complexes in the gas phase

Transition metal cations Co²⁺, Ni²⁺ and Zn²⁺ form 1 : 1 : 1 ternary complexes with 2,2′‐bipyridine (bpy) and peptides in aqueous methanol solutions that have been studied for tripeptides GGG and GGL. Electrospray ionization of these solutions produced singly charged [Metal(bpy)(peptide ? H)]⁺ and doubly charged [Metal(bpy)(peptide)]²⁺ ions (Metal = metal ion) that underwent charge reduction by glancing collisions with Cs atoms at 50 and 100 keV collision energies. Electron transfer to [Metal(bpy)(peptide)]²⁺ ions was less than 4.2 eV exoergic and formed abundant fractions of non‐dissociated charge‐reduced intermediates. Charge‐reduced [Metal(bpy)(peptide)]⁺ ions dissociated by the loss of a hydrogen atom, ammonia, water and ligands that depended on the metal ion. The Ni and Co complexes mainly dissociated by the elimination of ammonia, water, and the peptide ligand. The Zn complex dissociated by the elimination of ammonia and bpy. A sequence‐specific fragment was observed only for the Co complex. Electron transfer to [Metal(bpy)(peptide ? H)]⁺ was 0.6–1.6 eV exoergic and formed intermediate radicals that were detected as stable anions after a second electron transfer from Cs. [Metal(bpy)(peptide ? H)] neutrals and their anions dissociated by the loss of bpy and peptide ligands with branching ratios that depended on the metal ion. Optimized structures for several spin states, electron transfer and dissociation energies were addressed by combined density functional theory and Møller–Plesset perturbational calculations to aid interpretation of experimental data. The experimentally observed ligand loss and backbone cleavage in charge‐reduced [Metal(bpy)(peptide)]⁺ complexes correlated with the dissociation energies at the present level of theory. The ligand loss in ⁺CR^? spectra showed overlap of dissociations in charge‐reduced [Metal(bpy)(peptide ? H)] complexes and their anionic counterparts which complicated spectra interpretation and correlation with calculated dissociation energies. Copyright © 2009 John Wiley & Sons, Ltd.     

Charge transfer reactions of organic ions containing oxygen: Correlation between reaction energetics and cross sections

Cross sections for charge transfer reactions of organic ions containing oxygen have been obtained using time-of-flight techniques. Charge transfer cross sections have been determined for reactions of 2.0 to 3.4 keV ions produced by electron impact ionization of oxygen containing molecules such as methanol, ethanal and ethanol. Experimental cross section magnitudes have been correlated with reaction energy defects computed from ion recombination energies and target ionization energies. Large cross sections are observed for reacting systems with small energy defects.     

Charge Transfer Dissociation (CTD) Mass Spectrometry of Peptide Cations Using Kiloelectronvolt Helium Cations

A kiloelectronvolt beam of helium ions is used to ionize and fragment precursor peptide ions starting in the 1+ charge state. The electron affinity of helium cations (24.6 eV) exceeds the ionization potential of protonated peptides and can therefore be used to abstract an electron from—or charge exchange with—the isolated precursor ions. Kiloelectronvolt energies are used, (1) to overcome the Coulombic repulsion barrier between the cationic reactants, (2) to overcome ion-defocussing effects in the ion trap, and (3) to provide additional activation energy. Charge transfer dissociation (CTD) of the [M+H]⁺ precursor of Substance P gives product ions such as [M+H]^2+? and a dominant series of a ions in both the 1+ and 2+ charge states. These observations, along with the less-abundant a + 1 ions, are consistent with ultraviolet photodissociation (UVPD) results of others and indicate that C–C_α cleavages are possible through charge exchange with helium ions. Although the efficiencies and timescale of CTD are not yet suitable for on-line chromatography, this new approach to ion activation provides an additional potential tool for the interrogation of gas phase ions. Graphical Abstract

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Cross sections and ion kinetic energy analysis for the electron impact ionization of acetylene

Using a Nier-type electron impact ion source in combination with a double focusing two sector field mass spectrometer, partial cross sections for electron impact ionization of acetylene are measured for electron energies up to 1000 eV. Discrimination factors for ions are determined using the deflection field method in combination with a three-dimensional ion trajectory simulation of ions produced in the ion source. Analysis of the ion yield curves obtained by scanning the deflectors allows the assignment of ions with the same mass-to-charge ratio to specific production channels on the basis of their different kinetic energy distributions. This analysis also allows to determine, besides kinetic energy distributions of fragment ions, partial cross sections differential in kinetic energy. Moreover a charge separation reaction, the Coulomb explosion of the doubly charged parent ions C2H2++ into the fragment ions C2H+ and H+, is investigated and its mean kinetic energy release (KER=3.88 eV) is deduced.     

Charge transfer reactions in alkane and cycloalkane systems. Estimated ionization potentials

Rate constants of change transfer reactions k_CT, involving C₃? C₉ alkanes and cycloalkanes, have been determined in an ion cyclotron resonance mass spectrometer. The rate constants are significantly lower than the corresponding rate constants for collision when the reaction is less than about 0.5 eV exothermic for linear alkane ions, or less than about 0.2 eV exothermic for cycloalkane ions. In this region of low reaction efficiency, the efficiency of reaction with linear or branched alkanes seems to depend primarily on reaction exothermicity. (The efficiencies of reaction of a given ion with cyclic alkanes also depend on ΔH_rn, but are higher than for reactions with other compounds). Although the lowered reaction efficiencies probably result, at least in part, from unfavorable Franck–Condon factors in the energy range near the ionization onset, quantitative correlations between reaction efficiency and estimated relative Franck–Condon factors were not observed. When the enthalpy of reaction is small (less than about ?0.15 eV), it is seen that the reverse charge transfer can also occur, and equilibrium is established under the conditions of these experiments. From the observed equilibrium constants, values for the standard free energy change are derived, and assuming that ΔS is small for electron transfer equilibria, values of ΔH_rn are estimated. In the case of the equilibria involving cyclohexane ion, these values of ΔH_rn lead to estimates of the ionization potentials of methylcyclopentane, 3-methylpentane, n-octane, 2,2-dimethylbutane, and 2,3-dimethylbutane, which are lower than the ionization potentials of cyclohexane, that is, <9.88 eV, although all these compounds had previously been reported to have ionization potentials above 10.03 eV. That the ionization potentials are indeed lower than 10.03 eV is confirmed by determining the quantum yields of ionization with 10.03-eV photons. It is pointed out that the conclusions reached here apparently also apply to the charge transfer reactions of alkane ions in the liquid phase.     

Effect of monovalent cations (Li+, Na+, K+, Cs+) on self-assembly of thiol-modified double-stranded and single-stranded DNA on gold electrode

In this article we investigate the effect of monovalent cations (Li(+), Na(+), K(+), Cs(+)) on self-assembly of thiol-modified double-stranded DNA (ds-DNA) and single-stranded DNA (ss-DNA) on gold electrodes. Electrochemical characteristics (surface coverage, ion penetration and charge transfer) of ds-DNA and ss-DNA self-assembled monolayers (SAMs) formed with different monovalent cations are inspected based on six important interfacial parameters including surface coverage (Γ(m)), interfacial capacitance (C), phase angle (Φ(1 Hz)), ion transfer resistance (R(it)*), current density difference (Δj) and charge transfer resistance (R(ct)) from chronocoulometry (CC), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Three sections are included: (1) Investigation of the relationships of parameters (Γ(m), C, Φ(1 Hz), R(it)*, Δj and R(ct)) for ds-DNA-SAMs and ss-DNA-SAMs with cation types and concentrations; (2) confirmation and explanation of our experimental results combined with our recently proposed simple DNA model and literature reports; (3) exploration of the mechanism for the orders of monovalent cations (Li(+), Na(+), K(+), Cs(+)) on availing the adsorption of ds-DNA and ss-DNA molecules on gold based on their physicochemical parameters (ion size, solvation free energy and enthalpy, ion-water bond length and water exchange rate) and possible binding modes with DNA molecules. This work might provide a useful reference for understanding interactional mechanism of cations with DNA molecules.     

Vibrationally resolved rate coefficients and branching fractions in the dissociative recombination of O2+

We have studied the dissociative recombination of the first three vibrational levels of O(2) (+) in its electronic ground X (2)Pi(g) state. Absolute rate coefficients, cross sections, quantum yields and branching fractions have been determined in a merged-beam experiment in the heavy-ion storage ring, CRYRING, employing fragment imaging for the reaction dynamics. We present the absolute total rate coefficients as function of collision energies up to 0.4 eV for five different vibrational populations of the ion beam, as well as the partial (vibrationally resolved) rate coefficients and the branching fractions near 0 eV collision energy for the vibrational levels v=0, 1, and 2. The vibrational populations used were produced in a modified electron impact ion source, which has been calibrated using Cs-O(2)(+) dissociative charge transfer reactions. The measurements indicate that at low collision energies, the total rate coefficient is weakly dependent on the vibrational excitation. The calculated thermal rate coefficient at 300 K decreases upon vibrational excitation. The partial rate coefficients as well as the partial branching fractions are found to be strongly dependent on the vibrational level. The partial rate coefficient is the fastest for v=0 and goes down by a factor of two or more for v=1 and 2. The O((1)S) quantum yield, linked to the green airglow, increases strongly upon increasing vibrational level. The effects of the dissociative recombination reactions and super elastic collisions on the vibrational populations are discussed.     

Unimolecular dissociations of excited C3H6O+: A comparative photoelectron—photoion coincidence study of 1,2-epoxypropane and acetone

The unimolecular fragmentation of internal energy selected 1,2-epoxypropane cations has been studied by fixed-wavelength photoelectron—photoion coincidence spectroscopy. Branching ratios for the prominent fragment ions are reported up to an ionization energy of I = 14 eV. It is shown that 1,2-epoxypropane cations initially formed with none or only little vibrational excitation in the electronic ground state do not dissociate, though their excess energy with respect to the lowest energetic fragmentation pathway is 1.25 eV. As the internal energy is increased, slow fragmentation into several dissociation channels is observed. This is used to explain a comparably slow dissociation process observed in the case of acetone molecular ions initially excited to their electronic à state. CH₂C(OH)CH₃⁺ and/or CH₃CHCHOH⁺ are proposed as precursors for these low-rate unimolecular reactions.     

Gas-phase ion-molecule bimolecular and clustering reactions in dilute solutions of acetonitrile in xenon,krypton, argon,neon and helium

Gas-phase bimolecular and clustering reactions of acetonitrile in Xe, Kr, Ar, Ne and He were studied at high chemical ionization pressures in the new coaxial ion source at Auburn. With electron energies near the ionization threshold, the mass spectra are exceedingly simple and are comprised of [CH₄CH]⁺ and clusters of [CH₄CN]⁺ with various ligands such as H₂O and CH₃CN. At higher electron energies many other peaks appear. The intensities of the new peaks depend upon the ionization potential of the charge transfer gas, the ionizing electron energy and the ion source conditions, and are due to reactions of fragment ions. Residence time distributions at electron energies above the ionization threshold (∼ 30 eV) demonstrate that two molecular structures are present in the ion beam at m/z 42, one presumably is protonated acetonitrile ([CH₃CNH]⁺) while the evidence indicates that the second species does not contain acidic hydrogens. With ionizing electron energies near threshold (∼ 10. 5 eV) only one structure is observed. Studies with electron energies near the ionization threshold under high-pressure chemical ionization conditions result in greatly simplified mass spectra and are possible only because of the coaxial geometry of the ion source.     

Reorganization energy, activation energy, and mechanism of hole transfer process in DNA: a theoretical study

The density functional calculations with aug-cc-pVDZ basis sets on cationic guanine-cytosine (GC(+)) and adenine-thymine (AT(+)) base pairs suggest that the cationic charge is almost entirely localized on the G and A units with significant changes in the N-H and N...O distances around the H-bonded area. While the calculated intramolecular reorganization energy (lambda(v)) for a GC base pair (0.75 eV) is remarkably larger than that for an isolated G base (0.49 eV), for the AT base pairs these values (0.44 and 0.40 eV) are almost the same. The gas phase activation energies (E(a)) for GC(+)GC-->GCGC(+), AT(+)AT-->ATAT(+), and GC(+)AT-->GCAT(+) hole transfer processes are 0.19, 0.11, and 0.73 eV with rate constants of 1.69 x 10(11), 3.15 x 10(11), and 4.61(0.168) s(-1), respectively, at 298 K. An alternative mechanism of hole transfer has been proposed on the basis of energy barriers.