Ion Branching Ratios Research Articles

Selected ion flow tube studies of several siloxanes

People are using increasing amounts of siloxanes that ultimately end up in landfills and then in landfill gas and biogas digesters. Their presence poses difficulties for industries seeking to utilize the energy content of landfill and biogas, as the combustion process oxidises silicon to silicon dioxide that in turn damages engine parts. Rapid, efficient and accurate methods are needed to quantify their presence. Selected ion flow tube mass spectrometry (SIFTMS) is an emerging real-time technique that has found application for monitoring trace volatiles in air. Samples containing the trace volatiles are simply drawn into the flow tube and convected in a stream of helium. Chemical ionization reactions from mass-selected reagent ions with the volatiles ensue. To quantify the volatiles in the sample, the ion chemistry of the reagent ion with each volatile must be known. Rate coefficients and product ion branching ratios were found for the compounds dodecamethylpentasiloxane, decamethylcyclopentasiloxane, decamethyltetrasiloxane, octamethylcyclotetrasiloxane, triethylsilanol, tetramethylsilane and hexamethyldisilazane. The ion-molecule reactions of the seven silicon-containing compounds examined here were fast, occurring at or near the collision rate, thus allowing for detection at low levels. The very simple reaction chemistry found of proton transfer, electron transfer and methyl loss will enable easy quantitation of the siloxanes in landfill gas and biogas using the SIFTMS technique.

Relationships between DFT/RRKM Branching Ratios of the Complementary Fragment Ions [C5H5O]+ and [M – C5H5O]+ and Relative Abundances in the EI Mass Spectrum of N-(2-Furylmethyl)aniline

The energy-dependent branching ratios of the complementary fragment ions [C(5)H(5)O](+) and [HC(6)H(4)NH](+) ([M - C(5)H(5)O](+)), originating from the N-(2-furylmethyl)aniline molecular ion, [HC(6)H(4)NH-C(5)H(5)O](+•), were obtained from Rice-Ramsperger-Kassel-Marcus (RRKM) rate calculations based on density functional theory (DFT) energy profiles. The UB3LYP/6-311G+(3df,2p)//UB3LYP/6-31G(d) level of theory was used to model the competitive reaction mechanisms by which the molecular ion can be fragmented. Initially, eight pairs of products were taken into account, corresponding to the combination of two isomeric structures for each fragment ion and the concomitant radicals, which can be formed by direct dissociations or through some isomerization-fragmentation pathways. A great deal of the obtained pathways was discarded by looking over the kinetic barrier heights and the individual RRKM rate coefficients calculated for all the steps. This way, the potential energy profiles were simplified to only three reaction channels, two pathways to [C(5)H(5)O](+) and one to [M - C(5)H(5)O](+). The pre-equilibrium and steady-state approximations were then applied to different regions of the remaining potential energy profiles, allowing the branching ratios of the complementary fragment ions to be easily calculated and discriminated among the three rival processes. According to these results, the major fragment ion in the ion source is [C(5)H(5)O](+), which is produced as a mixture of two structures, the furfuryl and pyrylium cations, one formed by a direct C-N bond cleavage and the other through an isomerization-fragmentation channel. In turn, the direct fragmentation is the only mechanism to produce [M - C(5)H(5)O](+). To confront these results with the available experimental information, the model was broadened out to the 4-substituted analogues [4-R-C(6)H(4)NH-C(5)H(5)O](+•) in which R = F, Br, Cl, CH(3), and OCH(3), finding excellent correlations of the calculated branching ratios and the relative abundances in the electron ionization mass spectra.

Gas-Phase Reactions of Microsolvated Fluoride Ions: An Investigation of Different Solvents

The gas-phase reactions of F(-)(DMSO), F(-)(CH(3)CN), and F(-)(C(6)H(6)) with t-butyl halides were investigated. Reaction rate constants, kinetic isotope effects, and product ion branching ratios were measured using the flowing afterglow selected ion flow tube technique (FA-SIFT). Additionally, the structure of F(-)(DMSO) was investigated both computationally and experimentally, and two stable isomers were identified. The reactions generally proceed by elimination mechanisms; however, the reaction of F(-)(C(6)H(6)) with t-butyl chloride occurs by a switching mechanism. These reactions are compared to previous studies of microsolvated reactions of t-butyl halides where the solvent molecules were polar, protic molecules.

A selected ion flow tube study of the reactions of H3O+, NO+ and O2+• with seven isomers of hexanol in support of SIFT-MS

A selected ion flow tube, SIFT, study has been carried out of the reactions of H3O+, NO+ and O2+• with seven structural isomers of hexanol with the common molecular formula C6H14O and molecular weight 102 that are commonly met in food science studies. The main objective is to provide the kinetic data, i.e., the rate constants and the product ion branching ratios to be included in the SIFT-MS kinetics database that would allow the separate identification and quantification of these compounds. The specific compounds involved are the primary alcohols 1-hexanol, CH3(CH2)5OH; 2-ethyl-1-butanol, (C2H5)2CHCH2OH; 4-methyl-1-pentanol, (CH3)2CH(CH2)3OH; secondary alcohols 2-hexanol, CH3(CH2)3CH(OH)CH3; 4-methyl-2-pentanol, (CH3)2CHCH2CH(OH)CH3; 3-hexanol, CH3(CH2)2CH(OH)CH2CH3 and a tertiary alcohol 3-methyl-3-pentanol, (CH3CH2)2C(CH3)OH. The reactions of H3O+ proceed via dissociative proton transfer invariably producing C6H13+ hydrocarbon ions (m/z 85). The reactions of NO+ proceed predominantly via hydride ion transfer producing C6H13O+ ions (m/z 101) for the primary and secondary hexanols, with a minor fragmentation channel for the primary hexanols, when a H2O molecule is lost from the major product ion resulting in C6H11+ ions (m/z 83), and a minor channel representing the process of hydroxide ion transfer producing C6H13+ ions (m/z 85) for secondary alcohols. The latter reaction is also the dominant process for the tertiary alcohol. O2+• reacts by dissociative charge transfer in all reactions resulting in multiple product ions analogous to electron ionization. The differences in product ion branching ratios and fragmentation patterns can thus assist SIFT-MS identification of the different hexanol isomers in cases when they are not present in complex mixtures.

Auger decay of 3p-ionized krypton

A theoretical study of Auger cascades during the decay of 3${p}_{1/2}$ and 3${p}_{3/2}$ vacancies in krypton has been performed by level-by-level calculations using a wide configuration interaction basis. Auger spectra for all steps of the cascades are presented and are compared with the existing experimental data. Good agreement of our results with the branching ratios of ions measured by a coincidence technique is obtained.

Proton transfer reaction mass spectrometry investigations on the effects of reduced electric field and reagent ion internal energy on product ion branching ratios for a series of saturated alcohols

In this paper we report an investigation of the effects of E/ N over the range of 90–140 Td on the product ions resulting from the reactions of H 3O + with 12 saturated alcohols using a proton transfer reaction mass spectrometer (PTR-MS). The alcohols included in this study are methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-methyl-1-propanol, 2-methyl-2-propanol, 2-butanol, cyclopentanol, 1-pentanol, cyclohexanol, and 1-hexanol. Only in the cases of methanol and ethanol are any substantial amounts of the protonated parent observed at any E/ N. For the other saturated alcohols predominantly fragment ions are observed. This implies that attempts to identify and hence monitor saturated alcohols in trace concentrations in a complex chemical environment using PTR-MS will be fraught with difficulties because a given m/ z will not be unique to a particular chemical compound, i.e., multiple species could be present at a given m/ z. In addition to changes in E/ N we present preliminary results with regards to changing the conditions in the generation of the reagent ions via altering the operational conditions within the ion source (a hollow cathode). We present product ion branching ratios as a function of hollow cathode emission current for cyclohexanol, 1-propanol and 2-propanol at fixed E/ N. Although not part of the reaction chamber, we have found that changing the hollow cathode emission current results in modifications to the product ion branching ratios. We presume that these observed changes are a result of altering the internal energies of the reagent ions and thereby modify the reaction kinetics and dynamics occurring within the drift tube of a PTR-MS.

Reactions of Negative Ions with ClN3 at 300 K

The reactivity of ClN(3) with 17 negative ions has been investigated at 300 K. The electron affinity (EA) of ClN(3) was bracketed to be between that of NO(2) and N(3), giving EA(ClN(3)) = 2.48 +/- 0.20 eV, in agreement with an electronic structure calculation. Reaction rate constants and product ion branching ratios were measured. In nearly all cases the major product of the reaction was chloride ions. Charge transfer, N(3)(-) production, and O atom incorporation is also observed. DFT calculations of stable complexes and transition states are presented for two typical ions. Mechanistic details are discussed in terms of reaction coordinate diagrams.

Ion Chemistry of VX Surrogates and Ion Energetics Properties of VX: New Suggestions for VX Chemical Ionization Mass Spectrometry Detection

Room temperature rate constants and product ion branching ratios have been measured for the reactions of numerous positive and negative ions with VX chemical warfare agent surrogates representing the amine (triethylamine) and organophosphonate (diethyl methythiomethylphosphonate (DEMTMP)) portions of VX. The measurements have been supplemented by theoretical calculations of the proton affinity, fluoride affinity, and ionization potential of VX and the simulants. The results show that many proton transfer reactions are rapid and that the proton affinity of VX is near the top of the scale. Many proton transfer agents should detect VX selectively and sensitively in chemical ionization mass spectrometers. Charge transfer with NO(+) should also be sensitive and selective since the ionization potential of VX is small. The surrogate studies confirm these trends. Limits of detection for commercial and research grade CIMS instruments are estimated at 80 pptv and 5 ppqv, respectively.

Survey of the Reactivity of O2(a 1Δg) with Negative Ions

The reactivity of O(2)(a (1)Delta(g)) was studied with a series of anions, including (-)CH(2)CN, (-)CH(2)NO(2), (-)CH(2)C(O)H, CH(3)C(O)CH(2)(-), C(2)H(5)O(-), (CH(3))(2)CHO(-), CF(3)CH(2)O(-), CF(3)(-), HC(2)(-), HCCO(-), HC(O)O(-), CH(3)C(O)O(-), CH(3)OC(O)CH(2)(-), and HS(-). Reaction rate constants and product ion branching ratios were measured. All of the carbanions react through a common pathway to produce their major products. O(2)(a) adds across a bond at the site of the negative charge, resulting in the cleavage of this bond and the O=O bond. Oxyanions react through a hydride transfer to produce their major products. Proton transfer within these product ion-dipole complexes can occur, where the final branching ratios reflect the basicity of the resulting anions. Several of these anions (CF(3)(-), HC(2)(-), CH(3)OC(O)CH(2)(-)) were also found to undergo several sequential reactions within a single encounter. These three basic types of mechanisms are supported by calculations; a potential energy diagram for each type of reaction has been calculated. Additionally, six of these reactions had been qualitatively studied before; our results are in agreement with previous data.

Kinetics of Ion−Molecule Reactions with Dimethyl Methylphosphonate at 298 K for Chemical Ionization Mass Spectrometry Detection of GX

Kinetics studies of a variety of positive and negative ions reacting with the GX surrogate, dimethyl methylphosphonate (DMMP), were performed. All protonated species reacted rapidly, that is, at the collision limit. The protonated reactant ions created from neutrals with proton affinities (PAs) less than or equal to the PA for ammonia reacted exclusively by nondissociative proton transfer. Hydrated H(3)O(+) ions also reacted rapidly by proton transfer, with 25% of the products from the second hydrate, H(3)O(+)(H(2)O)(2), forming the hydrated form of protonated DMMP. Both methylamine and triethylamine reacted exclusively by clustering. NO(+) also clustered with DMMP at about 70% of the collision rate constant. O(+) and O(2)(+) formed a variety of products in reactions with DMMP, with O(2)(+) forming the nondissociative charge transfer product about 50% of the time. On the other hand, many negative ions were less reactive, particularly, SF(5)(-), SF(6)(-), CO(3)(-), and NO(3)(-). However, F(-), O(-), and O(2)(-) all reacted rapidly to generate m/z = 109 amu anions (PO(3)C(2)H(6)(-)). In addition, product ions with m/z = 122 amu from H(2)(+) loss to form H(2)O were the dominant ions produced in the O(-) reaction. NO(2)(-) underwent a slow association reaction with DMMP at 0.4 Torr. G3(MP2) calculations of the ion energetics properties of DMMP, sarin, and soman were also performed. The calculated ionization potentials, proton affinities, and fluoride affinities were consistent with the trends in the measured kinetics and product ion branching ratios. The experimental results coupled with the calculated ion energetics helped to predict which ion chemistry would be most useful for trace detection of the actual chemical agents.

The essential role of many-electron Auger transitions in the cascades following the photoionization of 3p and 3d shells of Kr

Using the relativistic configuration interaction method it is shown that many-electron Auger transitions—discrete double Auger effect and hypersatellite Auger transitions—play an essential and even dominant role in the de-excitation of 3d−1 and 3p−1 vacancy states in Kr. Also various other many-electron Auger transitions (transitions with a shift of vacancy into a deeper shell as well as triple Auger transitions and hypersatellite Auger transitions with additional excitation of electrons) manifest themselves during the cascades due to the correlation effects and appearance of configurations with several vacancies. Good correspondence with the branching ratios of ions measured by the coincidence technique is obtained, and thus the existing discrepancy with the previous calculation results is removed.

Unimolecular Dissociation Reactions of Methyl Benzoate Radical Cation

The blackbody infrared radiation induced dissociation of methyl benzoate (C8H8O2(+*)) radical cation was investigated by using a Fourier transfer ion cyclotron resonance mass spectrometer equipped with a resistively heated (wire temperatures of 400-1070 K) wire ion guide. We observed product ion branching ratios that are strongly dependent upon wire temperature. At low temperatures (670-890 K) the major product ion C7H8 (+*) (m/z 92), which is formed by loss of CO2, and at higher temperatures (above 900 K), loss of methoxy radical ((*)OCH3) competes with loss of CO2. The energies of the various reactant ions and transition states for product ion formation were estimated by using density functional theory molecular orbital calculations, and a proposed mechanism for the dissociation chemistry of C8H8O2 (+*) involving a multistep rearrangement reaction is tested using the Master Equation formalism.

Kinetics of Ion−Molecule Reactions with 2-Chloroethyl Ethyl Sulfide at 298 K: A Search for CIMS Schemes for Mustard Gas

The rate constants and product ion branching ratios have been measured in a selected ion flow tube (SIFT) at 298 K for a variety of positive and negative ions reacting with 2-chloroethyl ethyl sulfide (2-CEES), a surrogate for mustard gas (HD). This series of experiments is designed to elucidate ion-molecule reactions that have large rate constants and produce unique product ions to guide the development of chemical ionization mass spectrometry (CIMS) detection methods for the chemical weapon agent using the surrogate instead. The negative ions typically used in CIMS instruments are essentially unreactive with 2-CEES, that is, SF 6 (-), SF 4 (-), CF 3O (-), and CO 3 (-). A few negative ions such as NO 2 (-) and NO 3 (-) undergo three-body association to give a unique product ion, but the bimolecular rate constants are small in the SIFT. Positive ions typically react at the collisional limit, primarily by charge and proton transfer, some of which is dissociative. For ions with high proton binding energies, association with 2-CEES has also been observed. Many of these reactions produced ions with the 2-CEES intact, including the parent cation, the protonated cation, and clusters. G3(MP2) calculations of the thermochemical properties for 2-CEES and mustard have been performed, along with calculations of the structures for the observed product cations. Reacting a series of protonated neutral molecules with 2-CEES brackets the proton affinity (PA) to between 812 ((CH 3) 2CO) and 854 (NH 3) kJ mol (-1). G3(MP2) calculations give a PA for 2-CEES of 823 kJ mol (-1) and a PA for mustard of 796 kJ mol (-1), indicating that the present results for 2-CEES should be directly transferable to mustard to design a CIMS detection scheme.

Selected Ion Flow Tube Study of the Ion−Molecule Reactions of Monochloroethene, Trichloroethene, and Tetrachloroethene

Data for the rate coefficients and product cations of the reactions of a large number of atomic and small molecular cations with monochloroethene, trichloroethene, and tetrachloroethene in a selected ion flow tube at 298 K are reported. The recombination energy of the ions range from 6.27 (H3O(+)) through to 21.56 (Ne(+)) eV. Collisional rate coefficients are calculated by modified average dipole orientation theory and compared with experimental values. Thermochemistry and mass balance predict the most feasible neutral products. Together with previously reported results for the three isomers of dichloroethene ( Mikhailov, V. A. ; Parkes, M. A. ; Tuckett, R. P. ; Mayhew, C. A. J. Phys. Chem. A 2006, 110, 5760 ), the fragment ion branching ratios have been compared with those from threshold photoelectron photoion coincidence spectroscopy over the photon energy range of 9-22 eV to determine the importance or otherwise of long-range charge transfer. For ions with recombination energy in excess of the ionization energy of the chloroethene, charge transfer is energetically allowed. The similarity of the branching ratios from the two experiments suggest that long-range charge transfer is dominant. For ions with recombination energy less than the ionization energy, charge transfer is not allowed; chemical reaction can only occur following formation of an ion-molecule complex, where steric effects are more significant. The products that are now formed and their percentage yields are a complex interplay between the number and position of the chlorine atoms with respect to the C=C bond, where inductive and conjugation effects can be important.

Reactions of small negative ions with O 2(a 1Δ g) and O 2(X 3Σ g−)

The rate constants and product ion branching ratios were measured for the reactions of various small negative ions with O 2(X 3Σ g −) and O 2(a 1Δ g) in a selected ion flow tube (SIFT). Only NH 2 − and CH 3O − were found to react with O 2(X) and both reactions were slow. CH 3O − reacted by hydride transfer, both with and without electron detachment. NH 2 − formed both OH −, as observed previously, and O 2 −, the latter via endothermic charge transfer. A temperature study revealed a negative temperature dependence for the former channel and Arrhenius behavior for the endothermic channel, resulting in an overall rate constant with a minimum at 500 K. SF 6 −, SF 4 −, SO 3 − and CO 3 − were found to react with O 2(a 1Δ g) with rate constants less than 10 −11 cm 3 s −1. NH 2 − reacted rapidly with O 2(a 1Δ g) by charge transfer. The reactions of HO 2 − and SO 2 − proceeded moderately with competition between Penning detachment and charge transfer. SO 2 − produced a SO 4 − cluster product in 2% of reactions and HO 2 − produced O 3 − in 13% of the reactions. CH 3O − proceeded essentially at the collision rate by hydride transfer, again both with and without electron detachment. These results show that charge transfer to O 2(a 1Δ g) occurs readily if the there are no restrictions on the ion beyond the reaction thermodynamics. The SO 2 − and HO 2 − reactions with O 2(a) are the only known reactions involving Penning detachment besides the reaction with O 2 − studied previously [R.S. Berry, Phys. Chem. Chem. Phys., 7 (2005) 289–290].

Selected ion flow tube cation–molecule reaction studies and threshold photoelectron photoion coincidence spectroscopy of cyclic-C5F8

The product ion branching ratios and rate coefficients have been measured using a selected ion flow tube (SIFT) at 298 K for the bimolecular reactions of cyclic-C5F8 with several atomic and molecular cations. The majority of reactions occur at the collisional rate calculated by the modified average dipole orientation theory, with the exception of H2O+ for which the reaction efficiency is only 55%. Apart from H2O+ and N+, the similarity of the product ion branching ratios determined from threshold photoelectron photoion coincidence (TPEPICO) and ion-molecule data suggests that long-range electron transfer is the dominant mechanism for reactions involving ions with recombination energies between 12 and 17 eV. For N+, the product ion branching ratios are very different to those produced by photoionisation; this result may be explained if some of the N-atom products are formed electronically excited. The onset of an ionisation signal of c-C5F8 measured by TPEPICO spectroscopy occurs at 12.25 +/- 0.05 eV. This is much higher than the value of the first adiabatic ionisation energy determined from electron ionisation (11.24 +/- 0.10 eV), He (I) photoionisation (11.30 +/- 0.05 eV), and an independent high resolution threshold photoelectron spectrum (11.237 +/- 0.002 eV). The ground electronic state of c-C5F8+ has very weak intensity under threshold electron conditions. The TPEPICO spectrum of c-C5F8 recorded from 12-23 eV shows detection of the parent ion and the daughter ions C4F6+ and C5F7+, with their appearance energies increasing in this order. Ion yield curves and branching ratios have been determined. Using Gaussian 03, the enthalpy of formation of c-C5F8 at 298 K has been determined to be -1495 kJ mol(-1).

Determination of the Gas-Phase Acidities of Cysteine-Polyalanine Peptides Using the Extended Kinetic Method

We determined the gas-phase acidities of two cysteine-polyalanine peptides, HSCA3 and HSCA4, using a triple-quadrupole mass spectrometer through application of the extended kinetic method with full entropy analysis. Five halogenated carboxylic acids were used as the reference acids. The negatively charged proton-bound dimers of the deprotonated peptides with the conjugate bases of the reference acids were generated by electrospray ionization. Collision-induced dissociation (CID) experiments were carried out at three collision energies. The enthalpies of deprotonation (Delta(acid)H) of the peptides were derived according to the linear relationship between the logarithms of the CID product ion branching ratios and the differences of the gas-phase acidities. The values were determined to be Delta(acid)H(HSCA3) = 317.3 +/- 2.4 kcal/mol and Delta(acid)H (HSCA4) = 316.2 +/- 3.9 kcal/mol. Large entropy effects (Delta(DeltaS) = 13-16 cal/mol K) were observed for these systems. Combining the enthalpies of deprotonation with the entropy term yielded the apparent gas-phase acidities (Delta(acid)G(app)) of 322.1 +/- 2.4 kcal/mol (HSCA3) and 320.1 +/- 3.9 kcal/mol (HSCA4), in agreement with the results obtained from the CID-bracketing experiments. Compared with that in the isolated cysteine residue, the thiol group in HSCA3,4 has a stronger gas-phase acidity by about 20 kcal/mol. This increased acidity is likely due to the stabilization of the negatively charged thiolate group through internal solvation.

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.

Evidence of CH2O(ãA23) and C2H4(ãB1u3) produced from photodissociation of 1,3-trimethylene oxide at 193nm

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.

Threshold photoelectron photoion coincidence spectroscopy and selected ion flow tube reactions of CHF3: comparison of product branching ratios§

The threshold photoelectron and threshold photoelectron photoion coincidence spectra of CHF3 in the range 13.5–24.5 eV have been recorded. Ion yields and branching ratios have been determined for the three fragments , and CF+. The mean kinetic energy releases into fragment ions involving either C–H or C–F bond cleavage have been measured, and compared with statistical and impulsive models. behaves in a non-statistical manner characteristic of the small-molecule limit, with the ground electronic state and low-lying excited states of being largely repulsive along the C–H and C–F coordinates, respectively. The rate coefficients and product ion branching ratios have been measured at 298 K in a selected ion flow tube for the reactions of CHF3 with a large number of gas-phase cations whose recombination energies span the range 6.3 through 21.6 eV. A comparison between the branching ratios from the two experiments, together with an analysis of the threshold photoelectron spectrum of CHF3, shows that long-range charge transfer probably occurs for the Ar+ and F+ atomic ions whose recombination energies lie above ca. 15 eV. Below this energy, the mechanism involves a combination of short-range charge transfer and chemical reactions involving a transition state intermediate. §In honour of Professor John Simons’ 70th birthday Festschrift.