Selected-ion Flow Tube Technique Research Articles

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.

Collision-induced dissociation of fluoropyridinide anions

Collision-induced dissociation of ortho-fluoro, meta-fluoro, and 2,6-difluoropyridinide anions are studied using the selected ion flow tube technique. Structures and energetics of the reactants, transition states, and products are calculated at the MP4(SDQ)/6-31 + G(d) level of theory based on the B3LYP/6-311++G(d,p) and/or MP2/6-31 + G(d) optimized geometries. The monofluoropyridinide anions (C 5NH 3F −) dissociate almost exclusively via loss of an HF molecule, i.e., C 5NH 2 − + HF at low collision energies, in addition to loss of F − at higher energies. 2,6-Difluoropyridinide anions (C 5NH 2F 2 −) dissociate via successive loss of HF molecules to form C 5NHF − then C 5N − depending on the collision energy. The CID results strongly suggest formation of ring-intact pyridynide structures (C 5NH 2 −, C 5NHF −) with a bent triple bond embedded in the azine ring systems. Calculated reaction energy diagrams are totally consistent with the experimental observations. Didehydropyridynides C 5NH 2 − and C 5NHF − have substantial barriers to decomposition. Tetradehydropyridynide C 5N − is a highly strained ring system and metastable with a predicted barrier of about 5 kcal mol −1 (20 kJ mol −1) toward ring-opening to a linear NCCCCC − structure. The observed C 5N − species is most likely the linear anion under experimental conditions; however, the ring-intact C 5N − pyridynide is a highly energetic species releasing about 80 kcal mol −1 (340 kJ mol −1) of energy upon the ring-opening.

Temperature dependence of the oxide ion/ozone reaction in the gas phase

Rate coefficient and branching fraction data were determined for the gas phase reaction of O − + O 3 over the temperature range 123–500 K using the temperature variable selected ion flow tube technique. The reaction rate coefficient is large and relatively insensitive to temperature; reaction takes place on every collision. In contrast, the product distributions are sensitive to temperature. Charge transfer is the major reactive pathway (≥67%) at all temperatures, and its importance increases with increasing temperature. Atom transfer, forming O 2 − + O 2, constitutes approximately 33% of the products at 123 K. The temperature dependence of the branching fraction explains an apparent discrepancy between product data from room temperature flow tube experiments and low energy beam experiments. A reaction mechanism is proposed to qualitatively explain the product branching fraction temperature dependence.

Fundamental Aspects of Gas Phase Ion Chemistry Studied Using the Selected Ion Flow Tube Technique

Gas phase ion chemistry using the selected ion flow tube (SIFT) technique is discussed with the focus on fundamental studies on ion structure, energetics, reactivity, and dynamics: Laser-induced fluorescence (LIF) kinetics/dynamics of N2+(v) ions (vibrationally state-selected ion-molecule reactions; energy transfer processes), mechanisms of prototypical organic ion reactions (SN2 nucleophilic substitution and kinetic isotope effects; hydrogen/deuterium exchange; isotope effects in termolecular association), and ion-molecule reactions with biological implications (SN2 reactions at a nitrogen center; ECO2 elimination reactions of alkyl hydroperoxides). Anion chemistry of small nitroalkanes is also presented. A powerful combination of SIFT and negative ion photoelectron spectroscopy is used to study thermochemistry and structure of energetic and exotic ions (alkyl peroxides; anionic derivatives of diazo and azole compounds, cyclooctatetraene, and cyclopentanone). Finally, some of the new directions in SIFT research are discussed, i.e., reactions of ions with polyatomic organic radicals, and chemical ionization mass spectrometry with new detection schemes.

Coordination chemistry of Fe+, (c-C5H5)Fe+, and (c-C5H5)2Fe+ in the gas phase at room temperature: kinetics of sequential ligation with hydrogen cyanide and cyanoacetylene

Measurements with the selected-ion flow tube technique are reported for the room-temperature kinetics of the sequential ligation of Fe+ and (c-C5H5)Fe+ with HCN and HC3N in helium bath gas at 0.35 Torr. (c-C5H5)2Fe+ was observed not to react. Titration curves of the variation in the apparent bimolecular rate coefficient for ligation with the number of ligands provide cyanide coordination numbers for Fe+ in the absence (4) and presence (3) of c-C5H5. Multicollision induced dissociation experiments indicate the absence of intramolecular ligand–ligand interaction mediated by Fe+.

Gas phase reactions of NH 2Cl with anionic nucleophiles: nucleophilic substitution at neutral nitrogen

Reactions of chloramine, NH 2Cl, with HO −, RO − (R = CH 3, CH 3CH 2, CH 3CH 2CH 2, C 6H 5CH 2, CF 3CH 2), F −, HS −, and Cl − have been studied in the gas phase using the selected ion flow tube technique. Nucleophilic substitution ( S N 2) at nitrogen to form Cl − has been observed for all the nucleophiles. The reactions are faster than the corresponding S N 2 reactions of methyl chloride; the chloramine reactions take place at nearly every collision when the reaction is exothermic. The thermoneutral identity S N 2 reaction of NH 2Cl with Cl −, which occurs approximately once in every 100 collisions, is more than two orders of magnitude faster than the analogous reaction of CH 3Cl. The significantly enhanced S N 2 reactivity of NH 2Cl is consistent with a previous theoretical prediction that the barrier height for the S N 2 identity reaction at nitrogen is negative relative to the energy of the reactants, whereas this barrier height for reaction at carbon is positive. Competitive proton abstraction to form NHCl − has also been observed with more highly basic anions (HO −, CH 3O −, and CH 3CH 2O −), and this is the major reaction channel for HO − and CH 3O −. Acidity bracketing determines the heat of deprotonation of NH 2Cl as 374.4 ± 3.0 kcal mol −1.

Coordination chemistry of ( c-C 5H 5)Fe + in the gas phase at 294 ± 3 K: reactions with the inorganic ligands H 2, H 2O, NH 3, CO, N 2, NO, CO 2, N 2O, and NO 2

Experimental results are reported for primary and higher-order gas-phase reactions of ( c-C 5H 5)Fe + with H 2, H 2O, NH 3, CO, N 2, NO, CO 2, N 2O, and NO 2. Reaction rate coefficients and product distributions were measured with the selected-ion flow tube technique operating at 294 ± 3 K and a helium buffer-gas pressure of 0.35 ± 0.01 Torr. The measurements provide intrinsic efficiencies for ligation and lead to intrinsic coordination numbers based on ligation kinetics. The extent of sequential ligation was found to be strongly dependent on the nature of the ligand. Single ligation of the ( c-C 5H 5)Fe + cation was observed with H 2, N 2, N 2O, and CO 2 whereas multiple ligation was observed with NO, NO 2, CO, H 2O, and NH 3. With NO 2 oxygen-atom transfer was the dominant reaction channel. Structures have been proposed for the ligated ions and relative bond energies were investigated using multicollision induced dissociation.

Flow Tube Studies of Benzene Charge Transfer Reactions from 250 to 1400 K

Temperature dependent rate constants and product branching fractions are reported for reactions of the atmospheric plasma cations NO+, O2+, O+, N+, N2+, and N4+ with benzene, as measured from 250 to 500 K by the selected ion flow tube technique. For the reactions of O2+ and N2+, data have also been obtained between 500 and 1400 K in a high-temperature flowing afterglow. These are among the first determinations of ion−molecule branching fractions above 600 K. Temperature dependent rate constants and product branching fractions are also reported for the reactions of benzene with Kr+, Ar+, Ne+, and F+. All reactions were found to proceed at the collision rate at all temperatures studied. With increasing reactant ion recombination energy, the mechanism changed from association and nondissociative charge transfer to dissociative charge transfer. Primary and secondary dissociation products were observed. Some of the reactivity in the N+ and F+ reactions is attributed to chemical channels. The temperature dependent branching fractions are converted to product ion breakdown curves and compared to previous studies. The current results exhibit a kinetic shift, resulting from slow fragmentation of the C6H6+* complex, combined with collisional stabilization of the complex by the He buffer gas. The pressure dependence of the N+ reaction was examined from 0.35 to 0.8 Torr. The flow tube data provide the first breakdown curve for the C5H3+ product and further indicate that C5H3+ is relatively unreactive, consistent with it having the cyclic ethynyl cyclopropene ion structure. The C3H3+ product was shown to have a cyclic structure, while the C4H4+ product was found to be a mixture of linear and cyclic isomers. The isomeric mixture of C4H4+ products was quantified as a function of the C6H6+* excess energy. A schematic reaction coordinate diagram representing the primary dissociation channels of C6H6+ is constructed from previous experimental and theoretical work. A possible reaction pathway for the C5H3+ product is discussed.

Chlorination of charged buckyballs: reactions of C 60x+ cations ( x = 1–3) with Cl 2, CCl 4, CDCl 3, CH 2Cl 2, and CH 3Cl

The chlorination of singly and multiply charged C 60 cations has been investigated with the selected-ion flow tube technique. Observations are reported for the reactions of C 60 ·+, C 60 2+ and C 60 ·3+ with Cl 2, CCl 4, CDCl 3, CH 2Cl 2 and CH 3Cl at room temperature (295 ± 2 K) in helium at a total pressure of 0.35 ± 0.02 Torr. C 60 ·+ and C 60 2+ were observed not to chlorinate, or react in any other way, with these five molecules. Chlorine also did not react with C 60 ·3+, but bimolecular chloride transfer and electron transfer reactions, reactions that result in charge reduction/charge separation, were observed to occur with CCl 4, CDCl 3, CH 2Cl 2 and CH 3Cl. Chloride transfer was the predominant channel seen with CCl 4, CDCl 3 and CH 2Cl 2 while electron transfer dominates the reaction with CH 3Cl. These results are consistent with trends in chloride affinity and ionization energy. The reluctant chlorination of the first two charge states of C 60 is attributed to the energy required to distort the carbon cage upon bond formation, while the observed chloride transfer to C 60 ·3+ is attributed to the greater electrostatic interactions with this ion.

Quantitative analysis of ammonia on the breath of patients in end-stage renal failure

The distinctive odor of the breath of patients with renal failure, usually termed the fetor, is frequently described as ammoniacal, and its potency is generally believed to reflect the uremic state [11. Previous attempts at breath analysis in these patients has, however, been directed at compounds other than ammonia, such as dimethylamine and trimethylamine, and have been at best semiquantitative in their approach [2]. This reflects the considerable methodological difficulties surrounding the accurate measurement of these compounds on breath. When they are present in low concentrations as part of complex mixtures, the value of conventional methods such as gas chromatography and mass spectrometry is limited because they usually require that the breath is pre-concentrated [3—SI. The selected ion flow tube (SIFT) technique is ideally suited for the accurate measurement of trace gases, particularly when present in complex mixtures [6—81. In contrast to conventional mass spectrometry, in which ionization of the trace gas molecules is achieved by electron bombardment, resulting in molecular cracking that yields spectra that are difficult to interpret, the SIFT technique exploits the method of chemical ionization. Molecules (M) present on the breath are soft ionized utilizing a precursor ion, such as H3O, which usually results in a single product, MH, which can then be detected by a mass spectrometer, thus allowing identification and quantification of trace gases with a sensitivity of 10 to 20 ppb from a single breath exhalation

Validation of the SIFT technique for trace gas analysis of breath using the syringe injection technique

The selected ion flow tube (SIFT) is a technique originally developed for the determination of rate coefficients for gas phase reactions. Recently the SIFT has been applied to the detection of trace gases in air and breath. The SIFT method is capable of determining absolute partial pressures of trace gases in air without the need for calibration. However, it is necessary to show that SIFT determinations are of acceptable accuracy. This study reports the findings of a validation study in which eight trace gases in air, in the concentration range of 100 ppb-30 ppm, were simultaneously detected and quantified. In addition, the results of the measurement of the partial pressure of perchloroethylene (C2Cl4) on the breath of a volunteer some 16 h after a controlled exposure are reported. This study demonstrates that several gases in breath can be rapidly quantified from just a single exhalation.

Selected ion flow tube: a technique for quantitative trace gas analysis of air and breath.

The selected ion flow tube (SIFT) technique for trace gas analysis of air and breath is based on soft chemical ionisation of the trace gases to the exclusion of the major air and breath gases, in fast-flowing inert carrier gas, exploiting the ion-molecule reactions that occur between the trace gases and the pre selected precursor ions (H3O+, NO+ and O2+). The physics and ion chemistry involved in the SIFT technique are described, as are the kinetics of the ion-molecule reactions that are exploited to quantitatively analyse the trace gases. Fast on-line data-acquisition hardware and software have been developed to analyse the mass spectra obtained, from which partial pressures of the trace gases down to about 10 parts per billion can be measured. The time response of the instrument is 20 ms, allowing the profiles of the trace gas concentrations on breath to be obtained during a normal breathing cycle. Pilot results obtained with this SIFT technique include detection and quantification of the most abundant breath trace gases, analysis of cigarette smoke, detection of gases present on smokers' breath and accurate measurement of the partial pressures of NH3, NO and NO2 in air. The simultaneous analysis of several breath trace gases during a single exhalation is clearly demonstrated, and thus different elution times for isoprene and methanol along the respiratory tract are observed. This technique has great potential in many clinical and biological disciplines, and in health and safety monitoring.

Gas-phase chemistry of HSiS- and HSiNH-: ions related to silathioformaldehyde and the silaazomethine of formaldehyde

Anion related to silathioformaldehyde (H 2 Si=S) and the silaazomethine of formaldehyde (H 2 Si=NH) have been prepared and studied using flowing after glow selected ion flow tube techniques. The connectivities of these anions are shown in experimental studies to be [HSiS] - and [HSiNH] - . In addition, ab initio computational studies demonstrate that these are the most stable anion isomers. These anion are prepared by both collisionally-induced dissociation and direct source reactions.

Charge-transfer reactions of C 3+.60: “bracketing” the third ionization energy of C 60

The occurrence or absence of charge transfer from C 3+. 60 to several neutral molecules is reported. The results, which were obtained using the selected-ion flow tube technique operating at 294±2 K and a helium buffer gas pressure of 0.35±0.01 Torr, are in good agreement with earlier Fourier-transform ion cyclotron resonance experiments. Rapid charge transfer, often in competition with other product channels, was noted for the reactions of C 3+. 60 with several neutrals having IE⩽11.02 eV. Charge tranfer to OCS (IE=11.736 eV) was observed to be inefficient, and no charge transfer was seen to C 2H 2 (IE=11.40 eV) or to neutrals having higher ionization energy. Consideration of the effects of Coulombic repulsion upon the occurrence of charge transfer from a triply-charged ion indicates IE(C 2+ 60)=15.6±0.5 eV, in disagreement with a value IE(C 2+ 60)=17.0±0.75 eV determined by charge stripping. A value for the fourth ionization energy of C 60, 19.5 eV, is estimated by extrapolation of the first three ionization energies.

Advances in flowing afterglow and selected-ion flow tube techniques

New developments in flowing afterglow and selected-ion flow tube (SIFT) techniques are briefly reviewed. Particular emphasis is given to the new chemical and physical information that can be obtained with use of the tandem flowing afterglow-triple quadrupole apparatus developed in the author's laboratory. Several outstanding recent achievements in the design and utilization of flowing afterglow and SIFT instruments in other laboratories are briefly highlighted that illustrate the power and flexibility of flow-tube-based methods. These include isotope tracer experiments with the tandem flowing afterglow-SIFT instrument in Boulder, studies of large molecular cluster ions with the variable temperature facility at Penn State, and gas-phase metal ion reactions with the laser ablation/fast flow reactor in Madison. Recent applications of the flowing afterglow-triple quadrupole instrument in our laboratory have made use of collision-induced dissociation (CID) as a tool for synthesizing novel ions and for obtaining new thermo-chemical information from threshold energy measurements. Collision-induced decar☐ylation of organic car☐ylate ions provides access to a variety of unusual and highly basic carbanions that cannot be generated with conventional ion sources. The formation and properties of saturated alkyl ions and studies of gas-phase reactions of the methyl anion are briefly described. We have developed a new method for carrying out “preparative CID” in a flowing afterglow with use of a mini-drift tube; some recent applications of this new ion source are presented. Measurement of CID thresholds for simple cleavage reactions of thermalized ions can provide accurate measures of bond strengths, gas-phase acidities and basicities, and heats of formation for ions and reactive neutral species. Applications of this approach in the thermochemical characterization of carbenes, benzynes and biradicals are described. Future prospects for the continued development of flow reactor-based methods are also briefly discussed.

Reactions of Ne+ and Ne+2 ions with several molecular species at 300 K: the importance of energy resonance, Franck-Condon factors and electron correlation effects on reaction efficiencies

Using the selected ion flow tube technique, the rate coefficients and reaction pathways have been obtained at 300 K for the reactions of Ne+ and Ne+2 with a variety of molecules including NO, O2, CO, H2, N2, OCS, N2O, CO2, NO2, H2S, SO2, NH3, C2H2, CH4, C2H4, C2H6, C3H8 and n-C4H10. During the course of this work the rate coefficients for the reactions of HeNe+ with these molecules were also obtained and are therefore presented in this report. The majority of reactions proceed via non-dissociative and/or dissociative thermal energy charge transfer processes. Using available photoelectron spectra, the influences of energy resonances and Franck-Condon factors on the efficiency of these processes are examined. There are indications that for many of the reactions distortions of the Franck-Condon factors occur leading to more efficient reactions than would otherwise be expected. Evidence for the importance of electron correlation effects during thermal energy charge transfer is presented. This should have far reaching implications outside of this study.

Gas-phase reactions of silicon ion (Si+ with ammonia and the amines (CH3)xNH3-x(X = 1-3): possible ion-molecule reaction pathways toward SiH, SiCH, SiNH, SiCH3, SiNCH3, and H2SiNH

Rate constants and product distributions have been determined for gas-phase reactions of ground-state Si/sup +/(/sup 2/P) ions with ammonia and the amines (CH/sub 3/)/sub x/NH/sub 3-x/ (x = 1-3) at 296 +- 2 K with the selected-ion flow tube technique. All reactions were observed to be fast and can be understood in terms of Si/sup +/ insertion into N-H and C-N bonds to form ions of the type SiNR/sub 1/R/sub 2//sup +/ (r/sub 1/, R/sub 2/ = H, CH/sub 3/) proceeding in competition (in the case of the amines) with hydride ion transfer to form immonium ions of the type CH/sub 2/NR/sub 1/R/sub 2//sup +/ (R/sub 1/, R/sub 2/ = H, CH/sub 3/). C-N bond insertion appears more efficient than N-H bond insertion. The contribution of hydride ion transfer increases with increasing stability of the immonium ion. The latter reaction leads directly to SiH as a neutral product. Other minor reaction channels were seen which lead directly or indirectly to SiCH and SiCH/sub 3/. Rapid secondary proton transfer reactions were observed for SiNH/sub 2//sup +/ and SiNHCH/sub 3//sup +/ to produce gas-phase SiNH and SiNCH/sub 3/ molecules. With methylamine SiNH/sub 2//sup +/ also appears to produce H/sub 2/SiNH/sub 2//sup +/more » which may deprotonate to form the simplest silanimine, H/sub 2/SiNH, or aminosilylene, HSiNH/sub 2/. Reactions of this type are of interest in molecular synthesis and here are proposed to contribute to the formation of SiH, SiCH, SiNH, SiCH/sub 3/, and H/sub 2/SiNH or HSiNH/sub 2/ in partially ionized interstellar gas clouds containing silicon, ammonia, and methylamines.« less

Spectroscopy of N+2 Meinel bands and CO+ comet tail bands

The N+2[X2 Σ+g(v″=0)– A 2Πu(v′=5)] Meinel band and the CO+[X 2Σ+(v″=0)– A 2Π(v′=1,2)] comet tail bands have been measured with Doppler-limited resolution in a selected ion flow tube (SIFT) apparatus using laser induced charge transfer. The charge transfer medium was Ar, which does not react with those ions in their ground levels, but undergoes a strong charge transfer reaction with the excited states under consideration here. The use of a single mode laser and the SIFT technique led to much higher signal-to-noise than obtained in earlier work. These measurements are the first reported with Doppler-limited resolution on any of these bands, and are analyzed to provide substantially improved molecular constants for the excited states involved in the transitions.

ChemInform Abstract: DETERMINATION OF PROTON AFFINITIES FROM THE KINETICS OF PROTON TRANSFER REACTIONS. VII. THE PROTON AFFINITIES OF O2, H2, KR, O, N2, XE, CO2, CH4, N2O, AND CO

The flowing afterglow and selected ion flow tube techniques are applied in a room temperature study of the kinetics of proton‐transfer reactions of the type XH++Y?YH++X for X or Y=He, H2, O2, Kr, O, N2, Xe, CO2, CH4, CD4, N2O, OH, and CO, and for the deuteration of O2 by D3+. Equilibrium constants are determined and changes in thermodynamic state properties (ΔG°, ΔH°, ΔS°) are derived for the reactions with X/Y=H2/O2, O2/Kr, H2/Kr, H2/N2, O/N2, N2/Xe, Xe/CO2, CO2/CH4, CH4/N2O, and N2O/CO. Proton affinities are reported for O2, H2, Kr, O, N2, Xe, CO2, CH4, N2O, and CO, together with the heats of formation of their protonated species. Also a correlation is presented between the kinetic and thermodynamic results obtained in this study.

Determination of proton affinities from the kinetics of proton transfer reactions. VII. The proton affinities of O2, H2, Kr, O, N2, Xe, CO2, CH4, N2O, and CO

The flowing afterglow and selected ion flow tube techniques are applied in a room temperature study of the kinetics of proton-transfer reactions of the type XH++Y?YH++X for X or Y=He, H2, O2, Kr, O, N2, Xe, CO2, CH4, CD4, N2O, OH, and CO, and for the deuteration of O2 by D3+. Equilibrium constants are determined and changes in thermodynamic state properties (ΔG°, ΔH°, ΔS°) are derived for the reactions with X/Y=H2/O2, O2/Kr, H2/Kr, H2/N2, O/N2, N2/Xe, Xe/CO2, CO2/CH4, CH4/N2O, and N2O/CO. Proton affinities are reported for O2, H2, Kr, O, N2, Xe, CO2, CH4, N2O, and CO, together with the heats of formation of their protonated species. Also a correlation is presented between the kinetic and thermodynamic results obtained in this study.