Ion Flow Tube Technique Research Articles

Chemistry of HCNH+: mechanisms, structures, and relevance to Titan’s atmosphere

The chemistry of HCNH+ in Titan’s atmosphere is not completely understood despite previous experimental and theoretical studies. In response to recent suggestions in the literature, we have searched for specific products of the reactions of HCNH+ with H2, CH4, C2H2, and C2H4 using the flowing afterglow-selected ion flow tube technique. We have probed for an association mechanism for reaction with H2, and associative-H2 loss for the reactions involving CH4, C2H2, and C2H4. In all cases, these reaction mechanisms were found to be inefficient pathways for the depletion of HCNH+. Our ab initio computational studies characterize the structures and energies for these mechanisms and indicate that the proposed pathways are endothermic or possess reaction barriers. We compare our studies to previous experimental and computational work, and we suggest other ion-neutral reactions with HCNH+ that have not been included in previous models of Titan’s ionosphere.

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.

EXPERIMENTAL AND THEORETICAL STUDIES OF REACTIONS BETWEEN H ATOMS AND NITROGEN-CONTAINING CARBANIONS

Nitrogen-containing organic compounds are important in the interstellar medium (ISM). The detection of some corresponding anions, CnN− (n = 1, 3, 5), highlights the importance of laboratory studies of their chemistry, especially with the most abundant atomic species in the ISM, hydrogen atom. This work is a combined experimental and computational study of a series of nitrogen-containing carbanions reacting with H atoms. The anions include CnN− (n = 1–6), CnN2− (n = 1, 3–5), and CnN3− (n = 2, 4), and reactions mainly proceed through associative detachment (A− + H → AH + e−) or fragmentation pathways. Experimental measurements of the reaction rate constants were made with the flowing afterglow-selected ion flow tube technique. Ab initio theoretical calculations were carried out to explore the reaction mechanisms and investigate the factors determining reactivities. Reaction is mainly determined by the characteristics of the potential energy surfaces along the approach of the reactants. In addition, angular momentum conservation of the anion–H atom collision may have the universal effect of decreasing the reaction efficiencies. Our results indicate that CnN− (n = 1–6) and CnN2− (n = 1, 3–5) can be destroyed in reactions with H atoms by either forming the corresponding neutral monohydride compound or fragmenting into smaller anionic and neutral species. However, CnN3− (n = 2, 4) anions are unreactive, such that they may exist in H atom rich regions in the ISM.

Anions in Space and in the Laboratory

AbstractThe astronomical detection of molecular anions has prompted our study of their chemical reactions with atomic species that are abundant in the interstellar medium. We have recently explored the chemistry of a variety of Cx Ny− anions with hydrogen atoms and determined their reaction rate constants and products using the flowing afterglow-selected ion flow tube technique. Computational studies allow characterization of the structures of reactants and products, as well as the energetics along the reaction pathway. For anions containing one or two nitrogen atoms, reactions with hydrogen atoms are facile, and proceed primarily by associative detachment. In contrast, anions containing three nitrogen atoms are unreactive with hydrogen atoms due to reaction barriers and unfavorable thermodynamics.

EXPERIMENTAL AND THEORETICAL STUDIES OF REACTIONS BETWEEN H ATOMS AND CARBANIONS OF INTERSTELLAR RELEVANCE

The recent detection of molecular anions in the interstellar medium (ISM) has highlighted the need for laboratory studies of negative ion chemistry. Hydrogen atoms are the most abundant atomic species in the ISM, and the chemistry of H atoms with anions may contribute to molecular synthesis in interstellar clouds. This work is a combined experimental and computational study of a series of anions reacting with H atoms by associative detachment (A– + H → AH + e –). The anions include deprotonated nitriles (CH2CN–, CH3CHCN–, and (CH3)2CCN–), acetaldehyde (HC(O)CH2 –), acetone (CH3C(O)CH2 –), ethyl acetate (CH3CH2OC(O)CH2 –), methanol (CH3O–), and acetic acid (CH3CO2 –). Experimental measurements of the reaction rate constants were made with the flowing afterglow-selected ion flow tube technique. Ab initio theoretical calculations were carried out to explore the reaction mechanism and investigate the factors influencing reaction efficiencies, which are largely proportional to reaction exothermicities. Other factors influencing reaction efficiencies include the charge density on the reactive site of the anion, the characteristics of the potential energy surfaces along the approach of the reactants, and angular momentum conservation of the anion-H atom collision.

Detection and Quantification of Chemical Warfare Agent Precursors and Surrogates by Selected Ion Flow Tube Mass Spectrometry

The rate coefficients and branching ratios of 15 chemical warfare agent precursor and surrogate compounds reacting with H(3)O(+), NO(+), and O(2)(+) have been measured in the laboratory using the selected ion flow tube (SIFT) technique. Measurement of the relevant kinetic parameters for these agents has enabled quantitative monitoring using the SIFT-MS analytical technique. Thirteen of the 15 compounds studied were found to have real-time detection limits in the parts-per-trillion-by-volume concentration range when measured on a standard commercial Voice100 instrument, with specific compounds having detection limits below 100 parts-per-trillion-by-volume.

Gas Phase Study of C+Reactions of Interstellar Relevance

The current uncertainty in many reaction rate constants causes difficulties in providing satisfactory models of interstellar chemistry. Here we present new measurements of the rate constants and product branching ratios for the gas phase reactions of C+ with NH3, CH4, O2, H2O, and C2H2, using the flowing afterglow-selected ion flow tube (FASIFT) technique. Results were obtained using two instruments that were separately calibrated and optimized; in addition, low ionization energies were used to ensure formation of ground-state C+, the purities of the neutral reactants were verified, and mass discrimination was minimized.

Ion Chemistry of 1H-1,2,3-Triazole

A combination of experimental methods, photoelectron-imaging spectroscopy, flowing afterglow-photoelectron spectroscopy and the flowing afterglow-selected ion flow tube technique, and electronic structure calculations at the B3LYP/6-311++G(d,p) level of density functional theory (DFT) have been employed to study the mechanism of the reaction of the hydroxide ion (HO-) with 1H-1,2,3-triazole. Four different product ion species have been identified experimentally, and the DFT calculations suggest that deprotonation by HO- at all sites of the triazole takes place to yield these products. Deprotonation of 1H-1,2,3-triazole at the N1-H site gives the major product ion, the 1,2,3-triazolide ion. The 335 nm photoelectron-imaging spectrum of the ion has been measured. The electron affinity (EA) of the 1,2,3-triazolyl radical has been determined to be 3.447 +/- 0.004 eV. This EA and the gas-phase acidity of 2H-1,2,3-triazole are combined in a negative ion thermochemical cycle to determine the N-H bond dissociation energy of 2H-1,2,3-triazole to be 112.2 +/- 0.6 kcal mol-1. The 363.8 nm photoelectron spectroscopic measurements have identified the other three product ions. Deprotonation of 1H-1,2,3-triazole at the C5 position initiates fragmentation of the ring structure to yield a minor product, the ketenimine anion. Another minor product, the iminodiazomethyl anion, is generated by deprotonation of 1H-1,2,3-triazole at the C4 position, followed by N1-N2 bond fission. Formation of the other minor product, the 2H-1,2,3-triazol-4-ide ion, can be rationalized by initial deprotonation of 1H-1,2,3-triazole at the N1-H site and subsequent proton exchanges within the ion-molecule complex. The EA of the 2H-1,2,3-triazol-4-yl radical is 1.865 +/- 0.004 eV.

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.

Deuterium Kinetic Isotope Effects in Microsolvated Gas-Phase E2 Reactions

This work describes the first experimental studies of deuterium kinetic isotope effects (KIEs) for the gas-phase E2 reactions of microsolvated systems. The reactions of F(-)(H(2)O)(n) and OH(-)(H(2)O)(n), where n = 0, 1, with (CH(3))(3)CX (X = Cl, Br), as well as the deuterated analogs of the ionic and neutral reactants, were studied utilizing the flowing afterglow-selected ion flow tube technique. The E2 reactivity is found to decrease with solvation. Small, normal kinetic isotope effects are observed for the deuteration of the alkyl halide, while moderately inverse kinetic isotope effects are observed for the deuteration of the solvent. Minimal clustering of the product ions is observed, but there are intriguing differences in the nature and extent of the clustering process. Electronic structure calculations of the transition states provide qualitative insight into these microsolvated E2 reactions.

Ion-Molecule Reactions of Several Ions with Ethylene Oxide and Propenal in a Selected Ion Flow Tube

The selected ion flow tube (SIFT) technique has been used to investigate the ion-molecule reactions of several ions with the neutral molecules ethylene oxide, CH(2)OCH(2)-c, and propenal, CH(2)CHCHO. Both molecules have been identified in hot-core star forming regions [] and have significance to astrochemical models of the interstellar (ISM) and circumstellar medium (CSM). Moreover, the molecules contain functional groups, such as the epoxide group (ethylene oxide) and an aldehyde group, which are part of a conjugated pi-electron system (propenal) whose reactivities have not been studied in detail in gas-phase ion-molecule reactions. The larger recombination energy ions, Ar(+) and N(2)(+), were reacted with the neutrals to give insight into general fragmentation tendencies. These reactions proceeded via dissociative charge-transfer yielding major fragmentation products of CH(3)(+) and HCO(+) for ethylene oxide and CH(2)CH(+) and HCO(+) for propenal. The amino acids glycine and alanine are of particular interest to astrobiology, especially if they can be synthesized in the gas phase. In an attempt to synthesize amino acid precursors, ethylene oxide and propenal were reacted with NH(n)(+) (n = 1-4) and HCNH(+). As might be expected from the proton detachment energies, NH(+), NH(2)(+), and HCNH(+) reacted via proton transfer. NH(3)(+) reacted with each molecule via H-atom abstraction to produce NH(4)(+), and NH(4)(+) reacted via a ternary association. All binary reactions proceeded near the gas kinetic rate. Several associated molecule switching reactions were performed and implications of these reactions to the structures of the association products are discussed Ikeda et al. and Hollis et al.

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.

Gas-Phase Oligomerization of Propene Initiated by Benzene Radical Cation

Initiation of the radical cation polymerization of propene has been observed following the selective ionization of benzene in the gas phase by resonance two-photon ionization−high-pressure mass spectrometry (R2PI−HPMS) and by selected ion flow tube (SIFT) techniques. In this system, the aromatic initiator (C6H6) has an ionization potential (IP) between those of the reactant's monomer (C3H6) and its covalent dimer (C6H12), i.e, IP(C3H6) > IP(C6H6) > IP(C6H12). Therefore, direct charge transfer from C6H6•+ to C3H6 is not observed due to the large endothermicity of 0.48 eV, and only the adduct C6H6•+(C3H6) is formed. However, coupled reactions of charge transfer with covalent condensation are observed according to the overall process C6H6•+ + 2C3H6 → C6H12•+ + C6H6, which results in the formation of a hexene product ion, C6H12•+. The formation of this ion can make the overall process of charge transfer and covalent condensation significantly exothermic. At higher concentrations of propene, the reaction products are the propene oligomers (C3H6)n•+ with n = 2−7 and the adduct series C6H6•+(C3H6)n with n ≤ 6. The significance of the coupled reactions is that the overall process leads exclusively to the formation of the condensation product (C3H6)n•+ and avoids other competitive channels in the ion/molecule reactions of propene. Gas-phase nominal second-order rate coefficients for the overall reaction into both channels are in the range of (1−3) × 10-12 cm3 s-1. The rate coefficients into both channels, especially for the formation of the C6H12•+ dimer, have large negative temperature dependencies. Consistent with the gas-phase results, the intracluster reactions of C6H6•+ produced selectively by R2PI of mixed benzene/propene clusters also do not form the monomer ion C3H6•+ but form higher propene clusters (C3H6)n•+ that contain at least the C6H12•+ hexene ion. The similarity of the reaction mechanisms in the gas phase and in preformed clusters suggests that the mechanism may also apply in the condensed phase in common aromatic solvents such as benzene and toluene.

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.

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.

Gas phase hydrogen/deuterium exchange reactions of fluorophenyl anions

Hydrogen/deuterium (H/D) exchange reactions of fluorophenyl and difluorophenyl anions (C 6H 4F −, o-C 6H 3F 2 −, m-C 6H 3F 2 −, p-C 6H 3F 2 −) have been studied using the flowing afterglow-selected ion flow tube technique. The C 6H 4F − anion exchanges all hydrogens for deuterium upon reaction with D 2O. The difluorophenyl anions o-, m-, and p-C 6H 3F 2 − exchange three, two, and one hydrogen, respectively, with D 2O, whereas they undergo one, two, and three H/D exchanges, respectively, with CH 3OD. The structures of the anions and the isotope exchange dynamics within the intermediate ion–dipole complexes are discussed using ab initio molecular orbital calculations. Calculated values for the proton affinities of the most stable anions are 385.2, 378.0, 371.9, and 378.2 kcal/mol for C 6H 4F −, o-C 6H 3F 2 −, m-C 6H 3F 2 −, and p-C 6H 3F 2 −, respectively, in excellent agreement (within 2 kcal/mol) with the previous experimental values for the acidities of the corresponding fluorobenzenes. The H/D exchange results are explained by the energy differences of the intermediate DO − and CH 3O − species within the ion–dipole complexes; CH 3O − is mobile within the “hot” intermediate complex, whereas DO − is nearly “frozen” within the complex and cannot migrate across the barriers caused by the fluorine atoms or by the π electrons.

Gas-phase reactions of oxide and superoxide radical anions with C6F6

The reactions of O− and O2− with C6F6 and C6F6− with O2 were studied using the selected ion flow tube (SIFT) technique. The reaction rate constants and branching fractions were measured. The reactions of O− and O2− with C6F6 occur with near unit efficiency while that of C6F6− with O2 is about 2% efficient. The reaction of O− produces three ionic products, F−, C5F4−, and C6F5O−. The reaction of O2− with C6F6 produces five ionic decomposition products, C4F4−, F−, C4F2−, C5F4O−, C5F5O−, and the electron transfer product, C6F6−. The reaction of C6F6− with O2 produces products similar to those observed for the reaction O2− with C6F6.

Gas-phase reactions of fullerene cations C 60x+ ( x = 1–3) with pyridine and pyrrole: formation of “ball-and-chain” and “spindle” isomers and their interconversion

The chemistry of buckminsterfullerene cations with charge states of +1, +2, and +3 has been investigated with two nitrogen heterocycles, pyridine and pyrrole, in the gas phase at room temperature in helium buffer gas at 0.35 Torr using the selected ion flow tube (SIFT) technique. Adduct formation was observed with pyridine for all three charge states of C 60 x+ , although electron transfer leading to charge separation was the predominant channel with C 60 ·3+. The number of pyridine molecules sequentially added was equal to the number of charges on C 60 x+ . Equilibrium was achieved for the addition of pyridine to C 60 ·+ and a bond dissociation enthalpy of 19.5 kcal mol −1 was deduced from the measured equilibrium constant. The structures of the derivatized C 60 cations was explored by multicollision-induced dissociation in mixed He/Ar collision gas. All observed dissociations were heterolytic. With the multiply derivatized C 60 cations evidence was obtained for both a kinetically favored “ball-and-chain”-like structure and a thermodynamically favored “spindle”-like structure. Experiments with doubly derivatized C 60 2+ indicated the occurrence of a rapid conversion of the ball-and-chain to the spindlelike structure in a bimolecular reaction with pyridine. Only charge-separation reactions driven by electron transfer were observed with pyrrole. The results for the two nitrogen heterocycles are compared with those obtained under similar operating conditions for ammonia and several aliphatic amines. A correlation is presented between the observed reactivity of C 60 x+ and the proton affinity of the adduct molecule.

Structures, Energetics, and Chemical Reactions of Anions Derived from Cyclooctatetraene

Structures, energetics, and reactions of anions derived from cyclooctatetraene [C8H8- (1), C8H7- (2α), and C8H6- (3)] have been studied using the selected ion flow tube (SIFT) technique and molecul...