Variable Temperature-selected Ion Flow Tube Research Articles

Experimental study of the reaction of NO2− ions with CO2 molecules at temperatures and energies relevant to the Martian atmosphere

Comprehensive theoretical models of the Martian ionosphere require reliable kinetic data for ion – molecule reactions over a wide range of temperatures. In situ measurements of the negative ions have not yet been performed and so ion chemical modelling is relied upon, which indicate that nitrite anions, NO2−, are important. Thus, in the present work the gas phase reaction of NO2− with carbon dioxide molecules, CO2, has been studied at the temperatures and energies relevant to the Martian atmosphere. The rate coefficient was measured over the temperature range 220 ± 2 K to 334 ± 2 K using a variable temperature selected ion flow tube (VT-SIFT) instrument. Additionally, a commercial Tandem Quadrupole Mass Spectrometer (TQMS) has been exploited to explore the reaction occurring during NO2−/CO2 collisions over the centre-of-mass energies from near-thermal to 6 eV. Two different channels of the reaction were studied. Under the thermal conditions of the VT-SIFT, the reaction is seen to proceed slowly via ternary association ultimately producing CO3– ions, the measured effective ternary rate coefficient in helium being k3 = (3.8 ± 1.5) × 10−30 cm6 s−1 at 220 ± 2 K and k3 = (0.5 ± 0.2) × 10−30 cm6 s−1 at 334 ± 2 K. The effective ternary rate coefficient can be greater in the Martian atmosphere where CO2 is the stabilizing gas. In the TQMS experiment, CO3– production occurs via a binary reaction above a threshold energy of 1.8 ± 0.2 eV. These experimental data can be used to refine the existing ion-chemical models of the Martian atmosphere.

Selected-ion flow tube temperature-dependent measurements for the reactions of O₂⁺ with N atoms and N₂⁺ with O atoms.

The temperature variation of rate constants has been measured for the gas phase reactions of the oxycation O2(+) with N atoms and of N2(+) with O atoms from 120 to 400 K using a variable temperature-selected ion flow tube. Measured room temperature rate constants, 0.75 × 10(-10) cm(3) s(-1) (±30%) for O2(+) with N and 1.4 × 10(-10) cm(3) s(-1) (±30%) for N2(+) with O, are in agreement with previously reported values. A temperature dependence of T(-0.7(±0.3)) is observed for the O2(+) + N reaction; however, the N2(+) + O reaction is found to be independent of temperature. Calculations at varying levels of theory were used in tandem with experiments to evaluate likely pathways in potential energy surfaces for the reactions of concern.

Temperature-Dependent Kinetics of Charge Transfer, Hydrogen-Atom Transfer, and Hydrogen-Atom Expulsion in the Reaction of CO+ with CH4 and CD4

We have determined the rate constants and branching ratios for the reactions of CO(+) with CH4 and CD4 in a variable-temperature selected ion flow tube. We find that the rate constants are collisional for all temperatures measured (193-700 K for CH4 and 193-500 K for CD4). For the CH4 reaction, three product channels are identified, which include charge transfer (CH4(+) + CO), H-atom transfer (HCO(+) + CH3), and H-atom expulsion (CH3CO(+) + H). H-atom transfer is slightly preferred to charge transfer at low temperature, with the charge-transfer product increasing in contribution as the temperature is increased (H-atom expulsion is a minor product for all temperatures). Analogous products are identified for the CD4 reaction. Density functional calculations on the CO(+) + CH4 reaction were also conducted, revealing that the relative temperature dependences of the charge-transfer and H-atom transfer pathways are consistent with an initial charge transfer followed by proton transfer.

Oxidation of Alkyl Ions, CnH2n+1+ (n = 1−5), in Reactions with O2 and O3 in the Gas Phase

Rate constants and product ion branching fractions are reported for the reactions of CH3+, C2H5+, s-C3H7+, s-C4H9+, t-C4H9+, and t-C5H11+ with O2 and O3 at 300 K in a variable-temperature selected-ion flow tube (VT-SIFT). The reaction rate constant for CH3+ with O3 is large and approximately equal to the thermal energy capture rate constant given by the Su−Chesnavich equation. The C2H5+, s-C3H7+, and s-C4H9+ ions are somewhat less reactive, reacting at approximately 7−46% of the thermal capture rate. The HCO+ and C2H3O+ ions are the major products in these reactions. The t-C4H9+ and t-C5H11+ ions are found to be unreactive, with rate constants <5 × 10-12 cm3 s-1, which is the present detection limit of our apparatus using this ozone source. Ozone is a singlet in its ground state, and ab initio calculations at the B3LYP/6-31G(d) level of theory indicate that reactant complexes can be formed, decreasing in stability with the size of alkyl chains attached to the cationic carbon atom. The decreasing reactivit...

Investigation of the Reaction of O3+ with N2 and O2 from 100 to 298 K

The kinetics of the reaction of O3+ with N2 and O2 have been studied at 100 and 298 K in a variable temperature-selected ion flow tube (VT-SIFT). The rate constants for O3+ reacting with N2 measured in the VT-SIFT are slow, proceeding at <5 × 10-13 cm3 s-1 at both temperatures, in disagreement with recent measurements by Cacace et al.1 of the total rate constant of ca. 1.7 × 10-10 cm3 s-1 at 298 K. However, a N2O3+ intermediate postulated to be involved in the reaction has been observed at 100 K, in agreement with the previous experimental and theoretical results. Rate constants for the reaction of O3+ with O2 at 100 and 298 K have also been measured in the VT-SIFT. The O2 reaction rate constants are 3.1 × 10-10 and 2.9 × 10-10 cm3 s-1 at 100 and 298 K, respectively, which are ca. half of the Langevin collision rate constant. Discrepancies between the current results and those of Cacace et al. in regard to the rate constants measured and the N2O+ product ions observed are discussed in light of newer data for the O3+ chemistry and the contributions of excited-state species. [Cacace, F.; et al. Angew. Chem., Int. Ed. Engl. 2001, 40, 1938.]

Reactions of H3O+(H2O)0,1 with Alkylbenzenes from 298 to 1200 K

Rate constants and branching fractions have been measured for the reactions of H3O+ and H3O+(H2O) with toluene (C7H8), ethylbenzene (C8H10), and n-propylbenzene (C9H12) as a function of temperature using a variable temperature-selected ion flow tube (VT-SIFT) and a high-temperature flowing afterglow (HTFA). The reactions have been studied up to 1200, 1000, and 900 K for toluene, ethylbenzene, and n-propylbenzene, respectively. The measurements are the first for H3O+(H2O) with these reactants. The H3O+ + alkylbenzene reaction rate constants are equal to the collision rate constants given by the Su−Chesnavich equation at all temperatures. Both nondissociative and dissociative proton-transfer products are observed. H3O+ reacts with toluene above 900 K to produce C7H9+, C7H7+, and C6H5+ predominately, whereas only C7H9+ is observed at lower temperatures. Proton transfer from H3O+ to ethylbenzene produces C8H11+ exclusively at 300 K, and by 800 K, C6H7+ is the major product with only minor amounts of C8H11+ an...

Reactions of NO+ with Isomeric Butenes from 225 to 500 K

The rate constants and branching fractions have been measured for the reaction of NO+ with the four butene isomers of C4H8 from 225 to 500 K in a variable temperature-selected ion flow tube. The cis- and trans-2-butene isomers react at the collision rate via charge transfer over the entire temperature range. Isobutene also reacts at the collision rate at all temperatures, primarily through charge transfer; however, an association channel also competes effectively at the lowest temperatures studied because the charge transfer reaction is only 0.02 eV exothermic. The reaction of NO+ with 1-butene is much slower than observed for the other butenes and has a negative temperature dependence. The negative temperature dependence is consistent with the predominant product channel being association. Other products are also observed with 1-butene, including C4H7+ from hydride transfer, C4H8+ from charge transfer at higher temperatures, and several other products involving significant rearrangement and chemical bond formation.

Rate constants and branching ratios for the reactions of various positive ions with naphthalene from 300 to 1400 K

Temperature dependent rate constants and branching ratios are reported for the reactions of a variety of ions with recombination energies ranging from 9.26 eV (NO +) to 21.56 eV (Ne +) with naphthalene. For most ions, the measurements are made between 300 and 370 K in a variable temperature-selected ion flow tube (VT-SIFT). For the reactions of Ar + and N 2 +, data have also been measured between 300 and 500 K in the selected ion flow tube. In addition, for the reactions of O 2 + and N 2 +, data have been obtained between 500 and 1400 K in a high temperature flowing afterglow (HTFA). These are among the first determinations of branching ratios for ion–molecule reactions measured over 700 K. All reactions are found to proceed at the Langevin collision rate for all temperatures studied. The reactions proceed by nondissociative and dissociative charge transfer except for the reaction involving F + where some of the reactivity is attributed to chemical channels. No dissociative charge transfer is observed for ions with recombination energies equal to or less than that for N 2 + at room temperature. At higher temperatures in the N 2 + reaction and for ions with higher recombination energies (F +, Ne +), naphthalene cation dissociation is observed, implying a threshold over 16 eV. This value is substantially higher than the known thermodynamic threshold because of kinetic shifts and quenching of the excited state of C 10H 8 + by collisions with the helium buffer gas. The observed product thresholds and branching ratios are presented within the context of previous work and the implications for combustion chemistry are discussed.

Rate Constants and Product Branching Fractions for the Reactions of H3O+ and NO+ with C2−C12 Alkanes

Gas-phase ion molecule reactions of H3O+ and NO+ with a number of C2−C12 alkanes have been studied using a variable temperature-selected ion flow tube instrument. Reaction rate constants and product branching fractions were measured at 300 K, and temperature dependences of the rate constants and product branching fractions for reactions of C7 and C8 alkanes were examined from 300 to 500 K. The threshold size for H3O+ reaction occurs at hexane, with the reaction rate constants increasing steadily with alkane size. Association is the predominant reaction channel at 300 K for C3−C12 alkanes, although these products thermally dissociate back into initial reactants between 300 and 400 K. The proton-transfer reaction channel is expected to be exothermic for large alkanes; however, this reaction channel does not proceed at the collision rate, nor are any direct proton-transfer product ions observed. The presence of smaller alkyl product ions suggests proton-transfer product ions may dissociate into smaller alkyl fragment ions and neutral alkanes. The threshold size for NO+ reaction with n-alkanes also occurs at hexane, and the reaction proceeds primarily via hydride transfer as previous experiments have indicated. However, a number of smaller product ions are also observed for the reactions of NO+ with all n-alkanes larger than butane, with the branching fraction of the nonhydride transfer product ions increasing with increasing alkane size. Together with our previously reported results regarding the reactions of various atmospheric ions with octane isomers, the present results indicate that H3O+, NO+, and O2+ are not suitable chemical ionization agents for analysis of hydrocarbon emissions.

Rate constants for the reactions of CO3−(H2O)n=0–5 + SO2: Implications for CIMS detection of SO2

The rate constants for the CO3−(H2O)n + SO2 reactions were measured for n=0–5 using a variable temperature‐selected ion flow tube. Temperature dependences were measured for n=0–3 over the following temperature ranges: n=0, 158–550 K; n=1, 158–270 K; n=2, 158–193 K; n=3, 158–193 K. The n=4, 5 rate constants were measured exclusively at 158 K. Good agreement is found with previous measurements of the n=0 and 1 rate constants. Expressions are estimated for use in chemical ionization mass spectrometric detection of SO2 in the troposphere.

A study of the reactions of trihydrogen(1+), H2D+, HD2+, and D3+ with hydrogen molecule, HD, and D2 using a variable-temperature selected ion flow tube

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTA study of the reactions of trihydrogen(1+), H2D+, HD2+, and D3+ with hydrogen molecule, HD, and D2 using a variable-temperature selected ion flow tubeKevin Giles, Nigel G. Adams, and D. SmithCite this: J. Phys. Chem. 1992, 96, 19, 7645–7650Publication Date (Print):September 1, 1992Publication History Published online1 May 2002Published inissue 1 September 1992https://doi.org/10.1021/j100198a030RIGHTS & PERMISSIONSArticle Views152Altmetric-Citations46LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (792 KB) Get e-Alerts

Gas-phase reactions of oxide and superoxide anions with CF4, CF3Cl, CF3Br, CF3I, and C2F4 at 298 and 500 K

Rate constants and product branching fractions have been measured for the gas-phase reactions of oxide (O−) and superoxide (O2−) anions with the halocarbons CF4, CF3Cl, CF3Br, CF3I, and C2F4 using a variable temperature–selected ion flow tube (VT–SIFT) instrument operated at 298 and 500 K. The reactions of O− with CF3X (X=Cl, Br, I) are fast and produce F−, XF−, and XO− for all X. For CF3Cl and CF3Br, X− is also formed. For CF3I, CF3− and IOF− are minor products. O− reacts rapidly with C2F4 producing F− as the major ionic product, along with contributions from reactive detachment and minor formation of FCO−, CF3−, and C2F3O−. The reaction of O2− with CF3Cl is slow, and both clustering and X− formation were observed. For CF3Br and CF3I, the reactions with O2− are fast, and nondissociative charge transfer was observed in addition to X− formation. O2− reacts rapidly with C2F4 by reactive detachment, in addition to producing F− as the major ionic product with smaller amounts of F2−, FCO−, FCO2−, CF3O−, and C2F4O−. O− and O2− were both found to be unreactive with CF4 at 298 and 500 K. The efficiencies of the reactions of both O− and O2− with CF3X are greater for the heavier halides at both 298 and 500 K. The rate constants for the reactions of O2− with CF3X appear to correlate both with the rates of thermal electron attachment to CF3X and with the electron affinities of CF3X, indicating that the O2−+CF3X reaction mechanism may involve initial electron transfer followed by dissociation. Thus the negative electron affinity of CF3Cl may explain the very slow rate for reaction with O2− despite the available exothermic pathways.

Reactions of Ar+ with halocarbons and of I+ with CF3I

The gas phase reactions of Ar+ with the halocarbons CF3Cl, CF3Br, CF3I, CF4, C2F6, and C2F4 have been studied using a variable temperature-selected ion flow tube (VT-SIFT) instrument operated at 298 and 500 K. Rate constants and product branching percentages were measured at both temperatures. Ar+ reacts at the collisional rate with all of the above neutrals at both 298 and 500 K. The reactions with CF3X yield CF+3 and CF2X+ for all X (the reaction with CF4 produces only CF+3 ). For X=I, there is an additional channel leading to the ionic product I+. The reaction of Ar+ with C2F6 produces both CF+3 and C2F+5. The reaction of Ar+ with C2F4 forms a rich product spectrum consisting of the ions CF+, CF+2, CF+3, C2F+3, and C2F+4. The reaction product distributions are compared with results from ionization experiments such as photoion–photoelectron coincidence (PIPECO) and electron impact mass spectrometry, and in some cases excellent agreement is found. The reaction of I+ with CF3I, which is a secondary reaction in the Ar+/CF3I system, was investigated at 298 K in separate experiments. This reaction is rapid and forms four product ions: CF+3, CF2I+, CF3I+, and I+2. The results are compared with previously published information.

Chemistry of CFn+ (n = 1-3) ions with halocarbons

The gas-phase reactions of CF + , CF 2 + , and CF 3 + with the halocarbons CF 3 Cl, CF 3 Br, CF 3 I, CF 4 , and C 2 F 6 have been studied using a variable-temperature-selected ion flow tube (VT-SIFT) instrument at 300 and 496 K. The ion CF + reacts rapidly with CF 3 X (X=Cl, Br, I) producing the ions CF 2 X +

Variable-temperature selected ion flow tube studies of the reactions of Ar + [rad], Ar + [rad]2 and ArH +n ( n = 1-3) ions wi H 2, HD and D 2 at 300 K and 80 K

The reactions of Ar + , Ar + 2, ArH + (ArD +), ArH + 2 (ArHD +, ArD + 2) and ArH + 3 with H 2, HD and D 2 have been studied at 300 K and 80 K using our variable-temperature selected ion flow tube (VT SIFT). The Ar + reactions proceed via parallel H-atom abstraction (major channel) and charge transfer, and the rate coefficients, k, are about 0.55 times the collisional rate coefficients, k c, at both temperatures for the three reactions. The ArH + (ArD +) reactions proceed via proton (deuteron) transfer, and the k values are about 0.6 · k c at 300 K and about 0.75 · k c at 80 K. The Ar + 2 reactions proceed via two parallel channels, for example, the H 2 reaction produces ArH + (atom abstraction) and ArH + 2 (Ar atom/H 2 molecule switching); the analogous reactions occur with HD and D 2. Whilst the atom abstraction channel is favoured at 300 K (in the ratio of about 70:30) the two channels are comparable at 80K for all three reactions. The overall K value for these reactions is about 0.3· k c at 300 K increasing to about (0.5–0.6)· k c at 80 K; in effect, the switching reactions are significantly enhanced at the low temperature. The ArH + 2 reaction with H 2 produces ArH + 3; similarly ArH 2D +, ArHD + 2 and ArD + 3 are produced in the other reactions. The k values are about 0.7· k c at both temperatures. From these studies, the dissociation energy of Ar·H + 2 (to Ar + H + 2) is shown to be ⩾ 1.04 eV and that of ArH + 3 (to Ar + H + 3) is shown to be within the range 0.29–0.36 eV.

An absolute proton affinity scale in the ∼130–140 kcal mol−1 range

The dependences on temperature of the rate coefficients for the endothermic proton transfer reactions of HBr+ with CO2 and CH4 have been obtained in a variable-temperature selected ion flow tube. The measurements have been used to determine the 300 K proton affinity of CO2, P.A.(CO2), =128.5±1.0 kcal mol−1, utilizing the literature value of P.A.(Br)=131.8 kcal mol−1, obtained from the dissociation energy of HBr+, as a primary standard. The proton affinity difference between CO2 and CH4 has been substantiated by equilibrium constant measurements as a function of temperature for proton transfer between CO2 and CH4. Similar equilibrium constant measurements have been used to determine the proton affinities of HCl, N2 O, HBr, and CO, giving a proton affinity ladder ordered (in kcal mol−1) as CO(141.4), HBr(138.8), N2 O(137.3), HCl(133.0), Br(131.8), CH4 (130.0), and CO2(128.5). Proton affinities have also been determined for Br2(140.0), NO(127.0), and CF4(126.5), the last two values being obtained from selected ion flow drift tube measurements. An upper limit to P.A.(SF6) of 127 kcal mol−1 has been inferred from the instability of SF6 H+ towards dissociation into SF+5 and HF.

A study of the temperature dependences of the N 2+ + N 2 → N 4+ and O 2+ + O 2 → O 4+ association reactions using the selected-ion flow-tube and drift-tube techniques

The three-body ion/molecule association reactions N 2 + + N 2 + He → k 1 N 4 + + He and O 2 + + O 2 + He → k 1 O 4 + + He have been studied in a variable-temperature selected-ion flow tube (SIFT) and a variable-temperature ion drift tube (DT). The rate coefficients k 1 and k 2 were determined using both apparatuses over uniquely wide temperature ranges, i.e., 45–520 K for k 1 and 48–200 K for k 2. Accurate measurements of k 2 at higher temperatures could not be made because of thermal break-up of the weakly bonded O 4 + ion. The magnitudes and temperature dependences of k 1 and k 2 determined in the SIFT and DT experiments are in very good agreement, and can be represented by the analytical expressions k 1 = 1.8 × 10 −29(300/ T) 2.3±0.2 cm 6 s −1 k 2 = 5.5 × 10 −31(300/ T) 2.7±0.3 cm 6 s −1 However, noticeable deviations from these power-law dependences occur at the lowest temperatures. The results are compared with those obtained from previous studies of these reactions and for the analogous association reactions involving N 2 and O 2 third bodies.