Variable Temperature Selected Ion Flow 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.

Lanthanides as Catalysts: Guided Ion Beam and Theoretical Studies of Sm+ + COS.

Reactions of samarium cations with carbonyl sulfide are examined using a guided ion beam tandem mass spectrometer and a variable temperature selected ion flow tube apparatus. Formation of SmS+ + CO is observed in both instruments with a kinetic energy and temperature dependence demonstrating a barrierless reaction occurring with an efficiency of 26 ± 9%. Formation of SmO+ + CS is also observed at high kinetic energies and exhibits a threshold determined as 2.81 ± 0.32 eV, substantially higher than expected from known thermochemistry. The potential energy surfaces for these reactions along sextet and octet spin surfaces are also examined theoretically at the MP2 and CCSD(T) levels. The observed barrier for oxidation is shown to likely correspond to the energy of the crossing between surfaces corresponding to the ground state electronic configuration of Sm+ (8F,4f66s1) and an excited surface having two electrons in the valence space (excluding 4f), which are needed to form the strong SmO+ bond. In contrast, the S-CO bond is activated much more readily because this crossing occurs at much lower energies. This result is attributed to the much weaker S-CO bond energy as well as the ability of sulfur to bind effectively at different angles. Although both reactions are spin-forbidden, evidence for a more efficient spin-allowed process is also observed in the SmS+ + CO cross section.

Reactivity from excited state (4)FeO(+) + CO sampled through reaction of ground state (4)FeCO(+) + N2O.

The kinetics of the FeCO(+) + N2O reaction have been studied at thermal energies (300-600 K) using a variable temperature selected ion flow tube apparatus. Rate constants and product branching fractions are reported. The reaction is modestly inefficient, proceeding with a rate constant of 6.2 × 10(-11) cm(3) s(-1) at 300 K, with a small negative temperature dependence, declining to 4.4 × 10(-11) cm(3) s(-1) at 600 K. Both Fe(+) and FeO(+) products are observed, with a constant branching ratio of approximately 40:60 at all temperatures. Calculation of the stationary points along the reaction coordinate shows that only the ground state quartet surface is initially sampled resulting in N2 elimination; a submerged barrier along this portion of the surface dictates the magnitude and temperature dependence of the total rate constant. The product branching fractions are determined by the behavior of the remaining (4)OFeCO(+) fragment, and this behavior is compared to that found in the reaction of FeO(+) + CO, which initially forms (6)OFeCO(+). Thermodynamic and kinetic arguments are used to show that the spin-forbidden surface crossing in this region is efficient, proceeding with an average rate constant of greater than 10(12) s(-1).

Coupling an electrospray source and a solids probe/chemical ionization source to a selected ion flow tube apparatus.

A new ion source region has been constructed and attached to a variable temperature selected ion flow tube. The source features the capabilities of electron impact, chemical ionization, a solids probe, and electrospray ionization. The performance of the instrument is demonstrated through a series of reactions from ions created in each of the new source regions. The chemical ionization source is able to create H3O(+), but not as efficiently as similar sources with larger apertures. The ability of this source to support a solids probe, however, greatly expands our capabilities. A variety of rhenium cations and dications are created from the solids probe in sufficient abundance to study in the flow tube. The reaction of Re(+) with O2 proceeds with a rate constant that agrees with the literature measurements, while the reaction of Re2(2+) is found to charge transfer with O2 at about 60% of the collision rate; we have also performed calculations that support the charge transfer pathway. The electrospray source is used to create Ba(+), which is reacted with N2O to create BaO(+), and we find a rate constant that agrees with the literature.

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.

Experimental and theoretical kinetics for the H2O+ + H2/D2 → H3O+/H2DO+ + H/D reactions: observation of the rotational effect in the temperature dependence.

Thermal rate coefficients for the title reactions computed using a quasi-classical trajectory method on an accurate global potential energy surface fitted to ∼81,000 high-level ab initio points are compared with experimental values measured between 100 and 600 K using a variable temperature selected ion flow tube instrument. Excellent agreement is found across the entire temperature range, showing a subtle, but unusual temperature dependence of the rate coefficients. For both reactions the temperature dependence has a maximum around 350 K, which is a result of H2O(+) rotations increasing the reactivity, while kinetic energy is decreasing the reactivity. A strong isotope effect is found, although the calculations slightly overestimate the kinetic isotope effect. The good experiment-theory agreement not only validates the accuracy of the potential energy surface but also provides more accurate kinetic data over a large temperature range.

Further insight into the reaction FeO(+) + H2 → Fe(+) + H2O: temperature dependent kinetics, isotope effects, and statistical modeling.

The reactions of FeO(+) with H2, D2, and HD were studied in detail from 170 to 670 K by employing a variable temperature selected ion flow tube apparatus. High level electronic structure calculations were performed and compared to previous theoretical treatments. Statistical modeling of the temperature and isotope dependent rate constants was found to reproduce all data, suggesting the reaction could be well explained by efficient crossing from the sextet to quartet surface, with a rigid near thermoneutral barrier accounting for both the inefficiency and strong negative temperature dependence of the reactions over the measured range of thermal energies. The modeling equally well reproduced earlier guided ion beam results up to translational temperatures of about 4000 K.

Incorporating time-of-flight detection on a selected ion flow tube apparatus

A new ion detection scheme incorporating both time-of-flight and quadrupole instrumentation has been implemented on a variable temperature selected ion flow tube and is described here. The new detection region retains typical quadrupole detection for the study of most systems, and adds time-of-flight capability to study systems over a wide mass range while minimizing the effects of mass discrimination. Experiments verify the accuracy of the kinetics obtained with time-of-flight detection. We find excellent agreement with previously published rate constants and product branching for the reactions of Ar+ with SF6 and C2H4. Additionally, new results are presented for the reaction of Ar+ with WF6, observing a rate constant of 1.23×10−9cm3s−1, with product branching of 0.9 and 0.1 to WF5+ and WF6+ respectively.

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.

Iron cation catalyzed reduction of N2O by CO: gas-phase temperature dependent kinetics

The ion-molecule reactions Fe(+) + N2O → FeO(+) + N2 and FeO(+) + CO → Fe(+) + CO2, which catalyze the reaction CO + N2O → CO2 + N2, have been studied over the temperature range 120-700 K using a variable temperature selected ion flow tube apparatus. Values of the rate constants for the former two reactions were experimentally derived as k2 (10(-11) cm(3) s(-1)) = 2.0(±0.3) (T/300)(-1.5(±0.2)) + 6.3(±0.9) exp(-515(±77)/T) and k3 (10(-10) cm(3) s(-1)) = 3.1(±0.1) (T/300)(-0.9(±0.1)). Characterizing the energy parameters of the reactions by density functional theory at the B3LYP/TZVP level, the rate constants are modeled, accounting for the intermediate formation of complexes. The reactions are characterized by nonstatistical intrinsic dynamics and rotation-dependent competition between forward and backward fluxes. For Fe(+) + N2O, sextet-quartet switching of the potential energy surfaces is quantified. The rate constant for the clustering reaction FeO(+) + N2O + He → FeO(N2O)(+) + He was also measured, being k4 (10(-27) cm(6) s(-1)) = 1.1(±0.1) (T/300)(-2.5(±0.1)) in the low pressure limit, and analyzed in terms of unimolecular rate theory.

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...

An ab initio and experimental study of vibrational effects in low energy O++C2H2 charge-transfer collisions

Theoretical and experimental studies are performed to elucidate the low energy charge-transfer dynamics of the reaction, O+(4S)+C2H2(X 1Σg+)→O+C2H2+. In particular, the role of the low-frequency acetylene bending modes (612 and 730 cm−1) in promoting charge transfer was examined. High-temperature guided-ion beam measurements are carried out over the energy range from near-thermal to 3 eV at 310 and 610 K. The charge-transfer cross sections are found to decrease up to 0.5 eV, to have a constant value at intermediate energies between 0.5 and 1.5 eV, and then to dramatically increase above a threshold of a spin-allowed process determined to be at 1.7 eV. A bending vibrational enhancement of ∼8 is observed at intermediate energies. Thermal energy rate co-efficients are measured in a variable temperature-selected ion flow drift tube apparatus from 193 to 500 K. At each temperature, a negative energy dependence is observed. In order to elucidate the reaction mechanism in detail, high level ab initio calculations using Complete Active Space Self-Consistent Field and Multi-Reference Single- and Double-excitation Configuration Interaction methods have been performed. The results indicate that the charge transfer reaction occurs at an early stage via nonadiabatic transition between quartet and doublet states. There is a weak van der Waals minimum at the entrance channel between O+(4S) and C2H2 with the relative energy of −1.51 kcal/mol. The minimum of the quartet/doublet crossing seam (Q/D MSX), where the spin-forbidden nonadiabatic transition is most likely to take place, lies very near this minimum at RCO=4.06 Å, RCC=1.20 Å, and ∠CCH=166.6° with a relative energy of −1.48 kcal/mol. After the nonadiabatic transition, the system propagates on the doublet surface to reach the exothermic O(1D)+C2H2+(X̃ 2Πu) products. No energy barrier exists on the reaction pathway, strongly suggesting that the reaction should occur at low energy with a negative energy dependence, which is consistent with the experiment. The Q/D MSX has a bent acetylene moiety, which suggests that the excitation in bending modes will enhance the reaction, in agreement with the experiment.

Mobilities of NH4+(NH3)n clusters in helium from 100 K to 298 K

A variable temperature-selected ion flow drift tube (VT-SIFDT) has been used to measure the mobilities of NH4+(NH3)n clusters drifting in He. The mobilities have been measured for n=0–2 at 298 K, n=0–3 at 200 K, and n=0–5 at 100 K, marking the first mobilities experiments for these clusters below room temperature. The reduced mobilities measured at 298 K are compared to the previous SIFDT results of Krishnamurthy et al. [J. Chem. Phys. 106, 530 (1997)] for n=0–2. While the current results compare quite favorably for n=0–2, there is a discrepancy for n=3 which is addressed by current 100 K and 200 K data where these species are thermally stable. The trends in the mobilities from 100 K to 298 K as a function of E/N and effective temperature reveal that the repulsive part of the He–NH4+(NH3)n interaction potential is sampled predominantly for n=1–5. However, the attractive part of the potential is accessed at the lowest temperatures for He–NH4+.

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 and Branching Ratios for the Reactions of Selected Atmospheric Primary Cations with n-Octane and Isooctane (2,2,4-Trimethylpentane)

Gas-phase ion molecule reactions of the primary atmospheric cations (NO+, O2+, O+, N+, and N2+) with two isomers of octane, n-C8H18 and iso-C8H18 (2,2,4-trimethylpentane) have been studied using a variable temperature selected ion flow tube instrument. Reaction rate constants and product branching fractions were measured from 300 to 500 K. The reactions of O2+, O+, N+, and N2+ with n-C8H18 and iso-C8H18 proceed at the collision rate via dissociative and/or nondissociative charge transfer. The NO+ reactions occur primarily by hydride transfer; the reaction rate for the straight-chain isomer is only one-fourth the collision rate, while the reaction rate for the branched isomer is significantly enhanced. The reaction of n-octane with each atmospheric cation generates two to four major product ions and numerous minor species. The major ionic products are alkyl cations, CnH2n+1+, where the degree of fragmentation of the hydrocarbon chain is governed largely by the reactant ion recombination energy. The largest ionic products observed, n-C8H17+ and n-C8H18+, thermally decompose at temperatures above 300 K. The reaction of isooctane with each atmospheric cation generates fewer minor species than was observed in the n-octane reactions, and only one to three major product ions are observed. The main ionic product of the O2+, O+, N+, and N2+ reactions is the alkyl cation C4H9+, although several reactions also produce significant amounts of the related radical cation C4H8+. The main ionic product of the NO+ reaction, C8H17+, thermally decomposes into C4H9+ at temperatures above 300 K. Presumably, the specificity of the product ion formation in the iso-C8H18 reactions is a consequence of the neutral reactant structure. Except for thermal fragmentation of C8Hm+ product ions at temperatures above 300 K, there is little temperature dependence on the product ion branching fractions for the reported reactions.

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.

Rotational and Vibrational Energy Effects on Ion−Molecule Reactivity As Studied by the VT-SIFDT Technique

A method is described for measuring internal energy dependences of gas phase ion−molecule reactions in a variable temperature-selected ion flow drift tube (VT-SIFDT) instrument. Numerous studies have been conducted to examine the effects of both rotational and vibrational excitation on rate constants and branching ratios. Rotational and translational energy are found to be equally efficient at driving endothermic reactions. For exothermic reactions, large rotational effects are found only when one or both of the reagents have a large rotational constant. This indicates that changing from a low to moderate J value can affect reactivity but that changing from moderate to high J has little influence on reactivity. Vibrational effects are more varied. In some reactions, vibrational excitation in the anticipated reaction coordinate strongly affects reactivity, while in other cases it does not. Vibrational effects are often observed in charge transfer reactions, presumably due to energy resonances and Franck−Condon arguments. For the SN2 reactions studied to date, vibrations play no role in governing reactivity for halide ions reacting with methyl halides, while vibrations are equivalent to other types of energy in influencing the reactivity of non-methyl halide systems.