Thermal Activation of Methane by MgO+: Temperature Dependent Kinetics, Reactive Molecular Dynamics Simulations and Statistical Modeling

The kinetics of MgO + + CH 4 was studied experimentally using the variable ion source, temperature adjustable selected ion flow tube (VISTA-SIFT) apparatus from 300 − 600 K and computationally by running and analyzing reactive atomistic simula- tions. Rate coefficients and product branching fractions were determined as a function of temperature. The reaction proceeded with a rate of k = 5 . 9 ± 1 . 5 × 10 − 10 ( T/ 300 K) − 0 . 5 ± 0 . 2 cm 3 s − 1 . MgOH + was the dominant product at all temperatures, but Mg + , the co-product of oxygen-atom transfer to form methanol, was observed with a product branching fraction of 0 . 08 ± 0 . 03( T/ 300 K) − 0 . 8 ± 0 . 7 . Reactive molecular dynamics simulations using a reactive force field, as well as a neural network trained on thousands of structures yield rate coefficients about one order of magnitude lower. This underestimation of the rates is traced back to the multireference character of the transition state [MgOCH 4 ] + . Statistical modeling of the temperature-dependent kinetics provides further insight into the reactive potential surface. The rate limiting step was found to be consistent with a four-centered activation of the C-H bond, consistent with previous calculations. The product branching was modeled as a competition between dissociation of an insertion intermediate directly after the rate- limiting transition state, and traversing a transition state corresponding to a methyl migration leading to a Mg-CH 3 OH + complex, though only if this transition state is stabilized significantly relative to the dissociated MgOH + + CH 3 product channel. An alternative non-statistical mechanism is discussed, whereby a post-transition state bifurcation in the potential surface could allow the reaction to proceed directly from the four-centered TS to the Mg-CH 3 OH + complex thereby allowing a more robust competition between the product channels

Photochemical determination of O densities in the Martian thermosphere: Effect of a revised rate coefficient

We investigate the production and loss rates of in the photochemical equilibrium region of the Martian ionosphere near the subsolar point. We adopt neutral and ion densities measured by the Mars Atmosphere and Volatiles EvolutioN (MAVEN) Neutral Gas and Ion Mass Spectrometer (NGIMS), electron densities and temperatures measured by the Langmuir Probe and Waves, and ion temperatures measured by the Supra-Thermal and Thermal Ion Composition instruments on the MAVEN spacecraft. Contrary to the conventional wisdom, we find that loss of by dissociative recombination is balanced mainly by production due to the reaction of O+ with CO2, with a smaller contribution due to the reaction of with O. We find that the O densities derived from this calculation are larger than those measured by the NGIMS instrument by a factor that averages about 4 over the range of 130–155 km. This general conclusion is supported by a newly measured rate coefficient for the reaction of O with , which is smaller by a factor of about 6 than the only value in the literature, which was measured 47 years ago

Determining Rate Constants and Mechanisms for Sequential Reactions of Fe\u3csup\u3e+\u3c/sup\u3e with Ozone at 500 K

We present rate constants and product branching ratios for the reactions of FeOx+ (x = 0-4) with ozone at 500 K. Fe+ is observed to react with ozone at the collision rate to produce FeO+ + O2. The FeO+ in turn reacts with ozone at the collision rate to yield both Fe+ and FeO2+ product channels. Ions up to FeO4+ display similar reactivity patterns. Three-body clustering reactions with O2 prevent us from measuring accurate rate constants at 300 K although the data do suggest that the efficiency is also high. Therefore, it is probable that little to no temperature dependence exists over this range. Implications of our measurements to the regulation of atmospheric iron and ozone are discussed. Density functional calculations on the reaction of Fe+ with ozone show no substantial kinetic barriers to make the FeO+ + O2 product channel, which is consistent with the reaction\u27s efficiency. While a pathway to make FeO2+ + O is also found to be barrierless, our experiments indicate no primary FeO2+ formation for this reaction

Kinetics of CO\u3csup\u3e+\u3c/sup\u3e and CO2\u3csup\u3e+\u3c/sup\u3e with N and O atoms

We have measured reaction rate constants for CO+ and CO2+ reacting with N and O atoms using a selected ion flow tube apparatus equipped with a microwave discharge atom source. Experimental work was supplemented by molecular structure calculations. Calculated pathways show the sensitivity of kinetic barriers to theoretical methods and imply that high-level ab initio methods are required for accurate energetics. We report room-temperature rate constants of 1.0 ± 0.4 × 10-11 cm3 s-1 and 4.0 ± 1.6 × 10-11 cm3 s-1 for the reactions of CO+ with N and O atoms, respectively, and 8.0 ± 3.0 × 10-12 cm3 s-1 and 2.0 ± 0.8 × 10-11 cm3 s-1 for the reactions of CO2+ with N and O atoms, respectively. The reaction of CO2+ + O is observed to yield O2+ exclusively. These values help resolve discrepancies in the literature and are important for modeling of the Martian atmosphere

Reactions of Fe⁺ and FeO⁺ with C₂H₂, C₂H₄, and C₂H₆: Temperature-Dependent Kinetics

We present the first temperature-dependent rate constants and branching ratios for the reactions of Fe⁺ and FeO⁺ with C₂H₂, C₂H₄, and C₂H₆ from 170 to 700 K. Fe⁺ is observed to react only by association with the three hydrocarbons, with temperature dependencies of <i>T</i>^–2 to <i>T</i>^–3. FeO⁺ reacts with C₂H₂ and C₂H₄ at the collision rate over the temperature range, and their respective product branchings show similar temperature dependences. In contrast, the reaction with ethane is collisional at 170 K but varies as <i>T</i>^–0.5, while the product branching remains essentially flat with temperature. These variations in reactivity are discussed in terms of the published reactive potential surfaces. The effectiveness of Fe⁺ as an oxygen-transfer catalyst toward the three hydrocarbons is also discussed

Temperature and Pressure Dependences of the Reactions of Fe⁺ with Methyl Halides CH₃X (X = Cl, Br, I): Experiments and Kinetic Modeling Results

The pressure and temperature dependences of the reactions of Fe⁺ with methyl halides CH₃X (X = Cl, Br, I) in He were measured in a selected ion flow tube over the ranges 0.4 to 1.2 Torr and 300–600 K. FeX⁺ was observed for all three halides and FeCH₃⁺ was observed for the CH₃I reaction. FeCH₃X⁺ adducts (for all X) were detected in all reactions. The results were interpreted assuming two-state reactivity with spin-inversions between sextet and quartet potentials. Kinetic modeling allowed for a quantitative representation of the experiments and for extrapolation to conditions outside the experimentally accessible range. The modeling required quantum-chemical calculations of molecular parameters and detailed accounting of angular momentum effects. The results show that the FeX⁺ products come via an insertion mechanism, while the FeCH₃⁺ can be produced from either insertion or S_N2 mechanisms, but the latter we conclude is unlikely at thermal energies. A statistical modeling cannot reproduce the competition between the bimolecular pathways in the CH₃I reaction, indicating that some more direct process must be important

Temperature and Isotope Dependent Kinetics of Nickel-Catalyzed Oxidation of Methane by Ozone

The temperature dependent kinetics of Ni+ + O3 and of NiO+ + CH4/CD4 are measured from 300 to 600 K using a selected-ion flow tube apparatus. Together, these reactions comprise a catalytic cycle converting CH4 to CH3OH. The reaction of Ni+ + O3 proceeds at the collisional limit, faster than previously reported at 300 K. The NiO+ product reacts further with O3, also at the collisional limit, yielding both higher oxides (up to NiO5+ is observed) as well as undergoing an apparent reduction back to Ni+. This apparent reduction channel is due to the oxidation channel yielding NiO2+∗ with sufficient energy to dissociate. 4NiO+ + CH4 (CD4) (whereas 4NiO+ refers to the quartet state of NiO+) proceeds with a rate constant of (2.6 ± 0.4) × 10-10 cm3 s-1 [(1.8 ± 0.5) × 10-10 cm3 s-1] at 300 K and a temperature dependence of ∼T-0.7±0.3 (∼T-1.1±0.4), producing only the 2Ni+ + 1CH3OH channel up to 600 K. Statistical modeling of the reaction based on calculated stationary points along the reaction coordinate reproduces the experimental rate constant as a function of temperature but underpredicts the kinetic isotope shift. The modeling was found to better represent the data when the crossing from quartet to doublet surface was incomplete, suggesting a possible kinetic effect in crossing from quartet to doublet surfaces. Additionally, the modeling predicts a competing 3NiOH+ + 2CH3 channel to become increasingly important at higher temperatures

Comment on “Role of (NO)₂ Dimer in Reactions of Fe⁺ with NO and NO₂ Studied by ICP-SIFT Mass Spectrometry”

Comment on “Role of (NO)₂ Dimer in Reactions of Fe⁺ with NO and NO₂ Studied by ICP-SIFT Mass Spectrometry

Kinetics of Cations with C₂ Hydrofluorocarbon Radicals

Reactions of the cations Ar⁺, O₂⁺, CO₂⁺, and CF₃⁺ with the C₂ radicals C₂H₅, H₂C₂F₃, C₂F₃, and C₂F₅ were investigated using the variable electron and neutral density attachment mass spectrometry technique in a flowing afterglow–Langmuir probe apparatus at room temperature. Rate coefficients for observed product channels for these 16 reactions are reported as well as rate coefficients and product branching fractions for the 16 reactions of the same cations with each of the stable neutrals used as radical precursors (the species RI, where R is the radical studied). Reactions with the stable neutrals proceed by charge transfer at or near the collisional rate coefficient where energetically allowed; where charge transfer is endothermic, bond-breaking/bond-making chemistry occurs. While also efficient, reactions with the radicals are more likely to occur at a smaller fraction of the collisional rate coefficient, and bond-breaking/bond-making chemistry occurs even in some cases where charge transfer is exothermic. It is noted that unlike radical reactions with neutral species, which occur with rate coefficients that are generally elevated compared to those of stable species, ion–radical reactivity is generally decreased relative to that of reactions with stable species. In particular, long-range charge transfer appears more likely to be frustrated in the ion–radical systems