Production of and Dissociative Electron Attachment to the Simplest Criegee Intermediate in an Afterglow

The simplest Criegee intermediate, CH₂OO, has been produced in a flowing afterglow using a novel technique. CH₂I is produced by dissociative electron attachment to CH₂I₂, leading to the established reaction CH₂I + O₂ → CH₂OO + I. The presence of CH₂OO is established by observation of dissociative electron attachment to yield O^– using the variable electron and neutral density attachment mass spectrometry (VENDAMS) technique. The measurements establish the electron attachment rate coefficient of thermal electrons at 300 K to CH₂OO as 1.2 ± 0.3 × 10^–8 cm³ s^–1. Thermal electron attachment is solely dissociative and is not a promising route to producing stable CH₂OO^–. The results open the possibility of measuring ion–molecule chemistry involving Criegee intermediates, as well as the reactivity of other unstable radicals produced in an analogous manner

One- and Two-Dimensional Translational Energy Distributions in the Iodine-Loss Dissociation of 1,2-C₂H₄I₂⁺ and 1,3-C₃H₆I₂⁺: What Does This Mean?

Threshold photoelectron photoion coincidence (TPEPICO) has been used to study the sequential photodissociation reaction of internal energy selected 1,2-diiodoethane cations: C₂H₄I₂⁺ → C₂H₄I⁺ + I → C₂H₃⁺ + I + HI. In the first I-loss reaction, the excess energy is partitioned between the internal energy of the fragment ion C₂H₄I⁺ and the translational energy. The breakdown diagram of C₂H₄I⁺ to C₂H₃⁺, i.e., the fractional ion abundances below and above the second dissociation barrier as a function of the photon energy, yields the internal energy distribution of the first daughter, whereas the time-of-flight peak widths yield the released translational energy in the laboratory frame directly. Both methods indicate that the kinetic energy release in the I-loss step is inconsistent with the phase space theory (PST) predicted two translational degrees of freedom, but is well-described assuming only one translational degree of freedom. Reaction path calculations partly confirm this and show that the reaction coordinate changes character in the dissociation, and it is, thus, highly anisotropic. For comparison, data for the dissociative photoionization of 1,3-diiodopropane are also presented and discussed. Here, the reaction coordinate is expected to be more isotropic, and indeed the two degrees of freedom assumption holds. Characterizing kinetic energy release distributions beyond PST is crucial in deriving accurate dissociative photoionization onset energies in sequential reactions. On the basis of both experimental and theoretical grounds, we also suggest a significant revision of the 298 K heat of formation of 1,2-C₂H₄I₂(g) to 64.5 ± 2.5 kJ mol^–1 and that of CH₂I₂(g) to 113.5 ± 2 kJ mol^–1 at 298 K

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

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

Analysis of the Pressure and Temperature Dependence of the Complex-Forming Bimolecular Reaction CH₃OCH₃ + Fe⁺

The kinetics of the reaction CH₃OCH₃ + Fe⁺ has been studied between 250 and 600 K in the buffer gas He at pressures between 0.4 and 1.6 Torr. Total rate constants and branching ratios for the formation of Fe⁺O­(CH₃)₂ adducts and of Fe⁺OCH₂ + CH₄ products were determined. Quantum–chemical calculations provided the parameters required for an analysis in terms of statistical unimolecular rate theory. The analysis employed a recently developed simplified representation of the rates of complex-forming bimolecular reactions, separating association and chemical activation contributions. Satisfactory agreement between experimental results and kinetic modeling was obtained that allows for an extrapolation of the data over wide ranges of conditions. Possible reaction pathways with or without spin-inversion are discussed in relation to the kinetic modeling results

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

Determining Rate Constants and Mechanisms for Sequential Reactions of Fe⁺ with Ozone at 500 K

We present rate constants and product branching ratios for the reactions of FeO_<i>x</i>⁺ (<i>x</i> = 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’s efficiency. While a pathway to make FeO₂⁺ + O is also found to be barrierless, our experiments indicate no primary FeO₂⁺ formation for this reaction

Activation of Methane by FeO⁺: Determining Reaction Pathways through Temperature-Dependent Kinetics and Statistical Modeling

The temperature dependences of the rate constants and product branching ratios for the reactions of FeO⁺ with CH₄ and CD₄ have been measured from 123 to 700 K. The 300 K rate constants are 9.5 × 10^–11 and 5.1 × 10^–11 cm³ s^–1 for the CH₄ and CD₄ reactions, respectively. At low temperatures, the Fe⁺ + CH₃OH/CD₃OD product channel dominates, while at higher temperatures, FeOH⁺/FeOD⁺ + CH₃/CD₃ becomes the majority channel. The data were found to connect well with previous experiments at higher translational energies. The kinetics were simulated using a statistical adiabatic channel model (vibrations are adiabatic during approach of the reactants), which reproduced the experimental data of both reactions well over the extended temperature and energy ranges. Stationary point energies along the reaction pathway determined by ab initio calculations seemed to be only approximate and were allowed to vary in the statistical model. The model shows a crossing from the ground-state sextet surface to the excited quartet surface with large efficiency, indicating that both states are involved. The reaction bottleneck for the reaction is found to be the quartet barrier, for CH₄ modeled as −22 kJ mol^–1 relative to the sextet reactants. Contrary to previous rationalizations, neither less favorable spin-crossing at increased energies nor the opening of additional reaction channels is needed to explain the temperature dependence of the product branching fractions. It is found that a proper treatment of state-specific rotations is crucial. The modeled energy for the FeOH⁺ + CH₃ channel (−1 kJ mol^–1) agrees with the experimental thermochemical value, while the modeled energy of the Fe⁺ + CH₃OH channel (−10 kJ mol^–1) corresponds to the quartet iron product, provided that spin-switching near the products is inefficient. Alternative possibilities for spin switching during the reaction are considered. The modeling provides unique insight into the reaction mechanisms as well as energetic benchmarks for the reaction surface