Electron attachment to 14 halogenated alkenes and alkanes, 300-600 K

Anion Photoelectron Imaging Spectroscopy of C6F5X- (X = F, Cl, Br, I)

The photoelectron (PE) spectra of C6F5X- (X = Cl, Br, I) and computational results on the anions and neutrals are presented and compared to previously reported results on C6F6- [McGee, C. J. J. Phys. Chem. A 2023, 127, 8556-8565.]. The spectra all exhibit broad, vibrationally unresolved detachment transitions, indicating that the equilibrium structures of the anions are significantly different from the neutrals. The PE spectrum of C6F5Cl- exhibits a parallel photoelectron angular distribution (PAD), similar to that of the previously reported C6F6- spectrum, while the PE spectra of C6F5Br- and C6F5I- have isotropic PADs, and also exhibit a prominent X- PE feature due to photodissociation of C6F5X- resulting in X- formation. Identification of the C6F5X- detachment transition origins, which is equivalent to the neutral electron affinity (EA), in all three cases is difficult, since the broadness of the detachment feature is accompanied by vanishingly small detachment cross section near the origin. Upper limits on the EAs were determined to be 1.70 eV for C6F5Cl, 2.10 eV for C6F5Br, and 2.00 eV for C6F5I, all significantly higher than the 0.76 eV upper limit determined for C6F6 with the same experiment. The broad detachment transitions are consistent with computational results, which predict very large differences between the neutral and anionic C-X (X = Cl, Br, I) bond lengths. Based on differences between the MBIS atom charges in the anions and neutrals, the excess charge in the anion is on the unique C atom and X, in contrast to the nonplanar C2v structured C6F6- anion, for which the charge is delocalized over the molecule. In C6F5Cl-, the C-Cl bond is predicted to be bent out of the plane, while both C6F5Br- and C6F5I- are predicted to be planar on average. The impact of the interruption of the symmetry in the hexafluorobenzene neutral and anion on the molecular and electronic structure of C6F5X/C6F5X- is considered, as well as the possible dissociative state leading to X- (X = Br, I) formation, and the nature of the C-X bond.

Reactions of C+ + Cl-, Br-, and I--A comparison of theory and experiment

Rate constants for the reactions of C⁺ + Cl^-, Br^-, and I^- were measured at 300 K using the variable electron and neutral density electron attachment mass spectrometry technique in a flowing afterglow Langmuir probe apparatus. Upper bounds of <10^-8 cm³ s^-1 were found for the reaction of C⁺ with Br^- and I^-, and a rate constant of 4.2 ± 1.1 × 10^-9 cm³ s^-1 was measured for the reaction with Cl^-. The C⁺ + Cl^- mutual neutralization reaction was studied theoretically from first principles, and a rate constant of 3.9 × 10^-10 cm³ s^-1, an order of magnitude smaller than experiment, was obtained with spin-orbit interactions included using a semiempirical model. The discrepancy between the measured and calculated rate constants could be explained by the fact that in the experiment, the total loss of C⁺ ions was measured, while the theoretical treatment did not include the associative ionization channel. The charge transfer was found to take place at small internuclear distances, and the spin-orbit interaction was found to have a minor effect on the rate constant.

Mutual neutralization of H+ and D+ with the atomic halide anions Cl-,Br-, and I. J Chem Phys 2018;149:044303. [PMID: 30068160 DOI: 10.1063/1.5036522]

Mutual neutralization (MN) rate constants k_MN for the reactions of H⁺ and D⁺ with the atomic halide anions Cl^-, Br^-, and I^- were measured using the variable electron and neutral density attachment mass spectrometry technique in a flowing afterglow Langmuir probe apparatus. At 300 K, the rate constants for each reaction studied are on the order of 10^-8 cm³ s^-1. A trend for the rate constants of the systems in this work, k_MNCl^-<k_MNBr^-<k_MN(I^-), is consistent with prior studies of rare gas cation with atomic halide anion MN. A recent theoretical study involving ab initio quantum mechanical treatment of the H⁺+Cl^- and D⁺+Cl^- reactions reported rate constants significantly lower than the rates reported here. A previously proposed empirical model that predicts atom-atom k_MN as a simple function of the total reaction exothermicity shows good agreement with the newly measured rate constants.

Contrast between the mechanisms for dissociative electron attachment to CH3SCN and CH3NCS

The kinetics of thermal electron attachment to methyl thiocyanate (CH₃SCN), methyl isothiocyanate (CH₃NCS), and ethyl thiocyanate (C₂H₅SCN) were measured using flowing afterglow-Langmuir probe apparatuses at temperatures between 300 and 1000 K. CH₃SCN and C₂H₅SCN undergo inefficient dissociative attachment to yield primarily SCN^- at 300 K (k = 2 × 10^-10 cm³ s^-1), with increasing efficiency as temperature increases. The increase is well described by activation energies of 0.17 eV (CH₃SCN) and 0.14 eV (C₂H₅SCN). CN^- product is formed at <1% branching at 300 K, increasing to ∼30% branching at 1000 K. Attachment to CH₃NCS yields exclusively SCN^- ionic product but at a rate at 300 K that is below our detection threshold (k < 10^-12 cm³ s^-1). The rate coefficient increases rapidly with increasing temperature (k = 6 × 10^-11 cm³ s^-1 at 600 K), in a manner well described by an activation energy of 0.51 eV. Calculations at the B3LYP/def2-TZVPPD level suggest that attachment to CH₃SCN proceeds through a dissociative state of CH₃SCN^-, while attachment to CH₃NCS initially forms a weakly bound transient anion CH₃NCS^-* that isomerizes over an energetic barrier to yield SCN^-. Kinetic modeling of the two systems is performed in an attempt to identify a kinetic signature differentiating the two mechanisms. The kinetic modeling reproduces the CH₃NCS data only if dissociation through the transient anion is considered.

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.

Calculations of the active mode and energetic barrier to electron attachment to CF3 and comparison with kinetic modeling of experimental results

To provide a deeper understanding of the kinetics of electron attachment to CF₃, the six-dimensional potential energy surfaces of both CF₃ and CF₃^- were developed by fitting ∼3000 ab initio points per surface at the AE-CCSD(T)-F12a/AVTZ level using the permutation invariant polynomial-neural network (PIP-NN) approach. The fitted potential energy surfaces for CF₃ and CF₃^- had root mean square fitting errors relative to the ab initio calculations of 1.2 and 1.8 cm^-1, respectively. The main active mode for the crossing between the two potential energy surfaces was identified as the umbrella bending mode of CF₃ in C_3v symmetry. The lowest energy crossing point is located at R_CF = 1.306 Å and θ_FCF = 113.6° with the energy of 0.051 eV above the minimum of the CF₃ electronic surface. This value is only slightly larger than the experimental data 0.026 ± 0.01 eV determined by kinetic modeling of electron attachment to CF₃. The small discrepancy between the theoretical and experimentally measured values is analyzed.

Electron attachment and positive ion chemistry of monohydrogenated fluorocarbon radicals

Rate coefficients and product branching fractions for electron attachment and for reaction with Ar(+) are measured over the temperature range 300-585 K for three monohydrogenated fluorocarbon (HFC) radicals (CF3CHF, CHF2CF2, and CF3CHFCF2), as well as their five closed-shell precursors (1-HC2F4I, 2-HC2F4I, 2-HC2F4Br, 1-HC3F6I, 2-HC3F6Br). Attachment to the HFC radicals is always fairly inefficient (between 0.1% and 10% of the Vogt-Wannier capture rate), but generally faster than attachment to analogous perfluorinated carbon radicals. The primary products in all cases are HF-loss to yield C(n)F(m-1)(-) anions, with only a minor branching to F(-) product. In all cases the temperature dependences are weak. Attachment to the precursor halocarbons is near the capture rate with a slight negative temperature dependence in all cases except for 2-HC2F4Br, which is ∼10% efficient at 300 K and becomes more efficient, approaching the capture rate at higher temperatures. All attachment kinetics are successfully reproduced using a kinetic modeling approach. Reaction of the HFC radicals with Ar(+) proceeds at or near the calculated collisional rate coefficient in all cases, yielding a wide variety of product ions.

Dissociative recombination and mutual neutralization of heavier molecular ions: C10H8(+), WF5(+), and C(n)F(m)(+)

Dissociative recombination (DR) rate coefficients for the naphthalene cation, C10H8(+), and WF5(+), and mutual neutralization (MN) rate coefficients for these species and five CnFm(+) ions, were determined at 300 K using variable electron and neutral density attachment mass spectrometry (VENDAMS). DR proceeds at 9 ± 3 × 10(-7) cm(3) s(-1) for C10H8(+) and at 6.1 ± 1.4 × 10(-7) cm(3) s(-1) for WF5(+). Consistent with previous results, MN for the polyatomic cations with the halide anions Cl(-), Br(-), and I(-) exhibits an approximate μ(-1/2) reduced mass dependence of the reactant partners, demonstrating that ion collision velocities influence the rate coefficients. This work is an extension of VENDAMS to systems, where low reactant concentrations are necessary to avoid significant reaction of product ions with the neutral precursor, i.e., conditions not suitable for traditional flowing afterglow measurements, as well as to ions of masses > ∼ 100 Da, which are not amenable to the study of DR in magnetic storage rings. Our results expand the sparse literature on DR and MN of heavier ions.

Kinetics of ion-ion mutual neutralization: halide anions with polyatomic cations

The binary mutual neutralization (MN) of a series of 17 cations (O₂⁺, NO(+), NO₂⁺, CO(+), CO₂⁺, Cl(+), Cl₂⁺, SO₂⁺, CF₃⁺, C₂F₅⁺, NH₃⁺, H₃⁺, D₃⁺, H2O(+), H3O(+), ArH(+), ArD(+)) with 3 halide anions (Cl(-), Br(-), I(-)) has been investigated in a flowing afterglow-Langmuir probe apparatus using the variable electron and neutral density attachment mass spectrometry technique. The MN rate constants of atom-atom reactions are dominated by the chemical nature of the system (i.e., the specific locations of curve crossings). As the number of atoms in the system increases, the MN rate constants become dominated instead by the physical nature of the system (e.g., the relative velocity of the reactants). For systems involving 4 or more atoms, the 300 K MN rate constants are well described by 2.7 × 10(-7) μ(-0.5), where the reduced mass is in Da and the resulting rate constants in cm(3) s(-1). An upper limit to the MN rate constants appears well described by the complex potential model described by Hickman assuming a cross-section to neutralization of 11,000 Å(2) at 300 K, equivalent to 3.5 × 10(-7) μ(-0.5).

Mutual neutralization of atomic rare-gas cations (Ne(+), Ar(+), Kr(+), Xe(+)) with atomic halide anions (Cl(-), Br(-), I(-))

We report thermal rate coefficients for 12 reactions of rare gas cations (Ne(+), Ar(+), Kr(+), Xe(+)) with halide anions (Cl(-), Br(-), I(-)), comprising both mutual neutralization (MN) and transfer ionization. No rate coefficients have been previously reported for these reactions; however, the development of the Variable Electron and Neutral Density Attachment Mass Spectrometry technique makes it possible to measure the difference of the rate coefficients for pairs of parallel reactions in a Flowing Afterglow-Langmuir Probe apparatus. Measurements of 18 such combinations of competing reaction pairs yield an over-determined data set from which a consistent set of rate coefficients of the 12 MN reactions can be deduced. Unlike rate coefficients of MN reactions involving at least one polyatomic ion, which vary by at most a factor of ∼3, those of the atom-atom reactions vary by at least a factor 60 depending on the species. It is found that the rate coefficients involving light rare-gas ions are larger than those for the heavier rare-gas ions, but the opposite trend is observed in the progression from Cl(-) to I(-). The largest rate coefficient is 6.5 × 10(-8) cm(3) s(-1) for Ne(+) with I(-). Rate coefficients for Ar(+), Kr(+), and Xe(+) reacting with Br2 (-) are also reported.

Electron attachment to CF3 and CF3Br at temperatures up to 890 K: experimental test of the kinetic modeling approach

Thermal rate constants and product branching fractions for electron attachment to CF3Br and the CF3 radical have been measured over the temperature range 300-890 K, the upper limit being restricted by thermal decomposition of CF3Br. Both measurements were made in Flowing Afterglow Langmuir Probe apparatuses; the CF3Br measurement was made using standard techniques, and the CF3 measurement using the Variable Electron and Neutral Density Attachment Mass Spectrometry technique. Attachment to CF3Br proceeds exclusively by the dissociative channel yielding Br(-), with a rate constant increasing from 1.1 × 10(-8) cm(3) s(-1) at 300 K to 5.3 × 10(-8) cm(3) s(-1) at 890 K, somewhat lower than previous data at temperatures up to 777 K. CF3 attachment proceeds through competition between associative attachment yielding CF3 (-) and dissociative attachment yielding F(-). Prior data up to 600 K showed the rate constant monotonically increasing, with the partial rate constant of the dissociative channel following Arrhenius behavior; however, extrapolation of the data using a recently proposed kinetic modeling approach predicted the rate constant to turn over at higher temperatures, despite being only ~5% of the collision rate. The current data agree well with the previous kinetic modeling extrapolation, providing a demonstration of the predictive capabilities of the approach.