Multistate Dynamics and Kinetics of CO2 Activation by Ta+ in the Gas Phase: Insights into Single-Atom Catalysis

The activation of carbon dioxide (CO2) by a transition-metal cation in the gas phase is a unique model system for understanding single-atom catalysis. The mechanism of such reactions is often attributed to a "two-state reactivity" model in which the high-energy barrier of a spin state correlating with ground-state reactants is avoided by intersystem crossing (ISC) to a different spin state with a lower barrier. However, such a "spin-forbidden" mechanism, along with the corresponding dynamics, has seldom been rigorously examined theoretically, due to the lack of global potential energy surfaces (PESs). In this work, we report full-dimensional PESs of the lowest-lying quintet, triplet, and singlet states of the TaCO2+ system, machine-learned from first-principles data. These PESs and the corresponding spin-orbit couplings enable us to provide an extensive theoretical characterization of the dynamics and kinetics of the reaction between the tantalum cation (Ta+) and CO2, which have recently been investigated experimentally at high collision energies using crossed beams and velocity map imaging, as well as at thermal energies using a selected-ion flow tube apparatus. The multistate quasi-classical trajectory simulations with surface hopping reproduce most of the measured product translational and angular distributions, shedding valuable light on the nonadiabatic reaction dynamics. The calculated rate coefficients from 200 to 600 K are also in good agreement with the latest experimental measurements. More importantly, these calculations revealed that the reaction is controlled by intersystem crossing, rather than potential barriers.

The associative ionization of N(2P) + O(3P)

The rate constant of the associative ionization reaction N(2P) + O(3P) → NO+ + e- was measured using a flow tube apparatus. A flowing afterglow source was used to produce an ion/electron plasma containing a mixture of ions, including N2+, N3+, and N4+. Dissociative recombination of these species produced a population of nitrogen atoms, including N(2P). Charged species were rejected from the flow tube using an electrostatic grid, subsequent to which oxygen atoms were introduced, produced either using a discharge of helium and oxygen or via the titration of nitrogen atoms with NO. Only the title reaction can produce the NO+ observed after the introduction of O atoms. The resulting rate constant (8 ± 5 ×10-11 cm3 s-1) is larger than previously reported N(2P) + O disappearance rate constants (∼2 × 10-11 cm3 s-1). The possible errors in this or previous experiments are discussed. It is concluded that the N(2P) + O(3P) reaction proceeds almost entirely by associative ionization, with quenching to the 2D or 4S states as only minor processes.

Temperature-Dependent Kinetics for the Reactions of Fen- (n = 2-17) and FexNiy- (x + y = 3-9) with O2: Comparison of Pure and Mixed Metal Clusters with Relevance to Meteor Radio Afterglows and Surface Oxidation

Rate constants and product branching fractions were measured from 300-600 K for Fen- + O2 (n = 2-17) and for 300-500 K for FexNiy- + O2 (x + y = 3-9) using a selected-ion flow tube (SIFT) apparatus. Rate constants for 46 species are reported. All rate constants increased with increasing temperature, and several were in excess of the Langevin-Gioumousis-Stevenson (LGS) capture rate at elevated temperatures. As with previously studied transition metal anion oxidation reactions, the collision limit is treated as the sum of the LGS limit along with a hard-sphere contribution, allowing for determination of activation energies. These values are compared to each other along with previous results for Nin-. Measured rate constants for all three series (Fen-, Nin-, and FexNy-) vary over a relatively narrow range (1-5 × 10-10 cm3 s-1 at 300 K) being at least 15% of the collision rate constant. All reaction rate constants increase with temperature, described by small activation energies of 0.5-4 kJ mol-1. The data are consistent with an anticorrelation between the electron binding energy and rate constant, previously noted in other systems. The Fen- reaction produces a larger population of higher energy electrons than do the Nin- reactions, with FexNiy- producing an intermediate amount. The results suggest that the overall rate constant is limited by a small energetic barrier located at a large internuclear distance where electrostatic forces dominate, causing the potentials to be similar across systems, while the product formation is determined by the shorter-range, valence portion of the potential, which varies widely between systems.

Mutual Neutralization of NO^{+} with O^{-}

We have studied the mutual neutralization reaction of vibronically cold NO^{+} with O^{-} at a collision energy of ≈0.1  eV and under single-collision conditions. The reaction is completely dominated by production of three ground-state atomic fragments. We employ product-momentum analysis in the framework of a simple model, which assumes the anion acts only as an electron donor and the product neutral molecule acts as a free rotor, to conclude that the process occurs in a two-step mechanism via an intermediate Rydberg state of NO which subsequently fragments.

Kinetics of associative detachment of O- + N2 and dissociative attachment of e- + N2O up to 1300 K: chemistry relevant to modeling of transient luminous events

The rate constants of O- + N2 → N2O + e- from 800 K to 1200 K and the reverse process e- + N2O → O- + N2 from 700 K to 1300 K are measured using a flowing afterglow - Langmuir probe apparatus. The rate constants for O- + N2 are well described by 3 × 10-12 e-0.28 eV kT-1 cm3 s-1. The rate constants for e- + N2O are somewhat larger than previously reported and are well described by 7 × 10-7 e-0.48 eV kT-1 cm3 s-1. The resulting equilibrium constants differ from those calculated using the fundamental thermodynamics by factors of 2-3, likely due to significantly non-thermal product distributions in one or both reactions. The potential surfaces of N2O and N2O- are calculated at the CCSD(T) level. The minimum energy crossing point is identified 0.53 eV above the N2O minimum, similar to the activation energy for the electron attachment to N2O. A barrier between N2O- and O- + N2 is also identified with a transition state at a similar energy of 0.52 eV. The activation energy of O- + N2 is similar to one vibrational quantum of N2. The calculated potential surface supports the notion that vibrational excitation will enhance reaction above the same energy in translation, and vibrational-state specific rate constants are derived from the data. The O- + N2 rate constants are much smaller than literature values measured in a drift tube apparatus, supporting the contention that those values were overestimated due to the presence of vibrationally excited N2. The result impacts the modeling of transient luminous events in the mesosphere.

Kinetics for the Reactions of Ar+, O2+, and NO+ with Isoprene (2-Methyl-1,3-butadiene) as a Function of Temperature (300-500 K)

Rate constants and product branching fractions were measured for reactions of Ar+, O2+, and NO+ with isoprene (2-methyl-1,3-butadiene C5H8) as a function of temperature. The rate constants are large (∼2 × 10-9 cm3 s-1) and increase with temperature, exceeding the ion-dipole/induced dipole capture rate. Adding a hard sphere term to the collision rate provides a more useful upper limit and predicts the positive temperature dependences. Previous kinetic energy-dependent rate constants show a similar trend. NO+ reacts only by non-dissociative charge transfer. The more energetic O2+ reaction has products formed through both non-dissociative and dissociative charge transfer, or possibly through an H atom transfer. The very energetic Ar+ has essentially only dissociative products; assumption of statistical behavior in the dissociation reasonably reproduces the product branching fractions.

Electron and Ar+ interaction with Mo(CO)6 at thermal energies; energetic limit on removal of 5 ligands from Mo(CO)6

The rate constant for electron attachment to Mo(CO)6 was determined to be ka = 2.4 ± 0.6 × 10-7 cm3 s-1 at 297 K in a flowing-afterglow Langmuir-probe experiment. The sole anion product is Mo(CO)5-. A small decline in ka was observed up to 450 K, and decomposition was apparent at higher temperatures. The charge transfer reaction of Ar+ with Mo(CO)6 is exothermic by 7.59 ± 0.03 eV, which appears to be sufficient to remove the first 5 ligands from Mo(CO)6+.

The influence of fluorination on the dynamics of the F- + CH3CH2I reaction

The competition between the bimolecular nucleophilic substitution (S_N2) and base-induced elimination (E2) reaction and their intrinsic reactivity is of key interest in organic chemistry. To investigate the effect of suppressing the E2 pathway on S_N2 reactivity, we compared the reactions F^- + CH₃CH₂I and F^- + CF₃CH₂I. Differential cross-sections have been measured in a crossed-beam setup combined with velocity map imaging, giving insight into the underlying mechanisms of the individual pathways. Additionally, we employed a selected-ion flow tube to obtain reaction rates and high-level ab initio computations to characterize the different reaction pathways and product channels. The fluorination of the β-carbon not only suppresses the E2-reaction but opens up additional channels involving the abstraction of fluorine. The overall S_N2 reactivity is reduced compared to the non-fluorinated iodoethane. This reduction is presumably due to the competition with the highly reactive channels forming FHF^- and CF₂CI^-.

Ion-molecule studies of energetic oxygen allotropes in flow tubes: O 2 ( v ) , O 2 ( a Δ g 1 ) , O 3 , and O ${{\rm{O}}}_{2}({\rm{v}}),{{\rm{O}}}_{2}({\rm{a}}{}^{1}{\rm{\Delta }}_{{\rm{g}}}),{{\rm{O}}}_{3},\mathrm{and}{\rm{O}}$

Starting in the 1960s, flow tube apparatuses have played a central role in the study of ion-molecule kinetics, allowing for immense chemical diversity of cationic, anionic, and neutral reactants. Here, we review studies of oxygen allotropes, excluding ground state O₂ ( X 3 ∑ g - ${X}^{3}{<mpadded xmlns="http://www.w3.org/1998/Math/MathML">\sum </mpadded>}_{g}^{-}$ ), and focusing instead on reactions of cations, anions, and metal chemi-ionization reactions with ground state atomic oxygen (O ³ P), vibrationally excited molecular oxygen (O₂ (v)), electronically excited molecular oxygen (O₂ ( a 1 Δ g ${a}^{1}{{\rm{\Delta }}}_{g}$ )), and ozone (O₃ ). Historical outlines of work over several decades are given along with a focus on more recent work by our group at the Air Force Research Laboratory.

Kinetics of O3 with Ca+ and Its Higher Oxides CaOn+ (n = 1-3) and Updates to a Model of Meteoric Calcium in the Mesosphere and Lower Thermosphere

The room-temperature rate constants and product branching fractions of CaO_n⁺ (n = 0-3) + O₃ are measured using a selected ion flow tube apparatus. Ca⁺ + O₃ produces CaO⁺ + O₂ with k = 9 ± 4 × 10^-10 cm³ s^-1, within uncertainty equal to the Langevin capture rate constant. This value is significantly larger than several literature values. Most likely, those values were underestimated due to the reformation of Ca⁺ from the sequential chemistry of higher calcium oxide cations with O₃, as explored here. A rate constant of 8 ± 3 × 10^-10 cm³ s^-1 is recommended. Both CaO⁺ and CaO₂⁺ react near the capture rate constant with ozone. The CaO⁺ reaction yields both CaO₂⁺ + O₂ (0.80 ± 0.15 branching) and Ca⁺ + 2O₂. Similarly, the CaO₂⁺ reaction yields both CaO₃⁺ + O₂ (0.85 ± 0.15 branching) and CaO⁺ + 2O₂. CaO₃⁺ + O₃ yield CaO₂⁺ + 2O₂ at 2 ± 1 × 10^-11 cm³ s^-1, about 2% of the capture rate constant. The results are supported using density functional calculations and statistical modeling. In general, CaO_n⁺ + O₃ yield CaO_n+1⁺ + O₂, the expected oxidation. Some fraction of CaO_n+1⁺ is produced with sufficient internal energy to further dissociate to CaO_n-1⁺ + O₂, yielding the same products as the oxidation of O₃ by CaO_n⁺. Mesospheric Ca and Ca⁺ concentrations are modeled as functions of day, latitude, and altitude using the Whole Atmosphere Community Climate Model (WACCM); incorporating the updated rate constants improves agreement with concentrations derived from lidar measurements.

Near-thermo-neutral electron recombination of titanium oxide ions

While the dissociative recombination (DR) of ground-state molecular ions with low-energy free electrons is generally known to be exothermic, it has been predicted to be endothermic for a class of transition-metal oxide ions. To understand this unusual case, the electron recombination of titanium oxide ions (TiO⁺) with electrons has been experimentally investigated using the Cryogenic Storage Ring. In its low radiation field, the TiO⁺ ions relax internally to low rotational excitation (≲100 K). Under controlled collision energies down to ∼2 meV within the merged electron and ion beam configuration, fragment imaging has been applied to determine the kinetic energy released to Ti and O neutral reaction products. Detailed analysis of the fragment imaging data considering the reactant and product excitation channels reveals an endothermicity for the TiO⁺ dissociative electron recombination of (+4 ± 10) meV. This result improves the accuracy of the energy balance by a factor of 7 compared to that found indirectly from hitherto known molecular properties. Conversely, the present endothermicity yields improved dissociation energy values for D₀(TiO) = (6.824 ± 0.010) eV and D₀(TiO⁺) = (6.832 ± 0.010) eV. All thermochemistry values were compared to new coupled-cluster calculations and found to be in good agreement. Moreover, absolute rate coefficients for the electron recombination of rotationally relaxed ions have been measured, yielding an upper limit of 1 × 10^-7 cm³ s^-1 for typical conditions of cold astrophysical media. Strong variation of the DR rate with the TiO⁺ internal excitation is predicted. Furthermore, potential energy curves for TiO⁺ and TiO have been calculated using a multi-reference configuration interaction method to constrain quantum-dynamical paths driving the observed TiO⁺ electron recombination.

Activation of Methane by Zr+: A Deep-Dive into the Potential Surface via Pressure- and Temperature-Dependent Kinetics with Statistical Modeling

The kinetics of Zr⁺ + CH₄ are measured using a selected-ion flow tube apparatus over the temperature range 300-600 K and the pressure range 0.25-0.60 Torr. Measured rate constants are small, never exceeding 5% of the Langevin capture value. Both collisionally stabilized ZrCH₄⁺ and bimolecular ZrCH₂⁺ products are observed. A stochastic statistical modeling of the calculated reaction coordinate is used to fit the experimental results. The modeling indicates that an intersystem crossing from the entrance well, necessary for the bimolecular product to be formed, occurs faster than competing isomerization and dissociation processes. That sets an upper limit on the lifetime of the entrance complex to crossing of 10^-11 s. The endothermicity of the bimolecular reaction is derived to be 0.09 ± 0.05 eV, in agreement with a literature value. The observed ZrCH₄⁺ association product is determined to be primarily HZrCH₃⁺ not Zr⁺(CH₄), indicating that bond activation has occurred at thermal energies. The energy of HZrCH₃⁺ relative to separated reactants is determined to be -0.80 ± 0.25 eV. Inspection of the statistical modeling results under best-fit conditions reveals reaction dependences on impact parameter, translation energy, internal energy, and angular momentum. Reaction outcomes are heavily affected by angular momentum conservation. Additionally, product energy distributions are predicted.

Kinetics for the Reactions of H3O+(H2O)n=0-3 with Isoprene (2-Methyl-1,3-butadiene) as a Function of Temperature (300-500 K)

We report kinetics studies of H₃O⁺(H₂O)_n=0-3 with isoprene (2-methyl-1,3-butadiene, C₅H₈) as a function of temperature (300-500 K) measured using a flowing afterglow-selected ion flow tube. Results are supported by density functional (DFT) calculations at the B3LYP/def2-TZVP level. H₃O⁺ (n = 0) reacts with isoprene near the collision limit exclusively via proton transfer to form C₅H₉⁺. The first hydrate (n = 1) also reacts at the collision limit and only the proton transfer product is observed, although hydrated protonated isoprene may have been produced and dissociated thermally. Addition of a second water (n = 2) lowers the rate constant by about a factor of 10. The proton transfer of H₃O⁺(H₂O)₂ to isoprene is endothermic, but transfer of the water ligands lowers the thermicity and the likely process occurring is H₃O⁺(H₂O)₂ + C₅H₈ → C₅H₉⁺(H₂O)₂ + H₂O, followed by thermal dissociation of C₅H₉⁺(H₂O)₂. Statistical modeling indicates the amount of reactivity is consistent with the process being slightly endothermic, as is indicated by the DFT calculations. This reactivity was obscured in past experiments due to the presence of water in the reaction zone. The third hydrate is observed not to react and helps explain the past results for n = 2, as n = 2 and 3 were in equilibrium in that flow tube experiment. Very little dependence on temperature was found for the three species that did react. Finally, the C₅H₉⁺ proton transfer product further reacted with isoprene to produce mainly C₆H₉⁺ along with a small amount of clustering.

Thermal Electron Attachment to Pyruvic Acid, Thermal Detachment from the Parent Anion, and the Electron Affinity of Pyruvic Acid

The kinetics of electron attachment to pyruvic acid (CH₃COCOOH) and thermal detachment from the resulting parent anion were measured from 300-515 K using a flowing afterglow─Langmuir probe apparatus. An adiabatic electron affinity (EA) for pyruvic acid was derived, 0.84 ± 0.02 eV. Electron attachment rate constants to pyruvic acid of 2.1 × 10^-8 and 1.2 × 10^-8 were measured at 300 and 400 K, respectively. Rate constants at higher temperatures are less well-defined due to possible contributions from attachment to zymonic open ketone, an endemic impurity in pyruvic acid. Similarly, unimolecular detachment rates are complicated by secondary proton transfer reaction of the pyruvic acid anion with pyruvic acid to yield an 87 Da anion. The possible contributions from these chemistries are considered, and in all cases the equilibrium constant between attachment and detachment remains well-defined, allowing for determination of the EA.

Gas-Phase Reactivity of Ozone with Lanthanide Ions (Sm+, Nd+) and Their Higher Oxides

The kinetics of SmO_n⁺ (n = 0-2) and NdO_n⁺ (n = 0-2) with O₃ are measured using a selected-ion flow tube. Reaction of Nd⁺ to yield NdO⁺ + O₂ occurs rapidly, with a rate constant near the capture-controlled limit of ∼8 × 10^-10 cm³ s^-1. NdO⁺ reacts at ∼40% of the capture limit to yield NdO₂⁺ with little temperature dependence from 200 to 400 K. NdO₂⁺ likely reacts very slowly (k ∼ 10^-13 cm³ s^-1) to yield NdO⁺ + 2O₂, does not react to yield NdO₃⁺, and associates slowly (k ∼ 10^-12 cm³ s^-1) to yield NdO₂⁺(O₃)_1-3. Reaction of Sm⁺ also yields SmO⁺ at near the capture limit at all temperatures, but a significant fraction (∼50%) of the SmO⁺ is produced in excited states that are long-lived compared to the millisecond time scale of the experiment. These states are evidently resistant to both radiative and collisional relaxation. The excited-state production is likely due to a spin-conservation constraint on the reaction, despite the large spin-orbit coupling typical for lanthanide-containing species. Ground-state SmO⁺ reacts inefficiently (k = 2 × 10^-11 (T/300)^-2.5 cm³ s^-1) to yield SmO₂⁺ + O₂, while the excited-state SmO^+* reacts at the capture limit, with branching to yield Sm⁺ + 2O₂ (ΔH_r,0K = 148.7 ± 0.4 kJ mol^-1 for ground-state SmO⁺) approximately 60% of the time, the remainder forming SmO₂⁺, which further reacts with O₃ to yield SmO⁺ at about 1% of the collisional value.

Temperature and energy dependences of ion-molecule reactions: Studies inspired by Diethard Böhme

Diethard Böhme has had a long career covering many topics in ion-molecule reactivity. In this review, we describe the work done at the Air Force Research Laboratory (and its variously named preceding organizations) that was inspired by his studies. These fall into two main areas: nucleophilic displacement (S_N 2) and metal cation chemistry. In S_N 2 chemistry, we revisited many of the reactions Diethard pioneered and studied them in more detail. We found nonstatistical behavior, both competition and noncompetition between multiple channels. New channels were found as hydration occurred, with more solution-like behavior occurring as only a few ligands were added. Temperature-dependent studies revealed details that were not observable at room temperature. These and other highlights will be discussed. In metal cation reactions, Diethard's use of an inductively coupled ion source allowed him to systematically study the periodic table of elements with a number of simple neutrals. We have taken the most interesting of these and studied them in greater detail. In doing so, we were able to identify curve crossing rates, in a few instances information about product states, and the importance of multiple entrance channels.

Structures and Electron Affinities of Aluminum Hydride Clusters AlnH (n = 3-13)

Low-energy structures and electron affinities (EAs) for aluminum hydride clusters Al_nH (n = 3-13) have been calculated using ab initio and density functional calculations. Geometries were optimized at the PBE0/def-2-TZVPP level of theory, which has been shown to match the currently accepted lowest-energy structures for the all-aluminum clusters Al_n and their anions. Neutral hydride clusters with n = 4, 7, and 9-12 are predicted to adopt terminal structures with the hydrogen atom bound to only one aluminum atom and with only minor alterations of the aluminum atom arrangement from that of the all-aluminum cluster. Clusters with n = 3 and 13 are predicted to adopt "face-centered" geometries, and the n = 6 cluster is predicted to prefer an isomer with the hydrogen atom bridging two aluminum atoms, also with little or no distortion to the aluminum atom arrangement from the all-aluminum cluster. Addition of a hydrogen atom to clusters with n = 5 and 8 is predicted to distort the aluminum atom arrangement significantly from that of the corresponding all-aluminum cluster. In the anionic clusters, terminal clusters are preferred for all cluster sizes except for n = 6 that prefers a face-centered arrangement. Minor distortions in the aluminum scaffolding for Al₁₁ and Al₁₂ were found, while all other anionic clusters adopt structures with little or no deviation in the aluminum atom arrangement from the corresponding all-aluminum cluster. Raw adiabatic electron affinities were computed using CCSD(T)/aug-cc-pVTZ single-point energies for the anionic and neutral hydride clusters at their respective DFT geometries. Isodesmic electron affinities for the hydride clusters were computed relative to their all-aluminum counterparts and show an even-odd alternation with cluster size. Derived EAs alternate in magnitude between even- and odd-numbered clusters, with the even-numbered clusters having relatively larger EAs.

Excited-State N Atoms Transform Aromatic Hydrocarbons into N-Heterocycles in Low-Temperature Plasmas

The direct formation of N-heterocycles from aromatic hydrocarbons has been observed in nitrogen-based low-temperature plasmas; the mechanism of this unusual nitrogen-fixation reaction is the topic of this paper. We used homologous aromatic compounds to study their reaction with reactive nitrogen species (RNS) in a dielectric barrier discharge ionization (DBDI) source. Toluene (C₇H₈) served as a model compound to study the reaction in detail, which leads to the formation of two major products at "high" plasma voltage: a nitrogen-replacement product yielding protonated methylpyridine (C₆H₈N⁺) and a protonated nitrogen-addition (C₇H₈N⁺) product. We complemented those studies by a series of experiments probing the potential mechanism. Using a series of selected-ion flow tube experiments, we found that N⁺, N₂⁺, and N₄⁺ react with toluene to form a small abundance of the N-addition product, while N(⁴S) reacted with toluene cations to form a fragment ion. We created a model for the RNS in the plasma using variable electron and neutral density attachment mass spectrometry in a flowing afterglow Langmuir probe apparatus. These experiments suggested that excited-state nitrogen atoms could be responsible for the N-replacement product. Density functional theory calculations confirmed that the reaction of excited-state nitrogen N(²P) and N(²D) with toluene ions can directly form protonated methylpyridine, ejecting a carbon atom from the aromatic ring. N(²P) is responsible for this reaction in our DBDI source as it has a sufficient lifetime in the plasma and was detected by optical emission spectroscopy measurements, showing an increasing intensity of N(²P) with increasing voltage.

Effect of Intersystem Crossings on the Kinetics of Thermal Ion-Molecule Reactions: Ti+ + O2, CO2, and N2O

A selected-ion flow tube apparatus has been used to measure rate constants and product branching fractions of ²Ti⁺ reacting with O₂, CO₂, and N₂O over the range of 200-600 K. Ti⁺ + O₂ proceeds at near the Langevin capture rate constant of 6-7 × 10^-10 cm³ s^-1 at all temperatures to yield ⁴TiO⁺ + O. Reactions initiated on doublet or quartet surfaces are formally spin-allowed; however, the 50% of reactions initiated on sextet surfaces must undergo an intersystem crossing (ISC). Statistical theory is used to calculate the energy and angular momentum dependences of the specific rate constants for the competing isomerization and dissociation channels. This acts as an internal clock on the lifetime to ISC, setting an upper limit on the order of τ_ISC < 1e^-11 s. ²Ti⁺ + CO₂ produces ⁴TiO⁺ + CO less efficiently, with a rate constant fit as 5.5 ± 1.3 × 10^-11 (T/300)^{-1.1 ± 0.2} cm³ s^-1. The reaction is formally spin-prohibited, and statistical modeling shows that ISC, not a submerged transition state, is rate-limiting, occurring with a lifetime on the order of 10^-7 s. Ti⁺ + N₂O proceeds at near the capture rate constant. In this case, both Ti⁺ON₂ and Ti⁺N₂O entrance channel complexes are formed and can interconvert over a barrier. The main product is >90% TiO⁺ + N₂, and the remainder is TiN⁺ + NO. Both channels need to undergo ISC to form ground-state products but TiO⁺ can be formed in an excited state exothermically. Therefore, kinetic information is obtained only for the TiN⁺ channel, where ISC occurs with a lifetime on the order of 10^-9 s. Statistical modeling indicates that the dipole-preferred Ti⁺ON₂ complex is formed in ∼80% of collisions, and this value is reproduced using a capture model based on the generic ion-dipole + quadrupole long-range potential.

Mutual neutralisation of O+ with O-: investigation of the role of metastable ions in a combined experimental and theoretical study

The mutual neutralisation of O⁺ with O^- has been studied in a double ion-beam storage ring with combined merged-beams, imaging and timing techniques. Branching ratios were measured at the collision energies of 55, 75 and 170 (± 15) meV, and found to be in good agreement with previous single-pass merged-beams experimental results at 7 meV collision energy. Several previously unidentified spectral features were found to correspond to mutual neutralisation channels of the first metastable state of the cation (O⁺(²D^o), τ ≈ 3.6 hours), while no contributions from the second metastable state (O⁺(²P^o), τ ≈ 5 seconds) were observed. Theoretical calculations were performed using the multi-channel Landau-Zener model combined with the anion centered asymptotic method, and gave good agreement with several experimentally observed channels, but could not describe well observed contributions from the O⁺(²D^o) metastable state as well as channels involving the O(3s ⁵S^o) state.

Determination of the SmO+ bond energy by threshold photodissociation of the cryogenically cooled ion

The SmO⁺ bond energy has been measured by monitoring the threshold for photodissociation of the cryogenically cooled ion. The action spectrum features a very sharp onset, indicating a bond energy of 5.596 ± 0.004 eV. This value, when combined with the literature value of the samarium ionization energy, indicates that the chemi-ionization reaction of atomic Sm with atomic oxygen is endothermic by 0.048 ± 0.004 eV, which has important implications on the reactivity of Sm atoms released into the upper atmosphere. The SmO⁺ ion was prepared by electrospray ionization followed by collisional breakup of two different precursors and characterized by the vibrational spectrum of the He-tagged ion. The UV photodissociation threshold is similar for the 10 K bare ion and the He tagged ion, which rules out the possible role of metastable electronically excited states. Reanalysis and remeasurement of previous reaction kinetics experiments that are dependent on D₀(SmO⁺) are included, bringing all experimental results in accord.

Cyclotrimerization of Acetylene under Thermal Conditions: Gas-Phase Kinetics of V+ and Fe+ + C2H2

The kinetics of successive reactions of acetylene (C₂H₂) initiated on either vanadium or iron atomic cations have been investigated under thermal conditions using the variable-ion source and temperature-adjustable selected-ion flow tube apparatus. Consistent with the literature results, the reaction of Fe⁺ + C₂H₂ primarily yields Fe⁺(m/z = (C₂H₂)₃); however, analysis via quantum chemical calculations and statistical modeling shows that the mechanism does not form benzene upon the third acetylene addition. The kinetics are more consistent with successive addition of three acetylene molecules, yielding Fe⁺(C₂H₂)₃, followed by an addition of a fourth acetylene molecule, initiating cyclotrimerization, yielding either Fe⁺(C₂H₂) + neutral benzene or Fe⁺(Bz) + acetylene, where Bz is a benzene ligand. In contrast, the reaction of V⁺ + C₂H₂ yields products via successive associations V⁺(m/z = (C₂H₂)_n) either with or without a bimolecular step involving loss of one H₂ and V⁺C₂(m/z = (C₂H₂)_m), where n and m extend at least up to 11 under conditions of 0.32 Torr at 300 K. Stabilized V⁺(Bz) is not a significant intermediate in the association mechanism. We propose a plausible mechanism for the generation of neutral benzene in this reaction and compare with the Fe⁺ results. The reaction steps that produce benzene result in turnover of the single-atom catalyst, and the large hydrocarbons produced that remain associated to the catalyst are proposed to be polycyclic aromatic hydrocarbons.

Electronic structure of NdO via slow photoelectron velocity-map imaging spectroscopy of NdO--

Electronically excited NdO is a possible product of the chemistry associated with the release of Nd into the ionosphere, and emission from these states may contribute to the observations following such experiments. To better characterize the energetics and spectroscopy of NdO, we report a combined experimental and theoretical study using slow photoelectron velocity-map imaging spectroscopy of cryogenically cooled NdO^- anions (cryo-SEVI) supplemented by wave function-based quantum-chemical calculations. Using cryo-SEVI, we measure the electron affinity of NdO to be 1.0091(7) eV and resolve numerous transitions to low-lying electronic and vibrational states of NdO that are assigned with the aid of the electronic structure calculations. Additionally, temperature-dependent data suggest contributions from the (2)4.5 state of NdO^- residing 2350 cm^-1 above the ground anion state. Photodetachment to higher-lying excited states of NdO is also reported, which may help to clarify observations from prior release experiments.

Gas-Phase Anionic Metal Clusters are Model Systems for Surface Oxidation: Kinetics of the Reactions of Mn- with O2 (M = V, Cr, Co, Ni; n = 1-15)

The reactions of anionic metal clusters M_n^- with O₂ (M = V (n = 1-15), Cr (n = 1-15), Co (n = 1-12), and Ni (n = 1-14)) are investigated from 300 to 600 K using a selected-ion flow tube. All rate constants show a positive temperature dependence, well described by an Arrhenius equation. Rate constants exceed (or are extrapolated to exceed at higher temperatures) the Langevin-Gioumousis-Stevenson capture rate constant. Application of a capture model accounting for the finite size of the clusters reproduces the size-dependent trends in reactivity. The assumption that reactivity is further controlled by an energetic barrier early in the reaction coordinate is consistent with the experimental observations. An observed correlation of the derived barrier heights on the electron binding energy of M_n^- suggests the barrier may be formed at an avoided crossing between electronic states correlating to M_n^- + O₂ and M_n + O₂^- reactants, analogous to that previously proposed for Al_n^- + O₂ systems. The mechanism is analogous to that for reactions of O₂ with neutral metal surfaces, indicating that gas-phase reactions of anionic metal clusters can be an appropriate model systems for surface oxidation.

Thermal rate constants for electron attachment to N2O: An example of endothermic attachment

Rate constants for dissociative electron attachment to N₂O yielding O^- have been measured as a function of temperature from 400 K to 1000 K. Detailed modeling of kinetics was needed to derive the rate constants at temperatures of 700 K and higher. In the 400 K-600 K range, upper limits are given. The data from 700 K to 1000 K follow the Arrhenius equation behavior described by 2.4 × 10^-8 e^{-0.288 eV/kT} cm³ s^-1. The activation energy derived from the Arrhenius plot is equal to the endothermicity of the reaction. However, calculations at the CCSD(T)/complete basis set level suggest that the lowest energy crossing between the neutral and anion surfaces lies 0.6 eV above the N₂O equilibrium geometry and 0.3 eV above the endothermicity of the dissociative attachment. Kinetic modeling under this assumption is in modest agreement with the experimental data. The data are best explained by attachment occurring below the lowest energy crossing of the neutral and valence anion surfaces via vibrational Feshbach resonances.

Barrierless methane-to-methanol conversion: the unique mechanism of AlO. Phys Chem Chem Phys 2020;22:14544-14550. [PMID: 32589175 DOI: 10.1039/d0cp02316g]

The kinetics of AlO+ + CH4 are studied from 300-500 K using a selected-ion flow tube. At all temperatures the reaction proceeds near the Langevin-Gioumousis-Stevenson collision rate with two product channels: hydrogen atom abstraction (AlOH+ + CH3, 86 ± 5%) and methanol formation (Al+ + CH3OH, 14 ± 5%). Density functional calculations show the key Al-CH3OH+ intermediate is formed barrierlessly via a mechanism unique to aluminum, avoiding the rate-limiting step common to other MO+. The reaction of Al2O3+ + CH4 follows a similar mechanism to that for AlO+ through to the key intermediate; however, the conversion to methanol occurs only for AlO+ due to favorable energetics attributed to a weaker Al+-CH3OH bond. Importantly, that bond strength may be tuned independent of competing product channels by altering the acidity of the Al with electron-withdrawing or donating groups, indicating a key design criteria to develop a real world Al-atom catalyst.

Methane Adducts of Gold Dimer Cations: Thermochemistry and Structure from Collision-Induced Dissociation and Association Kinetics

The bond dissociation energies at 0 K (BDE) of Au₂⁺-CH₄ and Au₂CH₄⁺-CH₄ have been determined using two separate experimental methods. Analyses of collision-induced dissociation cross sections for Au₂CH₄⁺ + Xe and Au₂(CH₄)₂⁺ + Xe measured using a guided ion beam tandem mass spectrometer (GIBMS) yield BDEs of 0.71 ± 0.05 and 0.57 ± 0.07 eV, respectively. Statistical modeling of association kinetics of Au₂(CH₄)_0-2⁺ + CH₄ + He measured from 200 to 400 K and at 0.3-0.9 Torr using a selected-ion flow tube (SIFT) apparatus yields slightly higher values of 0.81 ± 0.21 and 0.75 ± 0.25 eV. The SIFT data also place a lower limit on the BDE of Au₂C₂H₈⁺-CH₄ of 0.35 eV, likely an activated isomer, not Au₂(CH₄)₂⁺-CH₄. Particular emphasis is placed on determining the uncertainty in the derivation from association kinetics measurements, including uncertainties in collisional energy transfer, calculated harmonic frequencies, and possible contribution of isomerization of the association complexes. This evaluation indicates that an uncertainty of ±0.2 eV should be expected and that an uncertainty of better than ±0.1 eV is unlikely to be reasonable.

Toward a quantitative analysis of the temperature dependence of electron attachment to SF6

New flowing afterglow/Langmuir probe investigations of electronic attachment to SF₆ are described. Thermal attachment rate constants are found to increase from 1.5 × 10^-7 cm³ s^-1 at 200 K to 2.3 × 10^-7 cm³ s^-1 at 300 K. Attachment rate constants over the range of 200-700 K (from the present work and the literature), together with earlier measurements of attachment cross sections, are analyzed with respect to electronic and nuclear contributions. The latter suggest that only a small nuclear barrier (of the order of 20 meV) on the way from SF₆ to SF₆ ^- has to be overcome. The analysis shows that not only s-waves but also higher partial waves have to be taken into account. Likewise, finite-size effects of the neutral target contribute in a non-negligible manner.

Redefining the Mechanism of O2 Etching of Aln- Superatoms: An Early Barrier Controls Reactivity, Analogous to Surface Oxidation

New insights into aluminum anion cluster reactivity with O₂ were obtained through temperature-dependent kinetics measurements. Overall reactivity is controlled by a barrier at an avoided crossing where charge is transferred from the cluster to the O₂, mechanistically similar to what occurs as O₂ approaches a bulk Al surface. Contrary to prior interpretations, spin conservation does not inhibit the reaction of clusters with an odd number of Al atoms. In fact, the only spin constraint in these systems is on the reactivity of even clusters due to repulsive surfaces, not previously recognized. Although the superatom nature of Al₁₃^- is manifest in its high electron binding energy (EBE), the mechanism of its reactivity is not special; it reacts with O₂ as if it were a small piece of bulk Al. These experiments highlight the sensitivity of Al cluster reactivity with O₂ to temperature and EBE, uncovering routes to industrial scale use of aluminum superatom-based materials.

Thermal activation of methane by MgO+: temperature dependent kinetics, reactive molecular dynamics simulations and statistical modeling

The kinetics methane activation (MgO⁺ + CH₄) was studied experimentally and computationally by running and analyzing reactive atomistic simulations.

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.

On the Competition Between Electron Autodetachment and Dissociation of Molecular Anions

We treat the competition between autodetachment of electrons and unimolecular dissociation of excited molecular anions as a rigid-/loose-activated complex multichannel reaction system. To start, the temperature and pressure dependences under thermal excitation conditions are represented in terms of falloff curves of separated single-channel processes within the framework of unimolecular reaction kinetics. Channel couplings, caused by collisional energy transfer and "rotational channel switching" due to angular momentum effects, are introduced afterward. The importance of angular momentum considerations is stressed in addition to the usual energy treatment. Non-thermal excitation conditions, such as typical for chemical activation and complex-forming bimolecular reactions, are considered as well. The dynamics of excited SF₆^- anions serves as the principal example. Other anions such as CF₃^- and POCl₃^- are also discussed.

Thermal Kinetics of Aln- + O2 (n = 2-30): Measurable Reactivity of Al13. J Phys Chem A 2019;123:6123-6129. [PMID: 31251615 DOI: 10.1021/acs.jpca.9b03552]

Mass-selected aluminum anion clusters, Al_n^-, were reacted with O₂. Rate constants (300 K) for 2 < n < 30 and product branching fractions for 2 < n < 17 are reported. Reactivity is strongly anticorrelated to Al_n^- electron binding energy (EBE). Al₁₃^- reacts more slowly than predicted by EBE but notably is not inert, reacting at a measurable 0.05% efficiency (2.5 ± 1.5 × 10^-13 cm³ s^-1). Al₆^- is also an outlier, reacting more slowly than expected after accounting for other factors, suggesting that high symmetry increases stability. Implications of observed Al₁₃^- reactivity, contributions of both electronic shell-closing and geometric homogeneity to Al_n^- resistance to O₂ etching, and future directions to more fully unravel the reaction mechanisms are discussed.

On the Role of Hydrogen Atom Transfer (HAT) in Thermal Activation of Methane by MnO+: Entropy vs. Energy

The temperature dependent kinetics and product branching fractions of first-row transition metal oxide cation MnO+ with CH4 and CD4 at temperatures between 200 and 600 K are measured using a selected-ion flow tube apparatus. Likely reaction mechanisms are determined by comparison of temperature dependent kinetics to statistical modeling along calculated reaction coordinates. The data is well-modeled with the reaction proceeding over a rate limiting four-centered transition state leading to an insertion intermediate, similar to reactions of NiO+ and FeO+, and showing characteristics of proton-coupled electron transfer (PCET). However, a more direct pathway traversing a transition state of hydrogen atom transfer (HAT) character to a hydroxyl intermediate is found to possibly be competitive, especially with increasing temperature. While uncertainties in calculated energetics limit quantitative assessment of the role of HAT at thermal energies, it is clear that this mechanism becomes increasingly prevalent in higher energy regimes.

Au2+ cannot catalyze conversion of methane to ethene at low temperature

The previously reported conversion of methane to ethene catalyzed by Au₂⁺ at thermal energies is investigated through a combination of experiment and theory. The conversion is found not to occur, in-line with well-established thermodynamics.

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.

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.

Probing the rate-determining region of the potential energy surface for a prototypical ion-molecule reaction

We report a joint experimental-theoretical study of the F^- + HCl → HF + Cl^- reaction kinetics. The experimental measurement of the rate coefficient at several temperatures was made using the selected ion flow tube method. Theoretical rate coefficients are calculated using the quasi-classical trajectory method on a newly developed global potential energy surface, obtained by fitting a large number of high-level ab initio points with augmentation of long-range electrostatic terms. In addition to good agreement between experiment and theory, analyses suggest that the ion-molecule reaction rate is significantly affected by shorter-range interactions, in addition to the traditionally recognized ion-dipole and ion-induced dipole terms. Furthermore, the statistical nature of the reaction is assessed by comparing the measured and calculated HF product vibrational state distributions to that predicted by the phase space theory.This article is part of the theme issue 'Modern theoretical chemistry'.