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.

Mutual Neutralization of NO

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.

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.

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.

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}}{}

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 - ${X}^{3}{\sum }_{g}^{-}$ ), 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.

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

The competition between the bimolecular nucleophilic substitution (SN2) 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 SN2 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 ß-carbon not only suppresses the E2-reaction but opens up additional channels involving the abstraction of fluorine. The overall SN2 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-.

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+.

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.

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 CaOn+ (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, CaOn+ + O3 yield CaOn+1+ + O2, the expected oxidation. Some fraction of CaOn+1+ is produced with sufficient internal energy to further dissociate to CaOn-1+ + O2, yielding the same products as the oxidation of O3 by CaOn+. 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.

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.

Elementary Reactions Leading to Perfluoroalkyl Substance Degradation in an Ar+/e- Plasma.

The reactivities of three perfluoroalkyl carboxylic acids (PFCAs) (perfluoropropanoic acid (C2F5COOH, PFPA), perfluorobutanoic acid (C3F7COOH, PFBA), and perfluorooctanoic acid (C7F15COOH, PFOA)) in a thermal, weakly ionized, argon/electron plasma were investigated from 300 to 600 K using a Langmuir probe-flowing afterglow apparatus. The results are supported by density functional theory calculations of the energetics of PFCA, CnF2n+1COOH, from n = 1 to 7. PFPA and PFBA attach electrons at a substantial fraction of the calculated capture rate; PFOA likely attaches electrons with similarly high efficiency, but the low vapor pressure of PFOA resulted in only qualitative results. All three compounds attach electrons dissociatively via HF elimination. The "acidity channel" (i.e., formation of H + CnF2n+1COO-), calculated to be slightly endothermic, is never observed even at higher temperatures where this channel would be energetically allowed. Attachment to perfluorooctanesulfonic acid (n-C8F17SO3H) is rapid, yielding the conjugate base n-C8F17SO3- as proton transfer to the electron is exothermic. At temperatures near 450 K (PFOA), 550 K (PFBA), or 600 K (PFPA), the parent neutrals thermally decompose as evidenced by abrupt changes in rate constants and branching ratios. PFPA and PFBA react with Ar+ close to the calculated capture rate, and PFOA likely does as well. The reaction mechanism starts via charge transfer, which can then lead to a range of product ions. Reactions with Ar+ yield fluorocarbon radicals, clarifying and supporting a previously proposed mechanism of PFCA degradation in an argon plasma.

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 SmOn+ (n = 0-2) and NdOn+ (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 (ΔHr,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.

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

Low-energy structures and electron affinities (EAs) for aluminum hydride clusters AlnH (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 Aln 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 τISC < 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.