Revisiting the Discrepancy between Experimental and Theoretical Predictions of the Adiabaticity of Ti + CHOH.

We revisit the naked transition metal cation (Ti) and methanol reaction and go beyond the standard Landau-Zener (LZ) picture when modeling the intersystem crossing (ISC) rate between the lowest doublet and quartet states. We use both (i) unconstrained Born-Oppenheimer molecular dynamics (BOMD) calculations with an approximate two-state method to estimate population transfer between spin diabats and (ii) constrained dynamics to explore energetically accessible portions of the - 1 crossing seam, where is the total number of internal degrees of freedom. Whereas previous LZ calculations (that necessarily relied on the Condon approximation to be valid) fell short and predicted much slower crossing probabilities than shown in experiment, we show that ISC can occur rapidly because the spin-orbit coupling (SOC) between the doublet and quartet surfaces can vary by 2 orders of magnitude (depending on where in the seam the crossing occurs during dynamics) and the crossing region is revisited multiple times during a dynamics run of a few hundred femtoseconds.

Dissociative electron attachment and Ar+ reaction with chromium hexacarbonyl, 296-400 K.

Dissociative electron attachment rate constants have been measured for Cr(CO)6 under thermal conditions, 296-400 K, yielding Cr(CO)5- product. At 296 K, 2.92 ± 0.

Intersystem Crossing Control of the Nb + CO → NbO + CO Reaction.

The transfer of an oxygen atom from carbon dioxide (CO) to a transition metal cation in the gas phase offers atomic level insights into single-atom catalysis for CO activation. Given that these reactions often involve open-shell transition metals, they may proceed through intersystem crossing between different spin manifolds. However, a definitive understanding of such spin-forbidden reaction requires dynamical calculations on multiple global potential energy surfaces (PESs) coupled by spin-orbit couplings.

A Common Bottleneck for Metal Oxidation by Molecular Oxygen Across Size Regimes: Kinetics of Atomic Lanthanide Cations (La-Lu) with O.

Kinetics of the lanthanide cations (Ln = La-Lu excluding ) reacting with molecular oxygen were measured in a selected-ion flow tube apparatus from 300 to 600 K. Where exothermic, these reactions occur efficiently, producing LnO + O. The reactions display positive temperature dependences consistent with Arrhenius equation behavior and show small activation energies (0-2 kJ mol) that are strongly correlated to promotion energies of the Ln atoms.

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

The activation of carbon dioxide (CO) 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).

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).

Ta and Nb + CO: intersystem crossing in ion-molecule reactions.

The reactions of Ta and Nb with CO proceed only by a highly efficient oxygen atom transfer reaction to the respective oxide at room temperature in the gas phase. Although the product spin states are not determined, thermochemistry dictates that they must be different from ground state quintet Ta and Nb, implying that intersystem crossing (ISC) has occurred. Recent reactive scattering experiments found dominant indirect dynamics for the reaction with Ta hinting at a bottleneck along the reaction path.

Temperature-Dependent Kinetics for the Reactions of Fe ( = 2-17) and FeNi ( + = 3-9) with O: 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 Fe + O ( = 2-17) and for 300-500 K for FeNi + O ( + = 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.

Kinetics of associative detachment of O + N and dissociative attachment of e + NO up to 1300 K: chemistry relevant to modeling of transient luminous events.

The rate constants of O + N → NO + e from 800 K to 1200 K and the reverse process e + NO → O + N from 700 K to 1300 K are measured using a flowing afterglow - Langmuir probe apparatus. The rate constants for O + N are well described by 3 × 10 e cm s. The rate constants for e + NO are somewhat larger than previously reported and are well described by 7 × 10 e cm s.

Kinetics for the Reactions of Ar, O, 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, O, and NO with isoprene (2-methyl-1,3-butadiene CH) as a function of temperature. The rate constants are large (∼2 × 10 cm s) 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.

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.

Ion-molecule studies of energetic oxygen allotropes in flow tubes: .

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 O ( ), and focusing instead on reactions of cations, anions, and metal chemi-ionization reactions with ground state atomic oxygen (O P), vibrationally excited molecular oxygen (O (v)), electronically excited molecular oxygen (O ( )), and ozone (O ). 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 O with Ca and Its Higher Oxides CaO ( = 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 ( = 0-3) + O are measured using a selected ion flow tube apparatus. Ca + O produces CaO + O with = 9 ± 4 × 10 cm s, within uncertainty equal to the Langevin capture rate constant. This value is significantly larger than several literature values.

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).

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 + CH 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.

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

The reactivities of three perfluoroalkyl carboxylic acids (PFCAs) (perfluoropropanoic acid (CFCOOH, PFPA), perfluorobutanoic acid (CFCOOH, PFBA), and perfluorooctanoic acid (CFCOOH, 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, CFCOOH, from = 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.

Kinetics for the Reactions of HO(HO) with Isoprene (2-Methyl-1,3-butadiene) as a Function of Temperature (300-500 K).

We report kinetics studies of HO(HO) with isoprene (2-methyl-1,3-butadiene, CH) 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. HO ( = 0) reacts with isoprene near the collision limit exclusively via proton transfer to form CH.

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 (CHCOCOOH) 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.

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

The kinetics of SmO ( = 0-2) and NdO ( = 0-2) with O are measured using a selected-ion flow tube. Reaction of Nd to yield NdO + O occurs rapidly, with a rate constant near the capture-controlled limit of ∼8 × 10 cm s. NdO reacts at ∼40% of the capture limit to yield NdO with little temperature dependence from 200 to 400 K.

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

Low-energy structures and electron affinities (EAs) for aluminum hydride clusters AlH ( = 3-13) have been calculated using 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 and their anions. Neutral hydride clusters with = 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.

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

The direct formation of -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 (CH) 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 (CHN) and a protonated nitrogen-addition (CHN) product.

Effect of Intersystem Crossings on the Kinetics of Thermal Ion-Molecule Reactions: Ti + O, CO, and NO.

A selected-ion flow tube apparatus has been used to measure rate constants and product branching fractions of Ti reacting with O, CO, and NO over the range of 200-600 K. Ti + O proceeds at near the Langevin capture rate constant of 6-7 × 10 cm s at all temperatures to yield TiO + 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).