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We show how the powerful combination of temperature-dependent kinetics coupled with detailed statistical modeling can be used to derive dynamical information about transition state barrier heights, the importance of multiple entrance channel complexes, crossings between spin surfaces, energetics, product states, and other information for metal-ion reactions. The methods are not new, but with improved computers, ion sources, ion transport, and better detection techniques, the ability to derive such parameters from the combination of methods has improved greatly. Temperature-dependent kinetics is very sensitive to the above list of parameters because the energy is varied in a controlled way that can be easily modeled. The present measurements, performed in our variable-ion source temperature-adjustable selected-ion flow tube (VISTA-SIFT), have been enabled by advances in ion transport and injection improvements so that dim sources can be used. Replacing the quadrupole mass spectrometer detector with a time-of-flight mass spectrometer solved additional problems. Quantum chemical calculations have improved greatly and provide details about the surfaces, as well as frequencies, to use as starting points for the statistical modeling. For ion-molecule reactions, incorporation of both energy and angular momentum effects are important and we have developed an in-house computer program, based on the work of Juergen Troe, to rapidly compare statistical modeling predictions to the experimental data. As we show, modeling the kinetics data can often determine the most important parameters controlling the reactivity and deriving them is much simpler and usually more accurate than detailed ab initio calculations or dynamical modeling. Additionally, we show that even without statistical modeling, temperature-dependent rate constants as a function of metal anion cluster size can be used to show that such species react by the same mechanism as surfaces. In this review, we discuss reactions of metallic atomic ions, small metal oxide ions, mixed metal oxide ions, and a series of metallic anionic cluster reactions with small molecules such as CO, O2, CO2, N2O, CH4, and several other species. Particular attention was paid to reactions involving bond activation pertinent to catalysis.

The reactions of anionic metal clusters Mn– 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 c...

The catalytic oxidation of CO by N2O promoted by Co+ was studied as a function of temperature in a variable-ion source temperature-adjustable selected-ion flow tube (VISTA-SIFT). Each step of the c...

The reactions of Al2O2+ + N2O and Al2O3+ + CO, forming a catalytic cycle oxidizing CO by N2O, have been investigated from 300 to 600 K in a variable ion source, temperature adjustable, selected-ion...

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.

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 w...

The kinetics of V+ + N2O and VO+ + N2O are studied using a selected-ion flow tube from 300-600 K at pressures of 0.25-0.70 Torr helium. V+ + N2O yields VO+ (k = 4.9 ± 1.0 (T/300 K)-0.3±0.2 × 10-10 cm3 s-1) in both ground and excited states. The secondary reaction VO+ + N2O → VO2+ + N2 proceeds near the collision rate at >10-10 cm3 s-1, whereas thermalized VO+ + N2O studied as a primary reaction proceeds more than 100× more slowly (k = 4.2 ± 1.0 (T/300 K)-1.4±0.2 × 10-12 cm3 s-1). The results are best explained by contributions of competing pathways in V+ + N2O: a spin crossing to the lower energy 3VO+ in the exit well and a spin-conserved reaction yielding an electronically excited 5VO+. The intersystem crossing occurs in 35 ± 20% and 37 ± 15% of reactive interactions at 300 and 600 K, respectively. Statistical modeling of relevant reaction coordinates supports the lack of a temperature dependence, indicates an intersystem crossing rate constant of 1011 s-1, and yields derived bond and transition state energies.

Mass-selected aluminum anion clusters, Aln–, 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 Aln– 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 Aln– resistance to O2 etching, and future directions to more fully unravel the reaction mechanisms are discussed.

The kinetics of V nO m+ + CCl4 ( n, m = 2, 5; 3, 6-8; 4, 9-11; 5, 12-13) have been measured under thermal conditions using a selected-ion flow tube equipped with a laser vaporization ion source. All reactions proceed at approximately the capture rate limit, yielding three dominant categories of products: CCl3+ + V nO mCl (i.e., chloride transfer), COCl2 (phosgene) formation, and CO2 formation. Both CO2 and COCl2 are products of CCl4 reaction on a bulk vanadium oxide surface, while chloride (or chlorine) transfer is not observed. The product branching fraction of CCl3+ approaches 100% for small (V2) reactants and generally decreases with increasing cluster size down to

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.

The temperature dependencies of the reactions MnO+ + H2/D2 from 150 to 600 K are measured in a selected- ion flow tube apparatus. The reactions are approximately 15% efficient at room temperature with rate constants (k = 2.7 ± 0.7 × 10−10 cm3 s-1 for H2 and 1.5 ± 0.4 × 10−10 cm3 s-1 for D2 at 300 K) that vary as T-1.1±0.2 and T-1.3±0.2, respectively. Both Mn+ and MnOH+/D+ products are observed, with Mn+ being the minority product in the H2 reaction, but the majority product in the D2 reaction. Reaction coordinates are explored using a variety of density functional methods, with quantitative but not qualitative differences in the results. Unlike the analogous FeO+ + H2 reaction, no evidence for spin inversion prior to the rate-limiting transition state is found. A crossing between septet and quintet potential energy surfaces in the product well exists, and the possibility of spin inversion on the way to products is discussed. The reaction coordinates inform a statistical treatment of the reaction, which well reproduces all kinetic results, including the product branching inversion observed due to isotope substitution.

The previously reported conversion of methane to ethene catalyzed by Au2+ at thermal energies, at odds with established thermodynamics, is investigated through a combination of experiments under both single-collision conditions using a guided ion beam tandem mass spectrometer (GIBMS) apparatus and at higher pressures using a selected-ion flow tube (SIFT) apparatus, as well as through density functional calculations. Production of Au2(C2H4)+ (or m/z 422) or Au2(C2D4)+ (or m/z 426) is significantly lower in the higher pressure SIFT experiments relative to previously reported results. The amount observed is consistent with a pathway initiating from a small fraction of a reactive species, potentially electronically excited Au2+* or Au2O+, isobaric with Au2(CH4)+. Extensive theoretical exploration of the potential energy surface for Au2(CH4)+ + CH4 shows no low-energy pathway that is consistent with ethene formation, with prohibitive barriers of 1–2 eV calculated along all identified reaction coordinates, consistent with previous calculations. GIBMS data do show the production of m/z 422 in the reaction of Au2(CH4)+ + CH4, consistent with the proposed key intermediate, but also provide evidence that this observed species is Au2(CO)+ arising from Au2O+, not Au2(C2H4)+ arising from Au2(CH4)+. The present results are consistent with the established thermochemistry for methane-to-ethene conversion, which unambiguously demonstrates that such conversion cannot proceed at thermal energies regardless of the presence of Au2+.

Diethard Bohme 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 (SN 2) and metal cation chemistry. In SN 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.

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.

The kinetics of MgO + + CH 4 was studied experimentally using the variable ion source, temperature adjustable selected ion flow tube (VISTA-SIFT) apparatus from 300 − 600 K and computationally by running and analyzing reactive atomistic simula- tions. Rate coefficients and product branching fractions were determined as a function of temperature. The reaction proceeded with a rate of k = 5 . 9 ± 1 . 5 × 10 − 10 ( T/ 300 K) − 0 . 5 ± 0 . 2 cm 3 s − 1 . MgOH + was the dominant product at all temperatures, but Mg + , the co-product of oxygen-atom transfer to form methanol, was observed with a product branching fraction of 0 . 08 ± 0 . 03( T/ 300 K) − 0 . 8 ± 0 . 7 . Reactive molecular dynamics simulations using a reactive force field, as well as a neural network trained on thousands of structures yield rate coefficients about one order of magnitude lower. This underestimation of the rates is traced back to the multireference character of the transition state [MgOCH 4 ] + . Statistical modeling of the temperature-dependent kinetics provides further insight into the reactive potential surface. The rate limiting step was found to be consistent with a four-centered activation of the C-H bond, consistent with previous calculations. The product branching was modeled as a competition between dissociation of an insertion intermediate directly after the rate- limiting transition state, and traversing a transition state corresponding to a methyl migration leading to a Mg-CH 3 OH + complex, though only if this transition state is stabilized significantly relative to the dissociated MgOH + + CH 3 product channel. An alternative non-statistical mechanism is discussed, whereby a post-transition state bifurcation in the potential surface could allow the reaction to proceed directly from the four-centered TS to the Mg-CH 3 OH + complex thereby allowing a more robust competition between the product channels .

The mechanisms involved in catalytic oxidation of CO by N2 O promoted by the AlVO3+ and AlVO4+ ions are evaluated using temperature-dependent rate constants and statistical modeling. Reactions were studied from 300-600 K using a selected ion flow tube (SIFT) apparatus, and the data compared to statistical modeling of proposed mechanisms previously identified by density functional theory (DFT) calculations. Reduction of N2 O by AlVO3+ was found to take place only by complexation to the Al site; however, attack on the V site also occurred and led to stable association complexes, reducing the overall efficiency of the reaction. As the AlVO3+ (N2 O) complex resulted from approach on the V site, it did not block the reactive Al site and was observed to further react with N2 O to form AlVO4+ (N2 O). The oxidation of CO by AlVO4+ was found to proceed solely by activation on the Al-O site; however, isomerization of complexes formed with CO initially complexed to the V site were found to add to the reactivity, especially at lower temperatures.

The temperature-dependent kinetics for reactions of V+, Fe+, and Co+ with OCS are measured using a selected ion flow tube apparatus heated to 300–600 K. All three reactions proceed solely by C–S activation at thermal energies, resulting in metal sulfide cation formation. Previously calculated reaction pathways were employed to inform statistical modeling of these reactions for comparison to the data. As surmised previously, all three reactions at thermal energies require spin crossing, with the Fe+ reaction crossing once circumventing a prohibitive transition state, before crossing again to form ground state products. The Fe+ and Co+ reaction efficiencies increase with energy. For the Co+ reaction, and to a lesser extent the Fe+ reaction, the apparent activation energies are less than the reaction endothermicities, possibly indicating increasing diabatic behavior of the spin crossings with energy. The V+ reaction was well modeled assuming an entirely adiabatic spin crossing, such that the resultant avoide...

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 exotherm...

The pressure and temperature dependences of the reactions of Fe+ with methyl halides CH3X (X = Cl, Br, I) in He were measured in a selected ion flow tube over the ranges 0.4 to 1.2 Torr and 300-600 K. FeX+ was observed for all three halides and FeCH3+ was observed for the CH3I reaction. FeCH3X+ adducts (for all X) were detected in all reactions. The results were interpreted assuming two-state reactivity with spin-inversions between sextet and quartet potentials. Kinetic modeling allowed for a quantitative representation of the experiments and for extrapolation to conditions outside the experimentally accessible range. The modeling required quantum-chemical calculations of molecular parameters and detailed accounting of angular momentum effects. The results show that the FeX+ products come via an insertion mechanism, while the FeCH3+ can be produced from either insertion or SN2 mechanisms, but the latter we conclude is unlikely at thermal energies. A statistical modeling cannot reproduce the competition between the bimolecular pathways in the CH3I reaction, indicating that some more direct process must be important.

The kinetics of 4Fe+(CO)n=0–2 + O2 are measured under thermal conditions from 300–600 K using a selected-ion flow tube apparatus. Both the bare metal and n = 2 cations are inert to reaction over this temperature range, but 4Fe+(CO) reacts rapidly (k = 3.2 ± 0.8 × 10−10 cm3 s−1 at 300 K, 52% of the collisional rate coefficient) to form FeO+ + CO2. This is an example of the oxidation of CO by O2 occurring entirely on a single non-noble metal atom. The reaction of the bare metal reaction is known to be endothermic, such that this result is expected; however, the n = 2 reaction has highly exothermic product channels available, such that the lack of reaction is surprising in light of the n = 1 reactivity. Stationary points along all three reaction coordinates are calculated using the TPSSh hybrid functional. These surfaces show that the n = 1 reaction is an example of two-state reactivity; the reaction proceeds initially on the sextet surface over a submerged barrier to a structure with an O–O bond distance longer than that in O2, but must cross to the quartet surface in order to proceed over a second submerged barrier to rearrange to form CO2. The n = 2 reaction does not proceed because, on all spin surfaces, the transition state corresponding to O–O separation is at higher energy than the separated reactants. The difference between the n = 1 and n = 2 reactions is not a result of steric effects, but rather because the O2 is more strongly bound to Fe in the entrance well of the n = 1 case, and that energy is available to overcome the rate-limiting barrier to O–O cleavage. Experimental verification of some of these details are provided by guided ion beam tandem mass spectrometry results. The kinetic energy dependence of the n = 1 reaction shows evidence for a curve crossing and yields relevant thermochemistry for competing reaction channels.

We present rate constants and product branching ratios for the reactions of FeOx+ (x = 0–4) with ozone at 500 K. Fe+ is observed to react with ozone at the collision rate to produce FeO+ + O2. The FeO+ in turn reacts with ozone at the collision rate to yield both Fe+ and FeO2+ product channels. Ions up to FeO4+ display similar reactivity patterns. Three-body clustering reactions with O2 prevent us from measuring accurate rate constants at 300 K although the data do suggest that the efficiency is also high. Therefore, it is probable that little to no temperature dependence exists over this range. Implications of our measurements to the regulation of atmospheric iron and ozone are discussed. Density functional calculations on the reaction of Fe+ with ozone show no substantial kinetic barriers to make the FeO+ + O2 product channel, which is consistent with the reaction’s efficiency. While a pathway to make FeO2+ + O is also found to be barrierless, our experiments indicate no primary FeO2+ formation for this...

The role of collisional energy transfer in ion-molecule association reactions is analyzed. Rate constants for the formation of adducts between HCl and NO3−(HNO3)x (H2SO4)y or HSO4−(HNO3)x (H2SO4)y (with x = 0–2 and y = 0–2) in the buffer gases H2, He, and N2 and at temperatures between 150 and 300 K are considered. Quantum-chemical calculations of molecular parameters and statistical unimolecular rate theory are combined to model low-pressure rate constants whereas ion-molecule capture theory provides high-pressure rate constants. The comparison with experimental results indicates that energy transfer is dominated by overall collision numbers while weak-collision effects are only of minor importance. On the other hand, often neglected falloff effects between termolecular and bimolecular reaction behavior have to be taken into account.

We present 300 K rate constants for the ion-ion reactions of O + a n d N + with the atomic halide anions: C l - , B r - , a n d I - measured using a flowing afterglow – Langmuir probe apparatus. All six rate constants were > 1 × 10 - 8 c m 3 s - 1 , larger than those typical for atomic ion – atomic ion reactions, approaching the rate constants of more complex ion-ion systems. Evaluation of literature atomic ion – atomic ion rate constants shows variation over orders of magnitude, but limited to below the expected value for a polyatomic ion-ion reaction of the same reduced mass, fitting into an expected physical picture. Unlike for polyatomic systems, estimation of rate constant for atomic ion – atomic ion reaction rates by simple means does not seem effective.

Limiting high pressure rate constants for the association of H with C6H5+ and of CH3, C2H5, and n-C3H7 radicals with C7H7+ molecular ions are analyzed in terms of “rigidity factors” arising from the anisotropy of the interaction potentials and “collision rate constants” in the absence of anisotropy (the latter corresponding to phase space theory, PST). Model calculations based on ab initio potential energy surfaces show that the PST rate constants kPST(T) exceed collision rate constants from conventional ion-molecule capture theory (in this case given by Langevin rate constants kL). They can be represented by kPST(T) = kL + kh.sph. where kh.sph. denotes hard-sphere collision numbers with collision radii r0. The r0 derived from the modelled kPST(T) are related to properties of the interaction potentials. Applications to other ion-molecule reactions are proposed.

Electron attachment to phosphoryl chloride (phosphorus oxychloride) POCl 3 has been studied in the gas phase by mass spectrometry at several low temperatures (47.7, 74.5, 169.7 and 199.5 K) with the CRESU method. By measuring over this temperature range and data from [8] , we have demonstrated the inversion of the branching ratio between the exothermic non-dissociative exit channel POCl 3 − and the thermo-neutral dissociative exit channel POCl 2 − + Cl. A kinetic model in terms of statistical theory is used to fit the experimental data.

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 C3v symmetry. The lowest energy crossing point is located at RCF = 1.306 A and θFCF = 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.

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.

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.

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

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 ...

Reactions of samarium cations with carbonyl sulfide are examined using a guided ion beam tandem mass spectrometer and a variable temperature selected ion flow tube apparatus. Formation of SmS+ + CO is observed in both instruments with a kinetic energy and temperature dependence demonstrating a barrierless reaction occurring with an efficiency of 26 ± 9%. Formation of SmO+ + CS is also observed at high kinetic energies and exhibits a threshold determined as 2.81 ± 0.32 eV, substantially higher than expected from known thermochemistry. The potential energy surfaces for these reactions along sextet and octet spin surfaces are also examined theoretically at the MP2 and CCSD(T) levels. The observed barrier for oxidation is shown to likely correspond to the energy of the crossing between surfaces corresponding to the ground state electronic configuration of Sm+ (8F,4f66s1) and an excited surface having two electrons in the valence space (excluding 4f), which are needed to form the strong SmO+ bond. In contrast,...

The kinetics of the FeCO(+) + N2O reaction have been studied at thermal energies (300-600 K) using a variable temperature selected ion flow tube apparatus. Rate constants and product branching fractions are reported. The reaction is modestly inefficient, proceeding with a rate constant of 6.2 × 10(-11) cm(3) s(-1) at 300 K, with a small negative temperature dependence, declining to 4.4 × 10(-11) cm(3) s(-1) at 600 K. Both Fe(+) and FeO(+) products are observed, with a constant branching ratio of approximately 40:60 at all temperatures. Calculation of the stationary points along the reaction coordinate shows that only the ground state quartet surface is initially sampled resulting in N2 elimination; a submerged barrier along this portion of the surface dictates the magnitude and temperature dependence of the total rate constant. The product branching fractions are determined by the behavior of the remaining (4)OFeCO(+) fragment, and this behavior is compared to that found in the reaction of FeO(+) + CO, which initially forms (6)OFeCO(+). Thermodynamic and kinetic arguments are used to show that the spin-forbidden surface crossing in this region is efficient, proceeding with an average rate constant of greater than 10(12) s(-1).

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

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

We have measured reaction rate constants for CO+ and CO2+ reacting with N and O atoms using a selected ion flow tube apparatus equipped with a microwave discharge atom source. Experimental work was supplemented by molecular structure calculations. Calculated pathways show the sensitivity of kinetic barriers to theoretical methods and imply that high-level ab initio methods are required for accurate energetics. We report room-temperature rate constants of 1.0 ± 0.4 × 10−11 cm3 s−1 and 4.0 ± 1.6 × 10−11 cm3 s−1 for the reactions of CO+ with N and O atoms, respectively, and 8.0 ± 3.0 × 10−12 cm3 s−1 and 2.0 ± 0.8 × 10−11 cm3 s−1 for the reactions of CO2+ with N and O atoms, respectively. The reaction of CO2+ + O is observed to yield O2+ exclusively. These values help resolve discrepancies in the literature and are important for modeling of the Martian atmosphere.

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’.

The chemi-ionization reaction of atomic samarium, Sm + O → SmO+ + e−, has been investigated by the Air Force Research Laboratory as a means to modify local electron density in the ionosphere for reduction of scintillation of high-frequency radio waves. Neutral SmO is a likely unwanted byproduct. The spectroscopy of SmO is of great interest to aid in interpretation of optical emission spectra recorded following atmospheric releases of Sm as part of the Metal Oxide Space Cloud (MOSC) observations. Here, we report a joint experimental and theoretical study of SmO using slow photoelectron velocity-map imaging spectroscopy of cryogenically cooled SmO− anions (cryo-SEVI) and high-level spin-orbit complete active space calculations with corrections from second order perturbation theory (CASPT2). With cryo-SEVI, we measure the electron affinity of SmO to be 1.0581(11) eV and report electronic and vibrational structure of low-lying electronic states of SmO in good agreement with theory and prior experimental work....

Rate constants and product branching fractions of reactions between diatomic interhalogens (ICl, ClF) and a series of anions (Br−, I−) and cations (Ar+, N2+) are measured using a selected ion flow tube apparatus and reported over the temperature range 200–500 K. The efficiency of both anion reactions with ICl is 2%-3% at 300 K to yield Cl−, increasing with temperature in a manner consistent with the small endothermicities of the reactions. The anion reactions with ClF are 10%–20% efficient at 300 K to yield Cl− and also show a positive temperature dependence despite being highly exothermic. The stationary points along the anion + ClF reaction coordinates were calculated using density functional theory, showing no endothermic barriers inhibiting reaction. The observed temperature dependence can be rationalized by a decreasing dipole attraction with increasing rotational energy, but confirmation requires trajectory calculations of the systems. All four cation reactions are fairly efficient at 300 K with sma...

The chemi-ionization rate coefficients of La, Pr, Tb, and Ho with O have been measured from 200 to 450 K using a thermalized flow tube apparatus. Both La and Tb were found to react near the calculated hard sphere collision limit, while the Pr and Ho reactions proceeded at roughly 40% of that limit at all temperatures. The efficiencies of these reactions are considered and the near thermoneutral character of the reaction with Ho can explain this case, whereas an explanation for the inefficiency of the Pr reaction remains elusive. The chemi-ionization reaction of La with N2O was also studied and found to proceed roughly 2 orders of magnitude slower than the competing neutral oxidation pathway. The latter result disagrees with previous literature reports.

The ion-molecule reactions F-e+ + N2O -> FeO+ + N-2 and FeO+ + CO -> Fe+ + CO2, which catalyze the reaction CO + N2O -> CO2 + N-2, have been studied over the temperature range 120-700 K using a variable temperature selected ion flow tube apparatus. Values of the rate constants for the former two reactions were experimentally derived as k(2) (10(-11) cm(3) s(-1)) = 2.0(+/- 0.3) (T/300)(-1.5(+/- 0.2)) + 6.3(+/- 0.9) exp(-515(+/- 77)/T) and k(3) (10(-10) cm(3) s(-1)) = 3.1(+/- 0.1) (T/300)(-0.9(+/- 0.1)). Characterizing the energy parameters of the reactions by density functional theory at the B3LYP/TZVP level, the rate constants are modeled, accounting for the intermediate formation of complexes. The reactions are characterized by nonstatistical intrinsic dynamics and rotation-dependent competition between forward and backward fluxes. For Fe+ + N2O, sextet-quartet switching of the potential energy surfaces is quantified. The rate constant for the clustering reaction FeO+ + N2O + He -> FeO(N2O)(+) + He was also measured, being k(4) (10(-27) cm(6) s(-1)) = 1.1(+/- 0.1) (T/300)(-2.5(+/- 0.1)) in the low pressure limit, and analyzed in terms of unimolecular rate theory.