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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 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+(D-2(o)), tau approximate to 3.6 hours), while no contributions from the second metastable state (O+(P-2(o)), tau approximate to 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+(D-2(o)) metastable state as well as channels involving the O(3s S-5(o)) state.

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.

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...

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.

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 wavefunction-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 suggests 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 clarify observations from prior release experiments.

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 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.

The rate constant for OH − (H 2 O) 2 + CH 3 I reaction was determined by selected ion flow tube (SIFT) experiments for temperatures in the range of 298–398 K. It is found to be an order of magnitude smaller than the collision capture rate constant, a result substantially different than found previously for the OH − + CH 3 I and OH − (H 2 O) + CH 3 I reactions. The rate constants for these reactions are only ∼25% and ∼two times smaller, respectively, than their collision capture rate constants. Only two product ions are observed experimentally, i.e. I − and I − (H 2 O), and their respective percentage yields are 90:10 and 83:17 at 298 and 348 K. The kinetics for the OH − (H 2 O) 2 + CH 3 I reaction were also studied by direct dynamics simulations using the DFT/B97-1/ECP/d electronic structure theory, the same theory used in previous direct dynamics simulations of the OH − + CH 3 I and OH − (H 2 O) + CH 3 I reactions. Simulations for OH − (H 2 O) 2 + CH 3 I at 387 K give respective percentage yields of 91:9 for I − and I − (H 2 O), in good agreement with the experimental results. For both the experiments and simulations, the microsolvated ion I − (H 2 O) 2 is not formed and the formation of I − dominates I − (H 2 O). For the OH − + CH 3 I and OH − (H 2 O) + CH 3 I reactions the experimental and direct dynamics simulation rate constants agree. However, this is not the case for OH − (H 2 O) 2 + CH 3 I, for which the simulation rate constant is 8–9 times larger than the experimental value. Comparisons of the experimental, simulation, and collision capture rate constants for the OH − (H 2 O) 2 + CH 3 I reaction indicate the height of the submerged S N 2 barrier for the reaction is an important feature of its potential energy surface. The actual barrier is expected to be higher than the value given by the DFT/B97-1 calculations. In future work it will be important to perform higher level electronic structure calculations and establish an accurate value for this barrier. Preliminary calculations reported here indicate the barrier height is sensitive to the electronic structure method.

Atomic samarium has been injected into the neutral atmosphere for production of electron clouds that modify the ionosphere. These electron clouds may be used as high-frequency radio wave reflectors or for control of the electrodynamics of the F region. A self-consistent model for the photochemical reactions of Samarium vapor cloud released into the upper atmosphere has been developed and compared with the Metal Oxide Space Cloud (MOSC) experimental observations. The release initially produces a dense plasma cloud that that is rapidly reduced by dissociative recombination and diffusive expansion. The spectral emissions from the release cover the ultraviolet to the near infrared band with contributions from solar fluorescence of the atomic, molecular, and ionized components of the artificial density cloud. Barium releases in sunlight are more efficient than Samarium releases in sunlight for production of dense ionization clouds. Samarium may be of interest for nighttime releases but the artificial electron cloud is limited by recombination with the samarium oxide ion.

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 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...

An orthogonally accelerated time-of-flight mass spectrometer has been added to a flowing afterglow–Langmuir probe apparatus, coupled by a rectilinear ion guide. The TOF has improved the signal-to-noise (S/N) ratio by about an order of magnitude over a quadrupole mass filter/electron multiplier detection typical for flow tube instruments of this type and used previously. The improved S/N reduces our data acquisition time commensurately. Additionally, mass discrimination is low enough that it is minimal for ions heavier than ∼10 amu. The lack of mass discrimination not only reduces experiment time but also leads to greater accuracy in the measurement of reaction rate coefficients. Finally, simultaneous detection of both heavy and light ions is improved. H + can now be detected and WF 5 + isotopes can be separated without reducing sensitivity. We have found that discrimination against low-mass ions can occur when certain electron-attaching reactants are used (e.g. SF 6 ), and possibly due to radicals formed after certain other reactants attach electrons. Evidence points to adsorption of these species on surfaces of the ion guide immediately behind the flowing afterglow sampling orifice, which separates the flow tube and TOF region. The adsorption follows a Langmuir isotherm profile and is greatly reduced by heating the gas in the flow tube.

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.

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.

The reactivity of microsolvated fluoride ions, F−(CH3OH)0–2, with methyl, ethyl, n-propyl, and t-butyl bromide is evaluated over a broad range of temperatures. Significant decreases in reactivity are observed as either solvation or temperature increases. Increasing solvation increases sensitivity to the reaction barrier as revealed by a larger temperature dependence. These reactions are dominated by an SN2 mechanism for the methyl bromide reaction, while the SN2 and E2 mechanisms compete for the reactions with ethyl and n-propyl bromide reactions. The elimination mechanism, with some association, dominates the t-butyl bromide reactions. In all cases the unsolvated bromide ion is the primary ionic product. Branching ratios are discussed in both qualitative and quantitative terms for all reactions at 300 K.

A new ion detection scheme incorporating both time-of-flight and quadrupole instrumentation has been implemented on a variable temperature selected ion flow tube and is described here. The new detection region retains typical quadrupole detection for the study of most systems, and adds time-of-flight capability to study systems over a wide mass range while minimizing the effects of mass discrimination. Experiments verify the accuracy of the kinetics obtained with time-of-flight detection. We find excellent agreement with previously published rate constants and product branching for the reactions of Ar+ with SF6 and C2H4. Additionally, new results are presented for the reaction of Ar+ with WF6, observing a rate constant of 1.23 × 10−9 cm3 s−1, with product branching of 0.9 and 0.1 to WF5+ and WF6+ respectively.

Thermal electron attachment to CH 3 Br has been studied over the temperature range 303–1100 K using two flowing-afterglow Langmuir-probe apparatuses. The reaction yielded only Br − product over this temperature range. The rate coefficient for electron attachment to CH 3 Br was measured to be 8 ± 4 × 10 −12 cm 3 s −1 at 303 K, and was observed to increase strongly with gas temperature. Rate coefficients for the reaction show Arrhenius behavior over the entire temperature range with an activation energy of 260 ± 20 meV. The results are in substantial agreement with earlier data covering a smaller temperature range. Kinetic modeling implies that this behavior and the small rate coefficient at room temperature are due to a barrier in the crossing from the neutral to the anionic potential surfaces of ∼280 meV that dominates other factors in the attachment reaction. There is a hint of the Arrhenius plot reaching saturation at the highest temperatures. While examining an electron-cation recombination correction, the rate coefficient (1.8 ± 0.4 × 10 −9 cm 3 s −1 ) of the reaction Ar + + CH 3 Br was measured at 302 K, and the ion products identified (80% CH 3 + and 20% CH 2 Br + ). A secondary reaction forming the adduct (CH 3 Br)CH 3 + was seen to occur with a rate coefficient of 2.8 ± 1.0 × 10 −9 cm 3 s −1 .

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

Direct dynamics simulations and selected ion flow tube (SIFT) experiments were performed to study the kinetics and dynamics of the OH(-) + CH3I reaction versus temperature. This work complements previous direct dynamics simulation and molecular beam ion imaging experiments of this reaction versus reaction collision energy (Xie et al. J. Phys. Chem. A 2013, 117, 7162). The simulations and experiments are in quite good agreement. Both identify the SN2, OH(-) + CH3I → CH3OH + I(-), and proton transfer, OH(-) + CH3I → CH2I(-) + H2O, reactions as having nearly equal importance. In the experiments, the SN2 pathway constitutes 0.64 ± 0.05, 0.56 ± 0.05, 0.51 ± 0.05, and 0.46 ± 0.05 of the total reaction at 210, 300, 400, and 500 K, respectively. For the simulations this fraction is 0.56 ± 0.06, 0.55 ± 0.04, and 0.50 ± 0.05 at 300, 400, and 500 K, respectively. The experimental total reaction rate constant is (2.3 ± 0.6) × 10(-9), (1.7 ± 0.4) × 10(-9), (1.9 ± 0.5) × 10(-9), and (1.8 ± 0.5) × 10(-9) cm(3) s(-1) at 210, 300, 400, and 500 K, respectively, which is approximately 25% smaller than the collision capture value. The simulation values for this rate constant are (1.7 ± 0.2) × 10(-9), (1.8 ± 0.1) × 10(-9), and (1.6 ± 0.1) × 10(-9) cm(3)s(-1) at 300, 400, and 500 K. From the simulations, direct rebound and stripping mechanisms as well as multiple indirect mechanisms are identified as the atomic-level reaction mechanisms for both the SN2 and proton-transfer pathways. For the SN2 reaction the direct and indirect mechanisms have nearly equal probabilities; the direct mechanisms are slightly more probable, and direct rebound is more important than direct stripping. For the proton-transfer pathway the indirect mechanisms are more important than the direct mechanisms, and stripping is significantly more important than rebound for the latter. Calculations were performed with the OH(-) quantum number J equal to 0, 3, and 6 to investigate the effect of OH(-) rotational excitation on the OH(-) + CH3I reaction dynamics. The overall reaction probability and the probabilities for the SN2 and proton-transfer pathways have little dependence on J. Possible effects on the atomistic mechanisms were investigated for the SN2 pathway and the probability of the direct rebound mechanism increased with J. However, the other atomistic mechanisms were not appreciably affected by J.

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).

We report for the first time temperature dependences (from 300 to 600 K) of the reactions of Fe(+) and FeO(+) with NO and NO(2). Both ions react quickly with NO(2), and their rate constants have weak negative temperature dependences. The former is consistent with the calculated energy profile along the Fe(+) + NO(2) reaction coordinate. Ground state Fe(+) reacts with NO(2) to produce only FeO(+), while FeO(+) reacts with NO(2) to produce NO(+) exclusively. Certain source conditions produce excited Fe(+), as evidenced by production of primary NO(+), which is endothermic with the ground state by 0.35 eV. The room temperature rate constants are in agreement with previous values. For the reactions of Fe(+) and FeO(+) with NO, we find an upper limit of

Recently, it was discovered that specific aluminum clusters (e.g., Al13–) that demonstrate enhanced resistance to reactivity with oxygen may do so not only because of a closed electronic jellium shell as originally supposed but also because of a forbidden spin-flip in the transition state of the reaction. Herein, we discuss an experiment using a multiple-species laminar flow reaction vessel coupled to a singlet oxygen generator. The present results suggest that all clusters react with singlet oxygen. Additionally, we observe Al9–, a cluster previously unidentified as having any notable stability, as being resistant to reaction with triplet oxygen. Furthermore, we discuss a means of estimating rate constants in a multiple-species flow tube where the products and reactants do not allow the use of traditional methods.

Room-temperature ionic liquids exert vanishingly small vapor pressures under ambient conditions. Under reduced pressure, certain ionic liquids have demonstrated volatility, and they are thought to vaporize as intact cation-anion ion pairs. However, ion pair vapors are difficult to detect because their concentration is extremely low under these conditions. In this Letter, we report the products of reacting ions such as NO(+), NH4(+), NO3(-), and O2(-) with vaporized aprotic ionic liquids in their intact ion pair form. Ion pair fragmentation to the cation or anion as well as ion exchange and ion addition processes are observed by selected-ion flow tube mass spectrometry. Free energies of the reactions involving 1-ethyl-3-methylimidazolium bis-trifluoromethylsulfonylimide determined by ab initio quantum mechanical calculations indicate that ion exchange or ion addition are energetically more favorable than charge-transfer processes, whereas charge-transfer processes can be important in reactions involving 1-butyl-3-methylimidazolium dicyanamide.

Mutual neutralization (MN) rate constants kMN 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, kMNCl

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’.

It's the Network Numerous reactions of small molecules and ions in the atmosphere take place in the confines of watery aerosols. Relph et al. (p. 308 ; see the Perspective by Siefermann and Abel ) explored the specific influence of a water cluster's geometry on the transformation of solvated nitrosonium (NO + ) to nitrous acid (HONO). The reaction involves (O)N–O(H) bond formation with one water molecule, concomitant with proton transfer to additional, surrounding water molecules. Vibrational spectroscopy and theoretical simulations suggest that certain arrangements of the surrounding water network are much more effective than others in accommodating this charge transfer, and thus facilitating the reaction.

Spectroscopic studies of the SF6- and c-C4F8- anions are reported to provide experimental benchmarks for theoretical predictions of their structures and electron binding energies. The photoelectron spectrum of SF6- is dominated by a long progression in the S-F stretching mode, with an envelope consistent with theoretical predictions that the anion preserves the Oh symmetry of the neutral, but has a longer S-F bond length. This main progression occurs with an unexpectedly strong contribution from a second mode, however, whose characteristic energy does not correspond to any of the neutral SF6 fundamental vibrations in its ground electronic state. The resulting doublet pattern is evident when the bare ion is prepared with low internal energy content (i.e., using N2 carrier gas in a free jet or liquid nitrogen-cooling in a flowing afterglow) but is much better resolved in the spectrum of the SF6-.Ar complex. The infrared predissociation spectrum of SF6-.Ar consists of a strong band at 683(5) cm(-1), which we assign to the nu3 (t1u) fundamental, the same mode that yields the strong 948 cm(-1) infrared transition in neutral SF6. One qualitatively interesting aspect of the SF6- behavior is the simple structure of its photoelectron spectrum, which displays uncluttered, harmonic bands in an energy region where the neutral molecule contains about 2 eV of vibrational excitation. We explore this effect further in the c-C4F8- anion, which also presents a system that is calculated to undergo large, symmetrical distortion upon electron attachment to the neutral. The photoelectron spectrum of this species is dominated by a long, single vibrational progression, this time involving the symmetric ring-breathing mode. Like the SF6- case, the c-C4F8- spectrum is remarkably isolated and harmonic in spite of the significant internal excitation of a relatively complex molecular framework. Both these perfluorinated anions thus share the property that the symmetrical deformation of the structural backbone upon photodetachment launches very harmonic motion in photoelectron bands that occur far above their respective adiabatic electron affinities.

A new ion source region has been constructed and attached to a variable temperature selected ion flow tube. The source features the capabilities of electron impact, chemical ionization, a solids probe, and electrospray ionization. The performance of the instrument is demonstrated through a series of reactions from ions created in each of the new source regions. The chemical ionization source is able to create H3O(+), but not as efficiently as similar sources with larger apertures. The ability of this source to support a solids probe, however, greatly expands our capabilities. A variety of rhenium cations and dications are created from the solids probe in sufficient abundance to study in the flow tube. The reaction of Re(+) with O2 proceeds with a rate constant that agrees with the literature measurements, while the reaction of Re2(2+) is found to charge transfer with O2 at about 60% of the collision rate; we have also performed calculations that support the charge transfer pathway. The electrospray source is used to create Ba(+), which is reacted with N2O to create BaO(+), and we find a rate constant that agrees with the literature.

The exothermicity of the chemi-ionization reaction Sm + O → SmO(+) + e(-) has been re-evaluated through the combination of several experimental methods. The thermal reactivity (300-650 K) of Sm(+) and SmO(+) with a range of species measured using a selected ion flow tube-mass spectrometer apparatus is reported and provides limits for the bond strength of SmO(+), 5.661 eV ≤ D0(Sm(+)-O) ≤ 6.500 eV. A more precise value is measured to be 5.725 ± 0.07 eV, bracketed by the observed reactivity of Sm(+) and SmO(+) with several species using a guided ion beam tandem mass spectrometer (GIBMS). Combined with the established Sm ionization energy (IE), this value indicates an exothermicity of the title reaction of 0.08 ± 0.07 eV, ∼0.2 eV smaller than previous determinations. In addition, the ionization energy of SmO has been measured by resonantly enhanced two-photon ionization and pulsed-field ionization zero kinetic energy photoelectron spectroscopy to be 5.7427 ± 0.0006 eV, significantly higher than the literature value. Combined with literature bond energies of SmO, this value indicates an exothermicity of the title reaction of 0.14 ± 0.17 eV, independent from and in agreement with the GIBMS result presented here. The evaluated thermochemistry also suggests that D0(SmO) = 5.83 ± 0.07 eV, consistent with but more precise than the literature values. Implications of these results for interpretation of chemical release experiments in the thermosphere are discussed.

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

The absolute rate coefficients and product ion branching percentages at 298 K for the reactions of several PO x Cl y - species with atomic nitrogen (N ( 4 S 3/2 )) and atomic oxygen (O ( 3 P)) have been determined in a selected-ion flow tube (SIFT) instrument. PO x Cl y - ions are generated by electron impact on POCl 3 in a highpressure source. O atoms are generated by quantitative titration of N atoms with NO, where N atoms are produced by microwave discharge on N 2 . The experimental procedure allows for the determination of rate coefficients for the reaction of the reactant ion with N ( 4 S 3/2 ) and O ( 3 P) as well as with N 2 and NO. None of the ions react with N 2 or NO, giving an upper limit to the rate coefficient of 85% of the products. PO 2 - is a minor product (≤4%) from POCl 2 - + O. Only PO 2 Cl - reacts with both N and O, directly giving PO 2 - and PO 3 - as major products. In addition, calculations of the structures and energies for PO 2 N, PO 2 N - , and NCl have been performed at the G3 level of theory to obtain estimates for the energetics of the PO 2 Cl - reactions. PO 2 - , PO 3 - , and PO 2 Cl 2 - are all unreactive with both N and O. Comparisons of the reactivity of PO x Cl y - ions with O atoms are made to previous reactivity studies of these ions. In particular, routes that yield the very stable PO 2 - and PO 3 - ions are discussed.

The dissociative recombination and excitation of CF+ have been measured at the ASTRID and CRYRING storage rings. Though examination of the available potential energy curves would suggest that the recombination rate would be large for this ion, in fact a rate constant of 5.2 ± 1.0 × 10−8 (Te/300)−0.8 cm3 s−1 was found. The recombination cross section at low energies falls off to a minimum at 0.5 eV centre-of-mass collision energy but exhibits resonances at energies above this. The dissociative excitation cross section leading to C+ + F was also measured and this displays an onset beginning at about 7 eV.

Rate constants and product distributions have been measured for a number of hydrocarbon ions at the CRYRING facility at the Stockholm University. Rate constants at 300 K are about 5×10−7 cm−3 s−1. The electron temperature dependences are also roughly constant and follow a power law. The products appear to correlate with reaction exothermicity.

The dissociative recombination of Na(+)(D(2)O) ion has been studied at the heavy-ion storage ring CRYRING (Manne Siegbahn Laboratory, Stockholm University). The cross section has been measured as a function of center-of-mass energy ranging from 1 meV to 0.1 eV and found to have an E(-1.37) dependence. The rate coefficient has been deduced to be (2.3+/-0.32)x10(-7)(T(e)/300)(-0.95+/-0.01) cm(3) s(-1) for T(e)=50-1000 K. The branching ratios have been measured at 0 eV. Of the four energetically accessible dissociation channels, three channels are found to occur although the channel that breaks the weak Na(+)-D(2)O bond is by far dominant.

Rate constants and product ion branching fractions for the gas-phase reactions of O2 and O3 with the anions (a) PO2Cl-, (b) POCl3-, (c) POCl2-, and (d) PO2Cl2- were measured in a selected-ion flow ...

We have studied the vibrationally relaxed C2H + ion in the heavy - ion storage ring CRYRING in Stockholm. We measured the dissociative recombination absolute cross section over center-of-mass energies in the range between 0 and 0.1 eV by scanning the electron energy. The rate of dierent neutral product channels of dissociative recombination was measured. We found the three-body chan- nel C2H + H + H, with a branching ratio of 59%, to be the dominant one. Finally, we compare C2H + and C2H + (Derkatch et al. 1999) results.

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....

A novel ozone source was developed to study the negative ion chemistry of ozone in the gas phase. Rate constants and product ion branching fractions are reported for 17 negative ion-molecule reactions involving ozone (O 3 ). This is the most comprehensive set of O 3 reactions with negative ions to date. The reactions proceed primarily through charge transfer and O atom transfer. The reaction rate constants for O - , O 2 - , and OH- are large and approximately equal to the thermal energy capture rate constant given by the Su-Chesnavich equation based on average dipole orientation theory. The negative ions NO 2 - , CO 4 - , SF 6 - , and PO 2 - are somewhat less reactive, reacting at approximately 20-50% of the thermal capture rate. The hydrofluorocarbon ions CF 3 - and C 2 F 5 - react at 80% of the thermal capture rate, and F- is the major product ion formed. NO 3 - , CO 3 - , PO 3 - , CF 3 O - , F - , Cl-, and Br- are found to be unreactive with rate constants < 5 × 10 - 1 2 cm 3 s - 1 , which is the present detection limit of our apparatus using this ozone source. The I- ion was observed to cluster with O 3 to form IO 3 - with a rate constant of approximately I x 10 - 1 1 cm 3 s - 1 , which is a factor of 2 above our detection limit, and no other product channels were observed. All of the anions listed above showed no reactivity, k < 5 x 10 - 1 3 cm 3 s - 1 , with O 2 .

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...

Rate constants and branching fractions are reported for the reactions of NO+, Xe2+, O2+, Xe+(2P3/2), and Kr+(2P3/2) with toluene, ethylbenzene, and n-propylbenzene. Ions with recombination energies...

Technologicaldevelopments of fast-flow tubes that led to major advances in the study of cluster ion reactions are reviewed, including the coupling of high-pressure cluster ion sources to flowing-afterglow and selected-ion flow tube (SIFT) instruments. Several areas of cluster ion chemistry that have been studied recently in our laboratory, using a SIFT instrument with a supersonic expansion ion source, are reviewed. Firstly the thermal destruction of cluster ions is highlighted by a discussion of the competition between electron detachment and thermal dissociationin hydrated electron clusters (H O) . Rates and activationenergies for the n thermal destruction (dissociation plus detachment) of these clusters are discussed. The reactivity of hydrated electron clusters with several neutral electron scavengers is also reviewed. Secondly cluster ion chemistry related to trace neutral detection of atmospheric species using chemical ionization mass spectrometry is discussed. Recent rate measurements needed for ch...

This paper presents non-intrusive measurements of sodium-argon and sodium-nitrogen cluster ion recombination with electrons based on coherent microwave scattering (Radar) from Resonance Enhanced Multi-Photon Ionization (REMPI). The number density of sodium, in a mixture of sodium vapor with argon or nitrogen buffer gas, was determined by direct absorption measurement. Sodium was resonantly ionized by a tunable laser beam in the 2+1 REMPI process. Since the plasma is mainly made of sodium ions in the mixture, the sodium ion clusters are thus formed by the Chaperone process. The dissociative recombination rate of sodium-argon and sodium-nitrogen and the neutral stabilized recombination rate of sodium ions were determined from a least-squares Monte Carlo algorithm (LSM) that fitted the plasma dynamic model to the direct measurement of the total electron number in the laser-induced plasma. At 300 o C, and , the dissociative recombination rate of Na + ∙(Ar) was determined to be and the neutral stabilized recombination rate was determined to be . At 300C, and , the dissociative recombination rate of Na + ∙(N2) was determined to be and the neutral stabilized recombination rate was determined to be .The method can be easily extended to measurements of other cluster ion species over a wide range of temperatures and pressures in various mixture compositions, such as Mg and Ca with N2 and CO2. The results are of fundamental importance to the plasma chemistry field and hypersonic research.

Mutual neutralization (MN) rate coefficients kMN for He(+) with the anions Cl(-), Br(-), I(-), and SF6 (-) are reported from 300 to 500 K. The measured rate coefficients may contain a contribution from transfer ionization, i.e., double ionization of the anion. The large rate coefficient for He(+) + SF6 (-) (2.4 × 10(-7) cm(3) s(-1) at 300 K) is consistent with earlier polyatomic MN results found to have a reduced mass dependence of μ(-1/2). Neutralization of He(+) by the atomic halides follows the trend observed earlier for Ne(+), Ar(+), Kr(+), and Xe(+) neutralized by atomic halides, kMN (Cl(-)) < kMN (Br(-)) < kMN (I(-)). Only an upper limit could be measured for the neutralization of He(+) by Cl(-). Predictions of the rate coefficients from a previously proposed simple model of atomic-atomic MN results are consistent with the present He(+)-halide rate coefficients. The temperature dependences are modestly negative for Br(-) and I(-), while that for SF6 (-) is small or negligible.

Rate coefficients kMN have been measured for a number of anion neutralization reactions with Ar+ and Kr+ over the temperature range 300–550 K. For the first time, the data set includes anions of radicals and other short-lived species. In the present paper, we review these results and make note of correlations with reduced mass, electron binding energy of the anion (equivalent to the electron affinity of the corresponding neutral), and temperature, and compare with expectations from absorbing sphere models. An intriguing result is that the data for diatomic anions neutralized by Ar+ and Kr+ have kMN values close to 3 × 10−8 cm3 s−1 at 300 K, a figure which is lower than those for all of the polyatomic anions at 300 K except for SF5− + Kr+. For the polyatomic anions studied here, neutralized by Ar+ and Kr+, the reduced mass dependence agrees with theory, on average, but we find a stronger temperature dependence of T−0.9 than expected from the theoretical E−0.5 energy dependence of the rate coefficient at th...

Rate coefficients k(MN) have been measured for a number of anion neutralization reactions with Ar(+) and Kr(+) over the temperature range 300-550 K. For the first time, the data set includes anions of radicals and other short-lived species. In the present paper, we review these results and make note of correlations with reduced mass, electron binding energy of the anion (equivalent to the electron affinity of the corresponding neutral), and temperature, and compare with expectations from absorbing sphere models. An intriguing result is that the data for diatomic anions neutralized by Ar(+) and Kr(+) have k(MN) values close to 3 × 10(-8) cm(3) s(-1) at 300 K, a figure which is lower than those for all of the polyatomic anions at 300 K except for SF(5)(-) + Kr(+). For the polyatomic anions studied here, neutralized by Ar(+) and Kr(+), the reduced mass dependence agrees with theory, on average, but we find a stronger temperature dependence of T(-0.9) than expected from the theoretical E(-0.5) energy dependence of the rate coefficient at thermal energies. The k(MN) show a weak dependence on the electron binding energy of the anion for the polyatomic species studied.