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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.

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.

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.

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.

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.

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.

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.

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

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

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’.

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.

The structure of sodium chloride salt-cluster dianions of the type Na n Cl m 2 - ( n = 7, 15, m = n + 2) is studied using collisional dissociation with static argon gas targets. Collisions leading to single electron detachment leaving Na n Cl m - are not observed for energies up to 400 eV (Lab). An argument is proffered that the Na n Cl m - ion is unstable toward dissociation. The loss of Cl − is only observed at relatively high energies (∼400 eV), well above the expected threshold. This cross section is also observed to be small compared to that of molecular “fission”. Dissociation (fission) into singly-charged conjugate ion-pairs, e.g., Na x Cl x + 1 - + Na y Cl y + 1 - ( x + y = n ) occurs at energies below ∼1 eV. Calculations show that the Na 7 Cl 9 2 - dianions are metastable as a result of a Coulomb barrier inhibiting dissociation. Experimentally estimated lifetimes against metastable decay are greater than 10 μsec and are consistent with a long tunneling time through the Coulomb barrier. Quantum chemical calculations establish that the Coulomb barrier leading to dissociation into singly-charged anions, e.g. Na 7 Cl 9 2 - → NaCl 2 - + Na 6 Cl 7 - , is ∼0.8 eV and is consistent with the observed thresholds.

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...

Rate coefficients and branching fractions have been measured for electron attachment to perfluoromethylcyclohexane, C(7)F(14), along with thermal detachment rate coefficients for C(7)F(14)(-), from 300 to 630 K, using a flowing-afterglow Langmuir-probe apparatus. The attachment rate coefficient at room temperature is 4.5 ± 1.2 × 10(-8) cm(3) s(-1) and increases with temperature at a rate described by an activation energy of 50 ± 25 meV. Thermal electron detachment is negligible at room temperature, but measurable at 600 K and above, reaching 2300 ± 1300 s(-1) at 630 K. Analysis of the attachment-detachment equilibrium yields EA(C(7)F(14)) = 1.02 ± 0.06 eV, in agreement with the literature value while more than halving the uncertainty. Implications of the measurement for the electron affinity of SF(6) are discussed. The dominant product of electron attachment is the parent anion, but C(6)F(11)(-) and C(7)F(13)(-) are also observed at very low levels (

A novel technique using a flowing afterglow-Langmuir probe apparatus for measurement of temperature dependences of rate constants for dissociative recombination (DR) is presented. Low (∼1011 cm−3) concentrations of a neutral precursor are added to a noble gas/electron afterglow plasma thermalized at 300–500 K. Charge exchange yields one or many cation species, each of which may undergo DR. Relative ion concentrations are monitored at a fixed reaction time while the initial plasma density is varied between 109 and 1010 cm−3. Modeling of the decrease in concentration of each cation relative to the non-recombining noble gas cation yields the rate constant for DR. The technique is applied to several species (O2+, CO2+, CF3+, N2O+) with previously determined 300 K values, showing excellent agreement. The measurements of those species are extended to 500 K, with good agreement to literature values where they exist. Measurements are also made for a range of CnHm+ (C7H7+, C7H8+, C5H6+, C4H4+, C6H5+, C3H3+, and C6...

Valence and dipole-bound negative ions of the nitroethane (NE) molecule and its clusters are studied using photoelectron spectroscopy (PES), Rydberg electron transfer (RET) techniques, and ab initio methods. Valence adiabatic electron affinities (EA(a)s) of NE, C(2)H(5)NO(2), and its clusters, (C(2)H(5)NO(2))(n), n=2-5, are estimated using vibrationally unresolved PES to be 0.3+/-0.2 eV (n=1), 0.9+/-0.2 eV (n=2), 1.5+/-0.2 eV (n=3), 1.9+/-0.2 eV (n=4), and 2.1+/-0.2 eV (n=5). These energies were then used to determine stepwise anion-neutral solvation energies and compared with previous literature values. Vertical detachment energies for (C(2)H(5)NO(2))(n)(-) were also measured to be 0.92+/-0.10 eV (n=1), 1.63+/-0.10 eV (n=2), 2.04+/-0.10 eV (n=3), and 2.3+/-0.1 eV (n=4). RET experiments show that Rydberg electrons can be attached to NE both as dipole-bound and valence bound anion states. The results are similar to those found for nitromethane (NM), where it was argued that the diffuse dipole state act as a "doorway state" to the more tightly bound valence anion. Using previous models for relating the maximum in the RET dependence of the Rydberg effective principle number n(max)(*), the dipole-bound electron affinity is predicted to be approximately 25 meV. However, a close examination of the RET cross section data for NE and a re-examination of such data for NM finds a much broader dependence on n(*) than is seen for RET in conventional dipole bound states and, more importantly, a pronounced [l] dependence is found in n(max)(*) (n(max)(*) increases with [l]). Ab initio calculations agree well with the experimental results apart from the vertical electron affinity value associated with the dipole bound state which is predicted to be 8 meV. Moreover, the calculations help to visualize the dramatic difference in the distributions of the excess electron for dipole-bound and valence states, and suggest that NE clusters form only anions where the excess electron localizes on a single monomer.