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Specific rate constants k(E) of the dissociation of the halobenzene ions C6H5X+ --C6H5+ + X* (X* = Cl, Br, and I) were measured over a range of 10(3)-10(7) s-1 by threshold photoelectron-photoion coincidence (TPEPICO) spectroscopy. The experimental data were analyzed by various statistical unimolecular rate theories in order to derive the threshold energies E0. Although rigid activated complex RRKM theory fits the data in the experimentally measured energy range, it significantly underestimates E0 for chloro- and bromobenzene. Phase space theory (PST) does not fit the experimentally measured rates. A parametrized version of the variational transition state theory (VTST) as well as a simplified version of the statistical adiabatic channel model (SSACM) incorporating an energy dependent rigidity factor provide excellent fits to the experimental data and predict the correct dissociation energies. Although both approaches have just two adjustable parameters, one of which is E0, SSACM is effective and particularly simple to apply.

The dissociation dynamics of energy-selected neopentylamine cations, (CH3)3CCH2NH2+, were studied using threshold photoelectron photoion coincidence (TPEPICO) spectroscopy in which ion time-of-flight (TOF) distributions are recorded as a function of the ion internal energy. The lowest energy pathway, producing the CH3NH3+ ion, involves a rearrangement of the molecular ion. The 0 K dissociation onsets for the production of the CH3NH3+, CH2NH2+, and (CH3)2CCH2NH2+ ions have been determined to be 9.54 ± 0.05, 9.647 ± 0.025 and 9.90 ± 0.1 eV, respectively. Because the heat of formation of the CH2NH2+ + t-C4H9 products are established, we can use the measured onset of 9.647 eV for this reaction to establish the neopentylamine Δ f H 298 ° = − 130.3 ± 3.3 kJ / mol . This result is used in a group additivity scheme to estimate the neopentyl alcohol heat of formation as Δ f H 298 ° = − 315.5 ± 4 kJ mo l − 1 . Both of these results are strongly supported by a series of isodesmic reactions calculated at the G3B3 and CBS-APNO levels. This is the first reported experimental heat of formation of the neopentylamine and the first reported heat of formation of the neopentyl alcohol.

The dissociative photoionization onsets for Cl and Br loss reactions were measured for HCCl3, HCCl2Br, HCClBr2, and HCBr3 by threshold photoelectron photoion coincidence (TPEPICO) in order to establish the heats of formation of the mixed halides as well as the following fragment ions: HCCl2(+), HCClBr(+), HCBr2(+). The first zero Kelvin onsets were measured with a precision of 10 meV. The second onsets, which are in competition with the lower energy onsets, were established with a precision of 60 meV. Because both the chloroform and bromoform have relatively well established heats of formation, these measurements provide a route for establishing the heats of formation of the mixed halomethanes within uncertainties of less than 5 kJ mol(-1).

A narrow bandwidth infrared (IR) laser selectively excites and activates one isotopic variant of a gas phase precursor to permit isotope-selective chemical vapor deposition. We show that selective vibrational excitation of the 12CH4 or 13CH4 isotopologue of methane controls the carbon isotope ratio of adsorbates deposited on a Ni substrate and permits significant isotope enrichment in a single reaction step. A model of the process based on known reaction probabilities and process variables predicts the enrichment we observe. While our current laser system permits a 9-fold enhancement of 12C or 13C deposition, optimization of deposition and detection strategies and use of a more powerful, commercially available laser source suggest isotope enrichment factors of 100-fold or more are readily attainable. Other gas−surface reactions activated by vibrational energy may also be candidates for this approach to isotope-selective deposition.

Energy redistribution, including the many phonon-assisted and electronically assisted energy-exchange processes at a gas-metal interface, can hamper vibrationally mediated selectivity in chemical reactions. We establish that these limitations do not prevent bond-selective control of a heterogeneously catalyzed reaction. State-resolved gas-surface scattering measurements show that the ν1C-H stretch vibration in trideuteromethane (CHD3) selectively activates C-H bond cleavage on a Ni(111) surface. Isotope-resolved detection reveals a CD3:CHD2product ratio > 30:1, which contrasts with the 1:3 ratio for an isoenergetic ensemble of CHD3whose vibrations are statistically populated. Recent studies of vibrational energy redistribution in the gas and condensed phases suggest that other gas-surface reactions with similar vibrational energy flow dynamics might also be candidates for such bond-selective control.

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 temperature variation of rate constants has been measured for the gas phase reactions of the oxycation O2(+) with N atoms and of N2(+) with O atoms from 120 to 400 K using a variable temperature-selected ion flow tube. Measured room temperature rate constants, 0.75 × 10(-10) cm(3) s(-1) (±30%) for O2(+) with N and 1.4 × 10(-10) cm(3) s(-1) (±30%) for N2(+) with O, are in agreement with previously reported values. A temperature dependence of T(-0.7(±0.3)) is observed for the O2(+) + N reaction; however, the N2(+) + O reaction is found to be independent of temperature. Calculations at varying levels of theory were used in tandem with experiments to evaluate likely pathways in potential energy surfaces for the reactions of concern.

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.

A novel technique is described for the measurement of rate constants and product branching fractions of thermal reactions between cation and radical species. The technique is a variant of the variable electron and neutral density attachment mass spectrometry (VENDAMS) method, employing a flowing afterglow-Langmuir probe apparatus. A radical species is produced in situ via dissociative electron attachment to a neutral precursor; this allows for a quantitative derivation of the radical concentration and, as a result, a quantitative determination of rate constants. The technique is applied to the reactions of Ar(+) and O2(+) with CH3 at 300 K. The Ar(+) + CH3 reaction proceeds near the collisional rate constant of 1.1 × 10(-9) cm(3) s(-1) and has three product channels: → CH3(+) + Ar (k = 5 ± 2 × 10(-10) cm(3) s(-1)), → CH2(+) + H + Ar (k = 7 ± 2 × 10(-10) cm(3) s(-1)), → CH(+) + H2 + Ar (k = 5 ± 3 × 10(-11) cm(3) s(-1)). The O2(+) + CH3 reaction is also efficient, with direct charge transfer yielding CH3(+) as the primary product channel. Several results needed to support these measurements are reported, including the kinetics of Ar(+) and O2(+) with CH3I, electron attachment to CH3I, and mutual neutralization of CH3(+) and CH2(+) with I(-).

The reactions of FeO(+) with H2 and of Fe(+) with N2O were studied with respect to the production and reactivity of electronically excited (4)Fe(+) cations. The reaction of electronic ground state (6)FeO(+) with H2 was found to predominantly produce electronically excited (4)Fe(+) as opposed to electronic ground state (6)Fe(+) corresponding to a spin-allowed reaction. (4)Fe(+) was observed to react with N2O with a rate constant of 2.3 (+0.3/-0.8) × 10(-11) cm(3) molecule(-1) s(-1), smaller than the ground state (6)Fe(+) rate constant of 3.2 (±0.5) × 10(-11) cm(3) molecule(-1) s(-1) (at room temperature). While the overall reaction of (6)FeO(+) with H2 within the Two-State-Reactivity concept is governed by efficient sextet-quartet spin-inversion in the initial reaction complex, the observation of predominant (4)Fe(+) production in the reaction is attributed to a much less efficient quartet-sextet back-inversion in the final reaction complex. Average spin-inversion probabilities are estimated by statistical modeling of spin-inversion processes and related to the properties of spin-orbit coupling along the reaction coordinate. The reaction of FeO(+) with H2 served as a source for (4)Fe(+), subsequently reacting with N2O. The measured rate constant has allowed for a more detailed understanding of the ground state (6)Fe(+) reaction with N2O, leading to a significantly improved statistical modeling of the previously measured temperature dependence of the reaction. In particular, evidence for the participation of electronically excited states of the reaction complex was found. Deexcitation of (4)Fe(+) by He was found to be slow, with a rate constant3 × 10(-14) cm(3) molecule(-1) s(-1).

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...

Thermal rate coefficients for the title reactions computed using a quasi-classical trajectory method on an accurate global potential energy surface fitted to ∼81,000 high-level ab initio points are compared with experimental values measured between 100 and 600 K using a variable temperature selected ion flow tube instrument. Excellent agreement is found across the entire temperature range, showing a subtle, but unusual temperature dependence of the rate coefficients. For both reactions the temperature dependence has a maximum around 350 K, which is a result of H2O(+) rotations increasing the reactivity, while kinetic energy is decreasing the reactivity. A strong isotope effect is found, although the calculations slightly overestimate the kinetic isotope effect. The good experiment-theory agreement not only validates the accuracy of the potential energy surface but also provides more accurate kinetic data over a large temperature range.

We have determined the rate constants and branching ratios for the reactions of CO(+) with CH4 and CD4 in a variable-temperature selected ion flow tube. We find that the rate constants are collisional for all temperatures measured (193-700 K for CH4 and 193-500 K for CD4). For the CH4 reaction, three product channels are identified, which include charge transfer (CH4(+) + CO), H-atom transfer (HCO(+) + CH3), and H-atom expulsion (CH3CO(+) + H). H-atom transfer is slightly preferred to charge transfer at low temperature, with the charge-transfer product increasing in contribution as the temperature is increased (H-atom expulsion is a minor product for all temperatures). Analogous products are identified for the CD4 reaction. Density functional calculations on the CO(+) + CH4 reaction were also conducted, revealing that the relative temperature dependences of the charge-transfer and H-atom transfer pathways are consistent with an initial charge transfer followed by proton transfer.

The temperature dependences of the rate constants and product branching ratios for the reactions of FeO(+) with CH4 and CD4 have been measured from 123 to 700 K. The 300 K rate constants are 9.5 × 10(-11) and 5.1 × 10(-11) cm(3) s(-1) for the CH4 and CD4 reactions, respectively. At low temperatures, the Fe(+) + CH3OH/CD3OD product channel dominates, while at higher temperatures, FeOH(+)/FeOD(+) + CH3/CD3 becomes the majority channel. The data were found to connect well with previous experiments at higher translational energies. The kinetics were simulated using a statistical adiabatic channel model (vibrations are adiabatic during approach of the reactants), which reproduced the experimental data of both reactions well over the extended temperature and energy ranges. Stationary point energies along the reaction pathway determined by ab initio calculations seemed to be only approximate and were allowed to vary in the statistical model. The model shows a crossing from the ground-state sextet surface to the excited quartet surface with large efficiency, indicating that both states are involved. The reaction bottleneck for the reaction is found to be the quartet barrier, for CH4 modeled as -22 kJ mol(-1) relative to the sextet reactants. Contrary to previous rationalizations, neither less favorable spin-crossing at increased energies nor the opening of additional reaction channels is needed to explain the temperature dependence of the product branching fractions. It is found that a proper treatment of state-specific rotations is crucial. The modeled energy for the FeOH(+) + CH3 channel (-1 kJ mol(-1)) agrees with the experimental thermochemical value, while the modeled energy of the Fe(+) + CH3OH channel (-10 kJ mol(-1)) corresponds to the quartet iron product, provided that spin-switching near the products is inefficient. Alternative possibilities for spin switching during the reaction are considered. The modeling provides unique insight into the reaction mechanisms as well as energetic benchmarks for the reaction surface.

Dissociative recombination of electrons with HCl+, H2Cl+, DCl+, and D2Cl+ has been measured under thermal conditions at 300, 400, and 500 K using a flowing afterglow–Langmuir probe apparatus. Measurements for HCl+ and DCl+ employed the variable electron and neutral density attachment mass spectrometry (VENDAMS) method, while those for H2Cl+ and D2Cl+ employed both VENDAMS and the more traditional technique of monitoring electron density as a function of reaction time. At 300 K, HCl+ and H2Cl+ recombine with kDR = 7.7±2.14.5 × 10−8 cm3 s−1 and 2.6 ± 0.8 × 10−7 cm3 s−1, respectively, whereas D2Cl+ is roughly half as fast as H2Cl+ with kDR = 1.1 ± 0.3 × 10−7 cm3 s−1 (2σ confidence intervals). DCl+ recombines with a rate coefficient below the approximate detection limit of the method (≲5 × 10−8 cm3 s−1) at all temperatures. Relatively slow dissociative recombination rates have been speculated to be responsible for the large HCl+ and H2Cl+ abundances in interstellar clouds compared to current astrochemical mod...

Rate constants have been measured from 300 to 700 K for thermal electron attachment to F${}_{2}$ using two flowing afterglow--Langmuir probe apparatuses. Dissociative attachment yielding F${}^{\ensuremath{-}}$ is observed with a rate constant of 5.0 \ifmmode\pm\else\textpm\fi{} 1.3 \ifmmode\times\else\texttimes\fi{} 10${}^{\ensuremath{-}9}$ cm${}^{3}$ s${}^{\ensuremath{-}1}$ at 300 K, rising to 9.6 \ifmmode\pm\else\textpm\fi{} 2.4 \ifmmode\times\else\texttimes\fi{} 10${}^{\ensuremath{-}9}$ cm${}^{3}$ s${}^{\ensuremath{-}1}$ at 700 K, well below the previously accepted values of McCorkle et al. [D. L. McCorkle, L. G. Christophorou, A. A. Christodoulides, and L. Pichiarella, J. Chem. Phys. 85, 1966 (1986)]. The absolute concentration of F${}_{2}$ reaching the afterglow is verified by measuring the near-collisional rate constant (4.5 \ifmmode\pm\else\textpm\fi{} 1.5 \ifmmode\times\else\texttimes\fi{} 10${}^{\ensuremath{-}10}$ cm${}^{3}$ s${}^{\ensuremath{-}1}$) for Ar${}^{+}$ $+$ F${}_{2}$\ensuremath{\rightarrow}ArF${}^{+}$ $+$ F. Prior attempts to apply $R$-matrix calculations to the F${}_{2}$ $+$ ${e}^{\ensuremath{-}}$ system have failed to explain previously reported thermal and nonthermal attachment rate constants along with high-resolution, low-energy attachment cross sections. The present results are reproduced exceptionally well by $R$-matrix calculations employing previously calculated resonance widths without adjustment.

Thermal electron attachment to the radical species C2F3 and C2F5 has been studied over the temperature range 300-890 K using the Variable Electron and Neutral Density Attachment Mass Spectrometry technique. Both radicals exclusively undergo dissociative attachment to yield F(-). The rate constant for C2F5 shows little dependence over the temperature range, remaining ~4 × 10(-9) cm(3) s(-1). The rate constant for C2F3 attachment rises steeply with temperature from 3 × 10(-11) cm(3) s(-1) at 300 K to 1 × 10(-9) cm(3) s(-1) at 890 K. The behaviors of both species at high temperature are in agreement with extrapolations previously made from data below 600 K using a recently developed kinetic modeling approach. Measurements were also made on C2F3Br and C2F5Br (used in this work as precursors to the radicals) over the same temperature range, and, for C2F5Br as a function of electron temperature. The attachment rate constants to both species rise with temperature following Arrhenius behavior. The attachment rate constant to C2F5Br falls with increasing electron temperature, in agreement with the kinetic modeling. The current data fall in line with past predictions of the kinetic modeling approach, again showing the utility of this simplified approach.

Rate constants for the reactions of O2(-) and O(-) with N and O atoms have been measured for the first time as a function of temperature from 173 to 500 K for O(-) reactions and 173 to 400 K for O2(-) reactions. Room temperature rate constants for O2(-) reacting with N and O are 3.1 × 10(-10) and 1.7 × 10(-10) cm(3) s(-1), respectively, and the corresponding O(-) rate constants are 1.7 × 10(-10) and 1.5 × 10(-10) cm(3) s(-1), in good agreement with previous values. Temperature dependences are about T(-1.7) for both O2(-) reactions and T(-0.6) and T(-1.3) for the reactions of O(-) with N and O, respectively. Branching for the O2(-) reaction with N is found to predominantly form O(-) (>85%) in contrast to previous measurements, which reported NO2 + e(-) as the main channel. Calculations point to the present results being correct. The potential energy surface for this reaction was calculated using density functional theory, coupled cluster with singles, doubles (triples), complete active space self-consistent field, and complete active space second-order perturbation methods and is found to be quite complex, with agreement between the calculated surface and the observed kinetic data only possible through the inclusion of dynamical correlation.

Thermal rate constants and product branching fractions for electron attachment to CF3Br and the CF3 radical have been measured over the temperature range 300-890 K, the upper limit being restricted by thermal decomposition of CF3Br. Both measurements were made in Flowing Afterglow Langmuir Probe apparatuses; the CF3Br measurement was made using standard techniques, and the CF3 measurement using the Variable Electron and Neutral Density Attachment Mass Spectrometry technique. Attachment to CF3Br proceeds exclusively by the dissociative channel yielding Br-, with a rate constant increasing from 1.1 x 10(-8) cm(3) s(-1) at 300 K to 5.3 x 10(-8) cm(3) s(-1) at 890 K, somewhat lower than previous data at temperatures up to 777 K. CF3 attachment proceeds through competition between associative attachment yielding CF3- and dissociative attachment yielding F-. Prior data up to 600 K showed the rate constant monotonically increasing, with the partial rate constant of the dissociative channel following Arrhenius behavior; however, extrapolation of the data using a recently proposed kinetic modeling approach predicted the rate constant to turn over at higher temperatures, despite being only similar to 5% of the collision rate. The current data agree well with the previous kinetic modeling extrapolation, providing a demonstration of the predictive capabilities of the approach.

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 (~10(11) 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 10(9) and 10(10) 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 C6H6(+)) derived from benzene and toluene neutral precursors. CnHm(+) DR rate constants vary from 8-12 × 10(-7) cm(3) s(-1) at 300 K with temperature dependences of approximately T(-0.7). Where prior measurements exist these results are in agreement, with the exception of C3H3(+) where the present results disagree with a previously reported flat temperature dependence.

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.

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.

The product energy disposal in the abstraction reaction H + c-C 3 H 6 → H 2 (v', j') + C 3 H 5 at 1.6 eV collision energy has been characterized by quantum-state-selective, Doppler-resolved REMPI detection of the H 2 (v', j')product. The H 2 (v', j') product is observed with total energy, translational plus rotational plus vibrational, in excess of the total available energy if the C 3 H 5 product is a cyclopropyl radical, but the total H 2 (v', j') energy matches the total available energy if the C 3 H 5 product is allyl. This implies ring-opening of cyclopropyl to allyl concerted with abstraction and the deposition of a large fraction of the ring-opening energy into the H 2 (v', j') product. Such behavior is unprecedented and entirely unexpected. Calculations indicate that both direct H-by-H abstraction and H-addition/H 2 -elimination should occur too quickly to allow the isomerization to occur before product separation. Even if ring-opening did occur, the appearance of the total isomerization energy in the H 2 product would seem dynamically forbidden. The reaction must follow an unanticipated and surprising path.

Thermal electron attachment to 14 alkenes and alkanes with bromine, fluorine, and iodine substituents has been studied over the temperature range 300-600 K using a flowing-afterglow Langmuir-probe apparatus. Rate coefficients and anion products are reported, most for the first time. Among these were 3 isomers of C(3)F(5)Br and the 2 isomers of C(3)F(7)I. Four dibromide compounds were studied, all of which yield Br(2)(-) product in addition to Br(-) product. The results are analyzed using a statistical kinetic modeling approach, which is able to reproduce both attachment rate coefficients and product branching ratios within experimental uncertainty. The kinetic modeling indicates that factor of 2 differences in attachment rate coefficients to the isomeric species can be explained by subtle variations in the potential surfaces.

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 (

The rate constants of thermal electron attachment at 300 and 400 K to Fe(CO)(n) (n = 0-5) have been measured using a flowing afterglow Langmuir probe apparatus. The stable species Fe(CO)(5) was studied using the traditional method of monitoring electron depletion as a function of reaction time, and the remaining short-lived species were studied using the variable electron and neutral density attachment mass spectrometry (VENDAMS) technique. Attachment to Fe(CO)(5) is purely dissociative and about 20% efficient with a rate constant of (7.9 ± 1.4) × 10(-8) cm(3) s(-1) at 300 K and (8.8 ± 2) × 10(-8) cm(3) s(-1) at 400 K. The attachment rate constants decrease significantly as each CO ligand is removed, with Fe(CO)(n) (n = 4 to 1) attaching with efficiencies on the order of 10%, 1%, 0.1%, and 0.01% respectively. Under the conditions here, attachment to Fe(CO)(4) and Fe(CO)(3) are likely entirely dissociative, whereas attachment to Fe(CO)(2) and Fe(CO) are almost entirely associative. A statistical kinetic modeling approach is used to explain the strong dependence of the attachment rate constant on the number of ligands present in the neutral species through a combination of increasing autodetachment rates and decreasing exothermicities to dissociative attachment. The VENDAMS data also define the 300 K mutual neutralization rate constant of Fe(CO)(4)(-) + Ar(+) to be (5.0 ± 0.8) × 10(-8) cm(3) s(-1) with an upper limit to branching fraction of 0.5 to yield Fe(CO)(4), indicating that significant fragmentation to smaller Fe(CO)(n) occurs.

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.

Threshold photoelectron photoion coincidence (TPEPICO) has been used to study the sequential photodissociation reaction of internal energy selected 1,2-diiodoethane cations: C(2)H(4)I(2)(+) → C(2)H(4)I(+) + I → C(2)H(3)(+) + I + HI. In the first I-loss reaction, the excess energy is partitioned between the internal energy of the fragment ion C(2)H(4)I(+) and the translational energy. The breakdown diagram of C(2)H(4)I(+) to C(2)H(3)(+), i.e., the fractional ion abundances below and above the second dissociation barrier as a function of the photon energy, yields the internal energy distribution of the first daughter, whereas the time-of-flight peak widths yield the released translational energy in the laboratory frame directly. Both methods indicate that the kinetic energy release in the I-loss step is inconsistent with the phase space theory (PST) predicted two translational degrees of freedom, but is well-described assuming only one translational degree of freedom. Reaction path calculations partly confirm this and show that the reaction coordinate changes character in the dissociation, and it is, thus, highly anisotropic. For comparison, data for the dissociative photoionization of 1,3-diiodopropane are also presented and discussed. Here, the reaction coordinate is expected to be more isotropic, and indeed the two degrees of freedom assumption holds. Characterizing kinetic energy release distributions beyond PST is crucial in deriving accurate dissociative photoionization onset energies in sequential reactions. On the basis of both experimental and theoretical grounds, we also suggest a significant revision of the 298 K heat of formation of 1,2-C(2)H(4)I(2)(g) to 64.5 ± 2.5 kJ mol(-1) and that of CH(2)I(2)(g) to 113.5 ± 2 kJ mol(-1) at 298 K.

Dissociative electron attachment to the reactive C(2)F(5) molecular radical has been investigated with two complimentary experimental methods; a single collision beam experiment and a new flowing afterglow Langmuir probe technique. The beam results show that F(-) is formed close to zero electron energy in dissociative electron attachment to C(2)F(5). The afterglow measurements also show that F(-) is formed in collisions between electrons and C(2)F(5) molecules with rate constants of 3.7 × 10(-9) cm(3) s(-1) to 4.7 × 10(-9) cm(3) s(-1) at temperatures of 300-600 K. The rate constant increases slowly with increasing temperature, but the rise observed is smaller than the experimental uncertainty of 35%.

Flowing tube apparatuses have been used to study a variety of electron and ion processes at thermal energies. We describe a new technique, variable electron and neutral density attachment mass spectrometry (VENDAMS), which has enabled measurement of rate coefficients for electron attachment to unstable radical molecules and other transient species and for mutual neutralization reactions of anions with noble gas cations, along with, in favorable cases, determination of neutral products of mutual neutralization. Additionally, the method has yielded evidence of a new ternary reaction in which an electron third body enhances anion–cation neutralization at high plasma densities.

The mutual neutralization of anions with Ar+ has been studied by variable electron and neutral density attachment mass spectrometry. Evidence of a previously unobserved plasma loss process, electron-catalyzed mutual neutralization (ECMN), e.g., SF6-+Ar+ + e-→neutrals + e-, is reported. Results for 10 species suggest that ECMN occurs generally and significantly affects the total ion-loss rate in plasmas with electron densities exceeding 10(10) cm-3. ECMN is discussed in the context of other known three-body plasma processes, the mechanisms for which appear insufficient to explain the observed effect. A mechanism for ECMN involving an incident electron facilitating energy transfer to the internal modes of the anion is proposed.

Thermal electron attachment to C(60) has been studied by relative rate measurements in a flowing afterglow Langmuir probe apparatus. The rate coefficients of the attachment k(1) are shown to be close to 10(-6) cm(3) s(-1) with a small negative temperature coefficient. These results supersede measurements from the 1990s which led to much smaller values of k(1) with a large positive temperature coefficient suggesting an activation barrier. Theoretical modeling of k(1) in terms of generalized Vogt-Wannier capture theory shows that k(1) now looks more consistent with measurements of absolute attachment cross sections sigma(at) than before. The comparison of capture theory and experimental rate or cross section data leads to empirical correction factors, accounting for "intramolecular vibrational relaxation" or "electron-phonon coupling," which reduce k(1) below the capture results and which, on a partial wave-selected level, decrease with increasing electron energy.

The dissociation dynamics of energy selected neopentane, t-butyl iodide, and t-butyl hydroperoxide ions have been investigated by threshold photoelectron-photoion coincidence (TPEPICO) spectrometry. Although the methyl loss reaction from neopentane ions producing the t-butyl ion is in competition with a lower energy methane loss channel, modeling these two channels with the statistical theory of unimolecular decay provides a 0 K dissociation onset for methyl loss of 10.564 +/- 0.025 eV. This leads to a 298 K t-butyl ion heat of formation of 714.3 +/- 2.5 kJ x mol(-1), which is some 3 kJ x mol(-1) higher than the previously accepted value. The Delta(f)H degrees (298K)(t-C(4)H(9)(+)) combined with the measured 0 K onsets for t-C(4)H(9)(+) formation from t-butyl iodide (9.170 +/- 0.007 eV) and from t-butyl hydroperoxide (9.904 +/- 0.012 eV), yields 298 K t-butyl iodide and t-butyl hydroperoxide heats of formation of -68.5 +/- 2.6 kJ x mol(-1) and -233.2 +/- 2.8 kJ x mol(-1), respectively. Finally, the new t-C(4)H(9)(+) heat of formation leads to a predicted adiabatic ionization energy for the t-butyl radical of 6.86 +/- 0.20 eV, and a 298 K proton affinity for isobutene of 798.8 +/- 2.5 kJ x mol(-1). The predicted ionization energy exceeds all measured values by 0.10 eV.

The 0 K dissociative ionization onsets of C2H3X --> C2H3(+) + X (X = Cl, I) are measured by threshold photoelectron-photoion coincidence spectroscopy. The heats of formation of C2H3Cl (Delta H(f,0K)(0) = 30.2 +/- 3.2 kJ mol(-1) and Delta(H f,298K)(0) = 22.6 +/- 3.2 kJ mol(-1)) and C2H3I (Delta(H f,0K)(0) = 140.2 +/- 3.2 kJ mol(-1) and Delta(H f,298K)(0) = 131.2 +/- 3.2 kJ mol(-1)) and C- X bond dissociation enthalpies as well as those of their ions are determined. The data help resolve a longstanding discrepancy among experimental values of the vinyl chloride heat of formation, which now agrees with the latest theoretical determination. The reported vinyl iodide heat of formation is the first reliable experimental determination. Additionally, the adiabatic ionization energy of C2H3I (9.32 +/- 0.01 eV) is measured by threshold photoelectron spectroscopy.

We have measured the temperature-dependent kinetics for the reactions of OH(+) with H2 and D2 using a selected ion flow tube apparatus. Reaction occurs via atom abstraction to result in H2O(+)/HDO(+) + H/D. Room temperature rate coefficients are in agreement with prior measurements and resulting temperature dependences are T(0.11) for the hydrogen and T(0.25) for the deuterated reactions. This work is prompted in part by recent theoretical work that mapped a full-dimensional global potential energy surface of H3O(+) for the OH(+) + H2 → H + H2O(+) reaction [A. Li and H. Guo, J. Phys. Chem. A 118, 11168 (2014)], and reported results of quasi-classical trajectory calculations, which are extended to a wider temperature range and initial rotational state specification here. Our experimental results are in excellent agreement with these calculations which accurately predict the isotope effect in addition to an enhancement of the reaction rate constant due to the molecular rotation of OH(+). The title reaction is of high importance to astrophysical models, and the temperature dependence of the rate coefficients determined here should now allow for better understanding of this reaction at temperatures more relevant to the interstellar medium.

Rate coefficients and product branching fractions for electron attachment and for reaction with Ar(+) are measured over the temperature range 300-585 K for three monohydrogenated fluorocarbon (HFC) radicals (CF3CHF, CHF2CF2, and CF3CHFCF2), as well as their five closed-shell precursors (1-HC2F4I, 2-HC2F4I, 2-HC2F4Br, 1-HC3F6I, 2-HC3F6Br). Attachment to the HFC radicals is always fairly inefficient (between 0.1% and 10% of the Vogt-Wannier capture rate), but generally faster than attachment to analogous perfluorinated carbon radicals. The primary products in all cases are HF-loss to yield C(n)F(m-1)(-) anions, with only a minor branching to F(-) product. In all cases the temperature dependences are weak. Attachment to the precursor halocarbons is near the capture rate with a slight negative temperature dependence in all cases except for 2-HC2F4Br, which is ∼10% efficient at 300 K and becomes more efficient, approaching the capture rate at higher temperatures. All attachment kinetics are successfully reproduced using a kinetic modeling approach. Reaction of the HFC radicals with Ar(+) proceeds at or near the calculated collisional rate coefficient in all cases, yielding a wide variety of product ions.

The binary mutual neutralization (MN) of a series of 17 cations (O₂⁺, NO(+), NO₂⁺, CO(+), CO₂⁺, Cl(+), Cl₂⁺, SO₂⁺, CF₃⁺, C₂F₅⁺, NH₃⁺, H₃⁺, D₃⁺, H2O(+), H3O(+), ArH(+), ArD(+)) with 3 halide anions (Cl(-), Br(-), I(-)) has been investigated in a flowing afterglow-Langmuir probe apparatus using the variable electron and neutral density attachment mass spectrometry technique. The MN rate constants of atom-atom reactions are dominated by the chemical nature of the system (i.e., the specific locations of curve crossings). As the number of atoms in the system increases, the MN rate constants become dominated instead by the physical nature of the system (e.g., the relative velocity of the reactants). For systems involving 4 or more atoms, the 300 K MN rate constants are well described by 2.7 × 10(-7) μ(-0.5), where the reduced mass is in Da and the resulting rate constants in cm(3) s(-1). An upper limit to the MN rate constants appears well described by the complex potential model described by Hickman assuming a cross-section to neutralization of 11,000 Å(2) at 300 K, equivalent to 3.5 × 10(-7) μ(-0.5).

We report thermal rate coefficients for 12 reactions of rare gas cations (Ne(+), Ar(+), Kr(+), Xe(+)) with halide anions (Cl(-), Br(-), I(-)), comprising both mutual neutralization (MN) and transfer ionization. No rate coefficients have been previously reported for these reactions; however, the development of the Variable Electron and Neutral Density Attachment Mass Spectrometry technique makes it possible to measure the difference of the rate coefficients for pairs of parallel reactions in a Flowing Afterglow-Langmuir Probe apparatus. Measurements of 18 such combinations of competing reaction pairs yield an over-determined data set from which a consistent set of rate coefficients of the 12 MN reactions can be deduced. Unlike rate coefficients of MN reactions involving at least one polyatomic ion, which vary by at most a factor of ∼3, those of the atom-atom reactions vary by at least a factor 60 depending on the species. It is found that the rate coefficients involving light rare-gas ions are larger than those for the heavier rare-gas ions, but the opposite trend is observed in the progression from Cl(-) to I(-). The largest rate coefficient is 6.5 × 10(-8) cm(3) s(-1) for Ne(+) with I(-). Rate coefficients for Ar(+), Kr(+), and Xe(+) reacting with Br2 (-) are also reported.

We have studied the dependence of several ion-ion mutual neutralization (MN) reactions on helium density in the range from 1.6 × 10(16) to 1.5 × 10(17) cm(-3) at 300 K, using the Variable Electron and Neutral Density Attachment Mass Spectrometry method. The rate coefficients of the reactions Ar(+) + Br2(-), Ar(+) + SF6(-), and Ar(+) + C7F14(-) were found to be independent of gas density over the range studied, in disagreement with earlier observations that similar MN reactions are strongly enhanced at the same gas densities. The cause of the previous enhancement with density is traced to the use of "orbital-motion-limit" theory to infer ion densities from the currents collected by ion-attracting Langmuir probes in a region where it is not applicable.

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...

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.

Electron attachment to unstable fluorocarbon molecules has been investigated; CF2 - no dissociative electron attachment was observed, C2F5 – electron attachment observed at close to zero electron energy with the observation of C2F5−.

Experimental data from the literature for cross sections and rate constants for dissociative electron attachment to CF(3)Br, with separately varied electron and gas temperatures, are analyzed by a kinetic modeling approach. The analysis suggests that electronic and nuclear contributions to the rate constants can be roughly separated, the former leading to a negative temperature coefficient, the latter to a positive temperature coefficient. The nuclear factor in the rate constant is found to be of Arrhenius form with an activation energy which is close to the energy of crossing of the CF(3)Br and CF(3)Br(-) potential curves along the CBr bond.

The electron attachment rate constant to nitric acid (HNO(3)) has been measured in a flowing afterglow-Langmuir probe (FALP) apparatus at 300 and 500 K using three independent methods: the traditional FALP technique of monitoring electron depletion, "one-gas" VENDAMS (variable electron and neutral density attachment mass spectrometry), and "two-gas" VENDAMS. The three measurements are in agreement with a 300 K weighted average of 1.4 ± 0.3 × 10(-7) cm(3) s(-1), 2 to 10 times higher than previously reported values. Attachment is primarily dissociative yielding NO(2)(-) as previously reported, but for the first time a small endothermic channel to produce OH(-) was also observed at 500 K. From the one-gas VENDAMS data, associative attachment to the OH produced in the primary attachment was found to occur with an effective two body rate constant of 1.2±(0.7) (3)×10(-11) cm(3) s(-1) at 300 K, the first reported rate constant for this radical species. Finally, ion-ion neutralization rate constants of NO(2)(-) and NO(3)(-) with Ar(+) were determined to be 5.2±(2.5) (1.5) × 10(-8) and 4.5 ± 2.5 × 10(-8) cm(3) s(-1) at 300 K, respectively.

The kinetics of electron attachment to CF(3) as a function of temperature (300-600 K) and pressure (0.75-2.5 Torr) were studied by variable electron and neutral density attachment mass spectrometry exploiting dissociative electron attachment to CF(3)Br as a radical source. Attachment occurs through competing dissociative (CF(3) + e(-) → CF(2) + F(-)) and non-dissociative channels (CF(3) + e(-) → CF(3)(-)). The rate constant of the dissociative channel increases strongly with temperature, while that of the non-dissociative channel decreases. The rate constant of the non-dissociative channel increases strongly with pressure, while that of the dissociative channel shows little dependence. The total rate constant of electron attachment increases with temperature and with pressure. The system is analyzed by kinetic modeling in terms of statistical theory in order to understand its properties and to extrapolate to conditions beyond those accessible in the experiment.