Thomas M. Miller

Negative ion chemistry of SF4

A selected ion flow tube was used to conduct an extensive study of negative ion–molecule reactions of SF4 and SF−4. Rate constants and product ion branching fractions were measured for 56 reactions. The reactions bracket both the electron affinity of SF4 (1.5±0.2 eV or 34.6±4.6 kcal mol−1) and the fluoride affinity of SF3 (1.84±0.16 eV or 42.4±3.2 kcal mol−1). These results may be combined to give the neutral bond energy D(SF3–F)=3.74±0.34 eV or 86.2±7.8 kcal mol−1, independent of other thermochemical data except for the accurately known electron affinity of F. The heat of formation of SF−4 is derived from the electron affinity of SF4: ΔfH(SF−4)=−9.2±0.3 eV or −212.9±7.5 kcal mol−1. Lower limits to EA(SF2) and EA(SF3) are deduced from observation of SF−2(35%) and SF−3(65%) ion products of the reaction S−+SF4. Rapid fluoride transfer from both SF−2 and SF−3 to SF4 places upper limits on the electron affinities of SF2 and SF3. The combined results are 0.2 eV≤EA(SF2)≤1.6 eV and 2.0 eV≤EA(SF3)≤3.0 eV. We revi...

Observation of thermal electron detachment from cyclo-C4F8− in FALP experiments

Abstract : The methodology for use of a flowing afterglow-Langmuir probe apparatus to measure thermal electron detachment rate coefficients is described. We determined the thermal detachment rate coefficient (1010 + or - 300/s) for cyclo-C4F(-)8 ions and the rate coefficient (1.6 + or - 0.5 x 10(exp -8)cu cm/s) for electron attachment of cyclo-C4F8 at 375 K. The sole ionic product of attachment is cyclo-C4F(-)8. The equilibrium constant for the attachment/ detachment reaction yields a free energy for attachment at 375 K of -0.63 +or - 0.02 eV, from which we estimate the electron affinity (O K value) Of Cyclo-C4F8 to be about 0.63 eV. Electron attachment, Electron detachment, Cyclo perfluorobutane

The formation and destruction of H3O

We report the first measurements of rate constants for formation and reaction of the hydrated‐hydride ion H3O−. We studied the Kleingeld–Nibbering reaction [Int. J. Mass Spectrom. Ion Phys. 49, 311 (1983)], namely, dehydrogenation of formaldehyde by hydroxide to form hydrated‐hydride ion and carbon monoxide. The OD−+H2CO reaction is about 35% efficient at 298 K, with OD−/OH− exchange occurring in about half the reactions. H3O− was observed to undergo thermal dissociation in a helium carrier gas at room temperature with a rate constant of 1.6×10−12 cm3 s−1. We also studied a new reaction in which H3O− is formed: The association of OH− with H2 in a He carrier gas at low temperatures. The rate coefficient for this ternary reaction is 1×10−30 cm6 s−1 at 88 K. Rate coefficients and product branching fractions were determined for H3O− reactions with 19 neutral species at low temperatures (88–194 K) in an H2 carrier. The results of ion‐beam studies, negative‐ion photoelectron spectroscopy, and ion‐molecule react...

Thermal electron attachment to NF3, PF3, and PF5

Abstract A flowing-afterglow Langmuir-probe apparatus was used to measure rate constants (ka) for electron attachment to NF3 and PF5 over the temperature range T = 300–550 K. Electron attachment to NF3 is dissociative and produces only F− ionic product in the temperature range studied. At room temperature, ka(NF3) = 7 ± 4 × 10−12 cm3 s−1. The temperature dependence of ka(NF3) above 340 K is characterized by an activation energy of 0.30 ± 0.06 eV. Attachment to PF5 is nondissociative in a helium buffer at pressures in the range 53–160 Pa (0.4–1.2 Torr). The rate constant ka(PF5) is 1.0 ± 0.4 × 10−10 cm3 s−1 at 300 K and is approximately temperature independent over much of the temperature range studied. PF3 does not attach electrons in this temperature range. Upper limits to ka(PF3) were determined (and attributed to impurities): ka

Reactions of Fe− with acids: gas-phase acidity and bond energy of FeH

Abstract Kinetics and products for the gas-phase reactions of Fe− with the acids CH3C(O)CH2C(O)CH3, HCO2H, CH3CO2H, CH3CH2CO2H, and H2S have been determined using a selected-ion flow drift tube. Electron detachment is the sole reaction channel for reaction with CH3C(O)CH2C(O)CH3, CH3CO2H, and CH3CH2CO2H, and a dominant reaction channel for reaction with HCO2H and H2S. Proton transfer from HCO2H to Fe− occurs and was studied as a function of increasing kinetic energy. These reactions are used to determine δH° acid,298 (FeH) = 345.2 ± 4.4 kcal mol−1, which determines the homolytic bond energy D°298(Fe−H) = 35.1 ± 4.4 kcal mol−1. The electron detachment reactions and reactions leading to ion products in reactions with HCO2 H and H2S are discussed in view of the available thermochemistry. A mechanism involving initial proton transfer within the collision complex is suggested for all reactions.

SF4: electron affinity determination by charge-transfer reactions

Abstract A selected-ion flow drift tube was used to conduct an extensive study of the negative ion/molecule reaction of SF4 and SF−4. Thirteen reactions proceed by electron transfer. Data from these reactions, and information from systems that do not react at all, are used to determine the electron affinity of SF4, EA(SF4) = 1.5 ± 0.2 eV. Additional thermochemical data are used to determine the fluoride affinity of SF3, D(SF3-F−) = 1.8 ± 0.3 eV.

CIONO−2: A strongly bound negative ion

The gas phase reaction of NO2− with ClONO2 has been investigated at 300 K using a selected ion flow tube. Reaction occurs efficiently, k=1.5 (±40%)×10−9 cm3 s−1. The major ionic products are NO3− (∼90%) and ClONO2− (∼10%). The cluster ion NO2− (ClONO2) is formed in <1% of the reactive collisions under our experimental conditions. The observation of ClONO2−, arising from charge transfer from NO2−, indicates that the electron affinity of ClONO2 is ≥2.1 eV. We have determined the apparent second‐order clustering rate coefficient for NO3−+ClONO2 to be 3.0 (±40%)×10−11 cm3 s−1 at 300 K in 0.5 Torr He.

Electron attachment to ClONO2 at 300 K

A flowing–afterglow Langmuir probe apparatus with mass spectral analysis has been used to measure the rate constant for electron attachment to ClONO2 at 300 K. Electron attachment is efficient with a rate constant of 1.1 (±50%)×10−7 cm3 s−1 and proceeds principally through dissociative channels to produce the major product ions NO−2 (∼50%), NO−3 (∼30%), and ClO− (∼20%). The parent ion ClONO−2 and Cl− are also observed in the mass spectra but are at most minor products in the attachment process, ≤2% and ≤6%, respectively. A description of the secondary ion–molecule chemistry that takes place following electron attachment is given.

Electron attachment to haloacetonitriles, 295–556 K

Electron attachment to the haloacetonitriles FCH2CN, ClCH2CN, and BrCH2CN have been studied over the temperature range 295–556K, using a flowing-afterglow Langmuir-probe apparatus. No attachment is observed for FCH2CN; dissociative electron attachment to FCH2CN is presumably endothermic. Electron attachment to CICH2CN produces only Cl− ion product over the temperature range studied. The attachment rate constant is ka(ClCH2CN) = 3.9 ± 1.3 × 10−8 cm3 s−1 at 295K, and increases with temperature to a value of 9.7 ± 3.4 × 10−8 cm3 s−1 at 556K. Electron attachment to BrCH2CN produces only Br− ion product over the temperature range studied. The attachment rate constant is ka(BrCH2CN) = 1.9 ± 0.7 × 10−7 cm−3 s−1 at 295 K, becoming temperature independent at 2.4 ± 0.8 × 10−7 cm3 s−1 in the range 475–556 K.

Chemistry of C−2 and HC−2 with nitrogen, oxygen and nitrogen oxides

Abstract We have investigated the reactions of C−2 and HC−2 with N2, N2O, NO, NO2 and O2 by using a selected-ion flow drift tube at 300 K. Rate coefficients and branching fractions were measured as a function of ion/neutral average center-of-mass kinetic energy (〈KEcm〉) over the range from about 0.04 to 0.25 eV. The reaction of C−2 with NO forms CN− (about 11%), NCO− (about 6%), and reactive electron detachment products (about 83%). The rate coefficient at 300K is 1.2 (±0.3) × 10−10 cm3 s−1 and decreases with energy as 〈KEcm〉−0.7, while the product branching fractions are energy independent. The reaction of C−2 with NO2 forms C2O− (about 74%), CNO− (or NCO−) (about 14%) and CN− (about 12%). The rate coefficient for this reaction is 4.8 (±1.2) × 10−10 cm3 s−1 at 300K, and both the rate coefficient and branching fractions are approximately independent of kinetic energy over the range investigated. The reaction of C−2 with O2 forms C2O− (about 70%) and reactive electron detachment products (about 30%). The rate coefficient for this reaction is 1.5 (±0.4) × 10−11 cm3 s−1 at 300K and increases as 〈KEcm〉+0.7 over the energy range studied. C−2 is unreactive towards N2 and N2O within our experimental sensitivity over the energy range studied. HC−2 reacts with NO2 to form HC2O− with a rate coefficient of 1.8 × 10−10 cm3 s−1 at 300 K; the rate coefficient decreases with energy as 〈KEcm〉−0.7. HC−2 does not react with N2, N2O, NO or O2 within our experimental sensitivity over the energy range studied. The measured parameters are discussed in terms of the characteristic reactivity of the anions and possible reaction mechanisms. Finally, CN− was found to be unreactive towards NO and O2 within experimental uncertainty at 300 K and no applied electric field, and reacts inefficiently with NO2 with a rate coefficient less than or equal to 6 × 10−12 cm3 s−1.