John F. Paulson

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 kcalu2009mol−1) and the fluoride affinity of SF3 (1.84±0.16 eV or 42.4±3.2 kcalu2009mol−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 kcalu2009mol−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 kcalu2009mol−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

Chemistry of atmospheric ions reacting with fully fluorinated compounds

Abstract Reactions of the atmospheric ions O+, O2+, O−, O2−, NO+, H3O−, CO3−, and NO3− with five fully fluorinated compounds were studied using the selected-ion flow tube (SIFT) technique at 300 K. Reaction rate constants and product branching fractions were measured for the following selected perfluoro compounds: CF4, C2F6, c-C4F8, n-C6F14, and SF6. The ion O+ reacted at the collisional rate with all five compounds, with the oxygen always being a part of the neutral products. The O2+ ion reacted with c-C4F8 at unit efficiency and with n-C6F14 at 39% efficiency, with no oxygen incorporation in the ionic products in either case. The reactions of O− with c-C4F8 and n-C6F14 proceeded at or near unit efficiency, featuring ten and eight product channels, respectively, including a component from reactive electron detachment in the c-C4F8 case. In both cases, many of the product channels appeared to be paired, that is, two channels differing only in the location of the negative charge. The reaction of O2 with c-C4F8 proceeded with 37% efficiency via non-dissociative charge transfer. The ions NO+, H3O+, CO3−, and NO3− were unreactive with all five perfluorinated compounds under investigation.

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 cm3u2009s−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 cm6u2009s−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

Rate constants and product distributions as functions of temperature for the reaction of OH−(H2O)0,1,2 with CH3CN

Abstract A selected ion flow tube was used to measure the rate constants and product distributions for the reactions of OH− (H2O)n with CH3CN over the temperature range 240–363 K for the case n = 1 and at 298 K for n = 0 and 2. Proton transfer was the only primary reaction channel observed; this process was found to be fast (efficiency ≅ 70%) for n = 0 and 1 but much slower (efficiency ≅ 4%) for n = 2. Interpreting OH− + CH3CN in the context of the general reaction OH− + CH3X, two features are important. First, CN has an abnormally large electron affinity. This gives CN− a large methyl cation affinity and nucleophilic displacement a large barrier: it is not observed, even though exothermic. Second, CH2CN has a large electron affinity and CH2CN− is delocalized. Thus (1) CH3CN shows a low heat of deprotonation, making proton transfer exothermic for OH− + CH3CN, but (2) less than 100% efficient since the product ion is delocalized, and (3) endothermic for OH− (H2O)3 + CH3CN because CH2CN− has a low hydration energy. Solvent switching with CH3CN and the thermal dissociation of the solvated product ions were observed as secondary reactions in the OH− (H2O) + CH3CN system. This study suggests that thermal dissociation can complicate the interpretation of product distributions in flow-tube and comparable experiments.

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.

Positive ion chemistry related to hydrocarbon flames doped with CF3 Br

Abstract Reactions of positive ions known to be present in hydrocarbon flames have been studied for their reactivity toward the fire suppressant CF3Br (Halon 1301) at 300 and 525 K. Rate constants and product branching percentages were measured at the two temperatures. The ions HCO+, CH+3, and CH+5 reacted rapidly with CF3Br producing CF+3 and CF2Br+ in all three cases. For CH+5, proton transfer was also observed at 300K. The ions H2COH+, H3COH+2, and H3O+ were unreactive with CF3Br at 300 and 525 K, and at ≈0.5 eV of collision energy supplied by a drift tube at 300 K. The product ions CF+3 and CF2Br+ were studied in separate experiments for reactivity toward selected hydrocarbons, and rate constants and branching percentages were determined. The hydrocarbons CH4, C2H6, C3H8, C2 H4, C3 H6, and C2H2 were selected for study (CF2] Br+ was studied with CH4, C2 H6, C2 H4, and C2H2 only). Neither CF+3 nor CF2Br+ reacted with CH4, but both ions reacted with other hydrocarbons. Hydrogen fluoride was among the inferred neutral reaction products in the reactions of CF+3 with C2H4 and C3H6. We found no evidence for any ionic process which could release Br atoms, any other free radicals, or the CF3Br+ molecular ion, and therefore no evidence was found to indicate that ions play a role in flame inhibition by CF3Br.

Effects of O−2 and SF6 vibrational energy on the rate constant for charge transfer between O−2 and SF61

Abstract Rate constants for the charge-transfer reaction of O−2 with SF6 have been measured as a function of the average kinetic energy (KEcm) at several temperatures in a selected-ion flow-drift tube. Increasing kinetic energy is found generally to decrease the reactivity. In the temperature range below 300 K, increasing temperature is found to decrease the rate constants; above 300 K the rate constants increase with temperature. Information on the internal energy dependences of the reactions is derived from the data. The data indicate that neither rotations nor low-frequency vibrations measurably affect the reactivity. Higher-frequency vibrations and/or overtones are found to enhance the reactivity greatly, presumably by distorting the geometry of SF6 to match more closely that of SF−6. Vibrations of O−2 are found to increase the reactivity by a factor of 2.9 at 162 K.

Threshold energies for the reactions OH− + CH3X → CH3OH + X− (X = Cl, Br) measured by tandem mass spectrometry: deprotonation energies (acidities) of CH3Cl and CH3Br

In a tandem mass spectrometer, a beam of OH− ions was reacted with methyl chloride and methyl bromide at collision energies in the range 0.2 < E, < 5 eV. For both of the methyl halides, excitation functions, σ(ET), were measured for the two competing channels, endoergic proton transfer and exoergic nucleophilic displacement: The threshold energies estimated for the proton transfer reactions are used to derive energies of deprotonation for CH3Cl and CH3Br of 1672 ± 10 and 1660 ± 10kJ mol−1 respectively.