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Notes: We describe two versions of a high temperature flowing afterglow apparatus. With a stainless steel flow tube wrapped with heating tape we have obtained data over the range 300–1300 K. In a version with a ceramic flow tube in a commercial furnace we have obtained data over the range 300–1600 K. The ceramic version is designed to take data up to 1800 K, but we have encountered experimental problems at the upper temperature range. The design modifications to a standard flowing afterglow needed to make measurements at elevated temperatures are described in detail, as are problems associated with operating at elevated temperatures. Samples of data are given. © 1996 American Institute of Physics.

Notes: 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 review the status of measurements of EA(SFn), n=1–7. © 1995 American Institute of Physics.

Notes: 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. © 1996 American Institute of Physics.

Notes: We report experimental studies of the formation of CF3O− by ion-molecule and electron attachment reactions, and theoretical investigations of the structure and energetics of CF3O− and its neutral counterpart CF3O. The anion CF3O− is formed from the rapid attachment of free electrons to its neutral dimer, (CF3O)2. Potential sources of CF3O− through ion-molecule reactions of CF3− and F− were surveyed. CF3O− is formed in the bimolecular ion-molecule reaction of CF3− with SO2 and the third-order association reaction of F− with CF2O. In addition, rate constants for the reactions of CF3− with a variety of neutral compounds were measured. A number of cases were found in which formation of CF3O− was energetically allowed but was not observed. The potential energy surfaces of CF3O and CF3O− have been investigated using a variety of density functional theory (DFT) techniques. The ground-state minimum energy structure of CF3O was found to be a 2A′ Jahn–Teller distorted Cs-symmetry structure, while for the anion the ground state is 1A1 with a C3v-symmetry minimum. A search for other low-energy minima for CF3O− was unsuccessful. The DFT methods support a value for the adiabatic electron affinity of CF3O near 4.1 eV. © 1999 American Institute of Physics.

Notes: Rate constants have been measured for the reactions of O− with CH4, CH2D2, and CD4 as a function of ion-neutral average center-of-mass kinetic energy, 〈KEcm〉, at several temperatures over the range 93 K–565 K using a selected ion flow drift tube apparatus. For the CH4 reaction we also report measurements made using a high-temperature flowing afterglow (HTFA) instrument over the temperature range 300 K–1313 K. The rate constants are found to have a very large isotope effect, with the CH4 rate constant a factor of 15 higher than the CD4 rate constant at 93 K. The rate constants generally have a minimum with respect to temperature and 〈KEcm〉, except for the higher-temperature data for CD4 where the rate constants show only an increase with increasing kinetic energy. The data indicate that increasing rotational temperature decreases the rate constants and that rotational energy behaves similarly to translational energy. Single excitations of bending and twisting vibrations have a negligible effect on the rate constant. Either the stretching vibrations or overtones of the bending vibrations increase the rate constants. If the stretches are responsible for the increase in the rate constants, the derived rate constant for a single quantum of stretch excitation (v=1) is 5×10−10 cm3 s−1, a factor of 6 larger than the rate constant for v=0. The CH2D2 rate constants are approximately equal to the averages of the rate constants for the pure isotopes. The product branching ratio (OH−/OD−) shows no dependence on CH2D2 rotational temperature or low-frequency CH2D2 vibrations. A theoretical study of the minimum energy reaction path was performed to help elucidate the reaction dynamics. The minimum energy reaction surface was characteristic of the standard double minimum pathway for ion molecule reactions. The height of the central barrier was found to be close to the energy of the reactants and varied with isotopic substitution. Conformationally different transition states are found for these isotopic reactions. Theoretical studies at the QCISD(T) level of theory find distinct transition states corresponding to O−+CH4, O−+H-CHD2, O−+D-CH2D, O−+CD4. The transition state barriers increase in the order O−+CH4, O−+H-CHD2, O−+D-CH2D, and O−+CD4, in agreement with experimental reaction rates. The main features of the reactivity are explained by the characteristics of the reaction surface. © 1997 American Institute of Physics.