[en] Rate coefficients have been measured in a selected ion flow tube (SIFT) for reactions of several negative ions with WF₆. With the exception of SF^-₅, all the reactant ions studied having an electron detachment energy less than 3.36 eV reacted rapidly by charge exchange. SF^-₅ transferred a fluoride ion producing WF^-₇. Ions with detachment energies greater than 3.36 eV associated rapidly with WF^-₆. Br^-, with a detachment energy of 3.36 eV, reacted with WF₆ both by ion-neutral association and by charge exchange. The branching ratio for these two channels was found to depend on temperature and pressure. All these data indicate that the electron affinity of WF₆ is nearly equal to that of Br

[en] Intriguing comparisons may be drawn between SF₄ and SF₆ in their interactions with photons, electrons, ions, and molecules. We have found that the temperature dependence of the rate coefficient for electron attachment to SF₄ closely parallels that for attachment to SF₆ over the temperature range of 300-550 K, but the attachment process is less efficient for SF₄ by approximately a factor of 10. Both reactions are dominated by non-dissociative attachment in this temperature range; we find an activation energy of 0.42 eV for the production of SF₅^- from attachment to SF₆. Equally interesting is a comparison between PF₃ and PF₅. PF₃ apparently does not attach electrons (and PF₃^- has never been observed), while PF₅ is found to undergo non-dissociative electron attachment at a rate 700 times slower than for SF₆. Attachment rate coefficients for SF₄, SF₆, and PF₅ are nearly independent of temperature in the range studied. A flowing-afterglow Langmuir-probe apparatus was used in this work

[en] The selected ion flow drift tube has proven valuable for isotopic studies of gas-phase ion-molecule reactions. Both mechanistic and dynamical studies can be performed. Both aspects will be covered in this presentation. The reaction of O^- with N₂O to produce NO^- and NO will be used to show how a labeling study might yield information on reaction mechanism. This study was initiated to determine whether the mechanism proceeded by attack on the terminal or central N. Even the totally labeled experiment, however, failed to reveal the mechanism conclusively. Also to be presented is the effect of various types of energy on reactions involving isotopes. Examples will be shown of the influence of kinetic, rotational, and vibrational energy on both rate constants and branching ratios of ion-molecule reactions

[en] This study of H₃^- employs a valence-configuration-interaction (VCI) wavefunction expansion with a double-zeta-plus-polarization-and-diffuse-orbital Slater-type orbital (STO) basis set. Calculations have been carried out for both triangular and linear geometries to define the lowest singlet potential energy surface (PES). It is found that the minimum energy reaction path corresponds to a linear conformation of H^- + H₂. The calculated barrier height is 15 kcal/mol which is close to that obtained using a similar quality basis set. The energy contours for the isosceles triangle arrangement of nuclei indicates that the least energy path leads to dissociation into H^- + H + H. A threshold of approximately 1 eV and a maximum in the exchange cross section between 2 and 3 eV collision energy are found

[en] A simple model is used to predict the fractionation of isotopes as a function of temperature via ion-molecule reactions. It is developed for reactions proceeding via a single intermediate where the isotopic mixing is completed within the lifetime of the intermediate. The fractionation efficiency is K/(K+1) where K is the equilibrium constant of the isotope-exchange reaction. The model is applied to deuterium and ¹⁸O fractionation via the prototypical reactions OH^-+H₂O and H₃O + H₂O; and satisfactory agreement is found with data available for 300 K. The model is also applied to ¹⁵N fractionation via H₂H⁺ + N₂ and comparison is made with the data available for 80 K. The model predicts simply the fractionation efficiencies for such isotope-exchange reactions at the temperatures of interstellar clouds (10 - 100K)

[en] Nucleophilic displacement reactions which are exothermic do not react on every collision in the gas phase. They exhibit a negative temperature dependence, rate constants decreasing with increasing temperature, and reaction at 300 K is quenched dramatically by the addition of only one-three solvate molecules. In each respect nucleophilic displacement differs from proton transfer, as contrasted in the companion paper. Here we report how hydration influences the rate constant and the product distribution of the nucleophilic displacement reaction OD^- + CH₂Cl = CH₃OD + Cl^- ΔH⁰ = -50 kcal/mol¹¹ (1) within the temperature range 200-500 K. Such data invite interpretation using hypersurfaces calculated for hydrated reactants

[en] The operation of an apparatus constructed to measure absolute photodissociation spectra of ions and to determine the kinetic energy spectrum of the photoproducts by time-of-flight analysis is described. Photodissociation spectra for the processes D⁺₂+hν→D⁺+D and N₂O⁺→NO⁺+N have been obtained, and the photodissociation cross sections for HD⁺+hν→H⁺+D and HD⁺+hν→D⁺+H have been measured. The photodissociation spectrum of D⁺₂, using an irradiation bandwidth of <0.05 to 0.1 nm, is in reasonable agreement with the structureless spectrum of von Busch and Dunn obtained with a 20 nm bandwidth. The two possible photoprocesses in HD⁺ were observed to have the same cross sections 2 x 10^-19 cm² at 589 nm. The photodissociation spectrum of N₂O⁺ over the range 295--342 nm was found to be very sharply structured, with sigma/sub NO/+ =2.6 x 10^-18 cm² for the largest peak, at 338.5 nm. Fifteen peaks in the N₂O⁺ spectrum were assigned to vibrational progressions in the A ²Σreverse arrowX ²Pisub ₁/₂, ₃/₂ transition. In making the assignments it was necessary to assume the ions possess from 0 to 2 quanta in the degenerate bending mode, leading to Renner effect splitting in the ²PI state

[en] A simple model is used to predict the fractionation of isotopes as a function of temperature via ion-molecule reactions. It is developed for reactions proceeding via a single intermediate where the isotopic mixing is completed within the lifetime of the intermediate. The fractionation efficiency is K/(K+1) where K is the equilibrium constant of the isotope-exchange reaction. The model is applied to deuterium and ¹⁸O fractionation via the prototypical reactions OH^- + H₂O and H₃O⁺ + H₂O; and satisfactory agreement is found with the data available for 300 K. the model is also applied to ¹⁵N fractionation via N₂H⁺ + N₂ and comparison is made with the data available for 80 K. The model predicts simply the fractionation efficiencies for such isotope-exchange reactions at the temperatures of interstellar clouds (10 - 100 K)

[en] The reactions of several negative ion species with up to three solvate molecules have been studied in a variable temperature selected ion flow tube. The reactions include association of some species of interest in the stratosphere and some proton transfer reactions of the strong and 'super' acids. (Author)

[en] Rate constants have been measured using the Selected Ion Flow Tube (SIFT) technique over the temperature range 200 - 500K for the selectively hydrated reactant OH^-.(H₂O) with hydrogen halides and methyl halides. The former are representative proton transfer reactions and the latter representative displacement reactions, each of which show different temperature dependences for unhydrated reactants: dk/ dT = O and dk/dT < O respectively. For both categories, the temperature dependence of most reaction rate constants is not changed significantly by hydration of the reactant ion