Rate constants for the reactions of Kr+(2P3/2) with HCl and DCl and of Ar+ with HCl have been measured as a function of reactant ion/reactant neutral average center-of-mass kinetic energy (〈KEc.m.〉 ) at several temperatures. The measurements were made using helium as the carrier gas. From these data we have derived the dependences of the rate constants on the rotational temperature of H(D)Cl. Rate constants for the reaction of Kr+(2P1/2) with HCl have also been measured as a function of temperature. The rate constants for all of the reactions were found to decrease with increasing temperature. The rate constants were also found to decrease with increasing 〈KEc.m.〉 at low 〈KEc.m.〉 but then to increase at higher 〈KEc.m.〉 . A significant rotational temperature dependence of the rate constant was derived for the reaction of Kr+(2P3/2) with H(D)Cl. The analogous derivation for Ar+ reacting with HCl showed the rate constant for this reaction to be independent of the rotational temperature of HCl within experimental uncertainty.

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Atmospheric lifetimes are evaluated for the fully fluorinated compounds CF4, C2F6, c‐C4F8, C6F14, and SF6 using a two‐dimensional transport and chemistry model which includes removal by electrons and ions in the mesosphere and lower thermosphere. Laboratory measurements of the pertinent reaction rates were carried out at thermal energy for free electrons and for the atmospheric ions O+, O2+, O−, O2−, NO+, H3O+, NO3−, and CO3−. Atmospheric removal by electrons reduces the lifetimes of c‐C4F8 and SF6 from about 3200 years to 1400 and 800 years, respectively, only if the respective product anions C4F8− and SF6− do not subsequently regenerate the parent neutral compounds. Atmospheric removal by ion reactions is minor or negligible, with the largest effect (∼5%) being removal of C6F14 by O2+. Removal of CF4 and C2F6 by O+ is probably the most important single destruction process in the atmosphere for these two compounds, but their lifetimes are governed by removal at the Earth's surface in high‐temperature combustors. While we show that the lifetimes of c‐C4F8 and SF6 may be significantly shorter than previously estimated, these compounds remain extremely long‐lived with significant global warming potentials.

Rate constants have been measured as a function of average kinetic energy at several temperatures for the reactions of F" with CH3C1, CH3Br, and CH3I. The rate constants at low energy approach but are not equal to the collision limiting value. The rate constants for all three reactions decrease slowly with increasing average kinetic energy for energies below 0.2 to 0.3 eV. At 0.2 to 0.3 eV, a break occurs in the slopes of the energy dependence curves. Above these energies the rate constants decrease more rapidly with increasing energy. Trajectory calculations that include a probability function which is dependent upon collision angle were performed in order to examine the effects of orientation on the reactions. The calculations were fitted to the low energy portion of the experimental data. A more severe orientation requirement is found for the lighter halides. The energies at which the breaks occur in the experimental curves are reproduced by the calculations. These energies correspond to the energies at which the hard sphere collision diameter of the colliding partners is equal to the capture radius.

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Nucleophilic displacement reactions which are exothermic do not react on every collision in the gas phase.6,7 They exhibit a negative temperature dependence, rate constants decreasing with increasing temperature,8 and reaction at 300 K is quenched dramatically by the addition of only one-three solvate molecules.9In each respect nucleophilic displacement differs from proton transfer, as contrasted in the companion paper.10 Here we report how hydration influences the rate constant and the product distribution of the nucleophilic displacement reaction OD" + CH3C1 = CH3OD + Cl' A//°= -50 kcal/mol11(1)within the temperature range 200-500 K. Such data invite interpretation using hypersurfaces calculated for hydrated reactants.12 Rate constants for reaction 1 have been measured with a selected ion flow tube (SIFT), using techniques similar to those used in the companion study.10 Because the OH"(H20) reactant and the 35C1' product have the same mass-to-charge ratio (m/e = 35), perdeuterated anions, produced from D20 in the ion source, were used throughout. Rate constants, for the process OD"(D20)" +

Rate coefficients for electron attachment to SF4 and SF6 have been measured over the temperature range 300–550 K using a flowing-afterglow Langmuir-probe apparatus. The 300 K rate coefficient for SF4 is 2.5±0.6×10−8 cm3 s−1, a value 11 times smaller than the corresponding rate for SF6. The attachment rate coefficients for both SF4 and SF6 are nearly independent of temperature up to 500 K, and decline somewhat at still higher temperatures. SF−4 is the only ionic product of attachment to SF4 observed over the entire temperature range. SF−6 and SF−5 are products of attachment to SF6; an ‘‘activation energy’’ of 0.42±0.02 eV is inferred for SF−5 production.