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We present experimental and theoretical evidence for the rapid gas-phase reaction of C1-with ClONO2 to form C12 and Nos-. The reaction was studied in a Fourier-transform ion-cyclotron-resonance mass spectrometer, and a reaction rate constant of k = (9.2 f 3.0) X 10-lo cm3 s-l molecule-' at 298 K was determined. This value was -60% of the rate constant estimated from ion-dipolecollision theory. We also performed ab initiocalculations at the level of second-order Maller-Plesset perturbation theory using diffuse basis sets and at the singlesand-doubles coupled cluster level to examine portions of the potential energy surface for this reaction. We found no barrier for reaction for the approach of C1-toward the C1 atom on ClON02, but we found a minimum along the reaction coordinate corresponding to an ion-molecule complex C12*N03-. The reaction enthalpy remains exothermic with theinclusion of ion hydration enthalpies, indicating that the reaction could proceed in condensedphase water. These considerations suggest that chloride ions may react directly with ClONO2 on water ice films and type I1 polar stratospheric cloud particles. From the rapidity of the reaction, we also infer that gas-phase chloride ion cannot serve as a sink for negative charge or active chlorine in the stratosphere.

Guided ion beam mass spectrometry is used to study the kinetic energy dependence of the reactions of groundstate Mg+ and Sr+ with molecular hydrogen and its isotopomers. The results for reactions of Sr+ are analogous to those previously observed for reactions of Ca+ and Ba+ with H2, D2, and HD. We observe qualitatively different cross sections for the reactions of Mg+ with H2, D2, and HD, even though the monopositive ions in this group all have an ns valence electron configuration above completely filled core orbitals. The different pattern of reactivity can be explained with molecular orbital concepts, focusing on the interaction of the empty orbitals of the reactant ion with those of the hydrogen molecule. The threshold behavior of these reactions is analyzed to yield 0 K bond dissociation energies for SrH+ of 2.17 i 0.06 eV (50.0 f 1.3 kcal/mol) and an estimate for MgH+ of 1.94 f 0.06 eV (44.7 f 1.5 kcal/mol).

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