Reaction of Chloride Ions with Chlorine Nitrate and Its Implications for Stratospheric Chemistry

We present experimental and theoretical evidence for the rapid gas-phase reaction of Cl^- with ClONO_2 to form Cl_2 and NO_3^-. The reaction was studied in a Fourier-transform ion-cyclotron-resonance mass spectrometer, and a reaction rate constant of k = (9.2 ± 3.0) X 10^(-10) cm^3 s^(-1) molecule^(-1) at 298 K was determined. This value was ≈60% of the rate constant estimated from ion-dipole collision theory. We also performed ab initio calculations at the level of second-order Møller-Plesset perturbation theory using diffuse basis sets and at the singles-and-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 Cl^- toward the Cl atom on ClONO_2, but we found a minimum along the reaction coordinate corresponding to an ion-molecule complex Cl_2*NO_3^-. The reaction enthalpy remains exothermic with the inclusion of ion hydration enthalpies, indicating that the reaction could proceed in condensed phase water. These considerations suggest that chloride ions may react directly with ClONO_2 on water ice films and type II 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