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 ClONO2 to form Cl-2 and NO3-. 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 approximate to 60% of the rate constant estimated from ion-dipole collision theory. We also performed ah initio calculations at the level of second-order Moller-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 ClONO2, but we found a minimum along the reaction coordinate corresponding to an ion-molecule complex Cl-2.NO3-. 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 ClONO2 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.