REACTION OF CHLORIDE-IONS WITH CHLORINE NITRATE AND ITS IMPLICATIONS FOR STRATOSPHERIC CHEMISTRY

We present experimental and theoretical evidence for the rapid gas-pha se reaction of Cl- with ClONO2 to form Cl-2 and NO3-. The reaction was studied in a Fourier-transform ion-cyclotron-resonance mass spectrome ter, 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 approximat e to 60% of the rate constant estimated from ion-dipole collision theo ry. We also performed ah initio calculations at the level of second-or der 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 fo r reaction for the approach of Cl- toward the Cl atom on ClONO2, but w e found a minimum along the reaction coordinate corresponding to an io n-molecule complex Cl-2.NO3-. The reaction enthalpy remains exothermic with the inclusion of ion hydration enthalpies, indicating that the r eaction could proceed in condensed-phase water. These considerations s uggest that chloride ions may react directly with ClONO2 on water ice films and type II polar stratospheric cloud particles. From the rapidi ty of the reaction, we also infer that gas-phase chloride ion cannot s erve as a sink for negative charge or active chlorine in the stratosph ere.