The direct ClONO2 + HCl --> Cl-2 + HNO3 reaction on ice, implicated in pola
r stratospheric ozone depletion, is studied quantum chemically on a model i
ce lattice comprising nine water molecules. The reaction path is calculated
at the HF/(HW*,3-21G) level, using the Hay-Wadt effective core potential f
or Cl. At these geometries, energies are recalculated at the MP2/(SBK+*,6-3
1+G*) level, with the Stevens-Bash-Krauss effective core potential for Cl.
HCl is found to be ionized in the reactant complex. The calculated reaction
internal energy barrier, including zero-point energy correction, is 6.4 kc
al/mol. The reaction mechanism involves proton transfer in the ice lattice,
accompanied by nucleophilic attack of Cl- on the Cldelta+ in ClONO2; the l
attice is an active participant in the reaction. Implications for heterogen
eous atmospheric chemistry are discussed.