Aspects of the heterogeneous reaction HCl + ClONO_2 -> Cl_2 + HNO_3 on ice and N_2O_5 + H_2O -> 2 HNO_3 on supercooled water are modeled via electronic-structure calculations involving reactant-water cluster systems. Reaction-path calculations for a HCl centre dot ClONO_2 centre dot (H_2O)_9 system indicate a coupled proton-transfer (PT)ucleophilic attack (S_N2) process, in which the ice lattice is an active participant. Analysis of the N_2O_5 centre dot (H_2O)_4 reaction system geometry and charge distribution suggests that a similar coupled PT/S_N2 mechanism could occur in a larger system.
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