Exploration of the atmospheric reactivity of N2O5 and HCl in small water clusters using electronic structure methods

High level electronic structure calculations have been used to study the ga s phase reactivity of N2O5 and HCl in small neutral water clusters containi ng one and two water molecules. The free energy barrier decreases from 61 ( uncatalysed) to 39 kJ mol(-1) when catalysed by two water molecules where t he reaction products involve ClNO2 and HONO2. A new pathway for the hydroly sis of N2O5 by a single water is identified with a barrier comparable to th at for the uncatalysed reaction of N2O5 with HCl. The calculations predict these reactions may be important in contributing to the ozone destruction c ycle. However, the calculations also suggest that heterogeneous catalysis w ill be favoured over the homogeneous process.