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The study on acid ionization process in small water clusters has gained much interest, due to the impact in atmospheric chemistry, given its role in ozone depletion in the Antartic. It has been showed indeed that the production of Cl2 and HNO3 (which will then react with ozone) in the stratosphere is due to the reaction of ClONO2 with HCl and H2O and that this reaction is catalized on the surface of micron-sized ice particles constituting polar stratospheric clouds (PSCs).

Given the importance of HCl dissociation for the nucleation in atmospheric clusters, recent studies focused on determining the minumum number of water molecules for the onset of dissociation. Both experimental and theoretical results suggest that, for acid dissociation, at least 4 water molecules are needed at low temperature. At this value the zwitterionic structure is more stable than the molecular one.

With our study we aim at clarifying which the dynamics of microsolvated clusters is, with particular attention on the hydrogen bond and vibrational dynamics, and on the mechanism for acid dissociation.