Dissociation chemistry of hydrogen halides in water

Abstract

To understand the mechanism of aqueous acid dissociation, which plays a fundamental role in aqueous chemistry, the ionic dissociation of HX acids (X5F, Cl, Br, and I) in water clusters up to hexamer is examined using density functional theory and M？ªller.Plesset second-order perturbation methods ~MP2!. Further accurate analysis based on the coupled clusters theory with singles and doubles excitations agrees with the MP2 results. The equilibrium structures, binding energies, electronic properties, stretching frequencies, and rotational constants of HX(H2O)n and X2(H3O)1(H2O)n21 are calculated. The issociated structures of HF and HCl can be formed for n>4, while those of HBr and HI can be formed for n>3. Among these, the dissociated structures of HX (X5Cl, Br, and I) are more stable than the undissociated ones for n>4, while such cases for HF would require much more than six water molecules, in agreement with previous reports. The IR spectra of stable...