Knowledge Bank

The molecular structures and vibrational frequencies of the hydrated clusters of the strong acids, $HCl\bullet (H2O)n$ and $H2SO4\bullet (H2O)n);n=1\sim 5$, are examined by employing the density functional molecular orbital method. When the number of water molecules is small, the hydrated clusters take the hydrogen-bonded structures without proton transfer. In the cases of $n\ge 4$, the proton transferred forms become dominant and there are many isomeric forms for both the direct ion-pair $X-\bullet H3O+\bullet (H2O)n-1$, and indirect ion-pair $X-1\bullet (H2O)n-1\bullet H3O+$ structures, where $X=Cl,HSO4$. The calculated IR spectra of the stable clusters clearly indicate the large red-shifts of the stretching frequencies of the H-Cl and the O-H bond of $H2SO4$. The difference of the hydrated clusters of $HCl,H2SO4$, and HF is also discussed.