Hydration of the Bisulfate Ion: Atmospheric Implications

Using molecular dynamics configurational sampling combined with ab initio energy calculations, we determined the low energy isomers of the bisulfate hydrates. We calculated the CCSD(T) complete basis set (CBS) binding electronic and Gibbs free energies for 53 low energy isomers of HSO ₄ ^– (H ₂ O) _{n =1–6} and derived the thermodynamics of adding waters sequentially to the bisulfate ion and its hydrates. Comparing the HSO ₄ ^– /H ₂ O system to the neutral H ₂ SO ₄ /H ₂ O cluster, water binds more strongly to the anion than it does to the neutral molecules. The difference in the binding thermodynamics of HSO ₄ ^– /H ₂ O and H ₂ SO ₄ /H ₂ O systems decreases with increasing number of waters. The thermodynamics for the formation of HSO ₄ ^– (H ₂ O) _{n =1–5} is favorable at 298.15 K, and that of HSO ₄ ^– (H ₂ O) _{n =1–6} is favorable for T < 273.15 K. The HSO ₄ ^– ion is almost always hydrated at temperatures and relative humidity values encountered in the troposphere. Because the bisulfate ion binds more strongly to sulfuric acid than it does to water, it is expected to play a role in ion-induced nucleation by forming a strong complex with sulfuric acid and water, thus facilitating the formation of a critical nucleus.