Experimental Thermodynamics of Cluster Ions Composed of H2SO4 and H2O. 2. Measurements and ab Initio Structures of Negative Ions

Gas-phase nucleation in the atmosphere may be initiated by the condensation of sulfuric acid and water around ions. The thermodynamics of stepwise cluster ion growth must be known in order to model ion-induced nucleation of the H 2 SO 4 /H 2 O system. In the companion paper, we measured temperature-dependent equilibrium constants for the reactions of H 2 O with the H + (H 2 SO 4 ) s (H 2 O) w cluster ions using an ion flow reactor. In this work, H 2 O bond enthalpies and entropies for the HSO 4 - (H 2 SO 4 ) s (H 2 O) w, s ≤ 6 and w ≤ 10, cluster ions were measured. The thermodynamics of H 2 SO 4 ligand bonding in these clusters were also determined using data from previous experiments. The small positive and negative cluster ions exhibit very different chemical characteristics, but both families show trends analogous to bulk solution thermodynamics as clusters grow in size. Small negative cluster ions have a lower affinity for H 2 O than positive ion species, but H 2 SO 4 bonding is considerably stronger in the negative clusters. Ion-induced nucleation of the negative H 2 SO 4 /H 2 O cluster ions is therefore thermodynamically favored. Addition of H 2 O to certain HSO 4 - (H 2 SO 4 ) s (H 2 O) w cluster ions results in unusually large reaction entropies. Ab initio calculations were performed to investigate ligand bonding in the HSO 4 - (H 2 SO 4 ) s (H 2 O) w cluster ions. Stable cluster geometries comprised of multiple ions were identified for s ≥ 2. Large experimental entropy changes upon H 2 O addition may be due to multiply ionic species formed by proton transfer within the product cluster.