Dissociation of SO4 2-(H2O)n clusters, n=3-17

The dissociation pathways and energetics of SO42-(H2O)(n) for n = 3-17 were studied with use of a combination of blackbody infrared radiative dissociation (BIRD), sustained off-resonance irradiation collisional activated dissociation (SORI-CAD), infrared multiphoton dissociation (IRMPD), and double resonance experiments. For n = 7-17, the loss of a single water molecule is the only process observed. For n = 6, loss of a single water molecule is the dominant reaction (>90%), but some charge separation products are observed. In contrast, loss of a water molecule from the n = 5 cluster is small (<10%) and two charge separation pathways are the dominant processes observed. For both n = 3 and 4, charge separation is the only process that occurs at low internal energy. For both n = 5 and 6, the branching ratio of water loss to charge separation increases with increasing internal energy deposited into the clusters. This demonstrates that the water loss process is entropically favored over the charge separation process. Rate constants for loss of a water molecule from n = 6-17 clusters were measured with BIRD at 21 degreesC. A large increase is observed between n = 6 and 7, indicating that the seventh water molecule may go into an outer solvation shell or it may disrupt an unusually stable arrangement of water molecules at n = 6. The n = 12 cluster is more stable than either n = 11 or 13. This "magic" number hydrate is consistent with filling of a shell structure at n = 12. One such structure in which all 12 water molecules are symmetrically bonded to SO42- is identified as a low-energy structure at the B3LYP 6-31 G**++ level, although this structure is entropically disfavored compared to those where one or two water molecules occupy a second solvation shell.