Stepwise solvation enthalpies of protonated water clusters: collision-induced dissociation as an alternative to equilibrium studies

Absolute reaction cross sections for interaction of H_3O^+(H_2O), (x = 1-5) with xenon are determined from thermal energies to 20 eV (CM) by using guided ion beam mass spectrometry. In all cases, the primary product is endothermic collision induced dissociation (CID) to lose one water molecule. At higher energies, additional water molecules are lost successively, indicating that evaporation is the primary pathway for dissociation. The proper interpretation of the primary CID thresholds is considered in detail and the adjustment of these values to enthalpies at standard conditions is discussed. When the effects of multiple ion-molecule collisions, internal energy of the clusters, and dissociation lifetimes are properly accounted for, we determine the following bond dissociation energies (in eV): D_0[H_3O^+-H_2O] = 1.35 ± 0.06, D_0[H_2O_2^+-H_2O] = 0.86 ± 0.06, D_0[H_7O_3^+-H_2O] = 0.71 ± 0.06, D_0[H_9O_4^+-H_2O] = 0.52 ± 0.06, and D_0[H_(11_O_5^+-H_2O] = 0.51 ± 0.08. These results are in very good agreement with enthalpies of solvation determined by equilibrium methods.