Hydration of gaseous copper dications probed by IR action spectroscopy

Clusters of Cu2+(H2O)(n), n = 6-12, formed by electrospray ionization, are investigated using infrared photodissociation spectroscopy, blackbody infrared radiative dissociation (BIRD), and density functional theory of select clusters. At 298 K, the BIRD rate constants increase with increasing cluster size for n >= 8, but the trend reverses for the smaller clusters where Cu2+ (H2O)(6) is less stable than Cu2+(H2O)(8). This trend in stability is consistent with a change in fragmentation pathway from loss of a water molecule for clusters with n >= 9 to loss of hydrated protonated water clusters and the formation of the corresponding singly charged hydrated metal hydroxide for n <= 7. The lowest-energy structures of Cu2+(H2O)(n), n = 6-8 and 10, identified at the B3LYP/LACV3P**++ levelof theory, all have coordination numbers (CN) of 4, although structures with a CN = 5 are within about 10 kJ/mol for all clusters except Cu2+(H2O)(8). IR action spectra indicate the presence of hydrogen bonding for all clusters, and results for Cu2+(H2O)(n), n = 6-8, are consistent with a CN = 4, although minor contributions from structures with higher CN cannot be ruled out. Bands in the action spectra of Cu2+(H2O)(n), n = 10-12, show the presence of water molecules that accept two hydrogen bonds and donate one hydrogen bond as well as single hydrogen bond acceptors clearly indicating the onset for formation of a third solvent shell at a relatively small cluster size.