Infrared Action Spectra of Ca2+(H2O)11-69 Exhibit Spectral Signatures for Condensed-Phase Structures with Increasing Cluster Size

Infrared laser action spectroscopy is used to characterize divalent calcium ions solvated by up to 69 water molecules. The spectrum for Ca2+(H2O)(12) indicates that in the predominant structure, eight inner-shell water molecules solvate the metal ion and donate one hydrogen bond to one of four second-shell, double-acceptor water molecules. Eight-coordinate solvation is consistent with results from many condensed-phase studies, and contrasts with results for smaller gas-phase clusters that are most consistent with six-coordinate solvation. Each water molecule in this structure of Ca2+(H2O)(12) coordinates with two other members of the cluster. With increasing cluster size, the number of two-coordinate water molecules decreases, whereas that of three-coordinate water molecules increases. The number of one-coordinate water molecules increases until n approximate to 18, but they are essentially depleted by n approximate to 30. Spectra of the largest clusters, which have effective concentrations of divalent calcium that are less than 1 M, exhibit only subtle changes with increasing cluster size. The bonded-OH regions of these spectra are similar to, but blue-shifted from that of bulk water, whereas the free-OH regions are well-resolved and indicate that the surfaces of these clusters are well-structured. These results comprise the most extensive vibrational spectroscopic study yet performed on metal ion hydration in the gas phase and provide insights into metal ion solvation in bulk and interfacial environments.