Hydrated Alkali Metal Ions: Spectroscopic Evidence for Clathrates

The origin of enhanced abundances for some hydrated alkali metal ions, M+(H2O)(n), where M = Cs, Rb, K, Na, and Li with between 17 and 21 water molecules attached was investigated with infrared photodissociation (IRPD) spectroscopy and by blackbody infrared radiative dissociation (BIRD) at 133 K. The abundances of clusters of Cs+, Rb+, and K+ with 18 and 20 water molecules are anomalously high compared to the corresponding clusters of Na+, and Li+ with 20 water molecules has only a slightly enhanced abundance. BIRD results indicate that the anomalous abundance at n = 20 for the larger ions is due to the high stability of this cluster, and the significant instability of the next largest cluster, consistent with a stable core structure with 20 water molecules. IRPD spectra in the free-OH region (similar to 3600-3800 cm(-1)) for Cs+, Rb+, and K+ with 18 and 20 water molecules indicates that water molecules with a free-OH stretch accept two hydrogen bonds and donate one hydrogen bond (acceptor-acceptor-donor water) to other water molecules. No acceptor-donor (AD) bands are observed, consistent with clathrate structures for these ions. In contrast, the AD band is significant for Na+, indicating that these clusters adopt different structures. Results for Li+ indicate a contribution from clathrate structures at n = 20, but not at other cluster sizes. This analysis is supported by the relative intensities of bands in the hydrogen-bonding region for n = 20.