Hydrated Alkali Metal Ions: Spectroscopic Evidence for Clathrates

Abstract

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 (∼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.

Site-specific vibrational spectral signatures of water molecules in the magic H 3 O + (H 2 O) 20 and Cs + (H 2 O) 20 clusters

“…In this paper, we contrast the behavior of the n = 20 hydrates of the H 3 O + and Cs + ions, both of which are preferentially generated in the M + (H 2 O) n cluster ion distributions (i.e., are magic numbers) and are thought to be based on a pentagonal dodecahedron motif (12 interconnected H-bonded pentagons). An important difference between these ions is that H 3 O + is predicted to reside on the surface of the cage (6,8,26), whereas Cs + is sequestered in its interior (27,28). We follow the evolution of the bands in the perdeuterated isotopologues, Cs + (D 2 O) 20 and D 3 O + (D 2 O) 20 , and find strong support for the earlier assignment of the spectral signature of the embedded H 3 O + ion.…”

“…Two reported minimum energy structures (29) of the metal ion cage are displayed in Fig. 5 along with the corresponding harmonic predictions (note that nondodecahedral structures have also been shown to be nearly isoenergetic (27,30) and high-level ab initio searches have yet to be performed on this system). Both arrangements have 10 AAD type molecules colored in orange.…”

Nanometer patterning of water by tetraanionic ferrocyanide stabilized in aqueous nanodrops

“…Although the structural origin of the magic number cluster stabilities is unknown, infrared photodissociation studies have suggested that complex clathrate structures may form for clusters of this size, which optimize hydrogen bonding and ultimately lead to enhanced stability. 21,53,54 …”

“…The spectra for clusters with n ≥ 162 have a feature centered at ∼3704 cm –1 characteristic of three-coordinate AAD (acceptor–acceptor–donor) water molecules. 53,54,56–60 The frequency of these oscillators depends on the charge state of the hydrated ion, the extent of hydration and to a lesser extent, the size of the ion. 61,62 The intensity of this band increases for n = 182 and 218, indicating that the surface of these clusters is populated with an increasing number of AAD water molecules.…”

Role of Water in Stabilizing Ferricyanide Trianion and Ion-Induced Effects to the Hydrogen-Bonding Water Network at Long Distance

“…55−59 There is a strong ADD stretch in the spectra of Cs + (H 2 O) 20 and some other alkali metal ions that is indicative of clathrate structures. 58 Patterning of Water Molecules at Long Distances. A free O−H band is commonly observed in the spectra of hydrated ions.…”