Water exhibits remarkable properties in confined spaces, such as nanometer-sized droplets where hundreds of water molecules are required for crystalline structure to form at low temperature due to surface effects. Here, we investigate how a single ion affects the crystallization of (H2O)n clusters with infrared photodissociation spectroscopy of size-selected La(3+)(H2O)n nanodrops containing up to 550 water molecules. Crystallization in the ion-containing nanodrops occurs at n ≥ 375, which is approximately 100 more water molecules than what has been reported for neutral water clusters. This frustration of crystallinity reveals that La(3+) disrupts the hydrogen-bonding network of water molecules located remotely from the ion, a conclusion that is supported by molecular dynamics simulations. Our findings establish that a trivalent ion can pattern the H-bond network of water molecules beyond the third solvation shell, or to a distance of ∼1 nm from the ion.
Structures and reactivities of gaseous Fe(CN)(6)(3-)(H(2)O)n were investigated using infrared photodissociation (IRPD) kinetics, spectroscopy, and computational chemistry in order to gain insights into how water stabilizes highly charged anions. Fe(CN)(6)(3-)(H(2)O)(8) is the smallest hydrated cluster produced by electrospray ionization, and blackbody infrared dissociation of this ion results in loss of an electron and formation of smaller dianion clusters. Fe(CN)(6)(3-)(H(2)O)(7) is produced by the higher activation conditions of IRPD, and this ion dissociates both by loss of an electron and by loss of a water molecule. Comparisons of IRPD spectra to those of computed low-energy structures for Fe(CN)(6)(3-)(H(2)O)(8) indicate that water molecules either form two hydrogen bonds to the trianion or form one hydrogen bond to the ion and one to another water molecule. Magic numbers are observed for Fe(CN)(6)(3-)(H(2)O)n for n between 58 and 60, and the IRPD spectrum of the n = 60 cluster shows stronger water molecule hydrogen-bonding than that of the n = 61 cluster, consistent with the significantly higher stability of the former. Remarkably, neither cluster has a band corresponding to a free O-H stretch, and this band is not observed for clusters until n ≥ 70, indicating that this trianion significantly affects the hydrogen-bonding network of water molecules well beyond the second and even third solvation shells. These results provide new insights into the role of water in stabilizing high-valency anions and how these ions can pattern the structure of water even at long distances.
The structures of hydrated guanidinium, Gdm(+)(H2O)n, where n = 1-5, were investigated with infrared photodissociation spectroscopy and with theory. The spectral bands in the free O-H (∼3600-3800 cm(-1)) and free N-H (∼3500-3600 cm(-1)) regions indicate that, for n between 1 and 3, water molecules bind between the NH2 groups in the plane of the ion forming one hydrogen bond with each amino group. This hydration structure differs from Gdm(+) in solution, where molecular dynamics simulations suggest that water molecules form linear H-bonds with the amino groups, likely a result of additional water-water interactions in solution that compete with the water-guanidinium interactions. At n = 4, changes in the free O-H and bonded O-H (∼3000-3500 cm(-1)) regions indicate water-water H-bonding and thus the onset of a second hydration shell. An inner shell coordination number of n = 3 is remarkably small for a monovalent cation. For Gdm(+)(H2O)5, the additional water molecule forms hydrogen bonds to other water molecules and not to the ion. These results indicate that Gdm(+) is weakly hydrated, and interactions with water molecules occur in the plane of the ion. This study offers the first experimental assignment of structures for small hydrates of Gdm(+), which provide insights into the unusual physicochemical properties of this ion.
Infrared spectroscopy of guanidinium confined in gaseous nanodrops shows hydration depends on local environment and lends new insights into its effectiveness as a protein denaturant.
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