Nanometer patterning of water by tetraanionic ferrocyanide stabilized in aqueous nanodrops.

Formation of the small, highly charged tetraanion ferrocyanide, Fe(CN)64-, stabilized in aqueous nanodrops is reported. Ion-water interactions inside these nanodrops are probed using blackbody infrared radiative dissociation, infrared photodissociation (IRPD) spectroscopy, and molecular modeling in order to determine how water molecules stabilize this highly charged anion and the extent to which the tetraanion patterns the hydrogen-bonding network of water at long distance. Fe(CN)64-(H2O)38 is the smallest cluster formed directly by nanoelectrospray ionization. Ejection of an electron from this ion to form Fe(CN)63-(H2O)38 occurs with low-energy activation, but loss of a water molecule is favored at higher energy indicating that water molecule loss is entropically favored over loss of an electron. The second solvation shell is almost complete at this cluster size indicating that nearly two solvent shells are required to stabilize this highly charged anion. The extent of solvation necessary to stabilize these clusters with respect to electron loss is substantially lower through ion pairing with either H+ or K+ (n = 17 and 18, respectively). IRPD spectra of Fe(CN)64-(H2O) n show the emergence of a free O-H water molecule stretch between n = 142 and 162 indicating that this ion patterns the structure of water molecules within these nanodrops to a distance of at least ∼1.05 nm from the ion. These results provide new insights into how water stabilizes highly charged ions and demonstrate that highly charged anions can have a significant effect on the hydrogen-bonding network of water molecules well beyond the second and even third solvation shells

Long distance ion-water interactions in aqueous sulfate nanodrops persist to ambient temperatures in the upper atmosphere.

The effect of temperature on the patterning of water molecules located remotely from a single SO42- ion in aqueous nanodrops was investigated for nanodrops containing between 30 and 55 water molecules using instrument temperatures between 135 and 360 K. Magic number clusters with 24, 36 and 39 water molecules persist at all temperatures. Infrared photodissociation spectroscopy between 3000 and 3800 cm-1 was used to measure the appearance of water molecules that have a free O-H stretch at the nanodroplet surface and to infer information about the hydrogen bonding network of water in the nanodroplet. These data suggest that the hydrogen bonding network of water in nanodrops with 45 water molecules is highly ordered at 135 K and gradually becomes more amorphous with increasing temperature. An SO42- dianion clearly affects the hydrogen bonding network of water to at least ∼0.71 nm at 135 K and ∼0.60 nm at 340 K, consistent with an entropic drive for reorientation of water molecules at the surface of warmer nanodrops. These distances represent remote interactions into at least a second solvation shell even with elevated instrumental temperatures. The results herein provide new insight into the extent to which ions can structurally perturb water molecules even at temperatures relevant to Earth's atmosphere, where remote interactions may assist in nucleation and propagation of nascent aerosols

Hydration of guanidinium depends on its local environment.

Hydration of gaseous guanidinium (Gdm+) with up to 100 water molecules attached was investigated using infrared photodissociation spectroscopy in the hydrogen stretch region between 2900 and 3800 cm-1. Comparisons to IR spectra of low-energy computed structures indicate that at small cluster size, water interacts strongly with Gdm+ with three inner shell water molecules each accepting two hydrogen bonds from adjacent NH2 groups in Gdm+. Comparisons to results for tetramethylammonium (TMA+) and Na+ enable structural information for larger clusters to be obtained. The similarity in the bonded OH region for Gdm(H2O)20+vs. Gdm(H2O)100+ and the similarity in the bonded OH regions between Gdm+ and TMA+ but not Na+ for clusters with <50 water molecules indicate that Gdm+ does not significantly affect the hydrogen-bonding network of water molecules at large size. These results indicate that the hydration around Gdm+ changes for clusters with more than about eight water molecules to one in which inner shell water molecules only accept a single H-bond from Gdm+. More effective H-bonding drives this change in inner-shell water molecule binding to other water molecules. These results show that hydration of Gdm+ depends on its local environment, and that Gdm+ will interact with water even more strongly in an environment where water is partially excluded, such as the surface of a protein. This enhanced hydration in a limited solvation environment may provide new insights into the effectiveness of Gdm+ as a protein denaturant

Sequential water molecule binding enthalpies for aqueous nanodrops containing a mono-, di- or trivalent ion and between 20 and 500 water molecules.

Sequential water molecule binding enthalpies, ΔHn,n-1, are important for a detailed understanding of competitive interactions between ions, water and solute molecules, and how these interactions affect physical properties of ion-containing nanodrops that are important in aerosol chemistry. Water molecule binding enthalpies have been measured for small clusters of many different ions, but these values for ion-containing nanodrops containing more than 20 water molecules are scarce. Here, ΔHn,n-1 values are deduced from high-precision ultraviolet photodissociation (UVPD) measurements as a function of ion identity, charge state and cluster size between 20-500 water molecules and for ions with +1, +2 and +3 charges. The ΔHn,n-1 values are obtained from the number of water molecules lost upon photoexcitation at a known wavelength, and modeling of the release of energy into the translational, rotational and vibrational motions of the products. The ΔHn,n-1 values range from 36.82 to 50.21 kJ mol-1. For clusters containing more than ∼250 water molecules, the binding enthalpies are between the bulk heat of vaporization (44.8 kJ mol-1) and the sublimation enthalpy of bulk ice (51.0 kJ mol-1). These values depend on ion charge state for clusters with fewer than 150 water molecules, but there is a negligible dependence at larger size. There is a minimum in the ΔHn,n-1 values that depends on the cluster size and ion charge state, which can be attributed to the competing effects of ion solvation and surface energy. The experimental ΔHn,n-1 values can be fit to the Thomson liquid drop model (TLDM) using bulk ice parameters. By optimizing the surface tension and temperature change of the logarithmic partial pressure for the TLDM, the experimental sequential water molecule binding enthalpies can be fit with an accuracy of ±3.3 kJ mol-1 over the entire range of cluster sizes

Structural Investigation of the Hormone Melatonin and Its Alkali and Alkaline Earth Metal Complexes in the Gas Phase

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

Structures and reactivities of gaseous Fe­(CN)₆^3–(H₂O)_<i>n</i> 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–(H₂O)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–(H₂O)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–(H₂O)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–(H₂O)_<i>n</i> for <i>n</i> between 58 and 60, and the IRPD spectrum of the <i>n</i> = 60 cluster shows stronger water molecule hydrogen-bonding than that of the <i>n</i> = 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 <i>n</i> ≥ 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

Hydration of Guanidinium: Second Shell Formation at Small Cluster Size

The structures of hydrated guanidinium, Gdm⁺(H₂O)_<i>n</i>, where <i>n</i> = 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 <i>n</i> 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 <i>n</i> = 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 <i>n</i> = 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