Cation-nitrate ion contact in aqueous solutions

A Combined QM/MM Molecular Dynamics Simulations Study of Nitrate Anion (NO3-) in Aqueous Solution

The structural and dynamical properties of NO3- in dilute aqueous solution have been investigated by means of two combined quantum mechanics/molecular mechanics (QM/MM) molecular dynamics simulations, namely HF/MM and B3LYP/MM, in which the ion and its surrounding water molecules were treated at HF and B3LYP levels of accuracy, respectively, using the DZV+ basis set. On the basis of both HF and B3LYP methods, a well-defined first hydration shell of NO3- is obtainable, but the shell is quite flexible and the hydrogen-bond interactions between NO3- and water are rather weak. With respect to the detailed analysis of the geometrical arrangement and vibrations of NO3-, the experimentally observed solvent-induced symmetry breaking of the ion is well reflected. In addition, the dynamical information, i.e., the bond distortions and shifts in the corresponding bending and stretching frequencies as well as the mean residence time of water molecules surrounding the NO3- ion, clearly indicates the "structure-breaking" ability of this ion in aqueous solution. From a methodical point of view it seems that both the HF and B3LYP methods are not too different in describing this hydrated ion by means of a QM/MM simulation. However, the detailed analysis of the dynamics properties indicates a better suitability of the HF method compared to the B3LYP-DFT approach.

Ultrasonic Velocities, Densities, Viscosities, Electrical Conductivities, Raman Spectra, and Molecular Dynamics Simulations of Aqueous Solutions of Mg(OAc)2 and Mg(NO3)2: Hofmeister Effects and Ion Pair Formation

The ultrasonic velocities, densities, viscosities, and electrical conductivities of aqueous solutions of magnesium nitrate and magnesium acetate have been measured from dilute to saturation concentrations at 0 < or = t/degrees C < or = 50. The temperature derivative of the isentropic compressibility, kappa(s), became zero at 2.28 and 2.90 mol kg(-1) for Mg(OAc)2 and Mg(NO3)2 solutions, respectively, at 25 degrees C. The total hydration numbers of the dissolved ions were estimated to be, respectively, 24.3 and 19.2 at these concentrations. Differences in kappa(s) for various M2+ salts, using the present and literature data, correlated with reported M2+-OH2 bond lengths and to a lesser extent with cationic charge densities (ionic radii). The influence of anions on kappa(s) appears to follow the Hofmeister series and also correlates approximately with the anionic charge density. Substantial differences between Mg(OAc)2(aq) and Mg(NO3)2(aq) occur with respect to their structural relaxation times (derived from compressibility and viscosity data) and their electrical conductivities. These differences were attributed to a much greater ion association in Mg(OAc)2 solutions. Raman spectra recorded at 28 degrees C confirmed the presence of various types of contact ion pairs including mono- and bidentate complexes in Mg(OAc)2(aq). In Mg(NO3)2(aq), only noncontact ion pairs appear to be formed even at high concentrations. The experimental results are supported by molecular dynamics simulations, which also reveal the much stronger tendency of OAc- compared to NO3- to associate with Mg2+ in aqueous solutions. The simulations also allow an evaluation of the ion-ion and ion-water radial distribution functions and cumulative sums and provide a molecular picture of ion hydration in Mg(OAc)2(aq) and Mg(NO3)2(aq) at varying concentrations.

Reversible Colorimetric Probes for Mercury Sensing

The selectivity and sensitivity of two colorimetric sensors based on the ruthenium complexes N719 [bis(2,2'-bipyridyl-4,4'-dicarboxylate)ruthenium(II) bis(tetrabutylammonium) bis(thiocyanate)] and N749 [(2,2':6',2' '-terpyridine-4,4',4' '-tricarboxylate)ruthenium(II) tris(tetrabutylammonium) tris(isothiocyanate)] are described. It was found that mercury ions coordinate reversibly to the sulfur atom of the dyes' NCS groups. This interaction induces a color change in the dyes at submicromolar concentrations of mercury. Furthermore, the color change of these dyes is selective for mercury(II) when compared with other ions such as lead(II), cadmium(II), zinc(II), or iron(II). The detection limit for mercury(II) ions--using UV-vis spectroscopy--in homogeneous aqueous solutions is estimated to be approximately 20 ppb for N719 and approximately 150 ppb for N749. Moreover, the sensor molecules can be adsorbed onto high-surface-area mesoporous metal oxide films, allowing reversible heterogeneous sensing of mercury ions in aqueous solution. The results shown herein have important implications in the development of new reversible colorimetric sensors for the fast, easy, and selective detection and monitoring of mercuric ions in aqueous solutions.

Diffuse reflectance FT-IR spectroscopic study of interactions of α-Al2O3/molten alkali nitrate coexisting systems

The diffuse reflectance spectra of alpha-Al(2)O(3)/molten MNO(3) (M=K(+), Li(+)) coexisting systems have been measured for the investigation of the physicochemical properties of molten nitrate at the interface of aluminum oxide. In the region of overtone and combination modes from 3000 to 1700 cm(-1), the absorption band nu(3)+nu(4) decreases with increasing specific surface area of alpha-Al(2)O(3) powder for the potassium mixtures. Gaussian resolution of nu(3)+nu(4) also exhibits changes in band positions of components with the variation in the alumina specific surface area and the nitrate melt content. The second-order derivative of the spectra in the region from 1700 to 950 cm(-1) shows a set of peaks at 1645, 1554-1516, and 1452 cm(-1) characteristic of a coordinated nitrate ion. The peak at 1337 cm(-1) suggests a possible formation of nitrite species. The maxima of nu(3)+nu(4), nu(1)+nu(2), and nu(1)+nu(4) shift toward lower frequencies with increasing temperature above the nitrate melting point. This information shows that the intramolecular bonds of NO(3)(-) are affected. These perturbations are attributed to the effect of the solid phase interacting with the liquid phase. The interaction between solid and liquid phases affects the geometry of the ionic species and, therefore, influences the electrical properties of nitrate ions.