Leonid P. Kazansky

Are the solvent effects critical in the modeling of polyoxoanions

DFT calculations were driven for a set of differently charged polyoxoanions in the gas phase and in solution. We have calculated and analyzed their geometries and orbital energies to trace simple rules of behavior regarding the modeling of anions in isolated form. We discuss the quality of the results depending on the molecular charge, q, and the size of the cluster in terms of the number of metal centers, m. When the q/m ratio reaches a value of ∼0.8, DFT calculations for the isolated anion fail to describe the gap between the band of occupied oxo orbitals and the set of unoccupied orbitals delocalized among the metal atoms. In these cases the incorporation of the stabilizing external fields generated by the solvent through continuum models improves the geometries and orbital energies.

Towards the Accurate Calculation of 183W NMR Chemical Shifts in Polyoxometalates: The Relevance of the Structure

DFT calculations were carried out to study (183)W NMR chemical shifts in the family of the Keggin anions with formula alpha-[XW(12)O(40)](q-) (X=B, Al, Si, P, Ga, Ge, As, Zn), in the beta- and gamma-[SiW(12)O(40)](4-) geometric isomers, in the derivative Dawson anion [P(2)W(18)O(62)](6-), and in the most symmetrical Lindqvist [W(6)O(19)](2-) anion and its derivative [W(10)O(32)](4-). In this article, we show that the geometry employed in the calculation of NMR chemical shifts in polyoxotungstates is extremely important if we want to be quantitative. Using very large basis sets of QZ4P quality and taking into account the conductor-like screening model (COSMO) to account for solvent effects (aqueous and organic solutions), good geometries were found for the polyoxoanions. From these optimal geometries the (183)W NMR chemical shifts were computed with the more standard basis sets of TZP quality and including spin-orbit corrections inside the zero-order regular approximation (ZORA) to describe the relativistic effects of the internal electrons. With this strategy the mean absolute error between experimental and theoretical values was found to be less than 10 ppm, which is similar to the experimental error. We also discuss how the geometry of the polyoxoanion influences on the shielding.

Heteropoly anions as corrosion inhibitors for aluminium in high temperature water

Abstract Corrosion of aluminium and its alloys in high temperature water (90–300°C) has been studied with and without addition of various oxotungstates by a number of methods. Substantial decrease (in some cases by 10 times) of the corrosion rate in the presence of polyoxotungstates was observed. The pitting potential was shown to shift to positive values depending on the heteropolytungstate added. According to XPS tungsten is incorporated into the surface oxide film formed during the treatment. SEM cross-sections reveal a marked decrease of the oxide film thickness in the presence of polytungstate. It is assumed that the heteropolytungstate on active site in the aluminium oxide film is preventing the recrystallization and growth protective alumina layer. Pretreatment of aluminium in solutions of heteropoly tungstates is also effective in preventing corrosion at high temperature.

17O and 183W NMR diamagnetic and paramagnetic shifts in heterodecatungstates XW10O36− (X = Ln, Th, U) in aqueous solutions

Abstract Heterodecatungstates of the general formula X W 10 O 36 − where X = Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er, Th and U) have been studied by 17O and 183W NMR spectroscopy. It is shown that the structure of the heteropolyanions, where X occupies a centre of a square antiprism formed by two W5O18 ligands and which was determined in the crystalline state, is retained in aqueous solution. Completely resolved 17O and 183W NMR spectra for all atoms, even for the nearest oxygen atoms, have been recorded. From the analysis of the isotropic shifts, the contact and dipolar contributions have been estimated for all constituting atoms. The possible σ- and π-mechanisms of the spin transfer are considered. The negative spin density determines the dominant contact shift for the oxygen and tungsten nearest to X. The large pseudo-contact (ligand centred) contributions are estimated for the nearest oxygen and tungsten atoms. The positive spin density due to the σ-mechanism and large dipolar contribution gives rise to the observed chemical shift for the internal oxygen atom. The other oxygen atoms experience a mostly dipolar contribution with other terms probably cancelling each other. Temperature dependences of the chemical shifts are discussed. Copyright

Analysis of 18;W and 17O NMR chemical shifts in polyoxometalates by extended Hückel mo calculations

Abstract The extended Huckel molecular orbital (EHMO) calculations have been carried out using cluster approach to polyoxo anions, i.e. calculations have been done for a single octahedron MO 6 of different symmetry and results have been used to analyze 183 W and 17 O NMR spectra. Using five d→d∗ energy differences for the individual WO 6 the sum Σ1/ ( E d → E d ∗) have been calculated and plotted against the 183 W chemical shift (from +258 to −670 ppm) for corresponding type of tungsten atom and practically a linear correlation between two parameters have been observed. This points out the electronic nature of the 183 W chemical shift. Similar correlation have been found for the 17 O NMR chemical shifts (−90 to +800 ppm) when the plotted against product of the R −3 Σ1/ ( E p → E p ∗) where the R −3 bond length of the corresponding W–O bond. Increasing the 183 W nuclear magnetic shielding with the calculated electron population on tungsten atom for closely related anions has been observed, but no general tendency between δ and the calculated electronic charge if the symmetry of polyhedron is changed is expected.

Phosphorus-31 and oxygen-17 n.m.r. evidence of trapped electrons in reduced 18-molybdodiphosphate(V), P2Mo18O628–

Reduction of the P2Mo18O626– anion by two electrons results in the mixed valence form and, by using 31P and 17O n.m.r. high resolution spectra, these electrons are deduced to be firmly trapped by two adjacent molybdenum atoms located in both half-units of the anion.

On the existence of a spin bipolaron. Evidence from the 183W NMR of mixed valence polyoxotungstate anions

Abstract The 183 W NMR chemical shifts of polyoxotungstates reduced by two electrons regarded as a bipolaron have been reconsidered. It is assumed that the bipolaron formed by two W induces a spin bipolaron in nearby tungsten atoms manifested in NMR spectra by their substantial shielding. A quantitative estimation of the chemical shifts resulting from the delocalization process is given.

Inhibiting the corrosion of MNZh 5-1 alloy in neutral solutions of 5-chloro-1,2,3-benzotrialzol

The adsorption and protective properties of 5-chloro-1,2,3-benzotriazol (5-chloro-BTA) are studied in relation to MNZh 5-1 alloy in a chloride borate buffer solution with pH 7.4. It is shown that this inhibitor can stabilize the passive state of the alloy at a concentration of 0.12 mmol/g. The adsorption of 5-chloro-BTA on a surface of MNZh 5-1 alloy is polymolecular; the free energy of adsorption is about 80 kJ/mol. The advantages of adsorption and protective properties of 5-chloro-BTA compared to BTA on both MNZh 5-1 alloy and the metals contained in the alloy (Ni, Cu) are shown. XPS data indicate a 5-chloro-BTA monolayer formed on the surface of the alloy. This monolayer was composed of inhibitor molecules, which are normally oriented toward a surface and are not removed during ultrasonic washing of the electrode.

A Study of Adsorption of 5-Mercaptopentyl-3-amino-1,2,4-triazole on Copper in Neutral Solutions

The inhibiting action by 5-mercaptopetyl-3-amino-1,2,4-triazole and its adsorption on copper in neutral borate buffer solutions were studied. The potentiodynamic curves were used to detect the influence of the inhibitor concentration on the anodic dissolution current and activation potential of passive state. X-ray photoelectron spectroscopy was applied to determine both the composition and thickness of surface films as a function of the exposure time of the sample in a solution containing the inhibitor. Quantum-chemical calculations made it possible to identify the most probable structure of a copper–inhibitor complex.

Adsorption of sodium flufenaminate in zinc from aqueous solutions

Adsorption of sodium flufenaminate (SFF) anions on a zinc surface in borate buffer solutions at pH 7.4 and 9.1 has been studied by the ellipsometry and XPS techniques. SFF adsorption was described by the Frumkin isotherm with high free adsorption energy. The SFF molecule adsorbs on the surface of oxidized zinc through the carboxyl group.