1,721,147 research outputs found
DFT calculations of Ru-99 chemical shifts with all-electron and effective core potential basis sets
No full textThe nuclear shielding of Ru-99 in several complexes has been
investigated computationally by DFT methods with effective core
potential and all-electron basis sets. Shieldings calculated with ECP bases correlate very satisfactorily with available experimental data, although they are ca. one order of magnitude lower than the experimental values. The influence of molecular geometry (semiempirical vs. DFT) on the nuclear shielding is also examined and discussed, particularly in the case of species containing Ru-S bonds {[RuCl2(DMSO)(4)] and alpha-[PW11RuO39(DMSO)](5-)}. It is shown that such calculations may help the assignment of signals in Ru-99 NMR spectra of mono- and polynuclear complexes
Relativistic DFT calculation of Ru-99 NMR parameters: Chemical shifts and spin-spin coupling constants
No full textThe nuclear shielding of Ru-99 in a wide variety of complexes was investigated computationally by DFT methods, including relativistic effects (by means of the Pauli and ZORA approximations), up to spin-orbit coupling and Slater all-electron or frozen-core basis sets. Mononuclear complexes, a trinuclear cluster and a ruthenium-substituted polyoxometalate were included. Chemical shifts calculated in this way correlated very satisfactorily with experimental values, with fit lines having slopes close to unity. In the few cases where a comparison was possible, spin-spin coupling constants involving Ru-99 were also successfully calculated
Effective core potential DFT calculations of nuclear shielding as a tool for the prediction and assignment of the tungsten chemical shift in mono- and polynuclear complexes
No full textThe shielding of the W-183 nucleus in mononuclear tungsten complexes and in the Keggin heteropolyoxotungstate PW12O403- has been investigated by a density functional theory (DFT) method with effective core potentials. Calculated [PW12O40]3- shieldings correlate with experimental data, although they are one order of magnitude lower than the experimental values, which is reflected in low slopes (< 0.1) of the correlation lines. The influence of molecular geometry (semi-empirical vs. DFT) on the nuclear shielding is examined. There is a fair correlation between atomic charges at W and nuclear shielding only for species sharing the same formal oxidation state
Computational modeling of polyoxotungstates by relativistic DFT calculations of W-183 NMR chemical shifts
No full textThe W-183 nuclear shielding in a variety of tungsten polyoxometalates (POM) (Lindqvist, Anderson, decatungstates, Keggin) of different shapes and charges has been modeled by DFT calculations that take into account relativistic effects, by means of the zeroorder regular approximation (ZORA), and solvent effects, by the conductor-like screening model (COSMO) continuum method. The charge/surface area ratio (q1A) is proposed as an indicator of the charge density to which the solvation energies of all POMs are correlated in a satisfactory way. Among the various theoretical levels tested (ZORA scalar or spin-orbit, frozen-core or all-electron basis set, geometry optimization in the gas phase or in the continuum solvent, etc.), the best results are obtained when both geometry optimization in solvent and spin-orbit shielding are included (mean absolute error of delta = 35 ppm). The quality of the computed chemical shifts depends systematically on the charge density as expressed by q/A; thus, POMs with low q/A ratios display the best agreement with experimental data. The performance of the method is such that computed values can aid the assignment of the W-183 NMR spectra of polyoxotungstates, as shown by the case of alpha-[PW11TiO40](5-), whose six signals are ranked computationally so as to almost reproduce the experimental ordering even though the signals are spaced by as little as 5 ppm
Vicinal tungsten-tungsten coupling constants in polyoxotungstates: DFT calculations challenge an empirical rule
No full textThe empirical rule that edge and corner junctions give rise to small and large vicinal NMR tungsten–tungsten coupling constants, respectively, in polyoxometalates has led to the assumption that the W-O-W angle is the sole factor involved. Why then is the corner 2JWW coupling in γ-[SiW10O36]8− (see picture) so small? Relativistic DFT calculations show why this is indeed expected
alpha,alpha',alpha"-[Nitrilo-N)tris(methylene)]tris[benzenemethanolato-O](2-propanolato)titanium
No full textThe title catalyst belongs
to a family of titanium complexes bearing C3-symmetric trialkanolamines
ligands, which bind tightly to the Ti(IV) center in
a tetradentate fashion providing very stable titanatranes. These
complexes effect the enantioselective oxidation of prostereogenic
sulfides in the presence of alkyl hydroperoxide
Hydrogen peroxide activation by fluorophilic polyoxotungstates for fast and selective oxygen transfer catalysis
No full textFluorophilic polyoxotungstates perform the selective epoxidations of internal and terminal double bonds by hydrogen peroxide (H2O2) activation in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), under mild temperature conditions. A hybrid synergy of supramolecular interactions, involving the inorganic cluster and the fluorinated solvent, is envisaged to boost H2O2 activation and the oxygen transfer mechanism. 1,2-Epoxides have been obtained with >99% selectivity and 98% yield at T = 40-70 °C
Photooxidation in water by new hybrid molecular photocatalysts integrating an organic sensitizer with a polyoxometalate core
No full textHybrid compounds consisting of an organic sensitizer and a polyoxometalate unit were synthesized following two strategies: a) the covalent functionalization of lacunary decatungstosilicate with organosilylfulleropyrrolidines; b) the charge interaction between cationic sensitizers and the polyoxoanions, yielding electrostatic aggregates. These hybrid complexes effect catalytic photooxygenation in water under heterogeneous conditions, 25degreesC and O-2 (1 atm), using visible light irradiation (lambda > 375 nm). As representative target substrates, phenol (4 mM) is oxidized in 150 min with a COD loss up to 30% (TON up to 50), while L-methionine methyl ester (15 mM) undergoes selective photooxygenation to the corresponding sulfoxide in 90 min (TON up to 200). The photocatalyst stability has been evaluated on the basis of system recycling along three oxidation runs. The inhibition exerted by sodium azide, a typical O-1(2) quencher, suggests the occurrence of a Type II photooxidation mechanism
Supramolecular Catalysis: Enantioselective Oxidation of Thioanisole in Water by Hydrogen Peroxide Catalyzed by Mo(VI) in the Presence of beta Cyclodextrin-based Ligand
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