S. V. Fedorov

The Oxygen Permeation of Solid/Melt Composite BiVO4 – 10 wt % V2O5 Membrane

The oxygen permeation fluxes through solid/melt composite BiVO 4 ― 10 wt % V 2 O 5 membrane have been measured by using the conventional gas flow technique in the temperature range 650―670°C under various oxygen partial pressure gradients. Results indicate that in the range of thickness 0.9―3.5 mm used in the present study, the membrane operates under mixed control of the bulk diffusion and surface exchange kinetics. The ambipolar conductivity, characteristic thickness, and surface exchange coefficient are estimated at values of about 2.8 x 10 ―3 S/cm, 0.5 mm, and 5.2 x 10 ―6 cm/s, respectively, at 650°C.

Transport Properties of BiVO4 – V2O5 Liquid-Channel Grain-Boundary Structures

Mixed-conducting BiVO 4 -5, 7, 10, 12 wt % V 2 O 5 liquid-channel grain-boundary structures were studied with respect to their electrochemical properties. The electrical conductivity and oxygen-ion transference number, measured by the four-probe dc and volumetric measurements of the faradaic efficiency techniques in the temperature range of 640-660°C, vary from 5 X 10 -3 to 3 × 10 -2 Ω -1 cm -1 and from 0.68 to 0.85, respectively. The apparent conductivity activation energy decreases from 0.86 to 0.56 eV with increasing content of V 2 O 5 .

Conformational analysis and stereochemical dependences of 31P–1H spin–spin coupling constants of bis(2-phenethyl)vinylphosphine and related phosphine chalcogenides

Theoretical energy‐based conformational analysis of bis(2‐phenethyl)vinylphosphine and related phosphine oxide, sulfide and selenide synthesized from available secondary phosphine chalcogenides and vinyl sulfoxides is performed at the MP2/6‐311G** level to study stereochemical behavior of their 31P–1H spin–spin coupling constants measured experimentally and calculated at different levels of theory. All four title compounds are shown to exist in the equilibrium mixture of two conformers: major planar s‐cis and minor orthogonal ones, while 31P–1 H spin–spin coupling constants under study are found to demonstrate marked stereochemical dependences with respect to the geometry of the coupling pathways, and to the internal rotation of the vinyl group around the P(X)‐C bonds (X = LP, O, S and Se), opening a new guide in the conformational studies of unsaturated phosphines and phosphine chalcogenides. Copyright

Towards the versatile DFT and MP2 computational schemes for 31P NMR chemical shifts taking into account relativistic corrections

The main factors affecting the accuracy and computational cost of the calculation of 31P NMR chemical shifts in the representative series of organophosphorous compounds are examined at the density functional theory (DFT) and second‐order Møller–Plesset perturbation theory (MP2) levels. At the DFT level, the best functionals for the calculation of 31P NMR chemical shifts are those of Keal and Tozer, KT2 and KT3. Both at the DFT and MP2 levels, the most reliable basis sets are those of Jensen, pcS‐2 or larger, and those of Pople, 6‐311G(d,p) or larger. The reliable basis sets of Dunnings family are those of at least penta‐zeta quality that precludes their practical consideration. An encouraging finding is that basically, the locally dense basis set approach resulting in a dramatic decrease in computational cost is justified in the calculation of 31P NMR chemical shifts within the 1–2‐ppm error. Relativistic corrections to 31P NMR absolute shielding constants are of major importance reaching about 20–30 ppm (ca 7%) improving (not worsening!) the agreement of calculation with experiment. Further better agreement with the experiment by 1–2 ppm can be obtained by taking into account solvent effects within the integral equation formalism polarizable continuum model solvation scheme. We recommend the GIAO‐DFT‐KT2/pcS‐3//pcS‐2 scheme with relativistic corrections and solvent effects taken into account as the most versatile computational scheme for the calculation of 31P NMR chemical shifts characterized by a mean absolute error of ca 9 ppm in the range of 550 ppm. Copyright

Trivinylphosphine and trivinylphosphine chalcogenides: stereochemical trends of 31P1H spin–spin coupling constants†

A combined theoretical and experimental study of the stereochemical behavior of 31P1H spin–spin coupling constants has been performed in the series of trivinylphosphine and related trivinylphosphine oxide, sulfide and selenide. Theoretical energy‐based conformational analysis of the title compounds performed at the MP2/6‐311G** level reveals that each of the four compounds of this series exists in the equilibrium mixture of five true‐minimum conformers, namely s‐cis‐s‐cis‐s‐cis, s‐cis‐s‐cis‐gauche, syn‐s‐cis‐gauche‐gauche, anti‐s‐cis‐gauche‐gauche and gauche‐gauche‐gauche, which were taken into account in the conformational averaging of 31P1H spin–spin couplings calculated at the second‐order polarization propagator approach/aug‐cc‐pVTZ‐J level of theory. All 31P1H spin–spin coupling constants involving phosphorus and either of the vinyl protons are found to demonstrate a marked stereochemical dependences with respect to the geometry of the coupling pathway and internal rotation of the vinyl group around the PC bond which is of major importance in the stereochemical studies of the unsaturated phosphines and phosphine chalcogenides. Copyright

Novel Molten Oxide Membrane for Ultrahigh Purity Oxygen Separation from Air.

We present a novel solid/liquid Co3O4-36 wt % Bi2O3 composite that can be used as molten oxide membrane, MOM ( Belousov, V. V. Electrical and Mass Transport Processes in Molten Oxide Membranes. Ionics 22 , 2016 , 451 - 469 ), for ultrahigh purity oxygen separation from air. This membrane material consists of Co3O4 solid grains and intergranular liquid channels (mainly molten Bi2O3). The solid grains conduct electrons, and the intergranular liquid channels predominantly conduct oxygen ions. The liquid channels also provide the membrane material gas tightness and ductility. This last property allows us to deal successfully with the problem of thermal incompatibility. Oxygen and nitrogen permeation fluxes, oxygen ion transport number, and conductivity of the composite were measured by the gas flow, volumetric measurements of the faradaic efficiency, and four-probe dc techniques, accordingly. The membrane material showed the highest oxygen selectivity jO2/jN2 > 10(5) and sufficient oxygen permeability 2.5 × 10(-8) mol cm(-1) s(-1) at 850 °C. In the range of membrane thicknesses 1.5-3.3 mm, the oxygen permeation rate was controlled by chemical diffusion. The ease of the MOM fabrication, combined with superior oxygen selectivity and competitive oxygen permeability, shows the promise of the membrane material for ultrahigh purity oxygen separation from air.

Relativistic Environmental Effects in 29Si NMR Chemical Shifts of Halosilanes: Light Nucleus, Heavy Environment

Relativistic calculations of (29)Si NMR shielding constants (chemical shifts) in the series of halosilanes SiX(n)H(4-n) (X = F, Cl, Br and I) are performed within a full four-component relativistic Diracs scheme using relativistic Dyalls basis sets. Three different theoretical levels are tested in the computation of (29)Si NMR chemical shifts in comparison with experiment: namely, four-component relativistic GIAO-DFT, four-component relativistic GIAO-RPA, and a hybrid scheme of a nonrelativistic GIAO-MP2 with taking into account relativistic corrections using the four-component relativistic GIAO-RPA. The DFT results give larger relativistic effects as compared to the RPA data which might be rationalized in terms of the manifestation of correlation effects taken into account at the DFT level and not accounted for at the uncorrelated RPA level. Taking into account solvent effects slightly improves agreement with experiment, however, being not a matter of principle. Generally, relativistic pure nonempirical wave function methods perform much better as compared to relativistic DFT methods when benchmarked to experiment.

Calculations of 29Si NMR shifts of organylsilanes by DFT taking into account solvent effects and relativistic corrections

The accuracy of calculations of the 29Si NMR chemical shifts of organylsilanes with different hybridization of C atoms in the substituents at the Si atom (sp3, sp2, and sp) was analyzed on the basis of the effective calculation scheme in terms of the density functional theory using the KT3 functional in combination with the ?-polarization consistent basis set pcS-3. Taking into account the influence of the medium in terms of the supermolecular solvate model and relativistic effects based on the full four-component Dirac scheme made it possible to achieve the extremely high accuracy of calculations of 29Si NMR chemical shifts of organylsilanes: 1 ppm for a span of 100 ppm. The contributions of solvent, relativistic, and vibrational corrections for the 29Si NMR chemical shifts are 3, 1, and 1 ppm, respectively, and taking them into account noticeably improves the agreement of calculation results with experiment. The relativistic shielding contribution to the absolute shielding constant of the Si atom is 12—13 ppm on the average, i.e., to ca. 3% in the relative expression.

Quantum-chemical calculations of NMR chemical shifts of organic molecules: XII. Calculation of the 13C NMR chemical shifts of fluoromethanes at the DFT level

Calculation was carried out of chemical shifts in 13C NMR spectra for a series of fluoromethanes CHnF4−n (n = 0–4) by the methods of the electron density functional theory GIAO-DFT taking in consideration the solvent effect in the framework of the polarizable continuum model Tomasi IEF-PCM. The best results were obtained at the use of Keal-Tozer KT3 functional combined with Pople standard basis sets 6-311G(d,p) and 6-311++G(d,p), and also with Jensen special set pcS-2 containing tight p-functions. The optimum reference in the calculation of chemical shifts in 13C NMR spectra for the fluoromethanes series is TMS.

Modeling oxygen Ion transport of molten oxide membranes based on V2O5

A polymer model for oxygen ion transport in molten oxide membranes (MOM) based on V2O5 was developed. The model adapts Wagner’s theory for molten oxides and provides an interpretation of oxygen mobility in the oxide melts. Within the framework of this model, the values of oxygen permeation fluxes through the MOM were calculated and compared with experimental data. The calculated and experimental values are of the same order of magnitude which shows an adequacy of the model. A dynamic polymer chain concept is proposed. It is shown that the transference of oxygen ions in the oxide melts may occur by a mechanism of “connection–disconnection” of the polymer chains.