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The BH3‐mediated reduction of P‐stereogenic hydroxyalkylphosphine oxides was investigated by Density Functional Theory calculations. By combining theoretical approaches with experimental observations, we explored two mechanisms that account for its stereospecificity, implying an inversion of the configuration at the phosphorus atom. One mechanism proceeds through a cyclic intermediate in which the coordination of borane to the oxygen atoms lowers the BH3.THF‐attack barrier. However, the free energy barrier for this combined process is too high for a reaction feasible at room temperature. The calculations rather support a second mechanism that proceeds through from the same cyclic intermediate and the in‐situ formation of BH4− as reducing agent. [ABSTRACT FROM AUTHOR]

The racemization mechanism of tert‐butylphenylphosphido‐borane is investigated experimentally and theoretically. Based on this converging approach, it is shown, first, that several phosphido‐borane molecular species coexist at the time of the reaction and, second, that one particular of both initially assumed reactive routes most significantly contribute to the overall racemization process. From our converging modeling and experimental measurement, it comes out that the most probable species to be here encountered is a phosphido‐borane‐Li (THF)2 neutral solvate, whose P‐stereogenic center monomolecular inversion through a Y‐shaped transition structure (ΔrG°≠: 81 kJ mol−1) brings the largest contribution to the racemization process. [ABSTRACT FROM AUTHOR]

Periodic density functional theory (DFT) calculations concerning VOx/κ-Al2O3(001) surfaces are used for parametrizing Monte Carlo simulations performed on a mesoscopic scale surface sample (41.95×48.80 nm2). New structural and chemical information are then obtained that are not accessible from DFT calculations: segregation, short- and long-range ordering, and the effect of the temperature on the reducibility. The reducibility of the surface is investigated for locating chemical potential regions where the catalyst is especially reactive for oxidation reactions. Comparison with the V2O5(001) surface at catalytic conditions (800 K, 1 bar) is performed. The reducibility exhibits an unexpectedly strong temperature dependence. [ABSTRACT FROM AUTHOR]

In this paper, we define the two-photon absorption strength, a new characterization tool, similar to the oscillator strength, but for two-photon absorption. It allows the quantification of the two-photon absorption properties of molecular systems which are one-photon transparent. Its definition is such that the corresponding numerical values are around 100 for small molecules. We also show that this new theoretical tool allows the direct comparison of experimental and theoretical data without requiring the introduction of any arbitrary band width. As an example, the experimental and theoretical (AM1+CNDO/S and HF+CIS/3-21G) two-photon absorption properties of the 2,2′-bi(9,9-dihexylfluorene) molecule are compared. [ABSTRACT FROM AUTHOR]

Abstract: The VO x /γ-Al2O3 catalyst VA-200, which was introduced in a previous article, is further characterized by XPS and visible Raman spectroscopy. The reoxidation of highly dispersed VO x species with gas phase oxygen is investigated in detail and is described by an empirical kinetic model. It is observed that the reoxidation of reduced VO x /γ-Al2O3 catalyst is strongly affected by the presence of water. The proposed kinetic model includes a distinct coverage of VO x species with water or hydroxyl groups in the investigated temperature range of 479–712K. Hydrated surface species are oxidized under release of water. Best fits of the experimental data can be achieved with first order rate laws with respect to oxygen concentration. Experiments are performed in an ideally mixed Berty-type reactor using oxygen step-marking over the reduced catalyst. The evolution of oxygen concentration using the kinetic model is in agreement with the experimentally observed behaviour. The signal of water released during the reaction can be modeled only qualitatively, which (presumably) stems from its sorption behaviour on the acidic alumina support. The oxidation of VO x species furthermore depends on the reaction temperature. The oxidation of V+III to V+V cannot be completely achieved at temperatures below 673K. However, the activation energy of this reaction is low, as suggested by the absence of strong variations of the response shapes with respect to temperature. [Copyright &y& Elsevier]

A new family of 2-hydroxyalk(en/yn)ylimidazoles has been evaluated as serine-histidine bare dyad models for the ring-opening reaction of L-lacOCA, a cyclic O-carboxyanhydride. These models were selected to unravel the implication of intramolecular hydrogen bonding and to substantiate its influence on the nucleophilicity of the alcohol moiety, as it is suspected to occur in enzyme active sites. Although designed to exclusively facilitate the preliminary step of proton transfer during the studied ring-opening reaction, these minimalistic models depicted a measureable increase in reactivity relative to the isolated fragments. A couple of reliable experimental and theoretical methods have been developed to readily monitor the strength of the intramolecular hydrogen bond in dilute solution. Results show that the folded conformers are the most nucleophilic species because of the intramolecular hydrogen bond., (Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

In this paper, we investigate the vibronic shape of the lowest UV-visible absorption band of biphenyl-like systems, using the bifluorene molecule as a workhorse. The molecule is here regarded as a one-dimensional two-level system, whose ground and excited states are simulated with time-dependent density functional theory and semiempirical methods. The vibrational Schrödinger equation is then numerically solved along the torsional coordinate, and the vibronic shape of the absorption band is modeled. Comparisons with the harmonic approximation, with or without the Franck-Condon approximation, are performed. This study confirms that a vibronic effect is most likely responsible for the strong dissymmetry of the lowest UV-visible absorption band of biphenyl-like systems and that, for such systems, the experimental data should be extracted using the whole absorption band, instead of a Gaussian fit on the first part of the band, as it is often done when a superposition between several electronic transitions is suspected.