Origin of diastereoselectivity in the synthesis of chiral bicyclic lactams: pi-facial selective attack of singlet oxygen induced by hindered internal rotation

Previously reported experimental results indicate that photooxygenation of homochiral N-(hydroxyalkyl)-2-methylpyrroles with singlet oxygen yields trans-rather than cis-bicyclic lactams as the major product. In this study, the origin of selectivity in this reaction has been investigated with computational methods. Relative stabilities of homochiral N-(hydroxyalkyl)-2-methylpyrrole conformers and their effect on pi-facial selectivity of 1O2 were extensively studied. Stepwise and concerted reaction mechanisms, starting from the endoperoxide intermediates, were proposed and modeled in vacuum using the UB3LYP method with the 6-31+G** basis set. Solvent calculations were carried out in CH2Cl2, by means of the integral equation formalism-polarizable continuum model (IEF-PCM) at the UB3LYP/6-31+G** level of theory. Free energies of activation leading to both diastereomers were analyzed in an effort to explain the stereoselectivity and product distribution. Steric interactions among the pyrrole substituents were shown to lead to a rotational barrier higher than 10 kcal/mol. Hence, hindered internal rotation is suggested to cause one pyrrole conformer to be substantially overpopulated. This in turn has a major effect on pi-facial selectivity of 1O2, thereby favoring one endoperoxide over the other and leading to the diastereoselective synthesis of trans-pyrrolooxazolones. The importance of hindered internal rotors, for an accurate calculation of the frequency factors of a chemical reaction, has already been mentioned in the literature many times; however, in this work hindered internal rotors also seem to dictate the diastereoselective outcome of the reaction.     

Three-dimensional mixed-mode stress intensity factors for cracks in functionally graded materials using enriched finite elements

Three-dimensional enriched finite elements are used to compute mixed-mode stress intensity factors (SIFs) for three-dimensional cracks in elastic functionally graded materials (FGMs) that are subject to general mixed-mode loading and constraint conditions. The method, which advantageously does not require special mesh configuration/modifications and post-processing of finite element results, is an enhancement of previous developments applied so far on isotropic homogeneous and isotropic interface cracks. The spatial variation of FGM material properties is taken into account at the level of element integration points. To validate the developed method, two- and three-dimensional mixed-mode fracture problems are selected from the literature for comparison. Two-dimensional cases include: inclined central crack in a large FGM medium under uniform tensile strain and stress loadings, a slanted crack in a finite-size FGM plate under exponentially varying tensile stress loading and an edge crack in a finite-size plate under shear traction load. The three-dimensional example models a deflected surface crack in a finite-size FGM plate under uniform tensile stress loading. Comparisons between current results and those from analytical and other numerical methods yield good agreement. Thus, it is concluded that the developed three-dimensional enriched finite elements are capable of accurately computing mixed-mode fracture parameters for cracks in FGMs.     

Modelling of 3‐D Linear Viscoelastic Swelling of Charged Tissues and Gels

The skeleton of hydrated tissues or gels exhibits flow‐independent viscoelastic properties [2] which are strongly coupled with the dissipative phenomena resulting from the interstitial fluid flow and the electrochemical swelling mechanisms [4]. Following this, it is the goal of this contribution to combine a linear viscoelasticity formulation with an electrochemical swelling theory in the framework of a well‐founded multiphasic concept. Proceeding from a macroscopic mixture approach, the governing equations can be expressed in terms of three primary variables, namely the solid displacement u _S, the effective pore‐fluid pressure p and the molar salt concentration c_m of the interstitial fluid.     

Poly(iminoborane)s: An Elusive Class of Main‐Group Polymers?

The significance of inorganic main‐group polymers is demonstrated most clearly by the commercial relevance of polysiloxanes (silicones). Organoboron‐based materials such as π‐conjugated organoborane polymers and BN‐doped polycyclic aromatic hydrocarbons are currently attracting considerable attention. Surprisingly, poly(iminoborane)s (PIBs; [BRNR′]_n), that is, the parent unsaturated BN polymers, which are formally isoelectronic to polyacetylene, have not been convincingly characterized thus far. Herein, we present the synthesis and comprehensive characterization of a linear oligo(iminoborane), which comprises a chain of 12–14 BN units on average. With our synthetic approach, unwanted side reactions that result in borazine formation are effectively suppressed. Supporting DFT and TD‐DFT calculations provide deeper insight into the microstructure and the electronic structure of the oligomer.     

[N,N′‐Bis(5‐bromo­salicyl­idene)‐1,3‐di­amino­propane]­nickel(II) and [N,N′‐­bis(5‐chloro­salicyl­idene)‐1,3‐­di­amino­propane]copper(II)

The title compounds, {4,4′‐di­bromo‐2,2′‐[1,3‐propane­diyl­bis(nitrilo­methyl­idyne‐N)]­diphenolato‐O,O′}nickel(II), [Ni(C₁₇­H₁₄­Br₂­N₂O₂)], and {4,4′‐di­chloro‐2,2′‐[1,3‐pro­pane­diyl­bis­(ni­trilo­methyl­idyne‐N)]­di­phen­ol­ato‐O,O′}­copper(II), [Cu­(C₁₇­H₁₄­Cl₂­N₂O₂)], lie on crystallographic twofold axes. In both structures, the metal coordination sphere is a tetrahedrally distorted square plane formed by the four‐coordinate N₂O₂ donor set of the Schiff base imine–phenol ligands. In the Ni compound, the Ni—O and Ni—N distances are 1.908 (3) and 1.959 (4) Å, respectively, while in the Cu compound, the Cu—O and Cu—N distances are 1.907 (2) and 1.960 (2) Å, respectively. The two Schiff base moieties, which themselves are nearly planar, are inclined at an angle of 29.26 (7)° for the Ni compound and 29.26 (5)° for the Cu compound.     

Experimental study and large Eddy simulation of a coaxial jet with perforated obstacles to control thermal mixing characteristics

Thermal mixing phenomena of a coaxial jet with perforated obstacles are analyzed both experimentally and numerically. Perforated obstacles are used in front of a coaxial jet that has different temperatures of the fluid to control the thermal mixing behavior. An experimental set-up was constructed to perform several cases and all cases are simulated by using large eddy simulation (LES) turbulence model. Results of the study presented that inserting perforated obstacle affects the mixing performance positively and the best mixing performance is obtained in the case that the obstacle has the highest permeability. Experimental and computational results were compared and a good agreement was obtained.     

Explicit solutions of nonlinear wave equation systems

We apply the (G’/G)-expansion method to solve two systems of nonlinear differential equations and construct traveling wave solutions expressed in terms of hyperbolic functions, trigonometric functions, and rational functions with arbitrary parameters. We highlight the power of the (G’/G)-expansion method in providing generalized solitary wave solutions of different physical structures. It is shown that the (G’/G)-expansion method is very effective and provides a powerful mathematical tool to solve nonlinear differential equation systems in mathematical physics.     

Determination of tocopherol contents of some olive varieties harvested at different ripening periods

The tocopherol contents of oils obtained from Ayval?k, Domat and Gemlik olive varieties harvested at different ripening periods were evaluated by high-performance liquid chromatography. α-Tocopherol was the major tocol detected in all the studied olive oil samples. The oils extracted from olive fruits composed of 130.54-180.43?mg?kg?1 α-tocopherol, 0.73-1.61?mg?kg?1 β-tocopherol and 0.53-2.28?mg?kg?1 γ-tocopherol for Ayval?k oil. The α-, β- and γ-tocopherol contents of Domat oil are in the ranges 95.60-125.56, 0.71-2.70 and 0.49-1.25?mg?kg?1 at different harvesting periods. The α-, β- and γ-tocopherol contents of Gemlik olive oil are in the ranges 112.59-168.19, 0.94-1.21 and 0.85-2.40?mg?kg?1, respectively. There were significant differences between the oils from cultivars grown in different environments.     

Refractive index changes and absorption coefficients in a spherical quantum dot with parabolic potential

In this study, we have calculated the linear, nonlinear and total refractive index changes and absorption coefficients for the transitions 1s–1p, 1p–1d and 1d–1f in a spherical quantum dot with parabolic potential. Quantum Genetic Algorithm (QGA) and Hartree–Fock–Roothaan (HFR) method have been employed to calculate the wavefuctions and energy eigenvalues. The results show that impurity, dot radius, stoichiometric ratio, incident optical intensity and carrier density of the system have important effects on the optical refractive index changes and absorption coefficients. Also, we find that as the transitions between orbitals with big l value move to lower energy region in case with parabolic potential, in case without parabolic potential these transitions move to higher energy region.